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# Trapped by MTBF

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A short presentation about the perils around using MTBF

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### Trapped by MTBF

1. 1. Trapped by MTBF Fred Schenkelberg Reliability Consultant fms@fmsreliability.com www.fmsreliability.com (408) 710-8248
2. 2. 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
3. 3. MTBF Calculation # hoursMTBF # failures MTBF 1
4. 4. Mean (M)• The mean in MTBF• What does it mean to you? (no pun intended!)• Average?
5. 5. Start 1000 units, MTBF = 100368 still alive at 101 hours
6. 6. Note the exponential decay
7. 7. Other Issues• Time – just because it is hours…• Between – not the duration of the failure free period!• Failure – use the customer definition
8. 8. History of Use• Remember Slide Rule Mechanical Adding Machines
9. 9. History of Use• Early Parts Count based on adding failure rates of components (50’s and early 60’s) 1t 2t nt R (t ) e e  e (  )t R (t ) e 1 2 n
10. 10. 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
11. 11. Use better models/distributions• Weibull ( t ) R W eibull ( t ) e• Type I Gumbel t (e )• Exponential R Gumbel ( t ) e• Log Normal t R exp onential ( t ) e t• Etc. ln T 50 R log normal ( t )
12. 12. 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
13. 13. www.noMTBF.com