Ansi Agma 1010-f14-14.pdf

ANSI/AGMA 1 01 0-F1 4

(Revi si on of AN SI /AG M A 1 01 0-E95)

American National Standard Appearan ce of G ear Teeth - Term i n ol og y of

ANSI/AGMA 1 01 0-F1 4

Wear an d Fai l u re

Copyright American Gear Manufacturers Association

AMERICAN NATIONAL STANDARD

American National Standard

ANSI/AGMA 1 01 0-F1 4

Appearance of Gear Teeth - Terminology of Wear and Failure

ANSI/AGMA 1 01 0-F1 4 [Revision of ANSI/AGMA 1 01 0-E95] Approval of an American National Standard requires verification by ANSI that the requirements for due process, consensus and other criteria for approval have been met by the standards developer. Consensus is established when, in the judgment of the ANSI Board of Standards Review, substantial agreement has been reached by directly and materially affected interests. Substantial agreement means much more than a simple majority, but not necessarily unanimity. Consensus requires that all views and objections be considered, and that a concerted effort be made toward their resolution. The use of American National Standards is completely voluntary; their existence does not in any respect preclude anyone, whether he has approved the standards or not, from manufacturing, marketing, purchasing or using products, processes or procedures not conforming to the standards. The American National Standards Institute does not develop standards and will in no circumstances give an interpretation of any American National Standard. Moreover, no person shall have the right or authority to issue an interpretation of an American National Standard in the name of the American National Standards Institute. Requests for interpretation of this standard should be addressed to the American Gear Manufacturers Association. CAUTION NOTICE : AGMA technical publications are subject to constant improvement, revision or withdrawal as dictated by experience. Any person who refers to any AGMA Technical Publication should be sure that the publication is the latest available from the Association on the subject matter. [Tables or other self-supporting sections may be referenced. Citations should read: See ANSI/AGMA 1 01 0-F1 4, Appearance of Gear Teeth - Terminology of Wear and Failure, published by the American Gear Manufacturers Association, 1 001 N. Fairfax Street, Suite 500, Alexandria, Virginia 2231 4, http://www.agma.org.] Approved August 8, 201 4 ABSTRACT This nomenclature standard identifies and describes the classes of common gear failures and illustrates degrees of deterioration. Published by American Gear Manufacturers Association 1 001 N. Fairfax Street, Suite 500, Alexandria, Virginia 2231 4 Copyright © 201 4 by American Gear Manufacturers Association All rights reserved. No part of this publication may be reproduced in any form, in an electronic retrieval system or otherwise, without prior written permission of the publisher. Printed in the United States of America ISBN: 978-1 -61 481 -089-6

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Contents Foreword ...................................................................................................................................................... vii 1 Scope ..................................................................................................................................................... 1 2 Normative references ............................................................................................................................. 1 3 Definitions .............................................................................................................................................. 1 3.1 Definitions ...................................................................................................................................... 1 3.2 Classes and modes of failure ........................................................................................................ 2 4 Wear....................................................................................................................................................... 3 4.1 Adhesion ....................................................................................................................................... 3 4.1 .1 Mild adhesion ............................................................................................................................ 4 4.1 .2 Moderate adhesion ................................................................................................................... 4 4.1 .3 Summary of methods to reduce the risk of adhesive wear ....................................................... 5 4.2 Abrasion ........................................................................................................................................ 5 4.2.1 Mild abrasion ............................................................................................................................. 5 4.2.2 Moderate abrasion .................................................................................................................... 6 4.2.3 Severe abrasion ........................................................................................................................ 6 4.2.4 Sources of particles that may cause wear ................................................................................ 8 4.2.5 Methods for reducing abrasive wear ......................................................................................... 8 4.3 Polishing ........................................................................................................................................ 9 4.3.1 Mild polishing ............................................................................................................................. 9 4.3.2 Moderate polishing .................................................................................................................... 9 4.3.3 Severe polishing ........................................................................................................................ 9 4.3.4 Summary of methods to reduce the risk of polishing wear ..................................................... 1 0 4.4 Corrosion ..................................................................................................................................... 1 0 4.4.1 Methods to reduce the risk of corrosion .................................................................................. 1 1 4.5 Fretting ........................................................................................................................................ 1 2 4.5.1 True brinelling .......................................................................................................................... 1 2 4.5.2 False brinelling ........................................................................................................................ 1 3 4.5.3 Fretting corrosion .................................................................................................................... 1 3 4.5.4 Summary of methods to reduce the risk of false brinelling and fretting corrosion .................. 1 3 4.6 Scaling ......................................................................................................................................... 1 4 4.7 White layer flaking ....................................................................................................................... 1 4 4.7.1 Summary of methods to reduce the risk of white layer flaking ............................................... 1 5 4.8 Cavitation .................................................................................................................................... 1 5 4.9 Erosion ........................................................................................................................................ 1 7 4.1 0 Electric discharge ........................................................................................................................ 1 8 4.1 0.1 Summary of methods to reduce the risk of electrical discharge damage ........................... 21 5 Scuffing ................................................................................................................................................ 21 5.1 Mild scuffing ................................................................................................................................ 21 5.2 Moderate scuffing ........................................................................................................................ 21 5.3 Severe scuffing ............................................................................................................................ 23 5.3.1 Methods for reducing the risk of scuffing ................................................................................ 25 5.3.2 Summary of methods to reduce the risk of scuffing ................................................................ 26 6 Plastic deformation .............................................................................................................................. 26 6.1 Indentation ................................................................................................................................... 26 6.2 Cold flow...................................................................................................................................... 27 6.3 Hot flow ....................................................................................................................................... 27 6.4 Rolling ......................................................................................................................................... 27 ©AGMA 201 4 – All rights reserved

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6.5 Tooth hammer ............................................................................................................................. 27 6.6 Rippling ....................................................................................................................................... 27 6.7 Ridging ........................................................................................................................................ 30 6.8 Burr.............................................................................................................................................. 30 6.9 Root fillet yielding ........................................................................................................................ 31 6.1 0 Tip-to-root interference ................................................................................................................ 31 6.1 1 Tight mesh ................................................................................................................................... 31 7 Hertzian fatigue .................................................................................................................................... 32 7.1 Macropitting ................................................................................................................................. 32 7.1 .1 Nonprogressive macropitting .................................................................................................. 32 7.1 .2 Progressive macropitting ......................................................................................................... 34 7.1 .3 Point-surface-origin macropitting ............................................................................................ 34 7.1 .4 Spall macropitting .................................................................................................................... 37 7.2 Micropitting .................................................................................................................................. 39 7.2.1 Summary of methods to reduce the risk of micropitting .......................................................... 43 7.3 Subsurface initiated failures ........................................................................................................ 44 7.3.1 Inclusion origin failures ............................................................................................................ 44 7.3.2 Origins of nonmetallic inclusions ............................................................................................. 44 7.4 Subcase fatigue........................................................................................................................... 45 7.4.1 Summary of methods to reduce the risk of subcase fatigue ................................................... 46 8 Cracking and other surface damage .................................................................................................... 46 8.1 Hardening cracks ........................................................................................................................ 46 8.1 .1 Thermal stresses ..................................................................................................................... 47 8.1 .2 Stress concentration ............................................................................................................... 47 8.1 .3 Quench severity ...................................................................................................................... 47 8.1 .4 Phase transformation .............................................................................................................. 48 8.1 .5 Steel grades ............................................................................................................................ 48 8.1 .6 Part defects ............................................................................................................................. 48 8.1 .7 Heat treating practice .............................................................................................................. 48 8.1 .8 Tempering practice ................................................................................................................. 48 8.1 .9 Summary of methods to reduce the risk of hardening cracks ................................................. 48 8.2 Grinding damage ......................................................................................................................... 49 8.2.1 Grinding cracks ....................................................................................................................... 49 8.2.2 Overheating due to grinding .................................................................................................... 49 8.2.3 Summary of methods to reduce the risk of grinding cracks .................................................... 50 8.3 Rim and web cracks .................................................................................................................... 50 8.3.1 Summary of methods to reduce the risk of rim or web cracks ................................................ 50 8.4 Case/core separation .................................................................................................................. 52 8.4.1 Summary of methods to reduce the risk of case/core separation ........................................... 54 8.5 Fatigue cracks ............................................................................................................................. 54 9 Fracture ................................................................................................................................................ 55 9.1 Brittle fracture .............................................................................................................................. 55 9.1 .1 Methods for reducing the risk of brittle fracture ....................................................................... 58 9.2 Ductile fracture ............................................................................................................................ 58 9.3 Mixed mode fracture ................................................................................................................... 59 9.4 Tooth shear ................................................................................................................................. 59 9.5 Fracture after plastic deformation ............................................................................................... 59 ©AGMA 201 4 – All rights reserved

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Bending fatigue ............................................................................................................................... 61 10 1 0.1 Low cycle fatigue ......................................................................................................................... 62 1 0.2 High cycle fatigue ........................................................................................................................ 62 1 0.2.1 Morphology of fatigue fracture surfaces .............................................................................. 63 1 0.2.2 Summary of methods to reduce the risk of high-cycle bending fatigue .............................. 64 1 0.2.3 Root fillet cracks .................................................................................................................. 65 1 0.2.4 Profile cracks ....................................................................................................................... 65 1 0.2.5 Tooth end cracks ................................................................................................................. 66 1 0.2.6 Subsurface initiated bending fatigue cracks ....................................................................... 66 1 0.2.7 Tooth interior fatigue fracture, TIFF .................................................................................... 73 Annexes Annex A Design considerations to reduce the chance of failure ................................................................ 75 Annex B Bibliography .................................................................................................................................. 79 Annex C Acknowledgements ...................................................................................................................... 81 Tables Table 1 - Nomenclature of gear failure modes .............................................................................................. 2 Table 2 - Failure modes that have subsurface origins ................................................................................ 44 Table 3 - Fracture classifications ................................................................................................................ 55 Table 4 - Differences between TIFF and subsurface initiated bending fatigue .......................................... 74 Figures Figure 1 - Moderate wear .............................................................................................................................. 3 Figure 2 - Severe wear.................................................................................................................................. 4 Figure 3 - SEM image - abrasion .................................................................................................................. 6 Figure 4 - Mild abrasion near the tip of a ground gear.................................................................................. 6 Figure 5 - Severe abrasion ............................................................................................................................ 7 Figure 6 - Severe abrasion, enlarged view of Figure 5 ................................................................................. 7 Figure 7 - Severe abrasion ............................................................................................................................ 7 Figure 8 - Severe polishing ......................................................................................................................... 1 0 Figure 9 - Severe polishing ......................................................................................................................... 1 0 Figure 1 0 - Extensive corrosion .................................................................................................................. 1 1 Figure 1 1 - Fretting corrosion ...................................................................................................................... 1 2 Figure 1 2 - Scaling ...................................................................................................................................... 1 4 Figure 1 3 - White layer flaking .................................................................................................................... 1 5 Figure 1 4 - Cavitation damage .................................................................................................................... 1 6 Figure 1 5 - Cavitation damage .................................................................................................................... 1 6 Figure 1 6 - SEM image - cavitation damage ............................................................................................... 1 7 Figure 1 7 - SEM image - cavitation damage ............................................................................................... 1 7 Figure 1 8 - Erosion of a high speed helical gear ........................................................................................ 1 8 Figure 1 9 - Electric discharge damage due to a small electric current ....................................................... 1 9 Figure 20 - Severe electric discharge damage due to an electric current of high intensity ........................ 1 9 Figure 21 - SEM image - typical electric discharge crater .......................................................................... 20 Figure 22 - SEM image - remelted metal and gas pockets near edge of crater ......................................... 20 Figure 23 - SEM image - electric discharge damage .................................................................................. 21 Figure 24 - Mild scuffing .............................................................................................................................. 22 Figure 25 - SEM image - scuffing damage showing rough, torn, and plastically deformed appearance .. 22 Figure 26 - SEM image - scuffing damage showing crater formed when welded material was torn from surface .............................................................................................................................. 23 Figure 27 - Moderate scuffing ..................................................................................................................... 23 Figure 28 - Severe scuffing ......................................................................................................................... 24 Figure 29 - Severe scuffing of a low speed gear lubricated with grease .................................................... 24 Figure 30 - Severe indentations .................................................................................................................. 27 Figure 31 - Hot flow ..................................................................................................................................... 28 Figure 32 - Plastic deformation by rolling .................................................................................................... 28 Figure 33 - Plastic deformation by tooth hammer ....................................................................................... 29 Figure 34 - Rippling ..................................................................................................................................... 29 ©AGMA 201 4 – All rights reserved

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Figure 35 - Rippling ..................................................................................................................................... 29 Figure 36 - Rippling ..................................................................................................................................... 30 Figure 37 - Ridging ...................................................................................................................................... 30 Figure 38 - Burr ........................................................................................................................................... 31 Figure 39 - Tip-to-root interference ............................................................................................................. 32 Figure 40 - Cross section through a tooth flank showing how a pit develops below the surface ............... 32 Figure 41 - SEM image - pitting damage caused by Hertzian fatigue, showing fatigue cracks near boundary of pit .......................................................................................................................... 33 Figure 42 - Nonprogressive macropitting .................................................................................................... 33 Figure 43 - Progressive macropitting .......................................................................................................... 34 Figure 44 - Point-surface-origin macropitting .............................................................................................. 34 Figure 45 - Point-surface-origin macropitting .............................................................................................. 35 Figure 46 - Point-surface-origin macropitting .............................................................................................. 35 Figure 47 - Point-surface-origin macropitting on carburized helical gear at 1 .5 × 1 0 77 cycles ..................... 36 Figure 48 - Point-surface-origin macropitting on carburized helical gear at 3.0 × 1 0 cycles ..................... 36 Figure 49 - Point-surface-origin macropitting on carburized helical driven pinion ...................................... 37 Figure 50 - Point-surface-origin macropitting .............................................................................................. 37 Figure 51 - Spall macropitting ..................................................................................................................... 38 Figure 52 - Micropitting on misaligned carburized gear .............................................................................. 39 Figure 53 - Micropitting on induction hardened spur gear with crowned teeth ........................................... 39 Figure 54 - Micropitting on nitrided and ground spur gear .......................................................................... 40 Figure 55 - Detail of tooth surface showing micropitting ............................................................................. 40 Figure 56 - Detail of tooth surface showing micropitting at 1 000X magnification ....................................... 41 Figure 57 - Regularly distributed micropitting ............................................................................................. 41 Figure 58 - Subcase fatigue ........................................................................................................................ 45 Figure 59 - Crack at a forging defect .......................................................................................................... 46 Figure 60 - Hardening cracks ...................................................................................................................... 47 Figure 61 - Grinding cracks with a crazed pattern ...................................................................................... 49 Figure 62 - Rim crack .................................................................................................................................. 51 Figure 63 - Rim cracks in through hardened annulus gear......................................................................... 51 Figure 64 - Fracture surface of rim crack shown in Figure 63 .................................................................... 52 Figure 65 - Case/core separation ............................................................................................................... 52 Figure 66 - Case/core separation ............................................................................................................... 53 Figure 67 - Bending fatigue crack ............................................................................................................... 54 Figure 68 - Brittle fracture ........................................................................................................................... 56 Figure 69 - SEM image of transgranular brittle fracture .............................................................................. 56 Figure 70 - SEM image of intergranular brittle fracture ............................................................................... 57 Figure 71 - SEM image of ductile fracture .................................................................................................. 59 Figure 72 - Mixed mode fracture ................................................................................................................. 60 Figure 73 - Tooth shear............................................................................................................................... 60 Figure 74 - Fracture after plastic deformation ............................................................................................. 61 Figure 75 - Two adjacent teeth on a helical pinion that failed by bending fatigue ...................................... 63 Figure 76 - Bending fatigue of spiral bevel tooth ........................................................................................ 64 Figure 77 - Bending fatigue of two helical teeth .......................................................................................... 65 Figure 78 - Bending fatigue of several spur gear teeth ............................................................................... 66 Figure 79 - Bending fatigue of two bevel pinion teeth ................................................................................. 67 Figure 80 - Fatigue of several teeth that were loaded on both flanks ......................................................... 68 Figure 81 - Profile cracks originating from severe pitting ............................................................................ 69 Figure 82 - Broken tooth ends .................................................................................................................... 69 Figure 83 - Bending fatigue initiation from subsurface nonmetallic inclusion ............................................. 70 Figure 84 - Bending fatigue due to nonmetallic inclusion ........................................................................... 70 Figure 85 - Fracture surface of loose fragment showing nonmetallic inclusion .......................................... 71 Figure 86 - BSE image of fracture surface showing scanned areas 1 , 2, and 3 ........................................ 71 Figure 87 - EDS spectrum of figure 86 area 1 showing chemistry of the inclusion .................................... 72 Figure 88 - EDS spectrum of figure 86 area 3 showing chemistry of the steel matrix ................................ 72 Figure 89 - TIFF failure on an idler gear ..................................................................................................... 73

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ANSI/AGMA 1 01 0-F1 4

Foreword [The foreword, footnotes and annexes, if any, in this document are provided for informational purposes only and are not to be construed as a part of ANSI/AGMA 1 01 0-F1 4, Appearance of Gear Teeth Terminology of Wear and Failure. ] This standard provides a means to describe the appearance of gear teeth when they wear or fail. The study of gear tooth wear and failure has been hampered by the inability of two observers to describe the same phenomenon in terms that are adequate to assure uniform interpretation. The term “gear failure” is subjective and a source of considerable disagreement. For example, a person observing gear teeth that have a bright, mirrorlike appearance may believe that the gears have “run-in” properly. However, another observer may believe that the gears have failed by polishing wear. Whether the gears should be considered failed or not depends on how much change from original condition is tolerable. This standard provides a common language to describe gear wear and failure, and serves as a guide to uniformity and consistency in the use of that language. It describes the appearance of gear tooth failure modes and discusses their mechanisms, with the sole intent of facilitating identification of gear wear and failure. The purpose of the standard is to improve communication between equipment users and gear manufacturers for failure and wear analysis. Since there may be many different causes for each type of gear tooth wear or failure, it is not possible in the standard to identify a single cause for each type of wear or failure, nor to prescribe remedies. AGMA Standard 1 1 0 was first published in 1 943. A revised standard, AGMA 1 1 0.03, was published in 1 979 with improved photographs and additional material. AGMA 1 1 0.04 was reaffirmed by the members in 1 989. ANSI/AGMA 1 01 0-E95 was a revision of AGMA 1 1 0.04. It was approved by the AGMA Membership in March 9, 1 995. It was approved as an American National Standard on December 1 3, 1 995. ANSI/AGMA 1 01 0-F1 4 is a revision of ANSI/AGMA 1 01 0-E95. It merges ANSI/AGMA 1 01 0-E95 and AGMA 91 2-A04. New failure modes and additional photos were added and the content was reorganized. The description of failure mode morphology and mechanism was expanded, and methods to reduce the risk of a particular failure mode were added to the description of many of the failure modes. The first draft of ANSI/AGMA 1 01 0-F1 4 was made in August, 201 0. It was approved by the AGMA membership in June, 201 4. It was approved as an American National Standard on August 8, 201 4. Suggestions for improvement of this standard will be welcome. [email protected].

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PERSONNEL of the AGMA Nomenclature Committee Chairman: Dwight Smith ..................................... Cole Manufacturing Systems Vice Chairman: J.M. Rinaldo ............................... Atlas Copco Comptec, LLC

ACTIVE MEMBERS J.B. Amendola, III ................................................. Artec Machine Systems K. Burris ................................................................ Caterpillar, Inc. R.L. Errichello ....................................................... Geartech O.A. LaBath .......................................................... Gear Consulting Services of Cincinnati, LLC M. Li ...................................................................... Lufkin Industries, Inc. P. Terry................................................................. P. Terry & Associates

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American National Standard -

Appearance of Gear Teeth - Terminology of Wear and Failure 1

Scope

This standard provides nomenclature for general modes of gear tooth wear and failure. It classifies, identifies, and describes the most common types of failure and provides information that will, in many cases, enable the user to identify failure modes and evaluate the degree or change from original condition. This standard is based on experience with steel gears; however, many of the failure modes discussed may apply to gears made from other materials. The solution to many gear problems requires detailed investigation and analysis by specialists and is beyond the scope and intent of this standard. This standard does not define “gear failure”. One observer's “failure” is another observer's “run-in”. There is no single definition of gear failure, since whether or not a gear has failed depends on the specific application. The methods given for reducing the risk of a failure mode are specific to the failure mode considered, and implementation may sometimes worsen, or create other failure modes or unintended consequences. Therefore, it is imperative that any remedy be evaluated prior to implementation and thoroughly tested and evaluated after implementation.

NOTE:

2

“gear” throughout the standard means gear or pinion unless the gear is specifically identified.

Normative references

The following documents contain provisions which, through reference in this text, constitute provisions of this standard. At the time of publication, the editions were valid. All publications are subject to revision, and the users of this standard are encouraged to investigate the possibility of applying the most recent editions of the publications listed: AGMA 901 -A92, A Rational Procedure for the Preliminary Design of Minimum Volume Gears AGMA 923-B05, Metallurgical Specifications for Steel Gearing ANSI/AGMA 1 01 2-G05, Gear Nomenclature, Definitions of Terms with Symbols ANSI/AGMA/AWEA 6006-A03, Standard for Design and Specification of Gearboxes for Wind Turbines ANSI/AGMA 601 1 -I03, Specification for High Speed Helical Gear Units ANSI/AGMA 601 3-A06, Standard for Industrial Enclosed Gear Drives ANSI/AGMA 9005-E02, Industrial Gear Lubrication ISO 1 41 04, Gears - Surface temper etch inspection after grinding

3

Definitions

3.1

Definitions

The terms used in this standard, wherever applicable, conform to the definitions given in ANSI/AGMA 1 01 2-G05 and AGMA 923-B05.

NOTE: The symbols and definitions used in this standard may differ from other AGMA Standards. The user should not assume that familiar symbols can be used without a careful study of these definitions.

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3.2

ANSI/AGMA 1 01 0-F1 4

Classes and modes of failure

Tabl e 1 g rou ps th e com mon m od es of g ear fai l u re i n to seven g en eral cl asses an d su bd i vi d es th e g en eral cl asses i n to g en eral an d speci fi c m od es. I t al so i n cl u d es com m on l y u sed , bu t n on -preferred n am es.

Table 1 - Nomenclature of gear failure modes Class Wear

Clause 4. 1

General mode Ad h esi on

Specific mode or degree

Non-preferred terminology

Mild

N orm al , ru n n i n g -i n wear

M od erate

Teari n g , M i crowel d i n g

Severe (see scu ffi n g )

Scori n g Scratch i n g

4. 2

Abrasi on

M i l d , M od erate, Severe

4. 3

Pol i sh i n g

M i l d , M od erate, Severe

4. 4

Corrosi on

4. 5

Fretti n g

Cu tti n g Bu rn i sh i n g

Tru e bri n el l i n g Fal se bri n el l i n g Fretti n g corrosi on

Scu ffi n g

4. 6

Scal i n g

4. 7

Wh i te l ayer fl aki n g

4. 8

Cavi tati on

4. 9

Erosi on

4. 1 0

El ectri cal d i sch arg e

5

Scu ffi n g

Arci n g

M i l d , M od erate, Severe

Scori n g Col d scu ffi n g H ot scu ffi n g Wel d i n g , M i croweld i n g G al l i n g Sei zi n g

Pl asti c

6. 1

Pl asti c d eform ati on

I n d en tati on

d eformati on

Bru i si n g Peen i n g Den ti n g Tru e bri n el l i n g

6. 2

Col d fl ow

Perm an en t d eform ati on

6. 3

H ot fl ow

Overh eati n g

6. 4

Rol l i n g

6. 5

Tooth h am mer

6. 6

Ri ppl i n g

6. 7

Ri d g i n g

6. 8

Bu rr

6. 9

Root fi l l et yi el d i n g

6. 1 0

Ti p-to-root i n terferen ce

6. 1 1 H ertzi an

7. 1

Fi sh scal i n g

Ti g h t m esh M acropi tti n g

fati g u e

N on prog ressi ve

Con tact fati g u e, i n i ti al

Prog ressi ve

Destru cti ve

Poi n t-Su rface-Ori g i n

Arrowh ead

Spal l 7. 2

M i cropi tti n g

Frosti n g G ray stai n i n g Peel i n g

7. 3

Su bsu rface i n i ti ated fai l u res

Cracki n g

7. 4

Su bcase fati g u e

Case cru sh i n g

8. 1

H ard en i n g cracks

Qu en ch i n g cracks

8. 2

G ri n d i n g d am ag e

G ri n d i n g bu rn

8. 3

Ri m an d web cracks

8. 4

Case/core separati on

8. 5

Fati g u e cracks

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I n tern al ru ptu re

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Class Fractu re

Clause

ANSI/AGMA 1 01 0-F1 4

General mode

Specific mode or degree

Non-preferred terminology

9. 1

Bri ttl e fractu re

Fast fractu re

9. 2

Du cti l e fractu re

Sm eari n g

9. 3

M i xed m od e fractu re

Sem i -bri ttl e

9. 4

Tooth sh ear

9. 5

Fractu re after pl asti c d eformati on

Ben d i n g

1 0. 1

Low-cycl e fati g u e

fati g u e

1 0. 2. 3

H i g h -cycl e fati g u e

Root fi l l et cracks

1 0. 2. 4

Profi l e cracks

1 0. 2. 5

Tooth en d cracks

1 0. 2. 6

Su bsu rface-i n i tated

Tooth fl an k fractu re

ben d i n g fati g u e cracks Tooth i n teri or fati g u e

1 0. 2. 7

fractu re (TI FF)

4

Wear

Wear i s a term d escri bi n g ch an g e to a g ear tooth su rface i n vol vi n g th e rem oval or d i spl acem en t of m ateri al , d u e to m ech an i cal , ch em i cal , or el ectri cal acti on . Fi g u res 1 an d 2 sh ow m od erate an d severe wear.

Th ey are n ot i n ten d ed to i n d i cate th e m od e of wear.

Wear can be categ ori zed as m i l d , m od erate or severe.

I n some appl i cati on s, n o wear i s acceptabl e.

H owever, i n m an y oth er appl i cati on s mi l d wear i s con si d ered n orm al .

M od erate an d someti m es even

severe wear m ay be acceptabl e i n some appl i cati on s.

4.1

Adhesion

Ad h esi on i s cau sed by tran sfer of m ateri al from on e tooth su rface to an oth er d u e to m i crowel d i n g an d teari n g . Ad h esi on can be categ ori zed as m i l d or m od erate i f i t i s con fi n ed to su rface fi l m s an d oxi d e l ayers on th e tooth su rface.

I f, h owever, th e oxi d e l ayers are d i sru pted an d bare m etal i s exposed , th e tran si ti on to

severe ad h esi ve wear (scu ffi n g ) m ay occu r.

Scu ffi n g i s d i scu ssed i n cl au se 5.

Figure 1 - Moderate wear Copyright American Gear Manufacturers Association ©AG M A 201 4 – Al l ri g h ts reserved

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ANSI/AGMA 1 01 0-F1 4

Figure 2 - Severe wear 4.1 .1

Mild adhesion

M i l d ad h esi on typi cal l y occu rs d u ri n g ru n n i n g -i n an d u su al l y su bsi d es after i t h as sm ooth ed th e tooth su rfaces by rem ovi n g m i n or i m perfecti on s th rou g h l ocal i zed wear. appears

u n d am ag ed

an d

th e

ori g i n al

m ach i n i n g

m arks

To th e u n ai d ed eye, th e tooth su rface

are

vi si bl e.

M i croscopi cal l y,

sm ooth

m i cropl ateau s can be seen between th e m ach i n i n g fu rrows.

4.1 .2 Moderate adhesion Ad h esi on i s cl assi fi ed as m od erate i f i t rem oves som e or al l of th e ori g i n al mach i n i n g m arks from th e acti ve su rface of th e tooth .

U n d er certai n con d i tion s, ad h esi on m ay cau se con ti n u ou s rem oval of su rface

fi l m s an d oxi d e l ayers, resu l ti n g i n severe wear [1 ] . Wh en n ew g ear u n i ts are fi rst operated th e con tact between th e g ear teeth may n ot be opti m u m becau se of m an u factu ri n g i n accu raci es.

I f th e tri bol og i cal con d i ti on s are favorabl e, m i l d ad h esi ve wear occu rs

d u ri n g ru n -i n an d su bsi d es wi th ti m e, resu l ti n g i n a sati sfactory l i feti m e for th e g ears.

Th e wear th at

occu rs d u ri n g ru n -i n i s ben efi ci al i f i t creates sm ooth tooth su rfaces (i n creasi n g th e speci fi c fi l m th i ckn ess) an d i n creases th e area of con tact by rem ovi n g m i n or i m perfecti on s th rou g h l ocal wear. perform ed i n accord an ce wi th th e m an u factu rer’ s recom men d ati on s. com bi n ati on of parti al l oad an d su ffi ci en t ti m e.

Ru n -i n sh ou l d be

An effecti ve ru n -i n req u i res a proper

Fol l owi n g ru n -i n , th e l u bri can t sh ou l d be d rai n ed an d th e

g earbox fl u sh ed to rem ove wear d ebri s, an d th e fi l ter, i f presen t, ch an g ed before refi l l i n g th e g earbox wi th th e recom m en d ed l u bri can t. An al tern ate i s to u se an extern al pu ri fi er to cl ean th e oi l .

See AN SI /AG M A

601 3-A06 cl au se 1 1 . 6. 1 , AN SI /AG M A 601 1 -I 03 cl au se 6. 4 an d AN SI /AG M A/AWEA 6006-A03 cl au se 6. 7. Th e am ou n t of wear th at i s con si d ered tol erabl e d epen d s on th e operati n g speed an d expected l i feti m e for th e g ears an d on th e req u i rem en ts for th e con trol of n oi se an d vi brati on .

Th e wear i s con si d ered

excessi ve wh en th e tooth profi l es wear to th e exten t th at h i g h d yn am i c l oad s are en cou n tered , or th e tooth th i ckn ess i s red u ced to th e exten t th at tooth fai l u re by ben d in g fati g u e becom es i m m i n en t, or th e g ears

g en erate

excessi ve

n oi se

or vi brati on .

M an y g ears,

becau se

of practi cal

l i m i ts

on

l u bri can t

vi scosi ty, speed an d tem peratu re, m u st operate u n d er bou n d ary-l u bri cated con d i ti on s wh ere some wear i s i n evi tabl e.

Referen ce [1 ] i n d i cates th at h i g h l y-l oad ed , sl ow speed (l ess th an 0. 5 m /s pi tch l i n e vel oci ty),

bou n d ary-l u bri cated g ears are especi al l y pron e to excessi ve wear.

Th e probl em can al so affect g ears

operati n g at u p to 2 m /s pi tch l i n e vel oci ty. Tests wi th sl ow-speed g ears [1 ] h ave sh own th at n i tri d ed g ears h ave g ood wear resi stan ce wh ereas carbu ri zed an d th rou g h -h ard en ed g ears h ave si m i l ar, l ower wear resi stan ce. ad h esi ve wear.

Referen ce [1 ] con cl u d ed th at l u bri can t vi scosi ty h as a l arg e i n fl u en ce on sl ow-speed , I t fou n d th at h i g h vi scosi ty l u bri can ts red u ce th e wear rate si g n i fi can tl y.

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I t al so fou n d th at

4

AMERICAN NATIONAL STANDARD som e

ch em i cal l y

ag g ressi ve

ad d i ti ves

ANSI/AGMA 1 01 0-F1 4 th at

con tai n

su l ph u r-ph osph orou s

an ti scu ff

ad d i ti ves

can

d etri m en tal wi th very sl ow-speed (l ess th an 0. 05 m /s) g ears, g i vi n g h i g h er wear rates th an expected . probl em can al so affect g ears wi th u p to 2 m /s pi tch l i n e vel oci ty.

be

Th i s

I n som e cases wi th l ow speed s,

ad h esi on m ay l ook l i ke pol i sh i n g wi th a resu l ti n g m i rrorl i ke fi n i sh . H owever, n ot al l su l ph u r-ph osph orou s con tai n i n g ad d i ti ves are d etri m en tal u n d er th ese same con d i ti on s, con su l t you r l u bri can t m an u factu rer to en su re th e proper l u bri can t appl i cati on . Som e g ear u n i ts operate u n d er i d eal con d i ti on s wi th sm ooth tooth su rfaces, h i g h pi tch l i n e speed , an d h i g h l u bri can t fi l m th i ckn ess.

I t h as been observed , for exam pl e, th at tu rbi n e g ears th at operated al m ost

con ti n u ou sl y at 1 50 m /s pi tch l i n e speed sti l l h ad th e ori g i n al m ach i n i n g m arks on th ei r teeth even after operati n g for 20 years.

M ost g ears h owever, operate between th e bou n d ary an d fu l l -fi l m l u bri cati on

reg i mes, u n d er el astoh yd rod yn am i c l u bri cati on (EH L) con d i ti on s. proper type an d

vi scosi ty of l u bri can t i s u sed ,

I n th e EH L reg i m e, provi d ed th at th e

th e wear rate u su al l y red u ces d u ri n g ru n n i n g -i n

ad h esi ve wear vi rtu al l y ceases on ce ru n n i n g -i n i s com pl eted .

an d

I f th e l u bri can t i s properl y m ai n tai n ed (kept

cool , cl ean an d d ry) th e g earset sh ou l d n ot su ffer an ad h esi ve wear fai l u re.

4.1 .3 Summary of methods to reduce the risk of adhesive wear -

-

Red u ce su rface rou g h n ess



U se sm ooth er tooth su rfaces;



Ru n -i n n ew g earsets by operati n g at l east th e fi rst 1 0 h ou rs at parti al l oad ;

Opti m i ze g eom etry



U se h i g h pi tch l i n e speed s i f possi bl e.

H i g h l y-l oad ed , sl ow-speed g ears are bou n d ary l u bri cated

an d especi al l y pron e to excessi ve wear; -

Opti m i ze m etal l u rg y

-

Opti m i ze l u bri can t properti es



U se n i tri d ed g ears i f th ey h ave ad eq u ate capaci ty.



Drai n an d fl u sh th e l u bri can t after th e fi rst 50 h ou rs of operati on to remove wear d ebri s from



For very sl ow-speed g ears (l ess th an 0. 05 m /s), u se l u bri can ts wi th ad d i ti ves th at h ave been



U se an ad eq u ate amou n t of cool , cl ean an d d ry (free of water) l u bri can t of th e h i g h est vi scosi ty



Lower m esh l u bri cati on tem peratu re wi th i m proved cool i n g .

ru n n i n g -i n , refi l l wi th recomm en d ed l u bri can t, an d i n stal l a n ew fi l ter el em en t i f th ere i s on e;

proven n ot to be ag g ressi ve to th e tooth su rfaces;

perm i ssi bl e for th e operati n g con d i ti on s;

4.2

Abrasion

Abrasi on i s th e removal or d i spl acem en t of m ateri al d u e to th e presen ce of h ard parti cl es:

for exam pl e,

m etal l i c d ebri s, scal e, ru st, san d , or abrasi ve powd er, su spen d ed i n th e l u bri can t or em bed d ed i n th e fl an ks of th e m ati n g teeth . Abrasi on cau ses scratch es or g ou g es on th e tooth su rface th at are ori en ted i n th e d i recti on of sl i d i n g . U n d er m ag n i fi cati on , th e scratch es appear as paral l el fu rrows th at are sm ooth an d cl ean .

See Fi g u re 3.

NOTE: Dam ag e from abrasi on i s n ot l i m i ted to g ear teeth ; i t al so can severel y d eg rad e beari n g s, seal s, an d oth er com pon en ts.

Abrasi on can

prom ote fai l u res of g ear teeth by cau si n g m i sal i g n m en t d u e to red u ced

beari n g

perform an ce.

Two-bod y abrasi on occu rs wh en em bed d ed parti cl es or asperi ti es on on e g ear tooth abrad e th e opposi n g tooth su rface.

Abrasi on d u e to l oose con tam i n an ts i s cal l ed th ree-bod y abrasi on .

I t i s g en eral l y m u ch

l ess severe th an two-bod y abrasi on becau se th e abrasi ve can rol l , sl i d e, an d vary i ts approach an g l e. G en eral l y, two-bod y abrasi on i s m u ch more d am ag i n g th an th ree-bod y abrasi on becau se th e abrasi ve i s fi xed i n on e bod y an d i t abrad es d i rectl y on th e oth er bod y. Based on th e severi ty of th e d am ag e, abrasi on can be categ ori zed as m i l d , m od erate, or severe.

4.2.1

Mild abrasion

Abrasi on i s cl assi fi ed as m i l d i f i t con si sts of fi n e scratch es th at are n ot n u m erou s or d eep en ou g h to rem ove si g n i fi can t am ou n ts of m ateri al from th e tooth su rface an d som e m ach i n i n g m arks are vi si bl e on th e tooth su rface.

See Fi g u re 4.

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The diagonal line is an abrasion furrow cut by a hard particle showing smooth, clean appearance. The vertical lines are the original grind marks.

Figure 3 - SEM image - abrasion

Figure 4 - Mild abrasion near the tip of a ground gear 4.2.2 Moderate abrasion

Abrasion is classified as moderate if remnants of the original machining marks are visible on the tooth surface.

4.2.3 Severe abrasion

Severe abrasion removes all of the original machining marks from the active surface of the tooth. There may be wear steps at the ends of the active face and in the dedendum. The tooth thickness may be reduced significantly, and in some instances the tooth tip may be reduced to a sharp edge. See Figures 5, 6 and 7.

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– All rights reserved

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ANSI/AGMA 1 01 0-F1 4

Figure 5 - Severe abrasion

Figure 6 - Severe abrasion, enlarged view of Figure 5

Figure 7 - Severe abrasion

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4.2.4 Sources of particles that may cause wear Con tam i n ati on en ters g ear u n i ts by bei n g presen t at assem bl y, i n tern al l y-g en erated , i n g ested th rou g h breath ers

an d

seal s,

carri ed

by

th e

l u bri can t

from

an

i m properl y

cl ean ed

l u bri cati on

system

or

i n ad verten tl y ad d ed d u ri n g m ai n ten an ce. San d , scal e, ru st, m ach i n i n g ch i ps, g ri n d i n g d u st, wel d spl atter or oth er d ebri s m ay fi n d th ei r way i n to n ew g ear u n i ts.

To rem ove bu i l t-i n con tami n ati on , i t i s g en eral l y worth wh i l e to d rai n an d fl u sh th e g earbox

l u bri can t after approxi m atel y 50 h ou rs of operati on , i n stal l a n ew oi l fi l ter, i f th ere i s on e, an d refi l l wi th th e recom m en d ed l u bri can t. I n tern al l y-g en erated parti cl es are u su al l y wear d ebri s from g ears, beari n g s or oth er compon en ts d u e to H ertzi an fati g u e, ad h esi ve wear, an d abrasi ve wear.

Th e wear parti cl es can be abrasi ve becau se th ey

becom e work h ard en ed wh en th ey are trapped between th e g ear teeth .

I n tern al l y-g en erated wear d ebri s

can be m i n i m i zed by u si n g accu rate, su rface-h ard en ed g ear teeth (wi th h i g h m acropi tti n g resi stan ce), sm ooth tooth su rfaces an d cl ean h i g h vi scosi ty l u bri can ts.

4.2.5 Methods for reducing abrasive wear Cl ean oi l i s absol u tel y essen ti al to preven t abrasi ve wear. Forei g n parti cl es i n th e oi l are d am ag i n g to g ears, beari n g s an d seal s an d wi l l cau se a d ecl i n e i n th e i n teg ri ty of th e g eared system . M ag n eti c pl u g s m ay be u sed to captu re ferrou s parti cl es th at are presen t at startu p, or are g en erated d u ri n g operati on .

Peri od i c i n specti on of th e m ag n eti c pl u g m ay be u sed to m on i tor th e d evel opm en t of

ferrou s parti cl es d u ri n g operati on .

M ag n eti c wear ch i p d etectors wi th al arms are al so avai l abl e.

Th e l u bri cati on system sh ou l d be carefu l l y m ai n tai n ed an d m on i tored to en su re th at th e g ears recei ve an ad eq u ate amou n t of cool , cl ean an d d ry (free of water) l u bri can t. h el ps to rem ove con tam i n ati on .

For ci rcu l ati n g -oi l systems, fi n e fi l trati on

Fi l ters as fi n e as 3 m i crom eters h ave been u sed to si g n i fi can tl y i n crease

g ear l i fe, wh ere th e pressu re l oss across th e fi l ter can be tol erated . system s) m ay al so be u sed to cl ean oi l . process

on l y abou t 1 0%

of th e

total

Offl i n e fi l ters (ki d n ey-l oop type

Th ey effi ci en tl y rem ove sm al l (1 -1 0 fl ow rate

an d

μm )

parti cl es becau se th ey

th ereby al l ow fi n er fi l trati on .

Th ey m ay u ti l i ze

el ectrostati c ag g l om erati on system s to red u ce th e am ou n t of very fi n e parti cl es th at n orm al l y wou l d pass th rou g h th e fi l ters.

Oth er system s m ay be u sed to rem ove water from th e oi l .

N ote th at fi n e fi l trati on m ay

rem ove som e ben efi ci al ad d i ti ves from some l u bri can ts, so th e l u bri can t su ppl i er sh ou l d be con su l ted reg ard i n g th e fi l trati on l evel an d fi l ter type. Th e l u bri can t m ay h ave to be ch an g ed or processed to rem ove water an d m ai n tai n ad d i ti ve l evel s.

For

oi l -bath g ear u n i ts, th e l u bri can t sh ou l d be ch an g ed freq u en tl y becau se th at i s th e on l y way to rem ove con tam i n ati on .

Th e l u bri can t sh ou l d be ch an g ed m ore freq u en tl y wh en th e operati n g tem peratu re i s

above 225° F. I n m an y cases th e l u bri can t sh ou l d be ch an g ed at l east every 2500 operati n g h ou rs or si x m on th s, wh i ch ever occu rs fi rst; h owever, th e m an u factu rer’ s recom m en d ati on s sh ou l d be fol l owed . AN SI /AG M A 9005

for ad d i ti on al

i n form ati on .

For cri ti cal

m on i tori n g can be u sed to assess l u bri can t con d i ti on .

g ear u n i ts

a

reg u l ar prog ram

See

of l u bri can t

Th e l u bri can t m on i tori n g m ay i n cl u d e su ch i tems

as spectrog raph i c an d ferrog raph i c an al ysi s of con tam i n ati on al on g wi th an al ysi s of aci d i ty, vi scosi ty, an d water con ten t.

U sed fi l ter el em en ts m ay be exam i n ed for wear d ebri s an d con tam i n an ts.

Breath er ven ts are u sed on g ear u n i ts to rel i eve i n tern al pressu re th at occu rs wh en ai r en ters th rou g h seal s or wh en th e ai r wi th i n th e g earbox expan d s an d con tracts d u ri n g n ormal h eati n g an d cool i n g .

Th e

breath er ven t sh ou l d be l ocated i n a cl ean , n on -pressu ri zed area an d i t sh ou l d h ave a fi l ter to preven t i n g ressi on of ai rborn e con tam i n an ts an d d esi ccan t to rem ove water.

I n especi al l y h arsh en vi ron m en ts,

th e g earbox can someti m es be com pl etel y seal ed , an d th e pressu re vari ati on can be accom m od ated by an expan si on ch am ber wi th a fl exi bl e d i aph rag m . Al l m ai n ten an ce proced u res th at i n vol ve open i n g an y part of th e g ear u n i t or l u bri cati on system sh ou l d be carefu l l y perform ed i n an en vi ron men t as cl ean as possi bl e to preven t con tam i n ati on of th e g ear u n i t. U n l ess th e tooth su rfaces of a su rface-h ard en ed g ear are sm ooth l y fi n i sh ed , th ey m ay act l i ke fi l es i f th e m ati n g g ear i s appreci abl y softer.

Th i s i s th e reason th at a worm i s pol i sh ed after g ri n d i n g before i t i s ru n

wi th a bron ze worm wh eel .

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ANSI/AGMA 1 01 0-F1 4

4.2.5.1 Summary of methods to reduce the risk of abrasive wear -

M i n i m i ze con tam i n ati on



Fl u sh u n i t th orou g h l y before i n i ti al operati on ;



Rem ove bu i l t-i n con tam i n ati on from n ew g ear u n i ts by d rai n i n g an d fl u sh i n g th e l u bri can t after approxi m atel y 50 h ou rs of operati on .

Refi l l wi th cl ean recom men d ed l u bri can t an d i n stal l a n ew

fi l ter i f th ere i s on e;



M i n i m i ze i n tern al l y-g en erated wear d ebri s by u si n g su rface-h ard en ed g ear teeth , sm ooth tooth



M i n i m i ze i n g ested con tam i n ati on by mai n tai n i n g oi l -ti g h t seal s an d u si n g fi l tered breath er ven ts

su rfaces an d h i g h vi scosi ty l u bri can ts;

l ocated i n cl ean , n on -pressu ri zed areas;



M i n i m i ze

con tam i n ati on

th at

is

ad d ed

d u ri n g

mai n ten an ce

by

u si n g

g ood

h ou sekeepi n g

proced u res;

-



For ci rcu l ati n g -oi l systems, u se fi n e fi l trati on i n con su l tati on wi th l u bri can t m an u factu rer;



U se an offl i n e (ki d n ey l oop) fi l ter to rem ove very sm al l parti cl es;



U se an ag g l om erati on system to rem ove very fi n e parti cl es;

M ai n tai n l u bri can t



Ch an g e or process th e l u bri can t to rem ove water;



For oi l -bath system s, ch an g e th e l u bri can t at l east every 2500 h ou rs or every si x m on th s, or as recom m en d ed by th e m an u factu rer, or d eterm i n ed by l u bri cati on sam pl i n g an al ysi s;



M on i tor th e l u bri can t wi th

spectrog raph i c an d

aci d i ty, vi scosi ty an d water con ten t.

ferrog raph i c an al ysi s tog eth er wi th an al ysi s of

Oi l sam pl i n g i s th e best m eth od for d eterm i n i n g l u bri cati on

ch an g i n g i n terval s.

4.3

Polishing

Pol i sh i n g i s fi n e-scal e abrasi on [2] th at cau ses g ear teeth to h ave a bri g h t m i rrorl i ke fi n i sh . tooth su rface m ay be sm ooth or wavy wi th l ocal bu m ps.

Th e g ear

U n d er m ag n i fi cati on , th e su rface appears to be

covered by fi n e scratch es th at are ori en ted i n th e d i recti on of sl i d i n g . Wh en a h ard su rface m ates wi th a soft su rface, pol i sh i n g i s more l i kel y to occu r on th e h ard su rface becau se th e abrasi ves embed i n th e soft su rface an d create two-bod y abrasi on on th e h ard su rface. Pol i sh i n g can be prom oted by ch em i cal l y ag g ressi ve ad d i ti ves wh en th e l u bri can t i s con tam i n ated wi th fi n e abrasi ves [2] .

4.3.1

Based on th e severi ty, pol i sh i n g can be categ ori zed as m i l d , m od erate, or severe.

Mild polishing

Pol i sh i n g i s cl assi fi ed as m i l d i f i t i s con fi n ed to th e peaks of th e su rface asperi ti es.

M i l d pol i sh i n g

typi cal l y occu rs d u ri n g ru n n i n g -i n an d ceases before th e ori g i n al m ach i n i n g marks are removed from th e tooth su rface.

4.3.2 Moderate polishing Pol i sh i n g i s cl assi fi ed as m od erate i f rem n an ts of th e ori g i n al m ach i n i n g m arks are vi si bl e on th e tooth su rface.

4.3.3 Severe polishing Severe pol i sh i n g removes al l of th e ori g i n al m ach i n i n g m arks from th e acti ve su rface of th e tooth .

Th e

pol i sh ed su rface may be wavy an d th ere m ay be wear steps at th e en d s of th e acti ve face an d i n th e d ed en d u m .

I f extrem e, pol i sh i n g m i g h t red u ce tooth th i ckn ess to wh ere th e topl an d i s a kn i fe-ed g e.

See

Fi g u res 8 an d 9. Th e g ear teeth m ay pol i sh to a bri g h t, m i rrorl i ke fi n i sh i f th e an ti scu ff ad d i ti ves i n th e l u bri can t are too ch em i cal l y ag g ressi ve, an d a fi n e abrasi ve i s presen t [2] .

Al th ou g h th e pol i sh ed g ear teeth m ay l ook

g ood , pol i sh i n g wear can be u n d esi rabl e i f i t red u ces g ear accu racy by weari n g th e tooth profi l es away from th ei r i d eal form .

An ti scu ff ad d i ti ves th at con tai n su l fu r or ph osph orou s are u sed i n l u bri can ts to

preven t scu ffi n g (see 5. 3).

Th ey fu n cti on by form i n g i ron -su l fi d e an d i ron -ph osph ate fi l m s on areas of

g ear teeth wh ere h i g h tem peratu res occu r. th ere i s a d an g er of wel d i n g .

I d eal l y, th e ad d i ti ves sh ou l d react on l y at tem peratu res wh ere

I f th e rate of reacti on i s too h i g h , an d th ere i s a con ti n u ou s rem oval of th e

su rface fi l ms cau sed by very fi n e abrasi ves i n th e l u bri can t, pol i sh i n g wear m ay becom e excessi ve [2] .

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Figure 8 - Severe polishing

Figure 9 - Severe polishing Polishing wear can be prevented by using less chemically active additives [3] and clean oil. The antiscuff additives should be appropriate for the service conditions. The use of any dispersed material, such as some antiscuff additives, should be monitored since it may precipitate or be filtered out. The abrasives in the lubricant should be removed by using fine filtration or frequent oil changes. 4.3.4 Summary of methods to reduce the risk of polishing wear - Use a less chemically aggressive additive system; - Remove abrasives from system, see 4.2.5 for methods. 4.4 Corrosion Corrosion is the chemical or electrochemical reaction between the surface of a gear and its environment. The tooth surfaces may appear stained or rusty and there may be reddish-brown deposits of rust. If the loose corrosion products are removed, rough irregular etch pits may be revealed. Corrosion commonly attacks the entire tooth surface and it may proceed intergranularly by preferentially attacking the grain boundaries of the tooth surfaces. See Figure 1 0.

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ANSI/AGMA 1 01 0-F1 4

Figure 1 0 - Extensive corrosion Identification of metal corrosion products is an indication of corrosion. For example, the identification of α -Fe 2 O 3 H 2 O by X-ray diffraction on pitted steel is evidence of rusting. Etch pits from corrosion on the active flanks of gear teeth cause stress concentrations that may initiate macropitting fatigue cracks. Etch pits on the root fillets of gear teeth may become initiation sites for bending fatigue cracks. Water is detrimental to lubricant properties and reduces fatigue life. The particles of rust are hard and they can cause abrasive wear of the gear teeth. Corrosion is often caused by contaminants in the lubricant such as water, or by changes in the lubricant pH. Overly reactive additives can also cause corrosion especially at high temperatures. Corrosive wear caused by contamination or formation of acids in the lubricant can be minimized by monitoring the lubricant acidity, viscosity and water content and by changing the lubricant when required. At times, gear tooth surfaces are chemically attacked during processing in the factory, for example, when copper plating is stripped from a gear after carburizing, or when acid is used as an etchant to inspect for surface temper from grinding. Proper processing procedures must be carefully followed to avoid damage when using such processes.

4.4.1

Methods to reduce the risk of corrosion

A gear lubricant should be changed if the neutralization number increases more than 75% over the baseline value of the unused product, the water content is greater than 0.1 %, or the viscosity increases or decreases to the next ISO viscosity grade. Corrosion easily occurs in gear units not properly protected during storage. If the gear unit must be stored, special precautions should be applied to prevent rusting of the components. Condensation occurs when humid air is cooled below its dew point and the air-water mixture releases water, which collects in the form of droplets on exposed surfaces. It may occur where there are frequent, wide temperature changes. To prevent condensation gearboxes should be stored indoors where humidity is controlled and temperature changes are minimized. For long term storage, it is best to completely fill the gear unit with oil and plug the breather vent. This minimizes the air space above the oil level and minimizes the amount of condensation. Where this is not practical, all exposed metal parts, both inside and outside, should be sprayed with a heavy duty water displacing rust preventative that has been proven to be compatible with the gear oil to be used in the gearbox. To be effective, the rust preventative must reach all bearings and stagnant areas. If stored outdoors, the gear unit should be raised off the ground and completely enclosed by a protective covering such as a tarpaulin. Plastic is not recommended because it accumulates Copyright American Gear Manufacturers Association ©AGMA 201 4

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condensation on its underside. The gears should be rotated periodically to distribute oil to the gears and bearings. If the gearbox has a circulating lubrication system, it should be activated periodically. It may also be necessary to periodically remove bearing caps, spray the bearings with oil, and replace the caps to ensure adequate protection.

4.5

Fretting

Fretting is localized wear of contacting gear and bearing surfaces caused by minute vibratory motion. It occurs between contacting surfaces that are pressed together and subjected to cyclic, relative motion of extremely small amplitude. Under these conditions, lubricant squeezes from between the surfaces, and motion of the surfaces is too small to replenish the lubricant. Natural, oxide films that normally protect surfaces are disrupted, permitting metal-to-metal contact and causing adhesion of surface asperities. Fretting commonly occurs in joints that are bolted, keyed, or press-fitted and in splines or couplings. It might occur on gear teeth and bearing raceways and rollers under specific conditions where the gears and bearings are not rotating and subjected to structure-borne vibrations such as those encountered during transport, or in parked wind turbines [4, 5]. Fretting can occur as two mechanisms; false brinelling and fretting corrosion. For lubricated contacts, under fretting conditions, false brinelling begins an incubation period of mild adhesive wear under boundary lubrication. If the contact becomes starved for lubrication, it may be subjected to severe adhesive wear known as fretting corrosion. See Figure 1 1 . True brinelling is a separate failure mode that is unrelated to false brinelling.

4.5.1

True brinelling

True brinelling occurs in contacts that are subjected to Hertzian stress that is high enough to cause permanent plastic deformation of the contacting surfaces. It is characterized by plastic deformation, without loss of material or change of surface texture that occurs during a single load event. True brinelling is characterized by dents that have raised shoulders. For example, true brinelling of a rolling element bearing occurs when the bearing is not rotating and subjected to an impact load great enough to plastically deform the raceway. The dents in the raceway occur at roller spacing, have raised shoulders, and the original grinding marks are visible microscopically in the bottoms of the dents.

Figure 1 1 - Fretting corrosion

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4.5.2 False brinelling Fretti n g beg i n s wi th an i n cu bati on peri od d u ri n g wh i ch th e wear m ech an i sm i s m i l d ad h esi on th at i s Th e wear d ebri s i s th e i ron oxi d e mag n eti te (Fe 3 O 4 ),

con fi n ed to th e n atu ral oxi d e l ayer th at covers steel . wh i ch i s bl ack an d h i g h l y m ag n eti c. a bl ack,

g reasy fi l m .

m orph ol og y th an sh ou l d ers. wear.

M ag n eti te d i scol ors th e l u bri can t su rrou n d i n g th e con tact an d forms

Dam ag e d u ri n g i n cu bati on i s fal se bri n el l i n g [4] ,

tru e bri n el l i n g .

Fal se bri n el l i n g i s ch aracteri zed

an d i t h as d i sti n ctl y d i fferen t

by d en ts th at d o n ot h ave rai sed

Fu rth erm ore, th e ori g i n al m ach i n i n g m arks wi th i n th e d en ts are worn away by m i l d ad h esi ve

Th e d en ts are created by th e weari n g off of pre-exi sti n g an d con ti n u al l y reform i n g oxi d e fi l m s [4] .

G en eral l y, th e d am ag e cau sed by fal se bri n el l i n g i s n eg l i g i bl e an d th e wear rate i s l ow. occu rs

on

g ear teeth

an d

beari n g

com pon en ts

wh en

th ey

are

n ot

rotati n g

bu t

Fal se bri n el l i n g

osci l l ati n g

th rou g h

extrem el y sm al l an g l es.

4.5.3 Fretting corrosion Wear d ebri s from fal se bri n el l i n g accu mu l ates i n th e oi l m en i scu s su rrou n d i n g th e con tact. is

su ffi ci en t to

d am

l u bri can t from

reach i n g

bou n d ary l u bri cati on to u n l u bri cated .

th e

con tact area,

th e

l u bri cati n g

reg i m e

I f th e am ou n t ch an g es

from

On ce th e l u bri can t wi th i n th e con tact area i s d epl eted by oxi d ati on

th e wear rate i n creases d ram ati cal l y u n ti l th e n atu ral oxi d e l ayer i s broken th rou g h .

Th en stron g wel d s

are form ed between th e asperi ti es of th e paren t i ron com pon en ts an d d am ag e escal ates to fretti ng corrosi on .

Rel ati ve

m oti on

breaks

stron g l y-wel d ed

asperi ti es

parti cl es th at oxi d i ze to form th e i ron -oxi d e h em ati te ( fi n en ess an d red d i sh -brown col or of cocoa.

α -Fe

2 O 3 );

an d

g en erates

extremel y

sm al l

wear

a n on -m ag n eti c powd er th at h as th e

Th e wear d ebri s i s h ard an d abrasi ve, an d i s i n fact th e sam e

com posi ti on as j ewel er’ s rou g e [4, 5] , an d pol i sh i n g wear [2] (fi n e scal e abrasi on ) i s freq u en tl y fou n d arou n d th e peri ph ery of a fretti n g corrosi on scar. an d form s rou g e-col ored paste.

H em ati te d i scolors th e l u bri can t su rrou n d i n g th e con tact

U su al l y, th e wear scar i s d i scol ored wi th bl ack or red d i sh fi l m s.

Fretti n g corrosi on d amag es g ear an d beari n g su rfaces by form i n g ru ts al on g l i n es of con tact.

Du ri n g

operati on , d am ag ed g ears an d beari n g s m i g h t g en erate sh arp, ham m eri n g n oi se as th e wear scars pass th rou g h th e con tact areas. For exam pl e, fretti n g corrosi on can occu r wh en g ears are i n m esh u n d er l oad an d vi brati n g wi th ou t si g n i fi can t rel ati ve rotati on .

Wh en th e ru ts are severe, th e g ears m ay be n oi sy wh en

th ey rotate. Pi ts from fretti n g corrosi on create l ocal stress con cen trati on s th at m i g h t cau se m acropi tti n g or i n i ti ate fati g u e cracks,

wh i ch

if in

high

ten si l e stress areas,

m i g h t propag ate to fai l u re.

G en eral l y,

fretti n g

corrosi on red u ces fati g u e stren g th si g n i fi can tl y. I f rol l i n g el emen t beari n g fi ts are i n ad eq u ate to stop rel ati ve m oti on between th e i n n er ri n g an d sh aft, or between th e ou ter ri n g an d h ou si n g , fretti n g corrosi on m i g h t d evel op at th ese i n terfaces. m an n er,

fretti n g

corrosi on

can

al so

occu r

between

a

g ear

bore

an d

sh aft

if

th ere

I n a si m i l ar

is

i n ad eq u ate

i n terferen ce. Th e best way to avoi d fal se bri n el l i n g an d fretti n g corrosi on i s to stop th e vi brati on , rotate th e com pon en ts to en trai n fresh oi l , or both .

Each ti m e th e com pon en ts en trai n fresh oi l , th e i n cu bati on peri od restarts,

an d th e wear reg i m e sh i fts to m i l d ad h esi ve wear.

Th e l en g th of th e i n cu bati on peri od d epen d s on th e

l u bri can t type an d h ow easi l y l u bri can t reach es th e con tact. For u n l u bri cated con tacts, th ere i s n o i n cu bati on peri od , an d fretti n g corrosi on m ay start i mm ed i atel y an d th e wear rate m ay be h i g h from th e beg i n n i n g .

4.5.4 Summary of methods to reduce the risk of false brinelling and fretting corrosion -

Stop th e vi brati on , rotate th e compon en ts to en trai n fresh oi l , or both ;

-

For reci procati n g system s su ch as yaw d ri ves or actu ators, en su re th e an g u l ar m oti on i s su ffi ci en t to wi pe fresh l u bri can t i n to th e con tact;

-

Avoi d parki n g wi n d tu rbi n es for exten d ed peri od s;

-

Avoi d d i th eri n g of wi n d tu rbi n e bl ad es; vary pi tch an g l e en ou g h to en trai n fresh oi l an d pi tch bl ad es freq u en tl y;

-

En su re ad eq u ate i n terferen ce fi t between

sh afts an d

cou pl i n g s,

g ears,

beari n g

ri n g s,

an d

oth er

i n terferen ce-fi t com pon en ts; -

U se case h ard en i n g or su rface h ard en i n g to obtai n ad h esi on -resi stan t su rfaces (n i tri d i n g i s best);

-

U se ph ysi cal or ch em i cal vapor d eposi ti on (PVD or CVD) h ard coati n g s to obtai n ad h esi on -resi stan t su rfaces;

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-

U se col d work, case h ard en i n g , or sh ot peen i n g to i n d u ce compressi ve resi d u al stresses.

-

U se a l u bri can t wi th an ti wear ad d i ti ves;

-

U se oi l rath er th an g rease an d u se a h i g h -pressu re j et to fl ood th e con tact an d fl u sh away wear d ebri s;

-

Store th e g earbox i n a vi brati on -free en vi ron men t;

-

Su pport th e g earbox on vi brati on i sol ators;

-

Sh i p th e g earbox wi th sh afts l ocked to preven t an y m oti on ;

-

Sh i p th e g earbox on an ai r-ri d e tru ck.

4.6

Scaling

Scal i n g can appear as patch y rai sed areas on th e tooth fl an ks th at are d u e to oxi d ati on d u ri n g h eat treatm en t. con d i ti on .

I n teg ral

q u en ch

con trol l ed

atm osph ere an d

vacu u m

h eat treatm en ts

d o n ot exh i bi t th i s

Wh en ru n n i n g u n d er l oad , th e tooth force i s i n i ti al l y tran sm i tted by way of th ese rai sed areas

th at rapi d l y acq u i re a m etal l i c sh een .

See Fi g u re 1 2.

Scal i n g i s an i ssu e on l y on g ears th at are n ot fi n i sh ed after h eat treatm en t an d on l y i f th e oxi d e l ayer i s overl y th i ck.

Wi th n orm al processi n g , th e oxi d e l ayer (as opposed to h ard en i n g scal e) i s th i n an d u n i form ,

an d i t u su al l y d oes n ot affect g ear perform an ce.

4.7

White layer flaking

Wh i te l ayer fl aki n g occu rs wh en th e com pou n d l ayer (wh i te l ayer) on n i tri d ed g ears ch i ps off l eavi n g sh al l ow scars th at h ave a wh i te appearan ce an d can be fel t wi th th e fi n g ern ai l .

See Fi g u re 1 3.

Figure 1 2 - Scaling

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Figure 1 3 - White layer flaking The compound layer is generally detrimental to fatigue strength. Therefore, it is often removed by grit blasting, grinding, honing, lapping or chemically assisted polishing. However, for some applications the compound layer is left on the gears and there is a risk of white layer flaking. The risk is greater for thick compound layers such as those that are developed on single-stage gas nitrided gears and less on twostage gas nitrided gears and ion nitrided gears. Generally the risk of white layer flaking is higher when the compound layer consists of both the epsilon phase and gamma-prime phase iron nitrides and less when the compound layer consists solely of the more ductile gamma-prime phase. As a general rule, white layer flaking is likely to occur when the compound layer is greater than 1 3 μm and unlikely when the compound layer is less than 1 0 μm. 4.7.1 Summary of methods to reduce the risk of white layer flaking - Remove the white layer with grit blasting, grinding, honing, lapping, or polishing; - Use two-stage nitriding or ion nitriding and keep the thickness of the compound layer less than 1 0 μm. 4.8 Cavitation Cavitation can occur in the lubricant film between mating gear teeth [6]. Cavitation is caused by relative motion between a solid surface and a liquid. Relative motion causes a pressure drop that nucleates vapor-filled bubbles within the liquid. When the bubbles travel into a region of high pressure, they collapse as they change state from gas to liquid. The implosion of the bubbles transmits localized forces to the surface and causes plastic deformation, work hardening, and ductile fracture of the surface asperities. This may cause damage in the gear tooth surface that appears to the unaided eye to be rough and clean as if it were sand blasted. Microscopically, the craters caused by cavitation are deep, rough, clean, and have a honeycomb appearance. See Figures 1 4 through 1 7.

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Figure 1 4 - Cavitation damage

Figure 1 5 - Cavitation damage

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Figure 1 6 - SEM image - cavitation damage

Figure 1 7 - SEM image - cavitation damage 4.9

Erosion

Erosion is the loss of material from a gear tooth surface due to the relative motion of a high velocity fluid. Figure 1 8 shows a high speed helical gear with erosion at tips of teeth caused by impingement of lubricant from oil nozzles. This may occur with clean fluids, but damage is much worse if there are solid contaminants in the fluid.

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Figure 1 8 - Erosion of a high speed helical gear 4.1 0 Electric discharge Gear teeth can be damaged if an electric current passes through the gear mesh. An electric current can also damage bearings and should be avoided in geared systems. Electric currents may result from faulty insulation, induction effects, or improper grounding. The electric discharge damage is caused by an electric arc discharge across the oil film between the active flanks of the mating gear teeth. The electric current may originate from many sources, including: -

electric motors; electric clutches or instrumentation; accumulation of static charge and subsequent discharge; during electric welding on or near the gear unit if the path to ground is not properly made around the gears rather than through them; during lightning strikes on wind turbines.

An electric arc discharge across the oil film between mating gear teeth produces temperatures that may be high enough to locally melt the gear tooth surface. To the unaided eye, a surface damaged by electric discharge appears as an arc burn similar to a spot weld. Microscopically, the damage appears as small, hemispherical craters. The edges of the craters are smooth and they may be surrounded by rounded particles that were once molten. A metallurgical section taken transversely through the craters and acid etched may reveal austenitized and rehardened areas in white, bordered by tempered areas in black. Sometimes very small cracks are found near the craters. The damage to the gear teeth is proportional to the number and size of the points of arcing. Depending on its extent, electric discharge damage can be destructive to the gear teeth. If electric discharge damage is found on the gears, all associated bearings should be examined for similar damage. See Figures 1 9 through 23.

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Figure 1 9 - Electric discharge damage due to a small electric current

Figure 20 - Severe electric discharge damage due to an electric current of high intensity

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Figure 21 - SEM image - typical electric discharge crater

Figure 22 - SEM image - remelted metal and gas pockets near edge of crater

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Figure 23 - SEM image - electric discharge damage 4.1 0.1 Summary of methods to reduce the risk of electrical discharge damage -

5

Provide adequate electrical insulation; Provide adequate grounding; Ensure proper welding grounding procedures are used.

Scuffing

Scuffing is severe adhesion that causes transfer of metal from one tooth surface to another due to welding and tearing. The damage typically occurs in the addendum, dedendum, or both, away from the operating pitch line, in narrow or broad bands that are oriented in the direction of sliding. Scuffing may occur in localized patches if it is due to load concentrations. The scuffed areas appear to have a rough or matte texture. Under magnification, the scuffed surface appears rough, torn, and plastically deformed. The term “scoring” which was incorrectly used in earlier gear nomenclature for scuffing, is in reality scratching and is now classified as a form of abrasive wear. Scuffing is not a fatigue phenomenon and it may occur instantaneously. Based on the severity of the damage, scuffing can be categorized as mild, moderate, or severe.

5.1

Mild scuffing

Scuffing is classified as mild if it occurs only on small areas of the teeth and is confined to the peaks of the surface asperities. It is generally nonprogressive. See Figures 24, 25 and 26.

5.2

Moderate scuffing

Moderate scuffing occurs in patches that cover significant portions of the teeth. conditions do not change, moderate scuffing may be progressive. See Figure 27.

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Figure 24 - Mild scuffing

Figure 25 - SEM image - scuffing damage showing rough, torn, and plastically deformed appearance

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Figure 26 - SEM image - scuffing damage showing crater formed when welded material was torn from surface

Figure 27 - Moderate scuffing 5.3

Severe scuffing

Severe scu ffi n g occu rs on si g n i fi can t porti on s of th e g ear tooth (for exam pl e, th e en ti re ad d en d u m , th e en ti re

d ed en d u m ,

or both ).

In

som e

cases

th e

su rface

m ateri al

d i spl aced over th e ti p of th e tooth or i n to th e root of th e tooth . severe scu ffi n g i s u su al l y prog ressi ve.

m ay be

pl asti cal l y d eform ed

an d

U n l ess correcti ve m easu res are taken ,

See Fi g u res 28 an d 29.

Scu ffi n g can occu r i n g ear teeth wh en th ey operate i n th e bou n d ary l u bri cati on reg i m e.

I f th e l u bri can t

fi l m i s i n su ffi ci en t to preven t si g n i fi can t m etal -to-m etal con tact, th e oxi d e l ayers th at n ormal l y protect th e g ear tooth su rfaces m ay be broken th rou g h , an d th e bare metal su rfaces m ay wel d tog eth er.

Th e sl i d i n g

th at occu rs between g ear teeth resu l ts i n teari n g of th e wel d ed j u n cti on s, m etal tran sfer an d d am ag e.

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Figure 28 - Severe scuffing

Figure 29 - Severe scuffing of a low speed gear lubricated with grease In contrast to Hertzian fatigue and bending fatigue, which only occur after a period of running time, scuffing may occur immediately upon start-up. In fact, gears are most vulnerable to scuffing when they are new and their tooth surfaces have not yet been smoothed by running-in. It is recommended that new gears be run-in under partial load to reduce the surface roughness of the teeth before the full load is applied. The gear teeth can be coated with iron-manganese phosphate or plated with copper or silver to reduce the risk of scuffing during the critical running-in period. The use of an oil with an antiscuff additive may be useful during running-in to both help prevent scuffing and to promote polishing. However, if a different oil is used for running-in, at the end of the running-in period the gearbox should be completely drained and flushed. Copyright American Gear Manufacturers Association ©AGMA 201 4

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Th e basi c m ech an i sm of scu ffi n g i s n ot cl earl y u n d erstood , bu t th ere i s g en eral ag reem en t th at i t i s cau sed by fri cti on al h eati n g g en erated by th e com bi n ati on of h i g h sl i d i n g vel oci ty an d i n ten se su rface pressu re.

Cri ti cal tem peratu re th eory [7] i s often u sed for pred i cti n g scu ffi n g .

I t states th at scu ffi n g wi l l

occu r i n g ear teeth th at are sl i d i n g u n d er bou n d ary-l u bri cated con d i ti on s, wh en th e m axi m u m con tact tem peratu re of th e g ear teeth reach es a cri ti cal m ag n i tu d e. For m i n eral oi l s wi th ou t an ti scu ff ad d i ti ves, each combi n ati on of oi l an d g ear tooth m ateri al h as a cri ti cal scu ffi n g tem peratu re th at i s con stan t reg ard l ess of th e operati n g con d i ti on s [8] .

Th e cri ti cal scu ffi n g

tem peratu re m ay n ot be con stan t for syn th eti c l u bri can ts an d l u bri can ts wi th an ti scu ff ad d i ti ves, an d sh ou l d be d eterm i n ed from tests th at cl osel y si m u l ate th e operati n g con d i ti on s of th e g ears or wi th i n -si tu tests on th e actu al g ears. M ost an ti scu ff ad d i ti ves are su l fu r-ph osph oru s compou n d s,

wh i ch

form

bou n d ary-l u bri cati n g

fi l m s by

ch em i cal l y reacti n g wi th th e m etal su rfaces of th e g ear teeth at l ocal poi n ts of h i g h tem peratu re. An ti scu ff fi l m s h el p preven t scu ffi n g by form i n g sol i d fi l m s on th e g ear tooth su rfaces an d i n h i bi ti n g tru e m etal -tom etal con tact.

Th e fi l m s of i ron su l fi d e an d i ron ph osph ate h ave h i g h m el ti n g poi n ts, al l owi n g th em to

rem ai n as sol i d s on th e g ear tooth su rfaces even at h i g h con tact tem peratu res. Th e rate of reacti on of th e an ti scu ff ad d i ti ves i s g reatest wh ere th e g ear tooth con tact tem peratu res are h i g h est.

Becau se of th e sl i d i n g acti on of th e g ear teeth , th e su rface fi l m s are repeated l y scraped off an d

reformed .

I n effect, scu ffi n g i s preven ted by su bsti tu ti n g m i l d corrosi on i n i ts pl ace.

m ay prom ote m i cropi tti n g . wear (see 4. 3).

An ti scu ff ad d i ti ves

Som e an ti scu ff ad d i ti ves m ay be too ch em i cal l y acti ve an d promote pol i sh i n g

Th i s m ay n ecessi tate a ch an g e to l ess ag g ressi ve an ti scu ff ad d i ti ves th at d eposi t a

bou n d ary fi l m wi th ou t reacti n g wi th th e m etal . Con su l t wi th a l u bri can t speci al i st for fu rth er g u i d an ce. U se cau ti on i n th e l u bri cati on of g ear u n i ts th at h ave fri cti on pl ate cl u tch es or backstops, si n ce som e ad d i ti ves m ay ch an g e th e coeffi ci en t of fri cti on .

Al ways con su l t wi th th e g earbox m an u factu rer an d

l u bri can t su ppl i er before maki n g an y ch an g es from on e l u bri can t to an oth er. For m i n eral oi l s wi th ou t an ti scu ff ad d i ti ves, th e cri ti cal scu ffi n g tem peratu re i n creases wi th i n creasi n g vi scosi ty, an d ran g es from 1 50° C to 300° C.

Th e i n creased scu ffi n g resi stan ce of h i g h -vi scosi ty l u bri can ts

i s bel i eved to be d u e to d i fferen ces i n ch em i cal composi ti on rath er th an i n creased vi scosi ty.

H owever, a

vi scosi ty i n crease al so h el ps red u ce th e ri sk of scu ffi n g by i n creasi n g EH L fi l m th i ckn ess an d red u ci n g con tact tem peratu re g en erated by m etal -to-m etal con tact. Accord i n g to [7] , th e cri ti cal tem peratu re i s:

T

c

 Tb  Tf

(1 )

wh ere

T T T

c

i s total con tact tem peratu re;

b

i s g ear bu l k tem peratu re;

f

i s fl ash tem peratu re.

Th e bu l k temperatu re i s th e eq u i l i bri u m tem peratu re of th e su rface of th e g ear teeth before th ey en ter th e m esh i n g zon e.

Th e fl ash tem peratu re i s th e l ocal an d i n stan tan eou s temperatu re ri se th at occu rs on th e

g ear teeth d u e to th e fri cti on al h eati n g as th ey pass th rou g h th e mesh i n g zon e.

5.3.1

Methods for reducing the risk of scuffing

An yth i n g th at red u ces ei th er th e bu l k tem peratu re or th e fl ash tem peratu re wi l l red u ce th e total con tact tem peratu re an d l essen th e ri sk of scu ffi n g .

H i g h er vi scosi ty l u bri can ts or sm ooth er tooth su rfaces h el p

by i n creasi n g th e speci fi c fi l m th i ckn ess, wh i ch i n tu rn red u ces th e fri cti on al h eat, an d th erefore th e fl ash tem peratu re. Th e l u bri can t performs th e i m portan t fu n cti on of removi n g h eat from th e g ear teeth .

Th e l u bri can t m u st

be su ppl i ed to th e g ear teeth su ch th at i t removes h eat rapi d l y an d m ai n tai n s a l ow bu l k tem peratu re. A h eat exch an g er can be u sed wi th a ci rcu l ati n g oi l system to cool th e l u bri can t before i t i s sprayed at th e g ears [9] . Scu ffi n g resi stan ce m ay be i n creased by opti m i zi n g th e g ear g eom etry su ch th at th e g ear teeth are as sm al l as possi bl e, con si sten t wi th ben d i n g stren g th req u i rem en ts, to red u ce th e tem peratu re ri se cau sed by sl i d i n g .

Th e am ou n t of sl i d i n g i s proporti on al to th e d i stan ce from th e pi tch poi n t an d i s zero wh en th e

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g ear teeth con tact at th e pi tch poi n t, an d l arg est at th e en d s of th e path of acti on .

Profi l e sh i ft can be

u sed to bal an ce an d m i n i m i ze th e temperatu re ri se th at occu rs i n th e ad d en d u m an d d ed en d u m of th e g ear teeth .

Th e tem peratu re ri se m ay al so be red u ced by m od i fyi n g th e tooth profi l es wi th sl i g h t ti p rel i ef,

root rel i ef, or both to ease th e l oad at th e start an d en d of th e en g ag em en t path wh ere th e sl i d i n g vel oci ti es are th e g reatest. Al so, th e g ear teeth sh ou l d be accu rate, h el d ri g i d l y i n g ood al i g n m en t, an d provi d ed wi th l ead m od i fi cati on to m i n i m i ze th e tooth l oad i n g an d tem peratu re ri se. Th e g ear m ateri al s sh ou l d be ch osen wi th th ei r scu ffi n g resi stan ce i n m i n d . n i tri d ed are g en eral l y fou n d to h ave h i g h resi stan ce to scu ffi n g . h ave th e h i g h est resi stan ce to scu ffi n g .

Steel s th at h ave been

N i tri d i n g steel s con tai n i n g al u m i n u m

Som e stai n l ess steel s m ay scu ff even u n d er n ear-zero l oad s.

Th e th i n oxi d e l ayer on th ese stai n l ess steel s i s h ard an d bri ttl e an d i t breaks u p easi l y u n d er sl i d i n g l oad s, exposi n g th e bare m etal , th u s prom oti n g scu ffi n g .

An od i zed al u m i n u m an d ti tan i u m al so h ave l ow

scu ffi n g resi stan ce. H ard n ess al on e d oes n ot seem to be a rel i abl e i n d i cati on of scu ffi n g resi stan ce. Th e i n i ti al ru n -i n of g eari n g can be cri ti cal to en su ri n g l on g term servi ce l i fe. I t i s stron g l y recom m en d ed to fol l ow

th e

ru n -i n

proced u res

see

proced u re

recom men d ed

AN SI /AG M A

601 3-A06

by

th e

cl au se

m an u factu rer. 1 1 . 6. 1 ,

For

m ore

AN SI /AG M A

i n formati on

601 1 -I 03

cl au se

on 6. 4

ru n -i n an d

AN SI /AG M A/AWEA 6006-A03 cl au se 6. 7.

5.3.2 Summary of methods to reduce the risk of scuffing -

U se sm ooth tooth su rfaces prod u ced by carefu l g ri n d i n g , h on i n g , pol i sh i n g or ch em i cal l y assi sted

-

Ru n -i n n ew g earsets fol l owi n g m an u factu rer’ s recomm en d ati on s;

-

Protect th e g ear teeth d u ri n g th e cri ti cal ru n -i n peri od by u se of a speci al l u bri can t, coati n g (su ch as

-

U se l u bri can ts of ad eq u ate vi scosi ty for th e operati n g con d i ti on s;

-

U se l u bri can ts th at con tai n an ti scu ff ad d i ti ves;

-

Cool th e g ear teeth by su ppl yi n g an ad eq u ate am ou n t of cool l u bri can t.

pol i sh i n g ;

i ron -m an g an ese ph osph ate), or by pl ati n g (su ch as copper or si l ver);

For ci rcu l ati n g -oi l system s,

u se a h eat exch an g er to cool th e l u bri can t; -

Opti m i ze th e g ear tooth g eom etry by u si n g sm al l teeth , profi l e sh i ft an d profi l e m od i fi cati on ;

-

U se

accu rate

g ear

teeth ,

ri g i d

g ear

m ou n ti n g s,

an d

l ead

m od i fi cati on

to

obtai n

u n i form

l oad

d i stri bu ti on d u ri n g operati on ; -

Avoi d u se of stai n l ess steel , al u m i n u m , or ti tan i u m al l oys si n ce th ey g reatl y i n crease th e ri sk of

-

U se n i tri d i n g for i m proved scu ffi n g resi stan ce.

scu ffi n g ;

6

Plastic deformation

Pl asti c d eform ati on i s perm an en t d eform ati on th at occu rs wh en th e stress exceed s th e yi el d stren g th of th e m ateri al .

I t m ay occu r at th e su rface or su bsu rface of th e acti ve fl an ks of th e g ear teeth d u e to h i g h

H ertzi an stress, or at th e root fi l l ets of th e g ear teeth d u e to h i g h ben d i n g stress (see 9. 5).

6.1

Indentation

Th e acti ve fl an ks of g ear teeth m ay be d am ag ed by i n d en tati on s cau sed by forei g n m ateri al th at becom es trapped between m ati n g teeth .

See Fi g u re 30.

Depen d i n g on th e n u mber, si ze, an d severi ty of th e i n d en tati on s, th e d am ag e m ay or may n ot i n i ti ate oth er types of fai l u re.

I f pl asti c d eform ati on associated wi th th e i n d en tati on s cau ses rai sed areas on th e

tooth su rface, i t creates stress con cen trati on s th at may l ead to su bseq u en t H ertzi an fati g u e.

For g ear

teeth su bj ected to H ertzi an stresses g reater th an 1 . 8 ti m es th e ten si l e yi el d stren g th of th e m ateri al , l ocal , su bsu rface yi el d i n g m ay occu r. Th e su bsu rface pl asti c d eform ati on cau ses g rooves (tru e bri n el l i n g , see 4. 5. 1 ) on th e su rfaces of th e acti ve fl an ks of th e teeth correspon d i n g to th e l i n es of con tact between th e m ati n g g ear teeth .

H i g h H ertzi an stress m i g h t resu l t from l arg e l oad s or g ear tooth i m pact (tooth h am mer,

see 6. 5) cau sed by vi brati on .

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Figure 30 - Severe indentations 6.2

Cold flow

Col d fl ow i s pl asti c d eformati on th at occu rs at a temperatu re l ower th an th e recrystal l i zati on tem peratu re. N ote,

for

steel

th i s

temperatu re

ran g es

from

450° C

to

900° C,

d epen d i n g

on

severi ty

of

pl asti c

d eform ati on , g rai n si ze pri or to pl asti c d eform ati on , tem peratu re at wh i ch pl asti c d eform ati on occu rs, ti m e for wh i ch th e pl asti cal l y d eform ed m etal i s h eated to attai n recrystal l i zati on , an d presen ce of d i ssol ved or u n d i ssol ved el em en ts [28] .

6.3

Hot flow

H ot fl ow i s pl asti c d eform ati on th at occu rs at a tem peratu re h i g h er th an th e recrystal l i zati on tem peratu re. See Fi g u re 31 .

At extrem e tem peratu res, bl ack ferrou s oxi d e, wu sti te (FeO), form s, an d i s i n d i cati ve of

h ot fl ow.

6.4

Rolling

Pl asti c d eform ati on may occu r on th e acti ve fl an ks of g ear teeth cau sed by h i g h H ertzi an stresses i n com bi n ati on wi th both th e rol l i n g an d sl i d i n g acti on of th e g ear m esh .

Di spl acemen t of su rface m ateri al

m ay form a g roove al on g th e pi tch l i n e an d bu rrs on th e ti ps an d i n th e roots of th e d ri vi n g g ear teeth . Th e su rface m ateri al of th e d ri ven g ear m ay be d i spl aced toward th e pi tch l i n e form i n g a ri d g e.

See

Fi g u re 32.

6.5

Tooth hammer

Local , su bsu rface yi el d i n g m ay occu r on g ear teeth th at are su bj ected to h i g h con tact stresses su ch as th ose

cau sed

by “tooth

h am m er”

(vi bratory i m pact wi th

i n term i tten t tooth

con tact separati on ).

Th e

su bsu rface pl asti c d eformati on cau ses sh al l ow g rooves (tru e bri n el l i n g , see 4. 5. 1 ) on th e su rfaces of th e acti ve fl an ks of th e g ear teeth al on g l i n es of con tact between m ati n g teeth .

6.6

See Fi g u re 33.

Rippling

Ri ppl i n g i s peri od i c, wavel i ke u n d u l ati on s [1 0] of th e su rfaces of th e acti ve fl an ks of g ear teeth . peaks or ri d g es of th e u n d u l ati on s ru n perpen d i cu l ar to th e d i recti on of sl i d i n g . th e l en g th of th e tooth , creati n g a fi sh scal e appearan ce. su rface or su bsu rface.

Th e

Th e ri d g es are wavy al on g

Ri ppl i n g i s cau sed by pl asti c d eform ati on at th e

I t u su al l y occu rs u n d er h i g h H ertzi an stress an d bou n d ary-l u bri cated con d i ti on s.

See Fi g u res 34, 35 an d 36.

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Figure 31 - Hot flow

Figure 32 - Plastic deformation by rolling

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Figure 33 - Plastic deformation by tooth hammer

Figure 34 - Rippling

Figure 35 - Rippling Copyright American Gear Manufacturers Association ©AGMA 201 4

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Figure 36 - Rippling 6.7 Ridging Ridging is the development of pronounced ridges and grooves on the active flanks of gear teeth. It frequently occurs on slow speed, heavily-loaded worm or hypoid gear teeth. See Figure 37. 6.8 Burr Burrs are rough, often sharp, extensions formed on the edges of components caused by heavy loading, high friction, rolling, or scuffing. Burrs are also sometimes caused by the manufacturing process.

Figure 37 - Ridging ©AGMA 201 4 – All rights reserved

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A pron ou n ced bu rr can be seen at th e ti p of th e worm th read 's worki n g fl an k i n Fi g u re 38.

Th i s bu rr was

g en erated by pl asti c d eform ati on d u e to th e pressu re an d th e sl i d i n g acti on al on g th e acti ve su rface of th e fl an ks.

6.9

Root fillet yielding

G ear teeth m ay be perm an en tl y ben t i f th e ben d i n g stress i n th e root fi l l ets exceed s th e yi el d stren g th of th e m ateri al . Th e ben d i n g d efl ecti on at i n i ti al yi el d i n g i s sm al l an d th ere i s a marg i n of safety before g ross yi el d i n g cau ses si g n i fi can t g ear tooth spaci n g error.

I f th e teeth h ave su ffi ci en t d u cti l i ty, i n i ti al yi el d i n g at th e root

fi l l ets red i stri bu tes th e stress an d l owers th e stress con cen trati on . resu l t i n rou g h er ru n n i n g an d a h i g h er n oi se l evel .

H en ce, root fi l l et yi el d i n g m ay on l y

H owever, i f th e yi el d i n g cau ses si g n i fi can t spaci n g

errors between l oad ed teeth th at are perm an en tl y ben t an d u n l oad ed teeth th at are n ot, su bseq u en t rotati on of th e g ears u su al l y resu l ts i n d estru cti ve i n terferen ce between th e pi n i on an d g ear teeth .

6.1 0 Tip-to-root interference Pl asti c d eform ati on , ad h esi on , abrasi on an d pi tti n g m ay occu r on th e roots of on e g ear an d i n th e tooth ti ps of th e m ati n g g ear teeth d u e to ti p-to-root i n terferen ce.

Th e i n terferen ce m ay be cau sed by g eometri c

errors i n th e profi l es of th e g ear teeth , en g ag em en t bel ow th e form d i am eter, i n ad eq u ate ti p or root rel i ef, spaci n g errors, or i n su ffi ci en t cen ter d i stan ce.

See Fi g u re 39.

As g ear teeth approach on e an oth er n ear th e start of en g ag emen t, th e corn ers of teeth on th e d ri ven g ear are very cl ose to th e d ed en d u m fl an ks of th e d ri vi n g teeth .

H i g h l oad s m i g h t d efl ect th e teeth al read y i n

m esh an d cl ose th e g ap between i n com i n g teeth , resu l ti n g i n ti p-to-root i n terferen ce.

Su bseq u en t cycl i c

con tact on areas wi th d am ag e from ti p-to-root i n terferen ce m i g h t l ead to H ertzi an fati g u e [1 1 ].

N ote th at

operati n g wi th ti p-to-root i n terferen ce can resu l t i n tooth fai l u re or catastroph i c bl an k fai l u re (typi cal l y th rou g h th e ri m ).

6.1 1 Tight mesh Typi cal l y wh en th e m esh i s ru n n i n g ti g h t, scu ffi n g wi l l appear on th e l oad fl an k as wel l as th e coast fl an k on th e m ati n g g ear.

Figure 38 - Burr

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Figure 39 - Tip-to-root interference

7

Hertzian fatigue

Repeated H ertzi an stresses may cau se su rface or su bsu rface fati g u e cracks an d th e d etach men t of m ateri al frag m en ts from th e g ear tooth su rface.

7.1

Macropitting

M acropi tti n g m ay occu r wh en fati g u e cracks i n i ti ate ei th er at th e su rface of th e g ear tooth or at a sh al l ow d epth bel ow th e su rface [1 2] .

Th e crack u su al l y propag ates for a sh ort d i stan ce i n a d i recti on rou g h l y

paral l el to th e tooth su rface before tu rn i n g or bran ch i n g to th e su rface.

Wh en th e cracks h ave g rown l on g

en ou g h to separate a pi ece of th e su rface materi al , a m acropi t i s form ed . u su al l y sh arp an d an g u l ar.

Th e ed g es of a m acropi t are

Cracks m ay be fou n d n ear th e bou n d ary of th e m acropi t an d fati g u e “beach

m arks” (see cl au se 1 0) may be evi d en t on th e crater bottom .

See Fi g u res 40 to 51 .

Based on th e n atu re an d severi ty of th e d amag e, m acropi tti n g can be categ ori zed as n on prog ressi ve, prog ressi ve, poi n t-su rface-ori g i n (PSO), or spal l .

7.1 .1

Nonprogressive macropitting

N on prog ressi ve m acropi tti n g n ormal l y con si sts of smal l m acropi ts th at occu r i n l ocal i zed areas. occu r i n l ocal i zed areas an d ten d to red i stri bu te th e l oad by removi n g h i g h asperi ti es. m ore even l y d i stri bu ted , th e m acropi tti n g stops.

NOTE:

Th ey

Wh en th e l oad i s

See Fi g u re 42.

Th e sh arp ed g es of n on prog ressi ve m acropi tti n g m ay wear over ti m e an d becom e sm ooth d u e to wear.

Figure 40 - Cross section through a tooth flank showing how a pit develops below the surface

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Figure 41 - SEM image - pitting damage caused by Hertzian fatigue, showing fatigue cracks near boundary of pit

Figure 42 - Nonprogressive macropitting

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Figure 43 - Progressive macropitting 7.1 .2 Progressive macropitting Prog ressi ve m acropi tti n g n ormal l y con si sts of m acropi ts th at g row at an i n creasi n g rate u n ti l a si g n i fi can t porti on of th e tooth su rface h as m acropi ts of vari ou s sh apes an d si zes.

See Fi g u re 43.

7.1 .3 Point-surface-origin macropitting Poi n t-su rface-ori g i n (PSO) m acropi tti n g con si sts of macropi ts th at are rel ati vel y sh al l ow bu t l arg e i n area. Th e fati g u e crack exten d s from an ori g i n at th e su rface of th e tooth i n a fan -sh aped m an n er u n ti l th i n fl akes of m ateri al break ou t an d form a tri an g u l ar crater [1 1 ] .

See Fi g u res 44 th rou g h 50.

Figure 44 - Point-surface-origin macropitting Copyright American Gear Manufacturers Association ©AG M A 201 4 – Al l ri g h ts reserved

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Figure 45 - Point-surface-origin macropitting

Figure 46 - Point-surface-origin macropitting

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Figure 47 - Point-surface-origin macropitting on carburized helical gear at 1 .5 × 1 0 7 cycles

Figure 48 - Point-surface-origin macropitting on carburized helical gear at 3.0 × 1 0 7 cycles

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Figure 49 - Point-surface-origin macropitting on carburized helical driven pinion

Figure 50 - Point-surface-origin macropitting 7.1 .4 Spall macropitting Spall macropitting is progressive macropitting that occurs when macropits coalesce and form irregular craters that cover a significant area of the tooth surface. See Figure 51 . There is no endurance limit for Hertzian fatigue, and macropitting occurs even at low stresses if the gears are operated long enough. Because there is no endurance limit, gear teeth must be designed for a suitable, finite lifetime.

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Figure 51 - Spall macropitting 7.1 .4.1 Methods for reducing the risk of macropitting To prol on g th e m acropi tti n g l i fe of a g earset, th e d esi g n er m u st keep th e H ertzi an stress l ow, m ateri al stren g th h i g h , m ateri al rel ati vel y free of i n cl u si on s, an d th e l u bri can t speci fi c fi l m th i ckn ess h i g h .

Th ere

are several g eom etri c vari abl es su ch as d i am eter, face wi d th , n u m ber of teeth , pressu re an g l e, an d h el i x an g l e th at m ay be opti m i zed to l ower th e H ertzi an stress. M ateri al al l oys an d h eat treatm en t are sel ected to obtai n h ard tooth su rfaces wi th h i g h stren g th , su ch as carbu ri zi n g or n i tri d i n g .

M axi m u m m acropi tti n g

resi stan ce i s obtai n ed wi th carbu ri zed g ear teeth becau se th ey h ave h ard su rfaces,

an d carbu ri zi n g

i n d u ces ben efi ci al com pressi ve resi d u al stresses th at effecti vel y l ower th e sh ear stresses.

H i g h l u bri can t

speci fi c fi l m th i ckn ess i s obtai n ed by u si n g sm ooth tooth su rfaces an d an ad eq u ate su ppl y of cool , cl ean an d d ry (free of water) l u bri can t th at h as h i g h vi scosi ty an d a h i g h pressu re-vi scosi ty coeffi ci en t. M acropi tti n g m i g h t i n i ti ate at th e su rface or at a su bsu rface d efect, su ch as a n on m etal l i c i n cl u si on .

Wi th

g ear teeth , m acropi ts are m ost often su rface-i n i ti ated becau se th e EH L fi l m th i ckn ess i s u su al l y l ow, resu l ti n g i n a rel ati vel y h i g h d eg ree of m etal -to-m etal con tact. I n teracti on between asperi ti es an d con tact at d efects, su ch as n i cks, fu rrows, or d en ts creates su rface-i n i ti ated , rath er th an su bsu rface i n i ti ated cracks. PSO m acropi tti n g i s often cau sed by g eom etri c stress con cen trati on (G SC) [1 1 ] . For h i g h -speed g ears wi th sm ooth tooth su rfaces, EH L fi l m th i ckn ess i s g reater an d su bsu rface i n i ti ated m acropi tti n g , rath er th an su rface-i n i ti ated m acropi tti n g , m i g h t pred om i n ate.

I n th ese cases, m acropi tti n g

u su al l y starts at a su bsu rface i n cl u si on , wh i ch acts as a poi n t of stress con cen trati on .

Cl ean er steel s

su ch as th ose prod u ced by vacu u m rem el ti n g , i n crease m acropi tti n g l i fe by red u ci n g th e si ze an d n u mber of i n cl u si on s. Con tam i n ati on from water i n l u bri can t promotes m acropi tti n g , an d sol i d parti cl es i n l u bri can t prom ote m acropi tti n g by i n d en ti n g tooth su rfaces, cau si n g stress con cen trati on s an d d i sru pti n g th e l u bri can t fi l m . At presen t, th e i n fl u en ce of l u bri can t ad d i ti ves on m acropi tti n g i s u n resol ved .

7.1 .4.2 Summary of methods to reduce the risk of macropitting -

Red u ce H ertzi an stresses by red u ci n g l oad s or opti mi zi n g g ear g eom etry;

-

U se cl ean steel , properl y h eat treated to h i g h su rface h ard n ess, preferabl y by carbu ri zi n g ;

-

U se sm ooth tooth su rfaces prod u ced by carefu l g ri n d i n g , h on i n g , or pol i sh i n g ;

-

U se an ad eq u ate am ou n t of cool , cl ean an d d ry (free of water) l u bri can t of ad eq u ate vi scosi ty;

-

For

su rface

processi n g .

h ard en ed

g eari n g ,

en su re

ad eq u ate

su rface

h ard n ess

an d

case

d epth

after

fi n al

N ote th at excessi ve su rface h ard n ess m ay l ead to oth er probl em s, su ch as ri sk of

g ri n d i n g cracks, see 8. 2. 1 .

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7.2

ANSI/AGMA 1 01 0-F1 4

Micropitting

Micropitting is Hertzian fatigue caused by cyclic Hertzian stresses and plastic flow on the asperity scale [1 , 1 3, 1 4, 1 5, 1 6, 1 7]. It results in ultra-small cracks at the surface and formation of micropits, resulting in loss of material. Ultra small cracks are different from microcracks as defined in AGMA 923-B05. See Figures 52 through 57.

Figure 52 - Micropitting on misaligned carburized gear

Figure 53 - Micropitting on induction hardened spur gear with crowned teeth Copyright American Gear Manufacturers Association ©AGMA 201 4

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Figure 54 - Micropitting on nitrided and ground spur gear

Figure 55 - Detail of tooth surface showing micropitting

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AMERICAN NATIONAL STANDARD

ANSI/AGMA 1 01 0-F1 4

Figure 56 - Detail of tooth surface showing micropitting at 1 000X magnification

Figure 57 - Regularly distributed micropitting Micropitting is influenced by: -

Operating conditions during run-in and service  Load  Speed  Temperature

Copyright American Gear Manufacturers Association ©AGMA 201 4

– All rights reserved

41

AMERICAN NATIONAL STANDARD -

G ear properti es



-

ANSI/AGMA 1 01 0-F1 4

M acrog eom etry (basi c tooth d i m en si on s)



M i crog eom etry (tooth m od i fi cati on s)



Su rface topog raph y (textu re)



Su rface m etal l u rg y



Su rface ch em i stry

Properti es of ru n -i n an d servi ce l u bri can ts



Rh eol og y



Ch em i stry



Cl ean l i n ess

I n ad d i ti on to H ertzi an stress d u e to n orm al l oad i n g , sl i d i n g between g ear teeth cau ses tracti on forces th at su bj ect asperi ti es to sh ear stresses. i n cu bati on

peri od

d u ri n g

wh i ch

Th e fi rst 1 0

d amag e

4

to 1 0

con si sts

6

cycl es of stress occu rri n g d u ri n g ru n -i n are an

pri m ari l y

of

pl asti c

d eform ati on

at

asperi ti es.

M acroscopi cal l y, su rfaces appear g l azed or g l ossy. M i croscopi cal l y, su rface asperi ti es appear pl asti cal l y d eform ed an d ori g i n al -m ach i n i n g marks m i g h t be parti al l y or total l y obl i terated .

Cycl i c H ertzi an an d sh ear stresses accu m u l ate pl asti c d eform ati on on

asperi ti es an d at sh al l ow d epth s bel ow asperi ti es.

Pl asti c fl ow prod u ces ten si l e resi d u al stresses, an d

wi th su ffi ci en t cycl es, fati g u e cracks i n i ti ate. After i n cu bati on , su rfaces wi th

m i cropi ts

a g ear tooth

rapi d l y n u cl eate, i n specti on

d am ag e can be extrem e after on l y 1 0

6

g row,

an d

coal esce.

Peri od i c i n specti on

of g ear tooth

m ach i n e d i scl oses a stead y rate of su rface d eteri orati on

an d

cycl es.

To th e u n ai d ed eye, m i cropi tted su rfaces appear d u l l , etch ed , frosted , m atte, or stai n ed wi th patch es of g ray. Th e i n cl i n ed crater fl oors refl ect l i g h t preferen ti al l y. m i cropi tti n g .

Th erefore, u se i n ten se d i recti on al l i g h ti n g to d i scl ose

Try d i fferen t l i g h ti n g an g l es to em ph asi ze featu res.

U n d er m ag n i fi cati on , th e su rface appears to be covered by very fi n e pi ts th at are typi cal l y l ess th an 1 0 20

μm

d eep.

M etal l u rg i cal secti on s cu t tran sversel y th rou g h m i cropi ts sh ow fati g u e cracks start at or n ear th e g ear su rface an d g row at a sh al l ow an g l e (typi cal l y 1 0 - 30° , bu t someti m es as steep as 45° ) to th e su rface. Th e cracks typi cal l y exten d d eeper th an th e vi si bl e m icropi ts an d su bsu rface crack n etworks are u su al l y m u ch m ore exten si ve th an wou l d be i mpl i ed from su rface featu res.

A m i cropi t form s wh en a bran ch crack

con n ects th e su bsu rface m ai n crack wi th th e su rface an d separates a smal l pi ece of m ateri al . resu l ti n g pi t m i g h t be on l y 1 0 - 20

μm

Th e

d eep an d n ot resol ved by th e u n ai d ed eye.

Li ke m acropi tti n g , m i cropi tti n g cracks g row opposi te to th e d i recti on of sl i d i n g at th e g ear tooth su rface. Becau se

sl i d e

d i recti on s

reverse

as

th e

d i recti on s above an d bel ow th e pi tch l i n e.

pi tch

line

is

crossed ,

m i cropi tti n g

cracks

g row i n

opposi te

I f m i cropi tti n g g rows across th e pi tch l i n e, i t m akes th e pi tch

l i n e read i l y d i scern i bl e becau se th e opposi te i n cl i n ati on s of th e fl oors of m i cropi t craters scatter l i g h t i n opposi te d i recti on s above an d bel ow th e pi tch l i n e.

See Fi g u re 55.

Al l g ears are su scepti bl e to m i cropi tti n g i n cl u d i n g extern al , i n tern al , spu r, h el i cal , an d bevel .

M i cropi tti n g

can occu r wi th al l h eat treatm en ts i n cl u d i n g th rou g h h ard en ed , carbu ri zed , n i tri d ed , fl am e h ard en ed , an d i n d u cti on h ard en ed .

See Fi g u res 52 th rou g h 54.

M i cropi tti n g m i g h t occu r m ore freq u en tl y on su rface

h ard en ed g ear teeth th an on th rou g h h ard en ed g ear teeth becau se l oad s are u su al l y h i g h er on su rface h ard en ed teeth . G rou n d teeth are especi al l y vu l n erabl e to m i cropi tti n g .

Experi men ts [1 ] h ave sh own th at

fl am e-h ard en ed an d i n d u cti on -h ard en ed g ears h ave l ess resi stan ce to m i cropi tti n g th an carbu ri zed g ears of th e sam e h ard n ess.

Th i s m i g h t be d u e to th e l ower carbon con ten t of th e su rface l ayers of fl am e-

h ard en ed an d i n d u cti on -h ard en ed g ears. G ear teeth d ed en d a are vu l n erabl e to m i cropi tti n g , especi al l y al on g th e start of acti ve profi l e (SAP) an d th e l owest poi n t of si n g l e tooth pai r con tact (LPSTC).

H owever, mi cropi tti n g can occu r an ywh ere on th e

acti ve fl an ks of g ear teeth [1 3] . Th ere can be m i cropi tti n g on l y on th e pi n i on , on l y on th e g ear, or on both .

G en eral l y, th e g ear wi th th e

rou g h est su rface cau ses m i cropi tti n g on th e m ati n g g ear, especi al l y i f i t i s h ard er th an th e mati n g g ear.

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42

AMERICAN NATIONAL STANDARD

ANSI/AGMA 1 01 0-F1 4

M i cropi tti n g m i g h t n ot cau se catastroph i c fai l u re.

I t m i g h t occu r on l y i n patch es an d m i g h t arrest after th e

tri bol og i cal con d i ti on s i m prove after ru n n i n g -i n . M i l d pol i sh i n g m i g h t rem ove m i cropi ts an d smooth tooth su rfaces d u e to wear.

H owever, arrest i s u n pred i ctabl e, an d m i cropi tti n g g en eral l y red u ces g ear tooth

accu racy, i n creases n oi se, an d can escal ate to fu l l -scal e macropi tti n g or oth er fai l u re mod es su ch as scu ffi n g or ben d i n g fati g u e [1 , 1 4, 1 5] .  

Lu bri can t speci fi c fi l m th i ckn ess i s an i m portan t param eter th at i n fl u en ces m i cropi tti n g . m ost read i l y on

g ear teeth

m esh i n g

wi th

l u bri cated wi th l ow-vi scosi ty l u bri can ts.

teeth

th at h ave rou g h

su rfaces,

Dam ag e occu rs

especi al l y wh en

th ey are

Su rface rou g h n ess i s th e m ost i m portan t parameter an d i t h as a

stron g er i n fl u en ce th an EH L fi l m th i ckn ess.

Gear pai rs fi n i sh ed wi th speci al g ri n d i n g wh eel s [1 6] or oth er

processes to m i rrorl i ke fi n i sh h ave effecti vel y el i m i n ated m i cropi tti n g . Sl ow-speed g ears are pron e to m i cropi tti n g becau se th ei r EH L fi l m th i ckn ess i s l ow. m i cropi tti n g ,

m axi m i ze

speci fi c

fi l m

th i ckn ess

by

l u bri can ts, an d i f possi bl e, h i g h pi tch l i n e vel oci ty.

u si n g

sm ooth

g ear

tooth

Th erefore, to preven t

su rfaces,

h i g h -vi scosi ty

AN SI /AG M A 9005-E02 g i ves recom m en d ati on s for

vi scosi ty as a fu n cti on of pi tch l i n e vel oci ty. Ru n -i n i s cri ti cal becau se i t i s th e i n cu bati on peri od for m i cropi tti n g .

Du ri n g i n cu bati on , con tacts between

asperi ti es on opposi n g su rfaces occu r freq u en tl y, cau si n g pl asti c d eform ati on of asperi ti es; th e pri n ci pl e cau se of m i cropi tti n g .

I n ad d i ti on , ad h esi on an d abrasi on at asperi ti es g en erate wear d ebri s.

U si n g a seri es of i n creasi n g l oad s al l ows prog ressi ve red u cti on of rou g h n ess th rou g h th e acti on of m i l d ad h esi on an d l i m i ted pl asti c d eform ati on .

Th i s con trol l ed ru n -i n m i n i m i zes pl asti c d eform ati on wh i l e

l i m i ti n g ad h esi on an d abrasi on to th e i ron oxi d e l ayer coveri n g asperi ti es.

M i l d ad h esi on con si sts of sm al l

j u n cti on s th at g en erate wear parti cl es sm al l er th an th e su rface rou g h n ess.

I f ad h esi on rem ai n s m i l d ,

asperi ti es are even tu al l y fl atten ed by ad h esi on an d pl asti c d eform ati on , an d su bseq u en t d eform ati on rem ai n s el asti c for th at parti cu l ar l oad .

Th en , wh en ru n -i n i s com pl ete, asperi ti es carry th e l oad sol el y by

el asti c d eform ati on . I f ad h esi on cau ses stron g bon d s th at break th rou g h oxi d e l ayers, ad h esi on escal ates to scu ffi n g , l arg e wear parti cl es are g en erated , an d su rfaces becom e rou g h er rath er th an sm ooth er. Th e ru n -i n properti es are l i kel y to d epen d on l u bri can t ch em i stry, tem peratu re, an d sl i d i n g vel oci ty, so experi men ts on actu al g ears are n ecessary to d eterm i n e a g ood ru n -i n l u bri can t. Experi m en ts [1 7] h ave sh own th at zi n c d i al kyl d i th i oph osph ate (Zn DTP) an ti wear ad d i ti ves can be d etri men tal to ru n -i n . Water

con tam i n ati on

prom otes

m i cropi tti n g

in

g ears

an d

beari n g s,

an d

si g n i fi can tl y

red u ces

th e

an ti corrosi on , EH L fi l m form ati on , an d fri cti on red u ci n g properti es of l u bri can ts.

7.2.1 Th e

Summary of methods to reduce the risk of micropitting fol l owi n g

g u i d el i n es

su m m ari ze

m eth od s

for m i ti g ati n g

an d

preven ti n g

m i cropi tti n g .

N ot every

m easu re m i g h t be ach i evabl e or appl i cabl e for a g i ven appl i cati on , bu t as man y as possi bl e sh ou l d be i m pl em en ted wh en appropri ate. -

M axi m i ze speci fi c fi l m th i ckn ess



I n crease oi l fi l m th i ckn ess

o o o o

U se h i g h est practi cal oi l vi scosi ty; Ru n g ears at h i g h speed i f possi bl e; Cool g ear teeth ; Revi ew l u bri can t operati n g vi scosi ty an d ch an g e l u bri can ts to ach i eve h i g h er operati n g fi l m th i ckn ess. Con su l t both th e g eari n g m an u factu rer an d l u bri can t su ppl i er before swi tch i n g l u bri can ts.



Red u ce su rface rou g h n ess

o o o o

o o o

Avoi d sh ot-peen ed fl an ks u n l ess th e fl an k su rface i s fi n i sh ed after sh ot peen i n g ; H on e or pol i sh g ear teeth , or bu rn i sh by ru n n i n g g ears ag ai n st a h ard , sm ooth m aster; M ake th e h ard est g ear as sm ooth as possi bl e; Coat teeth wi th i ron -m an g an ese ph osph ate, copper, or si l ver to l i m i t ad h esi on an d scu ffi n g ri sk; Ru n -i n wi th a speci al l u bri can t wi th ou t Zn DTP an ti wear ad d i ti ves; Pre-fi l ter l u bri can t an d u se a fi n e fi l ter (

≤

6

µ m ) d u ri n g

ru n -i n ;

Keep oi l cool d u ri n g ru n -i n ;

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43

AMERICAN NATIONAL STANDARD o o

ANSI/AGMA 1 01 0-F1 4

Ru n -i n g ears u si n g a seri es of i n creasi n g l oad s an d appropri ate speed ; Drai n l u bri can t an d fl u sh g earbox after ru n -i n , ch an g e th e fi l ter i f th ere i s on e, an d fi l l wi th th e servi ce l u bri can t.

-

Opti m i ze g ear g eometry



For paral l el axi s g ears, u se at l east 20 teeth i n th e pi n i on to i n crease m i cropi tti n g resi stan ce;



U se n on -h u n ti n g g ear rati o, especi al l y for g ears wi th l ow speci fi c fi l m th i ckn ess [29] ;



U se h el i cal g ears wi th axi al con tact rati o d i am eter rati o, m a

≤

m

F

≥

2. 0; U se aspect rati o, al so kn own as face wi d th to

1 . 0 for spu r an d si n g l e-hel i cal g ears (see AG M A 901 );

m

≤



U se aspect rati o, al so kn own as face wi d th to d i am eter rati o,



M i n i m i ze H ertzi an stress by speci fyi n g h i g h accu racy an d opti mi zi n g cen ter d i stan ce, face wi d th ,

a

2. 0 for d ou bl e-h el i cal g ears;

pressu re an g l e, an d h el i x an g l e;

-

-



U se profi l e sh i ft to m i n i m i ze speci fi c sl i d i n g ;



U se proper profi l e an d l ead m od i fi cati on ;



Avoi d ti p-to-root i n terferen ce.

Opti m i ze m etal l u rg y



M axi m i ze pi n i on h ard n ess;



M ake pi n i on 2 H RC poi n ts h ard er th an g ear;



U se approxi m atel y 20% retai n ed au sten i te.

Opti m i ze l u bri can t properti es



U se oi l wi th h i g h m i cropi tti n g resi stan ce as d eterm i n ed by tests on actu al g ears;



U se oi l wi th l ow tracti on coeffi ci en t;



U se oi l wi th h i g h pressu re-vi scosi ty coeffi ci en t;



Avoi d oi l s wi th ag g ressi ve an ti scu ff ad d i ti ves;



Avoi d oi l s wi th vi scosi ty i n d ex i m provers;



Keep oi l cool ;



Keep oi l cl ean of sol i d con tam i n an ts;



Keep oi l free of water.

7.3

Subsurface initiated failures

Tabl e 2 sh ows fai l u re mod es th at h ave su bsu rface ori g i n s.

7.3.1

Inclusion origin failures

N on metal l i c i n cl u si on s are often th e root cau se of cracks th at resu l t i n fai l u re m od es su ch as th e on es sh own i n Tabl e 2.

H arm fu l effects of n on m etal l i c i n cl u si on s d epen d on th e ch em i stry, si ze, l ocati on , an d

q u an ti ty of th e i n cl u si on s, ten si l e stren g th of th e steel an d resi d u al stresses i mm ed i atel y ad j acen t to th e i n cl u si on s.

Wi th case h ard en ed g ears, m an y fai l u res i n i ti ate at i n cl u si on s bel ow th e case/core bou n d ary,

wh ere resi d u al stresses from case h ard en i n g are ten si l e (see 1 0. 2. 6).

H ard , n on d eformabl e i n cl u si on s

su ch

n i tri d e,

as

cal ci u m

al u m i n ates,

especi al l y d am ag i n g , stress con cen trators.

si n g l e-ph ase

al u m i n a,

spi n el s,

ti tan i u m

an d

som e si l i cates

are

wh ereas m an g an ese su l fi d e i n cl u si on s are reg ard ed as bei n g th e l east poten t See [1 8] .

7.3.2 Origins of nonmetallic inclusions Steel i s refi n ed i n several stag es d u ri n g m an u factu re. Cal ci u m or mag n esi u m oxi d e sl ag s are u sed d u ri n g th e i n i ti al m el ti n g process to rem ove oxi d i zed i m pu ri ti es from th e m ol ten m etal .

Su bseq u en tl y, i n th e

l ad l e, al u m i n u m , si l i con , an d cal ci u m are i n j ected i n to th e m ol ten steel to promote fu rth er d eoxi d ati on an d d esu l fu ri zati on .

Table 2 - Failure modes that have subsurface origins Failure mode Clause Su bcase fati g u e

7. 4

Case/core separati on

8. 4

Su bsu rface i n i ti ated ben d i n g fati g u e cracks

1 0. 2. 6

Tooth i n teri or fati g u e fractu re, TI FF

1 0. 2. 7

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AMERICAN NATIONAL STANDARD

ANSI/AGMA 1 01 0-F1 4

I n d i g en ou s n on m etal l i c i n cl u si on s resu l t from th e d eoxi d ati on

th at occu rs d u ri n g steel m aki n g .

M ost

i n cl u si on s ori g i n ate i n th e m el t at h i g h tem peratu res, some form d u ri n g sol i d i fi cati on , an d som e to a l esser Al u m i n u m an d si l i con form i n cl u si on s of al u m i n u m oxi d e, Al 2 O 3 an d si l i con

exten t after sol i d i fi cati on . d i oxi d e, Si O 2 .

M ost of th e oxi d es fl oat off th e m el t i n to th e sl ag .

Arg on g as sti rri n g or i n d u cti ve sti rri n g

are u sed to en cou rag e th e i n cl u si on s to fl oat ou t of th e m el t.

Typi cal con cen trati on s of al u m i n u m i n th e

sol i d i fi ed steel are i n th e ran g e of 0. 02% -0. 04% by wei g h t.

M an g an ese d esu l fu ri zes steel by form i n g

m an g an ese su l fi d e, M n S.

Cal ci u m h as a stron g affi n i ty for su l fu r an d i s ad d ed to th e m el t to affect th e

com posi ti on , si ze, an d d i stri bu ti on of su l fi d e i n cl u si on s.

Th e i n cl u si on s th at d o n ot separate from th e m el t

i n to th e sl ag rem ai n i n th e m ateri al an d can affect th e perform an ce of th e m ateri al i n servi ce d epen d i n g on th ei r type, si ze, sh ape, q u an ti ty, l ocati on , resi d u al stresses i m med i atel y ad j acen t to th e i n cl u si on s, an d th e stresses i m posed on th e fi n al part. Exog en ou s non m etal l i c i n cl u si on s ari se from sl ag en trapm en t, con tam i n ati on from frag men ts of refractory m ateri al th at separate from fu rn ace l i n i n g s, l ad l es, ru n n ers, ri sers, an d i n g ots th at th e m ol ten steel com es i n con tact wi th , an d al so from oxi d ati on by th e ai r wh en mol ten steel i s pou red wi th ou t i sol ati on from th e en vi ron m en t.

7.4

Subcase fatigue

Su bcase

fati g u e

m ay occu r i n

h ard en ed , and fl am e h ard en ed ).

su rface

h ard en ed

g ears

(for exam pl e,

carbu ri zed ,

n i tri d ed ,

i n d u cti on

Th e ori g i n of th e fati g u e crack i s bel ow th e su rface of th e g ear teeth i n

th e tran si ti on zon e between th e case an d core wh ere cycl i c sh ear stresses exceed sh ear fati g u e stren g th . Typi cal l y, th e crack ru n s paral l el to th e su rface of th e g ear tooth fl an k before bran ch i n g to th e su rface. Th e bran ch ed cracks may appear at th e su rface as fi n e l on g i tu d i n al cracks on on l y a few teeth . su rface cracks j oi n tog eth er, l on g sh ard s of th e tooth su rface may break away. l on g i tu d i n al wi th a rel ati vel y fl at bottom an d sh arp, perpen d i cu l ar ed g es. evi d en t on th e crater bottom form ed by propag ati on of th e m ai n crack.

I f th e

Resu l ti n g craters are

Fati g u e beach m arks m ay be

See Fi g u re 58.

Su bcase fati gu e i s i n fl u en ced by H ertzi an stresses, resi d u al stresses an d materi al fati g u e stren g th .

Th e

su bsu rface d i stri bu ti on of resi d u al stresses an d fati g u e stren g th d epen d s on th e case h ard n ess, case d epth an d core h ard n ess [1 9] . Th e m axi m u m g ri n d stock th at wi l l be rem oved sh ou l d be accou n ted for wh en d esi g n in g case d epth to en su re fi n i sh ed case d epth i s ad eq u ate.

To preven t su bcase fati g u e,

steel s m u st h ave ad eq u ate h ard en abi l i ty to obtai n opti m u m case an d core properti es. i m portan t to

u se

cl ean

steel

becau se

i n cl u si on s

m ay i n i ti ate

fati g u e

cracks

I t i s especi al l y

i f th ey occu r n ear th e

case/core i n terface i n areas of ten si l e resi d u al stress.

Figure 58 - Subcase fatigue Copyright American Gear Manufacturers Association ©AG M A 201 4 – Al l ri g h ts reserved

45

AMERICAN NATIONAL STANDARD

ANSI/AGMA 1 01 0-F1 4

Overh eati n g g ear teeth d u ri n g operati on or man u factu ri n g , su ch as su rface tem per from g ri n d i n g , m ay l ower case h ard n ess, al ter resi d u al stresses, an d red u ce resi stan ce to su bcase fati g u e. See 8. 2. 2 for d i scu ssi on of su rface tem per from g ri n d i n g . Referen ces [1 9] th rou g h [24] g i ve m eth od s for an al yzi n g th e ri sk of su bcase fati g u e.

7.4.1

Summary of methods to reduce the risk of subcase fatigue

-

Red u ce H ertzi an stresses by red u ci n g l oad s or opti mi zi n g g ear g eom etry;

-

U se cl ean steel wi th ad eq u ate h ard en abi l i ty to obtai n acceptabl e case an d core properti es;

-

Ach i eve acceptabl e val u es of case h ard n ess, case d epth an d core h ard n ess to m axi m i ze resi stan ce to su bcase fati g u e;

-

Avoi d overh eati n g g ear teeth d u ri n g operati on or m an u factu ri n g ;

-

U se

an al yti cal

m eth od s

to

en su re

th at

su bsu rface

stresses

do

n ot

exceed

su bsu rface

fati g u e

stren g th s.

8

Cracking and other surface damage

Asi d e from cracks i n th e g ear tooth root fi l l ets cau sed by ben d i n g fati g u e, cracks m ay occu r el sewh ere on th e g ear d u e to m ech an i cal

stress,

th erm al

stress,

m ateri al

fl aws (for exam pl e,

see Fi g u re 59),

or

i m proper processi n g .

8.1

Hardening cracks

Cracki n g i n h eat treatm en t u su al l y occu rs d u ri n g or after q u en ch i n g . H ard en i n g cracks are g en eral l y i n terg ran u l ar wi th th e crack ru n n i n g from th e su rface toward th e cen ter of m ass i n a rel ati vel y strai g h t l i n e.

I f th e cracki n g occu rs pri or to tem peri n g , th e fractu re su rfaces wi l l be

d i scol ored by oxi d ati on wh en th e g ear i s exposed to th e fu rn ace atm osph ere d u ri n g tem peri n g .

See

Fi g u re 60.

Figure 59 - Crack at a forging defect

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Figure 60 - Hardening cracks Cracking in heat treatment occurs because of excessive localized stresses. These may be caused by nonuniform heating or cooling, or by volume changes due to phase transformation. Stress risers will make the part more susceptible to cracking. Crack formation may be related to some of the same factors that cause intergranular fracture in overheated steels. Cracks resulting from stress induced by heat treatment usually appear immediately, but may appear after a period of time or in operation.

8.1 .1

Thermal stresses

Thermal stresses are caused by temperature differences between the interior and exterior of the gear, and increase with the rate of temperature change. Cracking can occur either during heating or cooling. The cooling rate is influenced by the geometry of the gear, the agitation of the quench, quench medium, and temperature of the quenchant. The temperature gradient is higher and the risk of cracking greater with thicker sections, asymmetric gear blanks and variable thickness rims and webs.

8.1 .2 Stress concentration Features such as sharp corners, the number, location and size of holes, deep keyways, splines, and abrupt changes in section thickness within a part cause stress concentrations, which increase the risk of cracking. Surface and subsurface defects such as nonmetallic inclusions, forging defects such as hydrogen flakes, internal ruptures, seams, laps, and tears at the flash line increase the risk of cracking.

8.1 .3 Quench severity Quenching conditions and severity should be designed considering size and geometry of the gear, required metallurgical properties, and hardenability of the steel. Quench severity and the risk of cracking are greater with vigorously agitated, caustic, or brine quenchants and much less with quiescent, slow-oil or polymer quenchants. Therefore, quenching should be only as severe as required. Hardening cracks may occur after quenching if the gear is allowed to stand without proper tempering since hydrogen may diffuse to an inclusion where it can initiate a crack.

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8.1 .4 Phase transformation Tran sform ati on of au sten i te i n to m arten si te i s al ways accompan i ed by expan si on , an d m ay resu l t i n cracki n g .

See [24] .

8.1 .5 Steel grades I n g en eral ,

th e carbon con ten t of steel sh ou l d n ot exceed

cracki n g wi l l i n crease.

th e req u i red l evel ;

oth erwi se,

th e ri sk of

Th e su g g ested averag e maxi m u m carbon con ten t for fast q u en ch es su ch as

water, bri n e, an d cau sti c q u en ch i n g are g i ven bel ow: I n d u cti on h ard en i n g : Com pl ex sh apes

0. 40%

Si m pl e sh apes

0. 60%

Fu rn ace h ard en i n g : Com pl ex sh apes

0. 35%

Si m pl e sh apes

0. 40%

Very si m pl e sh apes (su ch as bars)

0. 50%

8.1 .6 Part defects Su rface d efect or weakn ess i n th e m ateri al may al so prom ote cracki n g , for exam pl e, d eep su rface seams or n on m etal l i c stri n g ers i n both h ot-rol l ed an d col d -fi n i sh ed bars. stam p i m pressi on s.

Oth er probl em s are i n cl u si on s an d steel

Forg i n g d efects i n sm al l forg i n g s, su ch as seam s, l aps, fl ash l i n e or sh eari n g cracks

as wel l as i n h eavy forg i n g s su ch as h yd rog en fl akes an d i n tern al ru ptu res, ag g ravate cracki n g .

Si m i l arl y,

som e casti n g d efects su ch as porosi ty, m ay prom ote cracki n g .

8.1 .7 Heat treating practice Th rou g h h ard en i n g al l oy steel s sh ou l d be n orm al i zed pri or to h ard en i n g or an y oth er h i g h -tem peratu re treatm en t,

su ch

as forg i n g

or wel d i n g ,

to prod u ce g rai n -refi n ed

m i crostru ctu re an d

rel i eve stresses.

Carbu ri zi n g al l oy steel s sh ou l d be n ormal i zed or n orm al i zed q u en ch ed an d tempered pri or to carbu ri zi n g . I m proper

h eat

treati n g

practi ces,

su ch

as

n on u n i form

h eati n g

or

cool i n g ,

con tri bu te

to

cracki n g .

H ard en i n g can cau se cracki n g i f th e steel i s n ot properl y processed .

8.1 .8 Tempering practice As-q u en ch ed m arten si te i s bri ttl e an d h i g h ten si l e resi d u al stresses are prod u ced by th e vol u m etri c expan si on associ ated wi th th e tran sform ati on of au sten i te to m arten si te.

Th erefore, th e l on g er steel i s

kept at a temperatu re between room tem peratu re (20°C) an d 1 00° C after q u en ch i n g , th e more l i kel y th e occu rren ce of q u en ch cracki n g . Al th ou g h th e parts sh ou l d be tem pered as soon as possi bl e to avoi d q u en ch cracki n g , care mu st be taken to en su re th at su ffi ci en t ti m e i s perm i tted for l arg e parts to fu l l y tran sform th rou g h to th e cen ter.

Two

tem peri n g practi ces can l ead to cracki n g probl em s: -

I f th e parts are tem pered too soon , before fu l l tran sform ati on h as taken pl ace, l ater tran sform ati on of th e core can i n d u ce su ffi ci en t stress d u e to th e vol u m etri c expan si on to crack th e su rface;

-

Su perfi ci al or “sn ap tem peri n g ” of th e su rface m ay n ot red u ce th e i n tern al stresses su ffi ci en tl y to preven t cracki n g .

Th i s probl em i s parti cu l arl y severe i f rapi d h eati n g m eth od s su ch as i n d u cti on ,

fl am e, or m ol ten sal t bath s are u sed , wh i ch can i n d u ce ad d i ti on al th erm al stresses between th e su rface an d th e core.

8.1 .9 Summary of methods to reduce the risk of hardening cracks -

Opti m i ze g eom etry:



Desi g n th e g ear bl an ks to be as sym m etri c as possi bl e an d keep secti on th i ckn ess u n i form ;



M i n i m i ze stress ri sers su ch as abru pt ch an g e i n cross secti on , h ol es, keyways, sh arp corn ers, an d steel stam p m arks.

U se ch am fers or rad i i on al l ed g es, especi al l y at th e en d s of th e teeth

an d at th e ed g es of th e g ear tooth topl an d s;

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Opti m i ze m etal l u rg y:



Sel ect steel type carefu l l y;



M i n i m i ze su rface an d su bsu rface fl aws su ch as n on m etal l i c i n cl u si on s, forg i n g fl aws su ch as



Desi g n th e q u en ch i n g m eth od , i n cl u d i n g th e ag i tati on , type of q u en ch an t an d tem peratu re of th e



Tem per th e g ear i m m ed i atel y after tran sform ati on to m arten si te h as fi n i sh ed ;



Li m i t carbon con ten t to th at sh own i n 8. 1 . 5 for al l oys i n ten d ed to be q u en ch ed by water, cau sti c,

h yd rog en fl akes, i n tern al ru ptu res, seam s, l aps, an d tears at th e fl ash l i n e;

q u en ch an t, for th e speci fi c g ear an d h ard en abi l i ty of th e steel ;

an d bri n e q u en ch an ts.

8.2 Grinding damage 8.2.1 Grinding cracks Cracks m ay d evel op on th e tooth su rfaces of g ears th at are fi n i sh ed by g ri n d i n g .

Th e cracks are u su al l y

sh al l ow an d m ay appear as a si n g l e crack, a seri es of paral l el cracks, or i n a crazed , m esh pattern .

Th e

cracks m ay appear i mm ed i atel y after g ri n d i n g , d u ri n g su bseq u en t h an d l i n g or storag e, or after ti m e i n servi ce.

M ag n eti c parti cl e or d ye pen etran t i n specti on

can be u sed

to d etect g ri n d i n g cracks.

See

Fi g u re 61 .

8.2.2 Overheating due to grinding Local i zed overh eati n g m ay resu l t from g ri n d i n g . tran sform ati on .

Th i s overh eati n g can resu l t i n over tem peri n g or ph ase

Areas of th e tooth su rface wh ere overh eati n g h as occu rred can be d etected by su rface

tem per etch i n specti on , see I SO 1 41 04. After etch i n g , tem pered areas appear brown or bl ack on a l i g h t brown or g ray backg rou n d .

Areas wh ere u n tem pered marten si te h as formed appear as wh i te areas

su rrou n d ed by bl ack, tem pered areas.

NOTE:

Barkh au sen

i n specti on

(measu rem en t of su d d en

som eti mes al so u sed to d etect overh eati n g from g ri n d i n g .

tran si ti on s

of m ag n eti sm

of th e

tooth

su rface)

is

I f ch emi cal l y en h an ced su rface i m provem en t i s u sed ,

i n som e cases overh eati n g m ay al so be d etected .

Cracks m ay be cau sed by th e g ri n d i n g tech n i q u e i f th e g ri n d i n g cu t i s too d eep, g ri n d i n g feed i s too h i g h , i n correct g ri n d i n g speed , g ri n d i n g wh eel g ri t or h ard n ess i s i n correct, or fl ow of cool an t i s i n su ffi ci en t. G ri n d i n g cracks may resu l t from tran sform ati on of retai n ed au sten i te to m arten si te i n respon se to th e h eat g en erated

by g ri n d i n g .

G ri n d i n g

cracks

m ay al so

be

possi bl e

au sten i te to m arten si te cau sed by th e pressu res of g ri n d i n g .

from

th e

tran sform ati on

of retai n ed

See [1 8] .

Figure 61 - Grinding cracks with a crazed pattern Copyright American Gear Manufacturers Association ©AG M A 201 4 – Al l ri g h ts reserved

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Steel s wi th h ard en abi l i ty provi d ed by carbi d e-form i n g el emen ts su ch as ch rom i u m are pron e to g ri n d i n g cracks.

Th i s i s especi al l y tru e for carbu ri zed g ears wi th a case th at h as h i g h carbon con ten t, parti cu l arl y i f

th ere are carbi d e n etworks.

Su rface h ard n ess above 60 H RC i n creases th e ri sk of cracki n g .

To avoi d

cracki n g d u ri n g g ri n d i n g , th e case m i crostru ctu re sh ou l d con si st pri m ari l y of tem pered m arten si te wi th even l y d i stri bu ted retai n ed au sten i te [1 8] an d be free of carbi d e n etworks. au sten i te l i m i ts vary d epen d i n g on appl i cati on . recom m en d ed .

Recom m en d ati on s for retai n ed

For ben d i n g fati g u e resi stan ce, a m axi m u m of 20% i s

For H ertzi an fati g u e resi stan ce, h i g h er l evel s may be n ecessary.

8.2.3 Summary of methods to reduce the risk of grinding cracks -

Con trol g ri n d i n g tech n i q u e to avoi d l ocal overh eati n g ;

-

For carbu ri zed g ears, con trol carbon con ten t an d en su re th at case m i crostru ctu re con si sts pri m ari l y of tem pered m arten si te wi th a con trol l ed am ou n t of even l y d i stri bu ted retai n ed au sten i te an d i s free of carbi d e n etworks;

-

For carbu ri zed g ears, l i m i t su rface h ard n ess to 60 H RC m axi m u m .

Depen d i n g on g ri n d i n g tech n i q u e,

h i g h er val u es of h ard n ess m ay be acceptabl e; -

U se su rface tem per etch i n specti on to d etect su rface tem per on g rou n d su rfaces , see I SO 1 41 04;

-

U se m ag n eti c parti cl e or d ye pen etran t i n specti on of g rou n d su rfaces to d etect g ri n d i n g cracks.

8.3

Rim and web cracks

I f th e g ear ri m i s th i n , i t m ay be su bj ected to si g n i fi can t al tern ati n g ri m ben d i n g stresses th at are ad d i ti ve to th e g ear tooth ben d i n g stresses.

Th ese stresses m ay resu l t i n fati g u e cracks i n th e ri m .

Ri m cracks are si m i l ar to tooth ben d i n g fati g u e cracks, except th at ri m cracks u su al l y propag ate rad i al l y th rou g h th e g ear ri m , wh ereas ben d i n g fati g u e cracks propag ate across th e base of th e teeth .

Ri m

cracks may g row i n to th e web of th e g ear. Web cracks m ay be cau sed by cycl i c stresses d u e to vi brati n g l oad s n ear a n atu ral freq u en cy of th e g ear bl an k.

A fati g u e crack m ay ori g i n ate i n th e web of th e g ear an d m ay g row i n to th e ri m of th e g ear.

Ri m an d web cracks g en eral l y ori g i n ate at stress con cen trati on s. on e or m ore of th e fol l owi n g :

Th ese con cen trati on s may ari se from

sh arp corn ers or n otch es i n th e root fi l l ets, keyways, spl i n es, h ol es, sh ri n k

fi ts, web-to-ri m or h u b-to-web fi l l ets or m etal l u rg i cal d efects su ch as i n cl u si on s. Oth er cau ses of ri m or web cracks i n cl u d e: -

ti p to root i n terferen ce, operati on i n ti g h t m esh ;

-

u se of l ower stren g th web m ateri al s i n fabri cated bl an ks;

-

i n correct wel d i n g proced u res, parti cu l arl y i n ad eq u ate stress rel i evi n g ;

-

g ear bl an ks wi th si g n i fi can t ch an g es i n secti on th i ckn ess th at l ead to ch an g es i n sti ffn ess an d a red i stri bu ti on of stress th at overl oad s th e ad j acen t th i n (weaker) secti on ;

-

i m pact l oad i n g .

Ri m or web cracks may cau se catastroph i c fai l u re i n h i g h speed g ears i f cen tri fu g al forces cau se th e fati g u e cracks to propag ate i n a bri ttl e fractu re m od e, open i n g th e ri m .

See Fi g u res 62, 63 an d 64.

M ag n eti c parti cl e or d ye pen etran t i n specti on sh ou l d be u sed to en su re th at th e g ear tooth fi l l ets, g ear ri m an d g ear web are free of fl aws.

8.3.1

Summary of methods to reduce the risk of rim or web cracks

-

U se ad eq u ate ri m th i ckn ess;

-

Desi g n

th e

g ear

bl an k

su ch

th at

i ts

n atu ral

freq u en ci es

do

n ot

coi n ci d e

wi th

th e

exci tati on

freq u en ci es; -

Pay atten ti on to d etai l s th at cau se stress con cen trati on s su ch as keyways, spl i n es, h ol es an d web-tori m fi l l ets;

-

U se m ag n eti c parti cl e or d ye pen etran t i n specti on to en su re th at th e g ear tooth fi l l ets, g ear ri m an d g ear web are free of fl aws;

-

Con trol m an u factu ri n g to avoi d n otch es i n th e root fi l l ets;

-

Con trol operati n g cen ter d i stan ce, tooth cl earan ce, an d avoi d ti p-to-root i n terferen ce.

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Figure 62 - Rim crack

Figure 63 - Rim cracks in through hardened annulus gear Copyright American Gear Manufacturers Association ©AGMA 201 4

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8.4

ANSI/AGMA 1 01 0-F1 4

Figure 64 - Fracture surface of rim crack shown in Figure 63 Case/core separation

Case/core separation may occur in case hardened gear teeth when internal cracks occur near the case/core interface near tips of teeth. The internal cracks may propagate causing corners, edges, or entire tips of the teeth to separate. The cracks may appear immediately after heat treatment, during subsequent handling or storage, or after time in service. See Figures 65 and 66.

Figure 65 - Case/core separation

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Figure 66 - Case/core separation Case/core separation is believed to be caused by high residual tensile stresses at the case/core interface when a case is very deep. If residual tensile stress is high and ductility is low, brittle fracture might occur and tips of teeth might separate explosively. If conditions are less severe, cracks might arrest before reaching the tooth surfaces. Hydrogen might accumulate at internal flaws and cause brittle fracture or stresses in service might cause cracks to grow by fatigue. Because cracks follow the case/core interface, tips of teeth have concave fracture surfaces, and remaining portions of teeth have convex fracture surfaces. Chevron marks may be apparent on fracture surfaces if the fracture was brittle. These marks are helpful because they point to the failure origin. Beach marks or fretting corrosion may be found on fracture surfaces if cracks grew by fatigue. Inclusions promote case/core separation especially when they occur near the case/core interface. When case/core separation is suspected as the cause of failure, intact teeth should be sectioned to determine if there are subsurface cracks near the tips of the teeth. On carburized gears, case depth at the tip can be controlled by: - avoiding narrow toplands; - masking the toplands with copper plate or stop off paint to restrict carbon penetration during carburizing; - remove carburized toplands by machining after carburizing but before quenching to harden. Steels with high fracture resistance have less risk of case/core separation. Material toughness depends on elemental composition, heat treatment, and mechanical processing. Many alloying elements increase hardenability of steel, but decrease toughness. Exceptions are nickel and molybdenum, which increase hardenability while improving toughness. Diesburg and Smith [25] tested impact fracture resistance of carburized steels and found the following: - High-hardenability steels have greater fracture toughness than low-hardenability steels; - High nickel content does not guarantee good fracture resistance, but nickel and molybdenum in the right combination give high fracture resistance; - High chromium and high manganese content give low fracture resistance. The best toughness properties are obtained with 3%NiCrMo steels with core hardness in the range of 30-40 HRC [1 8]. Toughness can be maximized by using vacuum-melted steel and keeping carbon, phosphorus, and sulfur content as low as possible. Most material properties are improved when grain size is uniform and fine. This is especially true for toughness; fine-grained steel has increased toughness and lower transition temperature. Steel-making practice, alloying elements, mechanical treatment, and heat treatment influence grain size. Steels ©AGMA 201 4 – All rights reserved

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containing nickel and molybdenum resist grain coarsening during austenitizing better than plain carbon steels. Aluminum, vanadium, or niobium are added to the steel melt to produce fine grain size. Shot peened flanks increase risk of case/core separation because in addition to increasing compressive residual stresses at surfaces of teeth, shot peening increases tensile residual stresses near the case/core interface. For carburized gearing, cold treatment can be used to reduce retained austenite. However, it increases risk of case/core separation by decreasing toughness and fatigue strength. Cold treatment may also increase the risk of microcracks within martensite platelets or needles, see AGMA 923. To minimize risk of case/core separation, gears should be tempered immediately after quenching and also after any cold treatment. Generous chamfers or radii on edges of gear teeth help avoid stress concentrations.

8.4.1 Summary of methods to reduce the risk of case/core separation - Control case depth especially at tips of gear teeth. On carburized gears, avoid narrow toplands and mask toplands of teeth to restrict carbon penetration or remove excessive case depth from toplands by machining after carburizing and before hardening; - Use steels with high nickel content. Nickel and molybdenum in the right combination maximizes toughness of carburized gears. Do not use steels with high chromium and manganese content. Keep carbon, phosphorus, and sulfur content as low as possible; - Use vacuum-melted steel; - Use fine-grained steel. Nickel and molybdenum steels resist grain coarsening during austenitizing; - Specify core hardness of 30-40 HRC; - Do not shot peen flanks; - Do not cold treat; - Temper gears immediately after quenching and also after any cold treatment; - Use generous chamfers or radii on edges of gear teeth to avoid stress concentrations. 8.5 Fatigue cracks Fatigue cracks are cracks that propagate under the influence of repeated alternating or cyclic stresses that are below the tensile strength of the material. These cracks can appear in tooth flanks and in tooth root fillets. See Figure 67. For fatigue fracture, see clause 1 0.

Figure 67 - Bending fatigue crack ©AGMA 201 4 – All rights reserved

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9

ANSI/AGMA 1 01 0-F1 4

Fracture

Wh en a g ear tooth i s overl oad ed becau se i t i s u n d er-d esi g n ed or th e l ocal l oad i s too h i g h , i t m ay fai l by pl asti cal l y d eform i n g or fractu ri n g .

I f i t fractu res,

th e fai l u re may be a d u cti l e fractu re preced ed by

appreci abl e pl asti c d eform ati on , a bri ttl e fractu re wi th l i ttl e pri or pl asti c d eform ati on , or a m i xed -m od e fractu re exh i bi ti n g both d u cti l e an d bri ttl e ch aracteri sti cs. Fati g u e

fai l u res

rem ai n i n g

tooth

u su al l y cu l m i n ate secti on

can

no

in

a

fractu re

wh en

l on g er su pport

th e

th e

l oad .

fati g u e In

cracks

th i s

sen se

g row to th e

a

si ze

remai n i n g

wh ere

th e

m ateri al

is

overl oad ed ; h owever, th e fractu re i s a secon d ary fai l u re m od e th at i s cau sed by th e pri m ary m od e of fati g u e cracki n g . G ear tooth fractu res wi th ou t pri or fati g u e cracki n g are i n freq u en t, bu t m ay resu l t from sh ock l oad s. sh ock l oad s m ay be g en erated by th e d ri vi n g or d ri ven eq u i pm en t.

Th e

Th ey m ay al so occu r wh en forei g n

obj ects en ter th e g ear m esh , or wh en th e g ear teeth are su d d en l y m i sal i g n ed an d j am tog eth er or operate i n ti g h t m esh after a beari n g or sh aft fai l s. Fractu res

are

cl assi fi ed

as

bri ttl e

or

d u cti l e

d epen d i n g

on

th ei r

m acroscopi c

an d

m i croscopi c

ch aracteri sti cs, as l i sted i n Tabl e 3.

9.1

Brittle fracture

Bri ttl e

fractu res

d eformati on .

are

ch aracteri zed

by

rapi d

crack

propag ati on

Bri ttl e fractu res h ave a bri g h t, g ran u l ar appearan ce.

an d perpen d i cu l ar to th e d i recti on of th e m axi m u m ten si l e stress.

wi th ou t

appreci abl e

g ross

pl asti c

Th e fractu re su rface i s g en eral l y fl at

Rad i al ri d g es or a ch evron pattern m ay

be presen t on th e fractu re su rface poi n ti n g toward th e ori g i n of th e crack. On a m i croscopi c l evel , bri ttl e fractu re typi cal l y con si sts of tran sg ran u l ar cl eavag e facets or i n terg ran u l ar facets.

See Fi g u res 68, 69 an d 70.

Th ree pri mary factors con trol th e su scepti bi l i ty of g ear teeth to bri ttl e fractu re: -

M ateri al tou g h n ess;

-

M ateri al fl aws;

-

Operati n g or resi d u al ten si l e stress l evel .

Bri ttl e fractu re occu rs wh en combi n ati on s of ten si l e stress an d fl aw si ze create a cri ti cal stress i n ten si ty for a parti cu l ar materi al tou g h n ess.

Part sh ape, m ach i n i n g m arks, an d materi al fl aws m ay l ead to stress

con cen trati on , wh i ch u su al l y pl ays a rol e i n bri ttl e fractu re.

Th e cri ti cal stress i n ten si ty i s a fu n cti on of th e

m ateri al tou g h n ess.

Table 3 - Fracture classifications Brittle fracture Characteristic of fracture surface l i g h t refl ecti on

textu re

ori en tati on

pattern

bri g h t

Ductile fracture g ray (d ark)

sh i n y

dull

crystal l i n e

si l ky

g rai n y

m atte

rou g h

sm ooth

coarse

fi n e

g ran u l ar

fi brou s (stri n g y)

fl at

sl an t or fl at

sq u are

an g u l ar or sq u are

rad i al ri d g es

sh ear l i ps

ch evron s pl asti c d eform ati on (n ecki n g or

n eg l i g i bl e

appreci abl e

cl eavag e (facets)

sh ear (d i m pl es)

d i storti on m i croscopi c featu res

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Figure 68 - Brittle fracture

Figure 69 - SEM image of transgranular brittle fracture

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– All rights reserved

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AMERICAN NATIONAL STANDARD

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Figure 70 - SEM image of intergranular brittle fracture Th e tou g h n ess of a g ear m ateri al d epen d s on man y factors especi al l y tem peratu re, l oad i n g rate an d con strai n t (state of pl an e stress or pl an e strai n ) at th e l ocati on of fl aws.

M an y steel s h ave a tran si ti on

tem peratu re wh ere th e fractu re m od e ch an g es from d u cti l e-to-bri ttl e as temperatu re d ecreases. tran si ti on temperatu re i s i n fl u en ced by th e l oad i n g rate an d con strai n t. be d etected wi th th e Ch arpy V-n otch i m pact test. steel s d o n ot exh i bi t a tran si ti on tem peratu re

is

of

pri mary

an d

tem peratu res bel ow th e servi ce tem peratu re. h ave l i m i ted fractu re tou g h n ess.

Som e h i g h stren g th , al l oyed , q u en ch ed an d tem pered

tem peratu re beh avi or.

i m portan ce,

Th e

Th e d u cti l e-to-bri ttl e tran si ti on can

g ear

For l ow tem peratu re servi ce,

m ateri al s

sh ou l d

be

ch osen

th at

th e tran si ti on

h ave

tran si ti on

Typi cal l y, al l oy steel s wi th a core h ard n ess above 40 H RC

Th e com pl i an ce of sh afts an d cou pl i n g s i n a d ri ve system h el ps to

cu sh i on sh ock l oad s an d red u ce th e l oad i n g rate d u ri n g i m pact. G ear d ri ves wi th cl ose-cou pl ed sh afts an d

ri g i d

cou pl i n g s h ave l ess com pl i an ce.

I f d ri ve system s wi th

l ow com pl i an ce m u st be u sed

in

appl i cati on s wh ere overl oad s are expected , th e g ears sh ou l d be l arg e en ou g h to absorb th e overl oad s wi th reason abl e stress l evel s.

Oth erwi se, g ears sh ou l d be i sol ated from sh ock l oad s by u si n g l oad -

l i m i ti n g cou pl i n g s em pl oyi n g sl i p cl u tch es or sh ear d evi ces.

H owever, l oad -l i m i ti n g cou pl i n g s can n ot be

u sed i n cri ti cal appl i cati on s su ch as h oi sts wh ere sl i p or sh ear d evi ces cou l d resu l t i n th e l oad bei n g d ropped . Fl aws or n otch es create stress con cen trati on s th at elevate th e stress l ocal l y ah ead of th e n otch . m ateri al , at l ower stress, con strai n s an d l i m i ts pl asti c d eform ati on .

Ad j acen t

For wi d e-face g ears wi th a fl aw or

n otch i n th e root d i stan t from th e en d face, tri axi al ten si l e stresses can d evel op at th at poi n t an d red u ce d u cti l i ty of th e m ateri al by d ecreasi n g sh ear stresses. Th e tou g h n ess of a m ateri al d epen d s on i ts el em en tal com posi ti on , processi n g .

h eat treatm en t an d m ech an i cal

M an y al l oyi n g el em en ts th at i n crease th e h ard en abi l i ty of steel al so d ecrease i ts tou g h n ess.

Excepti on s are n i ckel an d m ol ybd en u m th at i n crease h ard en abi l i ty wh i l e i m provi n g tou g h n ess.

Tests on

th e i m pact fractu re tou g h n ess of carbu ri zed steel h ave fou n d th e fol l owi n g , see [25] : -

H i g h -h ard en abi l i ty steel s h ave g reater i m pact fractu re tou g h n ess th an l ow-h ard en abi l i ty steel s;

-

H i g h n i ckel con ten t, above 3% , d oes n ot g u aran tee g ood i m pact fractu re tou g h n ess, bu t n i ckel an d

-

H i g h ch rom i u m an d h i g h m an g an ese con ten ts ten d to g i ve l ow i m pact fractu re tou g h n ess.

m ol ybd en u m i n th e ri g h t com bi n ati on resu l ts i n h i g h i mpact fractu re resi stan ce;

Tou g h n ess can be opti m i zed by keepi n g th e carbon , ph osph oru s an d su l fu r con ten t as l ow as possi bl e. Th e

m i crostru ctu re

Tem pered

of

steel

d epen d s

on

i n i ti al

m i crostru ctu re,

m arten si te g i ves th e h i g h est tou g h n ess.

Copyright American Gear Manufacturers Association ©AG M A 201 4 – Al l ri g h ts reserved

h ard en abi l i ty,

M i crostru ctu res con si sti n g

an d

h eat

of ferri te,

treatm en t. pearl i te,

or

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ANSI/AGMA 1 01 0-F1 4

bainite have lower fracture toughness. For maximum toughness, steel should have sufficient hardenability so that its heat-treated microstructure consists primarily of tempered martensite. Avoid embrittlement by selecting steel in which the desired hardness is achieved without tempering in the range of 250°C - 400°C. Most material properties are improved when grain size is uniform and fine. This is especially true for toughness; fine-grained steel has increased toughness and a lower transition temperature. Steel-making practice, alloying elements, mechanical treatment, and heat treatment influence grain size. Steels containing nickel and molybdenum resist grain coarsening during austenitizing better than plain carbon steels. Aluminum, vanadium, or niobium is alloyed with steel to produce fine grain size. Fracture initiates at flaws that cause stress concentrations. The flaw may be a notch, crack, surface tear, surface or subsurface inclusion, or porosity. The flaw size may be small initially, but it may initiate a fatigue crack that can grow until a critical size is reached, at which point the crack may extend in a brittle fracture. The critical flaw size is not constant, but depends on the geometry of the part, shape and orientation of the flaw, applied stress, and the fracture toughness of the material at the service temperature and loading rate. The root fillets of gear teeth are especially vulnerable to fracture because this is the location where tooth bending stresses are highest. Clean materials increase fracture resistance. The gear tooth geometry should be selected to reduce the tensile bending stress in the root fillets. The gear teeth may be cut with full-fillet tools to obtain large root fillets with minimum stress concentrations. If the gears are to be finished by shaving or grinding, protuberance tools should be used to reduce the risk of notching the root fillets. Case hardening by carburizing or nitriding can be beneficial because these hardening processes may induce compressive residual stresses that reduce the net tensile bending stresses. Also, controlled shot peening can be used to increase compressive residual stresses.

9.1 .1 Methods for reducing the risk of brittle fracture - Optimize design  Reduce tensile bending stresses by improving gear tooth geometry;  Reduce loading rates by using compliant shafts and couplings;  Protect gears from impact loads by using load limiting couplings; - Optimize metallurgy  Use materials with high cleanliness;  Use materials and heat treatments that give high toughness, such as steel with sufficient hardenability to obtain a microstructure of primarily tempered martensite. Avoid embrittlement by using steel in which the desired hardness will be achieved without tempering in the range of 250°C to 400°C;  Do not use steels at service temperatures below their transition temperature;  Use steels with high nickel content. For carburized gears, nickel and molybdenum in the right combination gives maximum toughness. Do not use steels with high chromium and manganese content. Keep the carbon, phosphorus and sulfur content as low as possible;  Avoid core hardness above 40 HRC;  Use fine grained steel;  Minimize flaws, especially in the root fillets of gear teeth. Use magnetic particle or dye penetrant inspection to detect flaws;  Use case hardening, or shot peening, or both to increase compressive residual stresses. 9.2 Ductile fracture Ductile fractures are characterized by tearing of metal accompanied by gross plastic deformation. Ductile fractures have a gray, fibrous appearance. The fracture surface may have a flat or slant orientation to the direction of the maximum tensile stress. The fracture surface may terminate with a shear lip that extends along the nonworking side of the gear tooth. Microscopically, ductile fractures are characterized by numerous dimples that are formed by the nucleation and growth of microvoids. See Figure 71 .

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ANSI/AGMA 1 01 0-F1 4

Figure 71 - SEM image of ductile fracture G ear tooth fai l u res th at occu r sol el y by d u cti l e fractu re are rel ati vel y i n freq u en t becau se m ost fractu res occu r at a pre-exi sti n g fl aw wh i ch ten d s to prom ote bri ttl e beh avi or.

I f th e fol l owi n g factors are presen t, a

fractu re i s m ore l i kel y to be d u cti l e rath er th an bri ttl e: -

h i g h m ateri al tou g h n ess;

-

h i g h g ear tooth tem peratu re;

-

sl ow l oad i n g rate;

-

n o si g n i fi can t m ateri al fl aws;

-

l ow operati n g or resi d u al ten si l e stress;

-

h i g h sh ear stress.

U n d er th ese con d i ti on s g ear teeth yi el d wh en th e ben d i n g stresses exceed th e yi el d stren g th of th e m ateri al , an d su bseq u en tl y sh ear off wi th si g n i fi can t pl asti c d eformati on before d u cti l e fractu re.

9.3

Mixed mode fracture

A l ocal

area of a fractu re su rface may exh i bi t both

con d i ti on s, th e fractu re i s term ed mi xed m od e.

d u cti l e an d

bri ttl e ch aracteri sti cs.

U n d er th ese

Th i s i s n ot to be con fu sed wi th a fractu re su rface h avi n g

featu res th at su g g est su ccessi ve crack propag ati on by d i fferen t m echan i sm s, for exam pl e a fati g u e crack cau si n g a d u cti l e fractu re.

9.4

See Fi g u re 72.

Tooth shear

Wh en

teeth

are

sh eared

m ach i n ed su rfaces.

9.5

from

g ears,

th e

appearan ce

of th e

sh eared

su rfaces

is

si m i l ar to th at of

Tooth sh ear i s al m ost al ways cau sed by a si n g l e severe overl oad , see Fi g u re 73.

Fracture after plastic deformation

Th ese fractu res beg i n wi th g ross pl asti c d eformati on s of th e teeth before fi n al breakag e.

See Fi g u re 74.

U su al l y, al l th e teeth su ffer d amag e th at occu rs becau se th e m ateri al i s u n abl e to su pport th e appl i ed l oad : -

wh en th e stress d u e to l oad exceed s th e m ateri al stren g th (col d fl ow fol l owed by fractu re);

-

wh en th e g ear m ateri al i s weaken ed by overh eati n g d u ri n g operati on (h ot fl ow fol l owed by fractu re).

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ANSI/AGMA 1 01 0-F1 4

Figure 72 - Mixed mode fracture

Figure 73 - Tooth shear Copyright American Gear Manufacturers Association ©AGMA 201 4

– All rights reserved

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AMERICAN NATIONAL STANDARD

ANSI/AGMA 1 01 0-F1 4

Figure 74 - Fracture after plastic deformation

10

Bending fatigue

Fati g u e i s a prog ressi ve fai l u re con si sti n g of th ree d i sti n ct stag es: -

Stag e 1 , Crack i n i ti ati on (pl asti c d eform ati on occu rs at stress con cen trati on s l ead i n g to mi croscopi c cracks);

-

Stag e 2, Crack propag ati on (cracks g row perpen d i cu l ar to m axi m u m ten si l e stress);

-

Stag e 3, Fractu re (wh en a crack g rows l arg e en ou g h , i t cau ses su d d en fractu re).

M ost of th e fati g u e l i fe i s occu pi ed by stag es 1 an d 2 u n ti l th e cracks g row to cri ti cal si ze wh ere su d d en fractu re occu rs i n stag e 3.

Th e fractu re may be d u cti l e, bri ttl e or m i xed m od e d epen d i n g u pon th e

tou g h n ess of th e m ateri al an d th e m ag n i tu d e of th e appl i ed stress. Du ri n g stag e 1

th e peak ben d i n g stress i s l ess th an th e yi el d stren g th of th e m ateri al an d n o g ross

yi el d i n g of th e g ear teeth occu rs. con cen trati on s i n cl u si on s.

or areas

H owever, l ocal pl asti c d eform ati on may occu r i n reg i on s of stress

of stru ctu ral

d i scon ti n u i ti es

su ch

as

su rface

n otch es,

g rai n

bou n d ari es,

or

Th e cycl i c, pl asti c d eform ati on u su al l y occu rs on sl i p pl an es th at coi n ci d e wi th th e d i recti on of

m axi m u m sh ear stress.

Th e cycl i c sl i p con ti n u es wi th i n th e sl i p pl an es of a few g rai n s, u su al l y n ear th e

su rface wh ere th e stress i s h i g h est, u n ti l very smal l cracks are i n i ti ated .

Th e cracks g row i n th e pl an es of

m axi m u m sh ear stress an d coal esce across several g rai n s u n ti l th ey form a m aj or crack. Th e

stag e

2

propag ati on

ph ase

beg i n s

wh en

th e

crack

tu rn s

an d

g rows

across

g rai n

(tran sg ran u l ar) i n a d i recti on approxi matel y perpen d i cu l ar to th e m axi m u m ten si l e stress.

bou n d ari es Du ri n g th e

propag ati on ph ase, th e pl asti c d eformati on i s con fi n ed to a sm al l zon e at th e l ead i n g ed g e of th e crack, an d th e su rfaces of th e fati g u e crack u su al l y appear sm ooth wi th ou t si g n s of g ross pl asti c d eform ati on . U n d er th e scan n i n g el ectron m i croscope, con tou rs, cal l ed fati g u e stri ati on s, may be seen on a fati g u e cracked su rface.

Th ey are th ou g h t to be associ ated wi th al tern ati n g bl u n ti n g an d sh arpen i n g of th e crack

ti p an d correspon d to th e ad van ce of th e crack d u ri n g each stress cycl e.

Th e ori en tati on of th e stri ati on s

i s at 90° to th e crack ad van ce. I f th e crack propag ates i n term i tten tl y, i t m ay l eave a pattern of m acroscopi cal l y vi si bl e “beach m arks”. Th ese

m arks

d ecreased .

correspon d

to

posi ti on s

of th e

crack

fron t

wh ere

th e

crack

stopped

becau se

stress

Th e ori g i n of th e fati g u e crack i s u su al l y on th e con cave si d e of cu rved beach m arks an d i s

often su rrou n d ed by several con cen tri c beach m arks.

Beach m arks m ay n ot be presen t, especi al l y i f th e

fati g u e crack g rows wi th ou t i n terru pti on u n d er cycl i c l oad s th at d o n ot vary i n mag n i tu d e.

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Th e presen ce

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of beach m arks i s a stron g i n d i cati on th at th e crack was d u e to fati g u e; bu t n ot absol u te proof, becau se oth er fai l u re m od es m ay l eave beach m arks (for exam pl e, stress corrosi on u n d er ch an g i n g en vi ron m en t). I f th ere are mu l ti pl e crack ori g i n s, each prod u ci n g separate crack propag ati on zon es, ratch et m arks m ay be form ed .

Th ey are cau sed wh en ad j acen t cracks, propag ati n g on d i fferen t crystal l og raph i c pl an es, j oi n

tog eth er to form a step.

Ratch et m arks are often presen t on fati g u e cracked su rfaces of g ear teeth

becau se th e stress con cen trati on i n th e root fi l l et freq u en tl y i n i ti ates m u l ti pl e fati g u e cracks. Th ere are several g eometri c vari abl es, su ch as d i am eter, face wi d th , n u m ber of teeth , pressu re an g l e, h el i x an g l e, an d profi l e sh i ft th at m ay be opti m i zed to l ower th e ben d i n g stress an d i n crease th e ben d i n g fati g u e l i fe. fi l l ets.

Th e g ear tooth g eom etry sh ou l d be d esi g n ed to red u ce th e ten si l e ben d i n g stress i n th e root

Th e g ear teeth sh ou l d be cu t wi th fu l l -fi l l et tool s to obtai n l arg e rad i u s root fi l l ets wi th m i n i m u m

stress con cen trati on s.

I f th e g ears are to be fi n i sh ed by sh avi n g or g ri n d i n g , th ey sh ou l d be fi n i sh ed

wi th ou t n otch i n g th e root fi l l ets.

1 0.1 Low cycle fatigue Low cycl e fati g u e occu rs wh en m acroscopi c pl asti c strai n occu rs i n every cycl e an d th e n u mber of cycl es to fai l u re i s l ess th an 1 0, 000. g ear

teeth

are

overl oad ed

I t i s an u n com mon fai l u re mod e for g ear teeth except for i n stan ces wh ere becau se

th ey

are

u n d er-d esi g n ed ,

severel y

m i sal i g n ed ,

or

th e

l oad

is

u n expected l y h i g h . Su rface con d i ti on s of a g ear tooth su bj ected to l ow-cycl e fati g u e are l ess i m portan t th an u n d er h i g h -cycl e fati g u e l oad i n g becau se cycl i c, pl asti c d eform ati on ten d s to rel ax both stress con cen trati on s an d resi d u al stresses.

Cracks m i g h t i n i ti ate wi th i n g ear teeth , as wel l as on th e su rface, an d a sm al l er fracti on of th e

l i fe i s spen t i n i ti ati n g rath er th an propag ati n g cracks. M axi m i ze d u cti l i ty an d tou g h n ess (see d i scu ssi on i n 9. 1 exten d l ow-cycl e fati g u e l i fe.

reg ard i n g factors th at prom ote tou g h n ess) to

Referen ce [24] recom m en d s th e fol l owi n g m eth od s to i n crease tou g h n ess

of carbu ri zed g ears: -

U se steel s th at con tai n n i ckel as a m aj or (m ore th an 1 % ) al l oyi n g el em en t;

-

Qu en ch to prod u ce 1 5% to 30% retai n ed au sten i te i n th e case m i crostru ctu re;

-

Tem per as-q u en ch ed

case

h ard n ess

from

58-62

H RC

d own

to

51 -55

H RC.

Avoi d

tem peri n g

tem peratu res of 250 ºC - 400 ºC becau se th i s tem peratu re ran g e can cau se em bri ttl em en t of th e core. Exerci se cau ti on wh en d esi g n i n g ag ai n st l ow-cycl e fati g u e becau se m an y of th e recom m en d ati on s th at i m prove l ow-cycl e fati g u e l i fe d ecrease h i g h -cycl e fati g u e l i fe.

I t i s better to avoi d l ow-cycl e fati g u e by

red u ci n g stresses.

1 0.2 High cycle fatigue H i g h cycl e fati g u e i s d efi n ed as fati g u e wh ere th e cycl i c stress i s bel ow th e yi el d stren g th of th e materi al an d th e n u m ber of cycl es to fai l u re i s h i g h .

M ost g ear tooth ben d i n g fai l u res are d u e to h i g h cycl e fati g u e

rath er th an l ow cycl e fati g u e.

NOTE:

Fretti n g on th e fractu re su rface i n d i cates h i g h cycl e fati g u e.

See Fi g u re 75.

Cracks u su al l y i n i ti ate at th e su rface of th e g ear tooth root fi l l ets an d a l arg e fracti on of th e l i fe i s spen t i n i ti ati n g rath er th an propag ati n g cracks.

H i g h -cycl e fati g u e l i fe can be exten d ed by maxi m i zi n g th e

u l ti m ate ten si l e stren g th of th e materi al an d en su ri n g th at th e m i crostru ctu re of th e su rface of th e g ear teeth i s opti m u m .

Referen ce [24] recom men d s th e fol l owi n g m eth od s to i n crease th e resi stan ce to

h i g h -cycl e ben d i n g fati g u e of carbu ri zed g ears: -

El i m i n ate bai n i te, pearl i te, an d n etwork carbi d es from th e case m i crostru ctu re;

-

El i m i n ate al l cracks especi al l y n ear th e su rface of th e root fi l l ets;

-

M axi m i ze resi d u al com pressi ve stress i n th e case by u si n g a steel wi th th e l owest possi bl e carbon con ten t;

-

El i m i n ate d efects on th e su rfaces of th e root fi l l ets.

Case h ard en i n g by carbu ri zi n g or n i tri d i n g can be ben efi ci al becau se th ese h ard en i n g processes m ay i n d u ce com pressi ve resi d u al stresses th at red u ce th e n et ten si l e ben d i n g stresses. peen i n g can be u sed to i n crease com pressi ve resi d u al stresses.

Al so, con trol l ed sh ot

For carbu ri zed g ears th ere are opti m u m

val u es of case h ard n ess, case d epth an d core h ard n ess [1 8] th at g i ve th e best bal an ce of resi d u al stresses an d fati g u e stren g th to m axi m i ze g ear tooth resi stan ce to ben d i n g fati g u e.

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Figure 75 - Two adjacent teeth on a helical pinion that failed by bending fatigue 1 0.2.1 Morphology of fatigue fracture surfaces Ratchet marks - High tensile stresses or high stress concentration might initiate several fatigue cracks on different planes. A ratchet mark forms where the cracks join to form a common plane. Ratchet marks help locate the crack origins. If there are no ratchet marks, it indicates there was a single crack origin. One or more ratchet marks indicate there were multiple crack origins. The higher the stress, or more acute the stress concentration is, the more likely there will be multiple ratchet marks. Beach marks - If a fatigue crack grows intermittently, marks form along lines of arrest where the crack stopped because the load decreased. If the range of cyclic load remains constant, there will be no beach marks. Fine, closely-spaced beach marks indicate slow growth. Beach marks surround, and help locate the crack origin and show the direction of crack growth. Case/core origins - Case hardened gears have tensile residual stress below the case/core boundary. Subsurface fatigue cracks may initiate at flaws such as nonmetallic inclusions if the flaws are near the case/core boundary in an area of high tensile residual stress. Polished areas - If a fatigue crack opens and closes repeatedly under alternating tension and compression, the surfaces of the crack may become polished. Polished areas are often found around subsurface fatigue origins caused by nonmetallic inclusions or other flaws. Fretting corrosion - Fretting corrosion often occurs on a fracture surface when the faces of the fatigue crack rub together during slow, high-cycle fatigue growth. Fretting corrosion is often found on the oldest, smoothest, and largest fatigue zone. Size of the fatigue zones on adjacent teeth - The first tooth to fail usually has the largest, smoothest fatigue zone because the tooth unloads as the crack grows and the tooth loses stiffness; decreasing the bending stress and the crack growth rate. Due to the loss in load sharing, adjacent teeth take on more load and crack sooner; have faster crack growth rate, and a rougher fracture surface. Adjacent teeth may have secondary distress such as macropitting. Ratio of fatigue/fracture surface area - A large fatigue zone and a small fracture zone indicates the nominal stress was low, whereas a small fatigue zone and a large fracture zone indicate the nominal stress was high. The size of the final fracture zone is an indication of the magnitude of the stress at final fracture. Figure 75 shows two adjacent teeth that failed by bending fatigue. The lower tooth in Figure 75 failed first. It has a single crack origin, the largest, smoothest fracture surface, and extensive fretting corrosion. The adjacent tooth failed next and it has a smaller, rougher fracture surface. Ratchet marks formed on ©AGMA 201 4 – All rights reserved

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AMERICAN NATIONAL STANDARD th e ad j acen t tooth

ANSI/AGMA 1 01 0-F1 4

becau se th e ben d i n g

stress was h i g h er an d

m ore fati g u e cracks i n i ti ated .

See

Fi g u re 76.

1 0.2.2 Summary of methods to reduce the risk of high-cycle bending fatigue -

Opti m i ze g eom etry



-

U se fu l l fi l l et roots, th e root sh ape h as a stron g i n fl u en ce on ben d i n g stress;



En su re th at th e su rfaces of th e root fi l l ets are free from si g n i fi can t n otch es an d tool m arks;



Red u ce ben d i n g stresses by red u ci n g l oad s;



U se l arg er mod u l e;



U se l arg er cen ter d i stan ce or face wi d th ;

Opti m i ze m etal l u rg y



U se cl ean er steel s, properl y h eat treated by carbu ri zi n g ;



U se

case

h ard en i n g ,

or

sh ot

com pressi ve resi d u al stresses.

peen i n g ,

or

both

wi th

proper

process

con trol

to

i n crease

For carbu ri zed g ears, m axi m i ze resi d u al com pressi ve stress i n

th e case by u si n g steel wi th th e l owest possi bl e core carbon con ten t;



For case h ard en ed g ears speci fy val u es of case h ard n ess, case d epth an d core h ard n ess to



U se steel wi th su ffi ci en t h ard en abi l i ty to obtai n a m i crostru ctu re of pri m ari l y tem pered m arten si te

m axi m i ze resi stan ce to ben d i n g fati g u e;

i n th e g ear tooth root fi l l ets;



Avoi d em bri ttl em en t by u si n g a steel i n wh i ch th e d esi red h ard n ess wi l l be ach i eved wi th ou t tem peri n g i n th e ran g e of 250° C to 400° C;



For carbu ri zed g ears, m ake su re th at th e m i crostru ctu re of th e case i s essen ti al l y free of bai n i te, pearl i te, n etwork carbi d es an d especi al l y m i crocracks wi th i n marten si te pl atel ets or n eed l es (see AG M A 923);



U se fi n e-g rai n steel ;



En su re th at th e su rfaces of th e root fi l l ets are free from si g n i fi can t cracks, n on metal l i c i n cl u si on s, d ecarbu ri zi n g , corrosi on , i n terg ran u l ar oxi d ati on , or oth er poten ti al stress ri sers;



U se vacu u m (l ow pressu re) carbu ri zi n g to preven t d ecarbu ri zi n g , i n terg ran u l ar oxi d ati on , an d u n even case d epth .

Figure 76 - Bending fatigue of spiral bevel tooth

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1 0.2.3 Root fillet cracks Although bending fatigue cracks may occur elsewhere, they usually initiate in the root fillet on the tensile side of the gear tooth. The geometry of the root fillets might cause significant stress concentrations, which combined with a high bending moment, might result in high bending stress and fatigue cracking. See Figure 77 through Figure 80.

1 0.2.4 Profile cracks Fatigue cracks may initiate on the active surface of the gear tooth if there are stress concentrations caused by macropits, material flaws, or pre-existing cracks from hardening or grinding. See Figure 81 .

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Figure 78 - Bending fatigue of several spur gear teeth If the origin is at the tooth flank surface in an area with macropitting, micropitting, corrosion, or fretting corrosion, the crack might be a secondary failure that initiated from one of these primary failure modes. If the origin is subsurface near the case/core boundary, and there are several parallel cracks on the flank, the profile crack might be a secondary crack that was caused by the primary failure mode of subcase fatigue. In contrast, if no other failure modes are apparent near the origin, the profile crack might be a primary failure that initiated from either a surface or subsurface flaw such as an inclusion, hardening crack, grinding crack, grinding temper, or incomplete hardening pattern.

1 0.2.5 Tooth end cracks Fatigue cracks may initiate at an end of the gear tooth if the load is concentrated at the tooth end. Stress concentrations or material flaws at the ends of the teeth may also be responsible for tooth end cracks. See Figure 82.

1 0.2.6 Subsurface initiated bending fatigue cracks Nonmetallic inclusions are often the root cause of cracks that result in failure modes such as subcase fatigue, case/core separation, or bending fatigue. See 7.3.1 for discussion of inclusions. Classic bending fatigue failures initiate at the surface of the root fillet on the tensile side of the gear tooth. However, when a bending fatigue crack initiates at a location significantly above the root fillet, where the nominal bending stress is much lower than at the root fillet, it is likely that the root cause of failure is a material flaw such as a nonmetallic inclusion, see Figure 83. Hard undeformable inclusions such as calcium aluminate have a lower thermal expansion coefficient than steel and they develop tensile residual stresses concentrated around each inclusion as a result of hardening heat treatments. The tensile residual stresses from the inclusions and the existing tensile residual stresses below the case/core boundary add to the nominal bending stress from the applied load. Therefore, a nonmetallic inclusion can shift the location of the crack origin from the surface of the root fillet to below the case/core boundary or other areas. Consequently, nonmetallic inclusions are often the root cause of bending fatigue cracks that initiate at a subsurface location below the case/core boundary. In some instances the severe stressraising effects of an inclusion might even initiate cracks on the compression side of the gear tooth.

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Figure 79 - Bending fatigue of two bevel pinion teeth

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Figure 80 - Fatigue of several teeth that were loaded on both flanks

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Figure 81 - Profile cracks originating from severe pitting

Figure 82 - Broken tooth ends 1 0.2.6.1 Metallurgical analysis for nonmetallic inclusions Fi g u re 83 i s an exam pl e of n on m etal l i c i n cl u si on fai l u re of a carbu ri zed bevel g ear (th e arrow poi n ts to th e i n cl u si on ).

An exam pl e of a n on m etal l i c i n cl u si on fai l u re from a paral l el axi s g ear i s sh own i n Fi g u re 84

th rou g h Fi g u re 88.

Fi g u re 84 sh ows a fractu red tooth wi th th e l oose frag men t set i n pl ace on th e g ear

bod y to sh ow th e posi ti on of cracks on th e d ri ve fl an k. su bsu rface i n cl u si on .

Th e red d ot m arks th e axi al l ocati on of th e

Wh en ever th e crack ori g i n i s h i g h on th e tooth fl an k i t i n d i cates th e root cau se i s

n ot cl assi c ben d i n g fati g u e bu t d u e to a m etal l u rg i cal fl aw.

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Figure 83 - Bending fatigue initiation from subsurface nonmetallic inclusion

Figure 84 - Bending fatigue due to nonmetallic inclusion

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Figure 85 - Fracture surface of loose fragment showing nonmetallic inclusion

Figure 86 - BSE image of fracture surface showing scanned areas 1 , 2, and 3

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Figure 87 - EDS spectrum of figure 86 area 1 showing chemistry of the inclusion

Figure 88 - EDS spectrum of figure 86 area 3 showing chemistry of the steel matrix Figure 85 shows the fracture surface of the loose fragment with the subsurface nonmetallic inclusion located about 5 mm below the gear tooth load flank, which is shown at the bottom of Figure 85. It is surrounded by a smooth fracture surface that was developed by rubbing of the opposing fracture surfaces of the initially tightly closed subsurface crack. Beach marks delineate successive arrests of the crack propagation as the crack expanded with an elliptically shaped crack front. Once the crack broke through to the surface of the load flank, the crack growth rate increased, then slowed as the crack reduced gear tooth stiffness and allowed the cracked tooth to bend away from the load and shed load to neighboring gear teeth. For helical gears with relatively large axial contact ratio, the growth rate near the exit end of the fatigue zone can be quite low and result in a very small overload zone of final fracture as shown by the less than 1 mm ligament at the top of Figure 85. In contrast, a spur gear with transverse contact ratio less than two usually has a much higher fatigue crack growth rate due to less load sharing. It typically has a larger overload zone of final fracture because a single tooth pair is subjected to a shock load when the lead pair of teeth leaves contact. Furthermore, helical gears tend to be more robust because they share load over more pairs of teeth and bending fatigue cracks tend to remove only ends of teeth, whereas spur gears usually fracture abruptly across the full-face width.

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Fi g u re 86 sh ows a scan n i n g el ectron m i croscopy (SEM ) i m ag e u si n g th e back scatter el ectron (BSE) i m ag i n g m od e.

BSE i s a h i g h -con trast m od e th at sh ows th e i n cl u si on i n th e d ark areas an d th e steel

m atri x i n th e l i g h t areas.

Th e th ree red rectan g l es d en oti n g areas (1 , 2, an d 3) were scan n ed wi th en erg y

d i spersi ve spectroscopy (EDS) to i d en ti fy ch em i stry at each l ocati on . Fi g u re 87 sh ows an EDS spectru m of area 1 th at shows h i g h con cen trati on of al u m i n u m (Al ) an d cal ci u m (Ca) th at i n d i cates th e i n cl u si on i s cal ci u m al u m i n ate (CaO-Al 2 O 3 ); th e m ost d an g erou s type.

Area 2

sh ows si m i l ar con cen trati on . Fi g u re 88 sh ows an EDS spectru m of area 3 of th e backg rou n d m atri x th at sh ows on l y th e expected el em en ts of th e steel al l oy an d n o traces of al u m i n u m or cal ci u m .

1 0.2.7 Tooth interior fatigue fracture, TIFF Referen ces [26,

27]

d escri be tooth

i n teri or fati g u e fractu re (TI FF).

TI FF fai l u res th at i n i ti ated

from

i n cl u si on s h ave been reported [26] , bu t TI FF fai l u res typi cal l y occu r at m od erate stress l evel s wh ere i n cl u si on s are l ess d am ag i n g .

At h i g h stress l evel s, i t i s m ore l i kel y to h ave a crack i n i ti ati n g at th e

su rface of th e root fi l l et (see 1 0. 2. 3) th an i n th e i n teri or [27] .

TI FF fai l u res ori g i n ate wi th i n th e tooth

i n teri or an d h ave a fl at pl ateau n ear th e cen terl i n e of th e tooth an d a terrace n ear each fl an k th at i s form ed wh ere th e m ai n core crack tu rn s to fol l ow th e case/core bou n d ary toward s th e roots. 5

m i g h t h ave a l i feti m e of on l y 1 0 - 1 0

6

TI FF fai l u res

cycl es [26] .

Referen ce [26] con cl u d es: -

TI FF h as been observed i n case h ard en ed i d l ers;

-

Th e fai l u re su rface of a TI FF h as a ch aracteri sti c sh ape wi th a d i sti n ct pl ateau i n th e cen ter at

-

Th e m ech an i cal d ri vi n g forces of th e crack are resi d u al ten si l e stresses i n th e i n teri or of th e tooth an d

approxi m atel y a m i d -h ei g h t posi ti on of th e tooth ;

al tern ati n g stresses d u e to th e i d l er u sag e of th e g ear; -

An an al ysi s tech n i q u e based on fi n i te el em en t com pu tati on s for th e stu d y of TI FF i s presen ted ;

-

Th e an al ysi s sh ows th at al tern ati n g stress d u e to th e i d l er u sag e of a g ear wh eel an d ten si l e stresses d u e to case h ard en i n g l ead to poten ti al fati g u e i n i ti ati on i n a l arg e reg i on i n th e i n teri or of th e tooth ;

-

Th e ri sk of fati g u e i n i ti ati on i n th e i n teri or of th e tooth i s i n creased by i d l er u sag e of th e g ear wh eel as com pared to si n g l e stag e u sag e.

Figure 89 - TIFF failure on an idler gear

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Referen ce [27] con cl u d es: -

TI FF i s a possi bi l i ty at l oad s l ower th an th e l oad wh ere tooth root ben d i n g fati g u e i s ach i eved an d at

-

By u si n g th e g ear wh eel as an i d l er i n stead of as a si n g l e stag e g ear, th e ri sk of TI FF i s i n creased by

l oad s h i g h er th an th e l oad wh ere con tact fati g u e occu rs;

20%; -

Th e m ore sl en d er th e tooth (g reater wh ol e d epth ) an d th e h i g h er th e l oad , th e g reater th e ri sk of TI FF;

-

Th e i n fl u en ce of th e carbu ri zi n g d epth on TI FF i s sm al l , an d th e ri sk of TI FF i s l ower for a h i g h carbu ri zi n g d epth th an for a l ow carbu ri zi n g d epth .

1 0.2.7.1 Comparison of TIFF to subsurface initiated bending fatigue Su bsu rface i n i ti ated ben d i n g fati g u e (1 0. 2. 6) fai l u res d i ffer si g n i fi can tl y from TI FF fai l u res an d exh i bi t th e fol l owi n g featu res: -

Cracks i n i ti ate n ear th e pi tch d i ameter an d h ave su bsu rface ori g i n s abou t 1 . 5-2. 5 ti m es th e effecti ve case d epth ;

-

Cracks u su al l y ori g i n ate at a n on m etal l i c i n cl u si on .

Very sm al l cracks m i g h t be g en erated d u ri n g or

sh ortl y after case h ard en i n g or i n i ti ate by fati g u e d u e to th e stress con cen trati on cau sed by th e i n cl u si on ; -

Fol l owi n g

i n i ti ati on ,

th e

fati g u e

crack

g rows

sl owl y

toward s

th e

l oad

fl an k

restrai n ed

by

th e

com pressi ve resi d u al stress i n th e case an d m ore rapi d l y toward s th e u n l oad ed fl an k accel erated by th e ten si l e stress fi el d i n th e core of th e tooth ; -

Th e traj ectory of th e fati g u e crack i s typi cal l y at an i n cl i n ati on of 45° to th e l oad fl an k;

-

Wh i te-etch i n g areas (WEAs),

wh i ch are evi d en ce of i n ten se pl asti c d eformati on ,

m i g h t be fou n d

paral l el to th e fl an k su rface wi th i n th e case zon e. Tabl e 4 su m m ari zes th e d i fferen ces between TI FF an d su bsu rface i n i ti ated ben d i n g fati g u e. th e m orph ol og y an d ben d i n g fati g u e.

operati n g

Fu rth erm ore, th e l oad l evel s at wh i ch th e two fai l u re m od es occu r, th ei r l i feti m es, an d

th ei r sen si ti vi ty to n on m etal l i c i n cl u si on s are d i fferen t. d efi n i ti on s,

I t sh ows th at

con d i ti on s of TI FF are si g n i fi can tl y d i fferen t from su bsu rface i n i ti ated

Th erefore, th e two fai l u re m od es d eserve separate

an d TI FF sh ou l d be reserved for th e fai l u re m od e d escri bed by referen ce [27] ,

wh ereas

su bsu rface i n i ti ated ben d i n g fati g u e sh ou l d be reserved for th e fai l u re m od e d escri bed by 1 0. 2. 6.

Table 4 - Differences between TIFF and subsurface initiated bending fatigue Tooth interior fatigue Subsurface initiated bending Parameter fracture, TIFF fatigue Fractu re pl an e

Pl ateau

perpen d i cu l ar

to

tooth

45° to su rface of l oad fl an k

cen terl i n e I n cl u si on at ori g i n

N ot n ecessari l y

Yes

Stress m ag n i tu d e

M od erate

High

Wh i te Etch i n g Areas, WEAs

U n l i kel y

N ot n ecessari l y

Li feti m e

10

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5

- 10

6

cycl es

Often >> 1 0

6

cycl es

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Annex A Design considerations to reduce the chance of failure [Th e foreword ,

footn otes an d an n exes,

i f an y,

con stru ed as a part of AN SI /AG M A 1 01 0-F1 4,

A.1

are provi d ed

for i n form ati on al

pu rposes on l y an d sh ou l d

Appearance of Gear Teeth - Terminology of Wear and Failure. ]

n ot be

General design considerations

As stated i n th e scope, th e m eth od s g i ven for red u ci n g th e ri sk of a fai l u re mod e are speci fi c to th e fai l u re m od e con si d ered .

Th ere are m an y th i n g s th at wi l l

i n creasi n g th e ri sk of an oth er fai l u re mod e.

red u ce th e ch an ce of on e type of fai l u re wh i l e

Si n ce d esi g n ch an g es m ay h ave u n i n ten d ed con seq u en ces,

an y ch an g e sh ou l d be eval u ated both pri or to an d after i m pl em en tati on . Som e d esi g n con si d erati on s are l i sted i n Tabl e A. 1 .

Al l of th ese sh ou l d be con si d ered d u ri n g d esi g n , wi th

th e real i zati on th at some d esi g n ch an g es wi l l h ave l i ttl e or n o i mpact on cost of m an u factu re, wh i l e oth ers m ay h ave a su bstan ti al i m pact th at sh ou l d be wei g h ed ag ai n st th e costs of fai l u re.

Table A.1 - Design considerations Advantages

Disadvantages

Cost impact

Geometry M od u l e

I n creased m od u l e i n creases

I n creased m od u l e

M od u l e g en eral l y h as l i ttl e

ben d i n g stren g th .

i n creases speci fi c sl i d i n g

i mpact on cost i f th e g ear

Decreased

m od u l e i n creases H ertzi an fati g u e

d i am eter d oes n ot ch an g e

resi stan ce an d scu ffi n g

(i . e. , i f th e n u m ber of teeth

resi stan ce.

ch an g es i n versel y wi th th e mod u l e), provi d ed th at

Decreased m od u l e m ay avoi d

tool i n g i s avai l abl e.

probl em s wi th l ow n u m bers of teeth on th e pi n i on . H i g h er n u m ber of teeth

H i g h er con tact rati o.

Lower ben d i n g stren g th .

Vari abl e.

Better preci si on i n g ear rati o.

Sm al l er mod u l e for g i ven

Lon g er i n specti on ti m e.

Lon g er tru e i n vol u te form . Qu i eter operati on .

cen ter d i stan ce. Profi l e sh i ft i s m ore sen si ti ve to tol eran ces.

Less ch an ce of u n d ercu t. H i g h er scu ffi n g resi stan ce d u e to l ower sl i d i n g vel oci ty. H i g h er effi ci en cy. Pressu re an g l e

Appropri ate pressu re an g l e can

I n creased pressu re an g l e

Cost i s on l y affected i f n ew

red u ce ch an ce of fai l u re.

i n creases rad i al l oad on

tool i n g h as to be

I n creased pressu re an g l e

beari n g s an d d ecreases

pu rch ased .

i n creases ben d i n g stren g th ,

tran sverse con tact rati o.

H ertzi an fati g u e resi stan ce, an d scu ffi n g resi stan ce. H el i x an g l e

I n creased h el i x an g l e i n creases

I n creased h el i x an g l e

tooth stren g th an d sm ooth n ess of

i n creases axi al forces on

tran smi ssi on , especi al l y i f th e

beari n g s.

M i n i m al

axi al con tact rati o i s sl i g h tl y above an i n teg er. Cen ter d i stan ce

I n creasi n g cen ter d i stan ce

I n creased cen ter d i stan ce

red u ces th e l oad s both on th e

req u i res l arg er g ears an d

g ear teeth an d on th e beari n g s.

h ou si n g s, an d m ay resu l t

Cost i n creases wi th si ze.

i n excessi ve peri ph eral speed .

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Advantages Face wi d th

Disadvantages

Cost impact

I n creased face wi d th i n creases

For m i cropi tti n g

l oad capaci ty provi d ed th e l oad

resi stan ce, F/d rati o

i n teg ral wi th sh aft,

d i stri bu ti on across th e face wi d th

sh ou l d be

proporti on al to face wi d th

rem ai n s u n i form .

an d si n g l e h el i cal g ears an d

≤

≤

1 . 0 for spu r

M i n i mal wh en g ear i s

wh en sh aft i s separate.

2. 0 for d ou bl e

h el i cal g ears. Profi l e sh i ft

Can red u ce ch an ce of fai l u re.

U su al l y can on l y be

M i n i m al

opti mi zed for on e fai l u re mod e. Cost

Gear fl an k

Better accu racy can red u ce

accu racy

d yn am i c l oad s, th at i n tu rn

Th ere are si g n i fi can t cost i mpacts i f ad d i ti on al steps

red u ces th e ch an ces of m an y

i n th e man u factu ri n g

fai l u re mod es.

process are req u i red to ach i eve th e req u i red accu racy.

Fl an k

Appropri ate fl an k m od i fi cati on s

Fl an k mod i fi cati on s

Depen d s on m an u factu ri n g

mod i fi cati on s

can red u ce d yn am i c l oad s an d

g en eral l y can on l y be

tech n i q u e.

resu l t i n better l oad d i stri bu ti on .

opti mi zed for on e l oad .

I m proved su rface fi n i sh i s al ways

Cost, al th ou g h ru n -i n m ay

Li ke fl an k accu racy, th e

ben efi ci al .

be u sed to i m prove

cost can be su bstan ti al i f

su rface fi n i sh at l ow cost.

ad d i ti on al man u factu ri n g

Su rface fi n i sh

steps are req u i red . Root fi l l et

Larg e sm ooth root fi l l ets red u ce

Cost i s affected i f n ew

g eom etry

ben d i n g stress.

tool i n g h as to be pu rch ased or m ach i n i n g practi ces ch an g ed .

Lubrication Lu bri can t

I n creased l u bri can t vi scosi ty i s

I n creased l u bri can t

g en eral l y better for th e g ear

vi scosi ty g en eral l y

m esh .

red u ces g earbox

Appropri ate ad d i ti ves

d esi g n ed i n to th e l u bri can t can

effi ci en cy an d may l ead to

i m prove l u bri can t performan ce.

ci rcu l ati on probl ems

Vari es

especi al l y d u ri n g col d starts or m ay cau se fi l ter bypass. Lu bri can t com pati bi l i ty sh ou l d be ch ecked wi th pai n t, seal s, g askets, etc. Lu bri cati on

Pressu ri zed l u bri cati on system s

Wh i l e som e l u bri can ts

Pressu ri zed l u bri cati on

system

th at cool an d fi l ter th e l u bri can t

h ave n o l i m i ts on th e l evel

system s are costl y, an d are

before sprayi n g i t i n to th e g ear

of fi l trati on , som e

n ot j u sti fi ed for m an y

m esh al l ow for rel i abl e operati on

l u bri can ts m ay h ave

appl i cati on s.

of th e g earbox.

ad d i ti ves th at can be

G ears n ot d i ppi n g i n l u bri can t can i m prove effi ci en cy.

fi l tered ou t i f too fi n e a fi l ter i s u sed .

Cost

Lu bri can t

Proper l u bri can t d i stri bu ti on i s

d i stri bu ti on (oi l

i m portan t both for cool i n g an d

com pl exi ty of th e

spray l ocati on ,

l u bri cati on .

d i stri bu ti on system .

d i recti on an d

vel oci ty g ears, fu l l y l u bri cati n g th e

pattern s)

teeth can be a ch al l en g e.

Wi th h i g h pi tch l i n e

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AMERICAN NATIONAL STANDARD Advantages

ANSI/AGMA 1 01 0-F1 4 Disadvantages Metallurgy

Cost impact

Hardness

Increased hardness generally is beneficial. Carburized gears have the greatest load capacity.

Excessively hard teeth may be brittle.

Increased hardness generally increases machining costs, and may require use of a different material and heat treatment.

Material

Use of a material appropriate for the heat treatment is critical.

Cost

The material, heat treatment and related manufacturing operations need to be considered together. The appropriate combination can reduce cost in some applications. The total system cost should be considered.

Material cleanliness

Improved material cleanliness is always beneficial.

Cost

Vacuum arc remelt can be used to improve the cleanliness for a price.

Shot peening

Shot peening may increase bending strength significantly.

Cost.

Limited availability with some materials and sizes Gear flanks should not be shot peened unless they are re-finished after shot peening.

Cost can be substantial especially if the flanks have to be manually masked.

A.2 Misalignment Misalignment is not a failure mode, but may be the root cause of many failure modes such as: -

Adhesion; Scuffing; Plastic deformation; Hertzian fatigue; Fracture; Bending fatigue.

Misalignment may result in end loading of the teeth, increasing the stresses in that section of the teeth and thereby increasing the risk of a failure. There are many possible causes of misalignment, including: -

Inaccurate lead, profile, spacing, or runout of pinion or wheel; Inappropriate lead or profile modifications; Bearing supports not parallel; Distortion of the gearbox housing or foundation due to applied stresses or thermal effects; Distortion of the gear teeth due to transmitted loads, centrifugal stresses, or thermal effects; Excessive radial space in the bearings, particularly those which do not have rolling elements; Excessive internal clearance in rolling-element bearings; Excessive tapered roller bearing endplay, see Figure A.1 .

Misalignment is always detrimental; proper alignment during operation is very important.

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Figure A.1 - Gear misalignment due to excessive endplay in tapered-roller bearings

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Annex B Bibliography Th e fol l owi n g d ocu m en ts are ei th er referen ced i n th e text of AN SI /AG M A 1 01 0-F1 4,

Teeth - Terminology of Wear and Failure, or i n d i cated for ad d i ti on al i n form ati on .

1.

Appearance of Gear

Wi n ter, H . an d Wei ss, T. , "Som e Factors I n fl u en ci n g th e Pi tti n g , M i cropi tti n g (Frosted Areas) an d Sl ow Speed Wear of Su rface H ard en ed G ears, " ASM E Pap. N o. 80-C2/DET-89, pp. 1 -7, 1 980.

2. 3.

M i l bu rn , A. , Erri ch el l o, R. , an d G od frey, D. , "Pol i sh i n g Wear, " AG M A Pap. N o. 90 FTM 5, Oct. , 1 990. Ad am s,

J.H.,

an d

G od frey,

D. ,

"Borate

G ear

Lu bri can t-EP

Fi l m

An al ysi s

an d

Perform an ce, "

Lu bri cati on En g i n eeri n g , Vol . 37, N o. 1 , pp. 1 6-21 , J an . 1 981 . 4.

G od frey, D. , "Fretti n g Corrosi on or Fal se Bri n el l i n g ?, " Tri bol og y & Lu bri cati on Tech n ol og y, Vol . 59, N o. 1 2, pp. 28-30, Dec. 2003.

5.

Erri ch el l o, R. , “An oth er Perspecti ve: Fal se Bri n el l i n g an d Fretti n g Corrosi on , ” Tri bol og y & Lu bri cati on Tech n ol og y, Vol . 60, N o. 4, pp. 34-36, Apri l , 2004.

6.

H u n t,

J . B. ,

Ryd e-Wel l er,

A. J . ,

an d Ash m ead ,

F. A. H . ,

"Cavi tati on Between M esh i n g G ear Teeth , "

Wear, Vol . 71 , pp. 65-78, 1 981 . 7.

Bl ok, H . , "Les Tem peratu res d e Su rface d an s Les Con d i ti on s d e Grai ssag e Son s Pressi on Extrem e, " Secon d Worl d Petrol eu m Con g ress, Pari s, J u n e, 1 937.

8.

Bl ok, H . , "Th e Postu l ate Abou t th e Con stan cy of Scori n g Tem peratu re, " I n terd i sci pl i n ary Approach to th e Lu bri cati on of Con cen trated Con tacts, N ASA SP-237, pp. 1 53-248, 1 970.

9.

Erri ch el l o,

R. ,

“Trou bl esh ooti n g

H ot

G ear

Dri ves, ”

Lu bri cati on

Excel l en ce

2003

Con feren ce

Proceed i n g s, N ori a, pp. 389-396, Apri l , 2003. 1 0. I sh i bash i , A. , an d M atsu m oto, S. , "U n d u l ati on of Su rfaces Cau sed by Rol l i n g Con tact, " Bu l l eti n of th e J SM E, Vol . 1 5, N o. 81 , pp. 387-400, 1 972. 1 1 . Erri ch el l o, R. L. , Eckert, R. , an d H ewette, C. , “Poi n t-Su rface-Ori g i n , PSO, M acropi tti n g Cau sed by G eom etri c Stress Con cen trati on , G SC, ” AG M A Pap. N o. 1 0FTM 1 1 , pp. 1 -1 1 , 201 0. 1 2. Li ttm an , W. E. , "Th e M ech an i sm of Con tact Fati g u e, " I n terd i sci pl i n ary Approach to th e Lu bri cati on of Con cen trated Con tacts, N ASA SP-237, pp. 309-377, 1 970. 1 3. Erri ch el l o, R. L. , "M orph ol og y of M i cropi tti n g , " AG M A Pap. N o. 1 1 FTM 7, pp. 1 -1 9, 201 1 . 1 4. U en o, T. , et al . , "Su rface Du rabi l i ty of Case-Carbu ri zed G ears - On a Ph en om en on of G rey - Stai n i n g of Tooth Su rface, " ASM E Pap. N o. 80-C2/DET-27, pp. 1 -8, 1 980. 1 5. Sh i pl ey, E. E. , "Fai l u re An al ysi s of Coarse-Pi tch , H ard en ed an d G rou n d G ears, " AG M A Pap. N o. P229. 26, pp. 1 -24, 1 982. 1 6. Tan aka, S. , et al , "Appreci abl e I n creases i n Su rface Du rabi l i ty of G ear Pai rs wi th M i rror-Li ke Fi n i sh , " ASM E Paper N o. 84-DET-223, pp. 1 -8, 1 984. 1 7. Ben yaj ati ,

C. ,

an d

Ol ver,

A. V. ,

“Th e

Effect

of a

Zn DTP

An ti wear

Ad d i ti ve

on

th e

M i cropi tti n g

Resi stan ce of Carbu ri zed Steel Rol l ers, ” AG M A Paper N o. 04FTM 6, pp. 1 -8, 2004. 1 8. Parri sh , G . , "Carbu ri zi n g : M i crostru ctu res an d Properti es, " ASM , 1 999. 1 9. Sh arm a, V. K. , Wal ter, G . H . , an d Breen , D. H . , "An An al yti cal Approach for Establ i sh i n g Case Depth Req u i rem en ts i n Carbu ri zed G ears, " ASM E pap. N o. 77-DET-1 52, pp. 1 -1 1 , 1 977. 20. Ped ersen , R. an d Ri ce, S. L. , "Case Cru sh i n g of Carbu ri zed an d H ard en ed G ears, " Tran s. SAE, Vol . 69, pp. 370-380, 1 961 . 21 . M u d d , G . C. , "A N u m eri cal M ean s of Pred i cti n g th e Fati g u e Performan ce of N i tri d e-H ard en ed G ears, " Proc. I n st. M ech . En g rs. , Vol . 1 84, Part 30, pap. 1 2, pp. 95-1 04, 1 969-1 970. 22. Kron , H . O. , "G ear Tooth Su b-Su rface Stress An al ysi s, " U n abri d g ed Text of Lectu res", Vol . 1 , Worl d Con g ress on G eari n g , Pari s, Fran ce, pp. 1 85-202, J u n e 22-24, 1 977. 23. San d berg , E. , "A Cal cu l ati on M eth od for Su bsu rface Fati g u e, " Proc. of I n tern ati on al Sym posi u m on G eari n g an d Power Tran sm i ssi on s, Vol . 1 , Au g . 30-Sep 3, pp. 429-434, Tokyo, 1 981 . 24. Kern , R. F. , an d Su ess, M . E. , "Steel Sel ecti on A Gu i d e for I m provi n g Perform an ce an d Profi ts, " J oh n Wi l ey, 1 979.

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25. Diesburg, D.E., and Smith, Y.E., "Fracture Resistance in Carburizing Steels," Metal Progress, Parts I, II and III, May, June and July, 1 979. 26. MackAldener, M., “Tooth Interior Fatigue Fracture & Robustness of Gears,” Doctoral Thesis, Department of Machine Design, Royal Institute of Technology, Stockholm, Sweden, 2001 . 27. MackAldener, M., and Olsson, M., “Design Against Tooth Interior Fatigue Fracture,” Gear Technology, Nov./Dec. 2000, pp. 1 8-24. 28. Clark, D.S., and Varney, W.R., "Physical Metallurgy- For Engineers," D. Van Nostrand Company, 1 962. 29. Radzevich, S.P., “Dudley’s Handbook of Practical Gear Design and Manufacture”, second edition, table 5.5, pp. 249, CRC Press, 201 2.

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Annex C Acknowledgements Th e Am eri can G ear M an u factu rers Associ ati on wou l d l i ke to th an k th e fol l owi n g org an i zati on s for th ei r con tri bu ti on s to th i s d ocu men t. 1.

G eartech (Fi g u res 1 -6, 8, 1 0, 1 1 , 1 4-1 8, 21 -28, 30-32, 35-37, 39-41 , 46, 50, 51 , 55, 56, 58, 66, 68-71 ,

2.

I n tern ati on al Stan d ard s Org an i zati on (Fi g u res 7, 1 2, 1 9, 20, 29, 33, 38, 42-44, 59, 61 , 62, 67, 72-74,

75, 77, 81 , 84-89)

78-80) 3.

Caterpi l l ar, I n c. (Fi g u res 9, 1 3, 45, 52, 65, 76, 82, 83)

4.

U n i versi ty of N ewcastl e-U pon -Tyn e (Fi g u res 47-49, 53, 54, 63, 64)

5.

AG M A 1 1 0 (Fi g u res 34, 60)

6.

Artec M ach i n e System s (Fi g u re 57)

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