Misuses of MTBF
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Transcript of Misuses of MTBF
Misuses of MTBF
Fred Schenkelberg Reliability Consultant
[email protected] 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
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
• 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
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