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Misuses of MTBF Fred Schenkelberg Reliability Consultant [email protected] www.fmsreliability.com (408) 710-8248
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Page 1: Misuses of MTBF

Misuses of MTBF

Fred Schenkelberg Reliability Consultant

[email protected] www.fmsreliability.com

(408) 710-8248

Page 2: Misuses of MTBF

Outline

•  MTBF – calculation •  MTBF – a very poor four letter acronym •  History of Use •  It’s Misleading •  A better metric •  Actually, we’ve been talking about MTTF

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MTBF Calculation

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Mean (M)

•  The mean in MTBF

•  What does it mean to you? (no pun intended!)

•  Average?

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Start 1000 units, MTBF = 100

368 still alive at 101 hours

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Note the exponential decay

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Other Issues

•  Time – just because it is hours…

•  Between – not the duration of the failure free period!

•  Failure – use the customer definition

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History of Use

•  Remember Slide Rule Mechanical Adding Machines

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History of Use

•  Early Parts Count based on adding failure rates of components (50’s and early 60’s)

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Use Reliability

•  R(t) is the probability that a random unit drawn from the population will still be operating by t hours

•  R(t) is the fraction of all units in the population that will survive by t hours

Applied Reliability, 2nd Ed., pg 29

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Use better models/distributions

•  Weibull •  Type I Gumbel •  Exponential •  Log Normal •  Etc.

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Actually… •  MTBF is or should be used for repairable systems •  MTTF is what I’ve been talking about •  MTTF is calculated the same as MTBF when we

assume –  negligible repair time –  Interarrival times as from an independent sample of

nonrepairable parts –  Expontential distribution for lifetime of parts

•  See Chap 10, Applied Reliability for more info

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