GMS 401 Chapter Notes - Chapter 3S: Standard Deviation, Mean Time Between Failures, Bathtub Curve
28 Jun 2018
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GMS – Chapter 3S – Reliability
Reliability – the ability of a product or part to perform its intended function under a prescribed
set of conditions (x% probability of functioning as intended)
Reliability of product is used in two ways: 1. Reliability when activated 2. Reliability for a given
length of time
Reliability management involves establishing, achieving and maintaining reliability objectives
for products
Determining that probability when the product consists of a number of independent
components requires the use of rules of probability for independent events. Independent
events have no relation to the occurrence or non-occurrence of each other
Redundancy – the use of backup components to increase reliability
Or another approach is to overdesign to avoid a particular failure, or to use simplification to
reduce the number of components in the product
Rule 1 if two or more events are independent and “success” is defined as the occurrence of all
of the events, then the probability of success Ps is equal to the product of the probabilities of
the events occurring i.e. Ps = P1 x P2 x P3
Rule 2 if two or more events are independent and “success” is defined as occurrence of at least
one of the events, then the probability of success Ps is equal to 1 – probability that none of the
events will occur. i.e. 1 – (1-P1)(1-P2)(1-P3) simplified… Ps = P1 +(1-P1)P2+(1-P1)(1-P2)P3+…
The products reliability is calculated in two stages: (1) first calculate the reliability of the parallel
components and then (2) use these to calculate the reliability of the resulting series system
The second way of looking at reliability considers a use factor, usually the time dimension
Failure rate per hour is defined as the number of failures divided by total operating hours
If failure rate is constant over time, time-quantity transposition is applicable (one can reduce
the test time but use more test items)
A typical failure rate over time is referred to as a bathtub curve
Mean time to failure (MTTF) – the average length of time before failure of a product or
component
MTTF = 1 / failure per hour
Meant time between failures (MTBF) – the average time from the uptime after the repair
following a failure to the next failure
find more resources at oneclass.com
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Document Summary
Reliability the ability of a product or part to perform its intended function under a prescribed set of conditions (x% probability of functioning as intended) Reliability of product is used in two ways: 1. Reliability management involves establishing, achieving and maintaining reliability objectives for products. Determining that probability when the product consists of a number of independent components requires the use of rules of probability for independent events. Independent events have no relation to the occurrence or non-occurrence of each other. Redundancy the use of backup components to increase reliability. Or another approach is to overdesign to avoid a particular failure, or to use simplification to reduce the number of components in the product. The products reliability is calculated in two stages: (1) first calculate the reliability of the parallel components and then (2) use these to calculate the reliability of the resulting series system. The second way of looking at reliability considers a use factor, usually the time dimension.
