Adding MTBF's
Adding MTBF's
(OP)
OK. First post and quite a simple query, that I really should know but something just doesn't sit well with me !
Is it acceptable to simply add sub-system MTBF's together to obtain the overall System MTBF ?
Eg.
A = 3000 miles MTBF
B = 2000 miles MTBF
C = 4000 miles MTBF
System MTBF = 9000 MTBF
Thanks in advance.
KB.
Is it acceptable to simply add sub-system MTBF's together to obtain the overall System MTBF ?
Eg.
A = 3000 miles MTBF
B = 2000 miles MTBF
C = 4000 miles MTBF
System MTBF = 9000 MTBF
Thanks in advance.
KB.
RE: Adding MTBF's
RE: Adding MTBF's
TTFN
FAQ731-376: Eng-Tips.com Forum Policies
RE: Adding MTBF's
So in essence, to obtain an overall System MTBF one would need to first convert the MTBF to a failure rate, then add the FR's before converting back to an MTBF !
Thanks.
KB.
RE: Adding MTBF's
If for example a vehicle has accrued a total of 4000 trials miles and has had say, 2 failures I have estimated the FR as 2000 miles MTBF for that system. This I have little issue with, however, for a sub-system that has not had any failures for the trial duration I have assumed that the MTBF is equal to the accrued mileage, i.e. 4000 miles accrued and no failures, MTBF is 4000 miles.
Does this sound acceptable ?
Thanks.
KB.
RE: Adding MTBF's
TTFN
FAQ731-376: Eng-Tips.com Forum Policies
RE: Adding MTBF's
I don't really follow what you mean. How else would you be able to determine an overall MTBF without converting the MTBF's to FR's prior to summing?. One ultimately ends up with an overall FR, which logically could be converted to an MTBF.
Cheers,
KB.
RE: Adding MTBF's
Do the math, assume 2 other components:
1/(2/4000 + 1/4000 + 1/4000) = 1000 mi MTBF
TTFN
FAQ731-376: Eng-Tips.com Forum Policies
RE: Adding MTBF's
Maybe I've misunderstood you or vice versa, but I agree with what you've laid out in your last post. That is precisely what I have done and another quick check on the internet has also confirmed the methodology I believe !
http:
Thanks once again. I get the feeling I shall be coming here on quite a frequent basis in the not too distant future !!
Cheers,
KB.
RE: Adding MTBF's
TTFN
FAQ731-376: Eng-Tips.com Forum Policies
RE: Adding MTBF's
By Robert A Dovich, Bill Wortman, Quality Council of Indiana, Published by Quality Council of Indiana, 1995
Page VI-9 says, ". . .in series systems, the failure rate are additive; MTBF values are not!"