01-introduction To Vibration.pdf

Lecture Note Introduction to Condition Monitoring

Prepared by Essam Abdel-Halim Moustafa

Abstract This lecture introduces the predictive maintenance concept of condition monitoring for industrial rotating machines. This makes it easier to understand how important the need for condition monitoring is.

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Introduction to Condition Monitoring

Condition monitoring (CM), by definition, is simply a technique for routinely evaluating the condition of equipment. This simple definition, however, has tremendous applications in improving the life of many industries and even our day-to-day existence. It is instrumental in increasing industrial production and profitability, improving product quality, reducing environmental pollution, improving safety, and reducing the waste of our limited natural resources. Spectacular gains can be made in all of these areas, if CM is properly employed. If not, it could equally generate losses. CM can be done on almost any kind of device from micro-sized circuit boards to huge 1000 MW hydroelectric turbines. Nearly every imaginable industry can use or is using some form of CM. This lecture will touch on several important topics which concern CM, but the focus will be on industrial applications using rotating machinery.

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Types of Maintenance Break-down

Corrective Maintenance

Cost

(Run-to-breakdown) Repair it when it fails

Preventive Maintenance (Time Based Maintenance) Maintenance at regular intervals

Cost

Predictive Maintenance (On Condition Maintenance) Problem detected before predicted failure. Maintenance planned ahead

Cost

Time

Time

Time

Maintenance has evolved from run-to-breakdown methods, to time-based methods to modern predictive maintenance practices. This has resulted in less spares and manpower to maintain machines, and higher machine availability.

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Corrective Maintenance -Run to BreakdownNot recommended for critical machines

Corrective CorrectiveMaintenance Maintenance leads leadsto: to: Secondary Secondarydamage damage Safety Safetyrisk risk

Cost

Unplanned Unplanneddowntime downtime Unplanned Unplanned maintenance maintenance Product Productwaste waste Spares inventory Spares inventory

Break -down

Time

Which is historically the first maintenance strategy employed. A machine is repaired only after a failure has occurred. This is a very expensive maintenance management scheme, since it requires: high spare parts inventory, high machine downtime, high overtime labor costs, and low production availability.

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Preventive Maintenance Time Based Maintenance

Not recommended for critical machines

Time-based Time-based Preventive Preventive Maintenance Maintenance involves: involves: More Morefrequent frequentoverhauls overhauls Risk of early failures Risk of early failures Tampering Tamperingwith withgood good machines machines Time Timeconsuming consumingoverhauls overhauls Experts needed for Experts needed foreach each overhaul overhaul

Where important machines are not fully duplicated or where unscheduled production stops can result in large losses, maintenance operations are often performed at fixed time-intervals. The advantage of this maintenance strategy is that it is planned strategy and is based on previous experience and the mean-time-between-failure (MTBF) statistic for the machine. The disadvantage of this maintenance scheme is that it is not based on the condition of the machine, but rather on the time elapsed since the previous maintenance occurred. Thus a failure may occur before a maintenance is performed, as in Run-ToBreakdown maintenance, or a perfectly operating machine may be maintained with a consequent waste in labor and material . This system is therefore called Time-based Predictive Maintenance.

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Predictive Maintenance On-condition -Maintenance

Recommended for critical machines

Monitor Monitorthe thecondition conditionof ofthe the machine and predict when machine and predict whenitit would wouldfail fail Plan maintenance Plan maintenanceahead aheadof of time and save money time and save money Repair Repairthe themachines machinesonly only when they need when they needto to Focus Focusoverhauls overhaulsonly onlyon onfaulty faulty parts parts ☺☺ Higher Higherplant plantavailability, availability,

performance performanceand andreliability reliability ☺☺ Greater safety Greater safety ☺☺ Better Betterproduct productquality quality ☺☺ Attention to environment Attention to environment ☺☺ Longer Longerequipment equipmentlife life ☺☺ Greater cost effectiveness Greater cost effectiveness

In which maintenance is performed on the basis of the machine condition. This is done by monitoring the machine condition. Any change in condition is detected, and the time to Failure is estimated . This is also accompanied by diagnosing the cause of the fault to actually pin point the defective components. With this method each machine is considered individually by making fixed-interval condition measurements to obtain a quantitative value of the “Health” of the machine. In this way maintenance is only allowed when measurements show it to be necessary.

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Comparison of Maintenance Strategies Run-to-breakdown

Preventive

Predictive

Method

Failure based

Time based

Condition Based

Premise

No maintenance

MTBF

As-Needed

Advantages

•No maintenance cost

•Planned Maintenance. •Structured program

Disadvantages

Use

•High spare parts inventory •High machine downtime •High overtime labor costs •low production availability

Used only on cheap, abundant and insignificant components

•Failure may occur before scheduled maintenance. •Maintenance may be performed unnecessarily. •Maintenance may cause failure. Used on all machines

• Lower maintenance costs . •Fewer machine failures. •Less repair downtime. •Reduced inventory. •Longer machine life. •Increased production. •Improved operator safety. •Verification of new equipment condition. •Improved overall profitability. •Initial investment in equipment

Used on all machines

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Why Make Condition Monitoring ?

Initial investigations, selection of monitoring points, establishment of limits Selection and purchase of instrumentation Training

Yes

No

Condition Monitoring

Longer time between overhauls Reduced repair duration Reduction of spare-part stock Less unexpected breakdowns Elimination of secondary damage Reduction in business interruption and damage insurance premiums

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Predictive Maintenance Flow Chart Start

Reference creation Regular Measurements No

Machine Specs. & Drawings

Fault detected Yes Fault diagnosis

Troubleshooting Chart

Fault correction

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Machine Potential Failures Analysis

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Yes

Out of Balance Misalignment / Bent Shaft

Yes

Damage of rolling elements bearings

Yes

Damage of journal bearing

Yes

Damage of gearboxes

Yes Yes Yes

Yes

Yes

Yes

Yes

Yes

Yes Yes

Belt Problems Motor Problems Mechanical Looseness Resonance

Vi br at io n

Cu rre nt

Oi la na ly si s

Fl ow

Type of machine fault

Pr es su re

Parameters

Te m p.

Parameters Used for Detection of machine Faults

Yes

Yes Yes Yes

Why Vibration ? Vibration is used as the fault detection parameter simply because it can give an early warnings of fault development for a wider variety of typical rotating machinery faults. Other detection techniques, if used in isolation, limit the variety of faults, and so unexpected breakdown by a fault type not included, is a real possibility.

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What is Vibration

Force •Forces caused by -Imbalance - Friction -Shock -Acoustic Input Forces

Mobility

Freq.

•Structural Parameters: -Mass -Stiffness -Damping

x System Response (Mobility)

=

x

Freq.

Vibration

= •Vibration Parameters: -Acceleration -Velocity -Displacement

Vibration Freq.

What is Vibration? Vibration is mechanical oscillation about a reference position. Vibration is an everyday phenomenon, we meet it in our homes, during transport and at work. Vibration is often a destructive and annoying side effect of a useful process, but is sometimes generated intentionally to perform a task. Vibration of machines Vibration is a result of dynamic forces in machines which have moving parts and in structures which are connected to the machine. Different parts of the machine will vibrate with various frequencies and amplitudes. Vibration causes wear and fatigue. It is often responsible for the ultimate breakdown of the machine.

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Signal level Descriptors Amplitude

T

Peak

RMS

Average

PeakPeak

RMS =

1 T 2 x ( t )dt T ∫0

Time

Average =

1 T x( t ) dt T ∫0

Crest Factor :

Peak RMS

Signal Level Descriptors The level of vibration signal can be described in different ways. Peak and peak-to-peak values are often used to describe the level of a vibration signal since they indicate the maximum excursion from equilibrium position. The RMS (Root Mean Square) level is a very good descriptor, since it is a measure of the energy content of the vibration signal.

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Time Signal Descriptors Amplitude

PeakPeak

Peak RMS Average Time

RMS =

1 T 2 x ( t )dt T ∫0

Average =

1 T x( t ) dt T ∫0

Crest Factor :

Peak RMS

Time Signal Descriptors These descriptors are not only used in conjunction with a single sinusoidal signal but also with normal machine vibration signals which are composed of many sinusoidal vibration components.

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FFT transformation Displacement

d = D sinωnt Displacement

D Time

Frequency

1 T

T Period, Tn in [sec] m

1

Frequency, fn= T in [Hz = 1/sec] n

k

ωn= 2 π fn =

k m

Mass and Spring Once a (theoretical) system of a mass and a spring is set in motion it will continue this motion with constant frequency and amplitude. The system is said to oscillate with a sinusoidal waveform. The Sine Curve The sine curve which emerges when a mass and a spring oscillate can be described by its amplitude (D) and period (T). Frequency is defined as the number of cycles per second and is equal to the reciprocal of the period. By multiplying the frequency by 2 π the angular frequency is obtained, which is again proportional to the square root of spring constant k divided by mass m. The frequency of oscillation is called the natural frequency fn. The whole sine wave can be described by the formula d = Dsin ωnt, where d = instantaneous displacement and D = peak displacement.

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Spectrum Analysis

We see that the longer the period of the sine wave, the lower the frequency. The magnitude of the peak in the spectrum corresponds to the energy content of the sine wave (RMS).

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Vibration Parameters Displacement

Velocity

Acceleration

m

d v

k

a

c

F=k×d

F=c×v

m

F=m×a

Mechanical Parameters Before going into a discussion about vibration measurement and analysis, we will examine the basic mechanical parameters and components and how they interact. All mechanical systems contain the three basic components: spring, damper, and mass. When each of these in turn is exposed to a constant force they react with a constant displacement, a constant velocity and a constant acceleration respectively.

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Acceleration, Velocity and Displacement

a v

M d

a v

Acceleration

d M M

a

a

Velocity m

v

v

d m

m

Displacement

d

Time (Simple vibration)

Frequency (real machine)

Vibration may be measured in acceleration, velocity or displacement. For higher frequencies the velocity is higher than the displacement, and acceleration is higher than velocity. Note that the peaks in the three spectra from a real machine are situated at the same frequencies. For very high frequencies, the peaks may not be seen in the displacement spectra due to noise.

Which Parameter to Choose? If the type of measurement being carried out does not call for a particular parameter to be measured e.g. due to some standard, the general rule is that the parameter giving the flattest response over the frequency range of interest should be chosen. This will give the biggest dynamic range of the whole measurement set up. If the frequency response is not known start by choosing velocity.

An advantage of the accelerometer is that its electrical output can be integrated to give velocity and displacement signals. This is important since it is best to perform the analysis on the signal which has the flattest spectrum. If a spectrum is not reasonably flat, the contribution of components lying well below the mean level, will be less noticeable. In the case of overall measurements, smaller components might pass completely undetected. Use the Flattest Spectrum In most cases this will mean that velocity is used as the detection parameter on machine measurements. On some occasions acceleration may also be suitable, although most machines will exhibit large vibration accelerations only at high frequencies. It is rare to find displacement spectra which are flat over a wide frequency range, since most machines will only exhibit large vibration displacements at low frequencies. In the absence of frequency analysis instrumentation to initially check the spectra, it is safest to make velocity measurements (but still using the accelerometer, of course, since even the integrated accelerometer signal gives a better dynamic and frequency range than the velocity transducer signal).

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Man-Machine Link Correct & Effective Maintenance Decision

M/C Information & Operation

Diagnostic Knowledge

+ + +

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Example of Machine Information table

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Example of Machine Component

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Frequency Spectrum Motor Speed

: 600 r.p.m

Rigid Coupling-1: No. of bolts 4 Co First gear

:50 :50 teeth

Second gear

:20 :20 teeth

Rigid Coupling-2: No. of bolts 6 Co :5

Vib.

Fan blades

r.p.m or (Hz)

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Frequency Spectrum interpretation Motor Speed

: 600 r.p.m

Rigid Coupling-1 : No. of bolts 4 Co First gear

: 50 teeth

Second gear

: 20 teeth

Rigid Coupling-2 : No. of bolts 6 Co :5 5 blades

5 75 Bla 00 d e rp s X m (1 15 25 00 Hz ) 6B 90 00 olt X rpm 15 (15 00 0H z)

4 24 Bo 00 lt X r p 60 m 0 (4 0H z)

600 r.p.m (10 Hz) 15 (2 00 5 r.p Hz .m )

Vib.

Fan blades

Gear Meshing Freq. 50 teeth x600 Or 20 teeth x 1500 30,000 rpm(500Hz)

r.p.m or (Hz)

Motor Running Speed Freq.

: 600 r.p.m (10 Hz)

First Coupling defect Frequency

: 600 r.p.m x 4 bolt = 2400 r.p.m(40 Hz)

Gear Meshing Frequency

: 600 r.p.m x 50 teeth=30,000 r.p.m(500Hz) : or 1500 r.p.m x 20 teeth=30,000 r.p.m (500 Hz)

Second shaft speed

: 600x50/20 =1500 r.p.m

Second Coupling defect Frequency :1500 r.p.m x 6 bolt = 9000 r.p.m(150 Hz) Blade Passing Frequency

: 1500 r.p.m x 5 Blade=7500 r.p.m (125Hz)

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Measuring Points

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Vib.

Reference Spectrum

r.p.m or (Hz)

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Overall Alarm

Vib.

Frequency Spectrum With Overall Alarm & Danger

r.p.m or (Hz)

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Band Alarm

Vib.

Frequency Spectrum With Band Alarm & Danger

r.p.m or (Hz)

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Vib.

Example of Bands alarm Sheet

r.p.m or (Hz)

Band No. Band 1

Band 2

Definitions BAND Freq. MIN BAND Freq. MAX BAND ALARM BAND DANGER BAND Freq. MIN BAND Freq. MAX BAND ALARM BAND DANGER

Suggested Setup

!? !? !? !?

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Profile or Narrow band alarm

Vib.

Frequency Spectrum With Profile Alarm & Danger

r.p.m or (Hz)

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Vib.

Overall Analysis

r.p.m or (Hz)

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Frequency Spectrum or Overall Level Frequency Spectrum Vibration

Overall Level 4

5 4 3 2 1

2

5

3

1

Fan Frequency

Date 5 1 2 3 4

Frequency

Date

Vibration 5 4 3 2 1

Gearbox

Frequency Spectrum or Overall Level To decide whether monitoring or testing of the overall level is sufficient or a complete frequency spectrum is required, the engineer must know his machine and something about the most likely faults to occur or which part of the object is of interest. The illustration shows two different situations in monitoring, but it might as well be testing: Monitoring of a fan: The most likely fault to occur is unbalance, which will give an increase in the vibration level at the speed of rotation. This will normally also be the highest level in the spectrum. To see if unbalance is developing, it is therefore sufficient to measure the overall level at regular intervals. The overall level will reflect the increase just as well as the spectrum. Monitoring of a gearbox: Damaged or worn gears will show up as an increase in the vibration level at the tooth meshing frequencies (shaft RPM number of teeth) and their harmonics. The levels at these frequencies are normally much lower than the highest level in the frequency spectrum, so it is necessary to use a full spectrum comparison to reveal a developing fault. A general rule is overall measurements are permissible for simple, non critical machines, while more complex, more critical machinery requires spectral analysis.

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END

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