Mttr, Mtbf,mttf,oee

WHAT IS MTTR, MTBF, MTTF, OEE, AVAILABILITY Maintenance Concept

What is MTBF • MTBF (Mean Time Between Failure) is ametric that concerns the average time elapsed between a failure and the next time it occurs. • Mean time between failures (MTBF) describes the expected time between two failures for a repairable system. For example, three identical systems starting to function properly at time 0 are working until all of them fail. The first system failed at 100 hours, the second failed at 120 hours and the third failed at 130 hours. The MTBF of the system is the average of the three failure times, which is 116.667 hours. If the systems are non-repairable, then their MTTF would be 116.667 hours. • These lapses of time can be calculated by using a formula

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Case study for MTBF A system should operate correctly for 9 hrs in a shift and 4 failures occured Adding to all failures we have 60 min (1 hr) So MTBF = (9-1)/4 = 2 Hrs

What is MTTR • MTTR (Mean Time To Repair) is the time it takes to run a repair after the occurrence of failure. • It is the time spent during the intervention in a given process • Formula

• Case study • A system should operate correctly for 9 hrs in a shift and 4 failures occured • Adding to all failures we have 60 min (1 hr) So • MTBF = 60 min/4 = 15 mins.

What is MTTF • MTTF (Mean Time To Failure) is the length of time a device or other product is expected to last in operation. MTTF is one of many ways to evaluate the reliability of pieces of hardware or other technology. • MTTF is a very basic measure of reliability used for non-repairable systems. When using MTTF as a failure metric, repair of the asset is not an option • MTTF is what we commonly refer to as the lifetime of any product or a device • Formula : MTTF :- Total hours of operation / total number of units. • Lets assume we tested 3 identical PC’s until all of them failed. The first PC fail after 10 hrs the second one failed at 11 hrs & third failed at 12 hrs. MTTF would be (10 + 11 + 12) / 3 = 11 hours. • This is particular type & model of the PC will need to be replaced on average every 11 hours.

What is the difference between MTBF and MTTF? • MTBF (Mean Time Between Failures) describes the time between to failures. • MTTF (Mean Time To Failure) describes the time up to the first failure.

Benifits in the use of these performance indicators • MTTR & MTBF are two indicators used for more than 60 years as points of reference for decision making • MTBF is a basic measure of the reliability of a system while MTTR indicates efficiency on corrective action of a process • If the MTBF has increased after a preventive maintenance process this indicates a clear improvement in the quality of your process • The MTBF increase will show that your maintenance & verification methods are being well run • In the case of MTTR the effort should be exactly the opposite to reduce it as much as possible to avoid loss of productivity for your unavailability.

What is OEE • Overall equipment effectiveness (OEE) is a measure of how well a manufacturing operation is utilized (facilities, time and material) compared to its full potential, during the periods when it is scheduled to run. it identifies the percentage of manufacturing time that is truly productive. An OEE of 100% means that only good parts are produced (100% quality), at the maximum speed (100% performance), and without interruption (100% availability) • Total effective equipment performance (TEEP) is a closely related measure which quantifies OEE against calendar hours rather than only against scheduled operating hours. A TEEP of 100% means that the operations have run with an OEE of 100% 24 hours a day and 365 days a year (100% loading). • TEEP = Loading * OEE • OEE is calculated with the formula (Availability)*(Performance)*(Quality)

The OEE of a manufacturing unit are calculated as the product of three separate components: • Availability: percentage of scheduled time that the operation is available to operate. Often referred to as Uptime. • Performance: speed at which the Work Center runs as a percentage of its designed speed. • Quality: Good Units produced as a percentage of the Total Units Started. It is commonly referred to as the first pass yield (FPY). • To calculate the TEEP, the OEE is multiplied by a fourth component: • Loading: percentage of total calendar time that is actually scheduled for operation Total Time: 8 hour shift or 28,800 seconds, producing 14,400 parts, or one part every 2 seconds. Fastest possible cycle time is 1.5 seconds, hence only 21,600 seconds would have been needed to produce the 14,400 parts. The remaining 7,200 seconds or 2 hours were lost. The OEE is now the 21,600 seconds divided by 28,800 seconds (same as minimal 1.5 seconds per part divided by 2 actual seconds per part), or 75%.

 Loading • The Loading portion of the TEEP Metric represents the percentage of time that an operation is scheduled to operate compared to the total Calendar Time that is available. The Loading Metric is a pure measurement of Schedule Effectiveness and is designed to exclude the effects how well that operation may perform. • Calculation: Loading = Scheduled Time / Calendar Time Example: • A given Work Center is scheduled to run 5 Days per Week, 24 Hours per Day. • For a given week, the Total Calendar Time is 7 Days at 24 Hours. • Loading = (5 days x 24 hours) / (7 days x 24 hours) = 71.4%

 Availability • The Availability portion of the OEE Metric represents the percentage of scheduled time that the operation is available to operate. The Availability Metric is a pure measurement of Uptime that is designed to exclude the effects of Quality and Performance. The losses due to wasted availability are called availability losses. Example: • A given Work Center is scheduled to run for an 8-hour (480 minute) shift with a 30-minute scheduled break and during the break the lines stop, and unscheduled downtime is 60 minutes. • The scheduled time = 480 minutes - 30 minutes = 450 minutes. • Operating Time = 480 Minutes – 30 Minutes Scheduled Downtime – 60 Minutes Unscheduled Downtime = 390 Minutes • Calculation: Availability = operating time / scheduled time • Availability = 390 minutes / 450 minutes = 86.6%

 Performance and productivity • The Performance Metric is a pure measurement of speed that is designed to exclude the effects of Quality and Availability. The losses due to wasted performance are also often called speed losses. • Calculation: Performance (Productivity) = (Parts Produced * Ideal Cycle Time) / Operating time Example: • A given Work Center is scheduled to run for an 8-hour (480 minute) shift with a 30-minute scheduled break. • Operating Time = 450 Min Scheduled – 60 Min Unscheduled Downtime = 390 Minutes • The Standard Rate for the part being produced is 40 Units/Hour or 1.5 Minutes/Unit • The Work Center produces 242 Total Units during the shift. Note: The basis is Total Units, not Good Units. The Performance metric does not penalize for Quality. • Time to Produce Parts = 242 Units * 1.5 Minutes/Unit = 363 Minutes • Performance (Productivity) = 363 Minutes / 390 Minutes = 93.1%

 Quality • The Quality Metric is a pure measurement of Process Yield that is designed to exclude the effects of Availability and Performance. The losses due to defects and rework are called quality losses and quality stops. Reworked units which have been corrected are only measured as unscheduled downtime while units being scrapped can affect both operation time and unit count. • Calculation: Quality = (Units produced - defective units) / (Units produced) Example: • 242 Units are produced. 21 are defective. • (242 units produced - 21 defective units) = 221 units • 221 good units / 242 total units produced = 91.32%

Availability In reliability theory and reliability engineering, the term availability has the following meanings: • The degree to which a system, subsystem or equipment is in a specified operable and committable state at the start of a mission, when the mission is called for at an unknown, i.e. a random, time. Simply put, availability is the proportion of time a system is in a functioning condition. This is often described as a mission capable rate. Mathematically, this is expressed as 100% minus unavailability. • The ratio of (a) the total time a functional unit is capable of being used during a given interval to (b) the length of the interval. For example :- A unit that is capable of being used 100 hours per week (168 hours) would have an availability of 100/168. However, typical availability values are specified in decimal (such as 0.9998). In high availability applications, a metric known as nines, corresponding to the number of nines following the decimal point, is used. With this convention, "five nines" equals 0.99999 (or 99.999%) availability.

• Availability of a system is typically measured as a factor of its reliability – as reliability increases, so does availability • Availability of a system may also be increased by the strategy of focusing on increasing testability, diagnostics and maintainability and not on reliability. Improving maintainability during the early design phase is generally easier than reliability (and Testability & diagnostics). Maintainability estimates (item Repair [by replacement] rates) are also generally more accurate. • when reliability is not under control, then many and different sorts of issues may arise, for example 1. The need for complex testability (built in test sensors, hardware and software) requirements, 2. The need for detailed diagnostic procedures, 3. Manpower (maintainers / customer service capability) availability, 4. Spare part availability, 5. Dead on arrival issues (non-quality impact on system availability), 6. Logistic delays of spares or manpower due to any reason, 7. Lack of repair knowledge and expert-personnel 8. Extensive retro-fit and complex configuration management costs and others.

Methods and techniques to model availability Fault tree analysis and related software are developed to calculate (analytic or by simulation) availability of a system or a functional failure condition within a system including many factors like: • Reliability models • Maintainability models • Maintenance concepts • Redundancy • Common cause failure • Diagnostics • Level of repair • Repair status (as good as new, as good as old) • Dormancy • Test coverage • Active operational times / missions / sub system states • Logistical aspects like; spare part (stocking) levels at different depots, transport times, repair times at different repair lines, manpower availability and more. • Uncertainty in parameters

Definitions within systems engineering  Availability, inherent (Ai) The probability that an item will operate satisfactorily at a given point in time when used under stated conditions in an ideal support environment. It excludes logistics time, waiting or administrative downtime, and preventive maintenance downtime. It includes corrective maintenance downtime. Inherent availability is generally derived from analysis of an engineering design: • The impact of a repairable-element (refurbishing/remanufacture isn't repair, but rather replacement) on the availability of the system, in which it operates, equals mean time between failures MTBF/(MTBF+ mean time to repair MTTR). • The impact of a one-off/non-repairable element (could be refurbished/remanufactured) on the availability of the system, in which it operates, equals the mean time to failure (MTTF)/(MTTF + the mean time to repair MTTR). • It is based on quantities under control of the designer

• Availability, achieved (Aa) The probability that an item will operate satisfactorily at a given point in time when used under stated conditions in an ideal support environment (i.e., that personnel, tools, spares, etc. are instantaneously available). It excludes logistics time and waiting or administrative downtime. It includes active preventive and corrective maintenance downtime. • Availability, operational (Ao) The probability that an item will operate satisfactorily at a given point in time when used in an actual or realistic operating and support environment. It includes logistics time, ready time, and waiting or administrative downtime, and both preventive and corrective maintenance downtime. This value is equal to the mean time between failure (MTBF) divided by the mean time between failure plus the mean downtime (MDT). This measure extends the definition of availability to elements controlled by the logisticians and mission planners such as quantity and proximity of spares, tools and manpower to the hardware item.

• Basic example If we are using equipment which has a mean time to failure (MTTF) of 81.5 years and mean time to repair (MTTR) of 1 hour: • MTTF in hours = 81.5 × 365 × 24 = 713940 (This is a reliability parameter and often has a high level of uncertainty!) • Inherent availability (Ai) = 713940 / (713940+1) = 713940 / 713941 = 99.999860% • Inherent unavailability = 1 / 713940 = 0.000140% • Outage due to equipment in hours per year = 1/rate = 1/MTTF = 0.01235 hours per year.