A short presentation about the perils around using MTBF

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

3. MTBF Calculation
# hours
MTBF
# failures
MTBF 1

4. Mean (M)
• The mean in MTBF
• What does it mean to you?
(no pun intended!)
• Average?

5. Start 1000 units,
MTBF = 100
368 still alive at 101 hours

6. Note the exponential decay

7. Other Issues
• Time – just because it is hours…
• Between – not the duration of the failure
free period!
• Failure – use the customer definition

8. History of Use
• Remember Slide Rule
Mechanical Adding Machines

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

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

15. www.noMTBF.com