1. Trapped by MTBF
Fred Schenkelberg
Reliability Consultant
fms@fmsreliability.com
www.fmsreliability.com
(408) 710-8248
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
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.
11.
12. 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
13. 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 )
14. 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