Misuses of MTBF
 

Misuses of MTBF

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A presentation about the common misuses of mean time between failures (MTBF).

A presentation about the common misuses of mean time between failures (MTBF).

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    Misuses of MTBF Misuses of MTBF Presentation Transcript

    • Misuses of MTBF Fred Schenkelberg Reliability Consultant fms@fmsreliability.com www.fmsreliability.com (408) 710-8248
    • 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
    • MTBF Calculation
    • Mean (M)•  The mean in MTBF•  What does it mean to you? (no pun intended!)•  Average?
    • Start 1000 units, MTBF = 100368 still alive at 101 hours
    • Note the exponential decay
    • Other Issues•  Time – just because it is hours…•  Between – not the duration of the failure free period!•  Failure – use the customer definition
    • History of Use•  Remember Slide Rule Mechanical Adding Machines
    • History of Use•  Early Parts Count based on adding failure rates of components (50’s and early 60’s)
    • 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
    • Use better models/distributions•  Weibull•  Type I Gumbel•  Exponential•  Log Normal•  Etc.
    • 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