SRC Forum - Message Replies
Forum: Reliability & Maintainability Questions and Answers
Topic: Reliability & Maintainability Questions and Answers
Topic Posted by: Reliability & Maintainability Forum
Organization: System Reliability Center
Date Posted: Mon Aug 31 12:47:36 US/Eastern 1998
Posted by: Gary
Date posted: Tue Sep 10 16:27:57 US/Eastern 2002
Subject: Conversion from kilometers to hours
Is there a standard method for converting failures per million miles to failures per million hours in order to integrate a known value (i.e MMBF = 1500 miles) into a system level prediction (MTBF)?
Subject: Converting MTBF units
Reply Posted by: Larry George
Organization: Problem Solving Tools
Date Posted: Fri Sep 13 17:33:39 US/Eastern 2002
Converting MTBF = MMBF/28.8 miles per hour is the standard, correct conversion, as long as all vehicles go exactly 28.8 mph all the time they operate.
If velocity is random, the conversion requires:
1. Probability distribution of velocity, e.g., P[MPH > z] for reasonable z values. This requires data.
2. Probability distribution of miles at failure, typically assumed to be exponential in MTBF prediction,
P[Miles to failure > x] = exp[-x/MMBF]
3. Probability calculus to compute the probability distribution of MBF/MPH and then integration to get
MTBF = E[TBF] = E[MBF/MPH],
where both MBF (miles to failure) and MPH are random variables.
A method for transforming reliability units is in "Convert Calendar Time to Operating Time Reliability," Reliability Review, ASQ-RD, Vol. 18, No. 3, Sept. 1998, in a section entitled "CAUTION NEUROHAZARD! PROBABILITY AHEAD! CONVERT TO OPERATING TIME."
If you dare risk the neurohazard, send an email to firstname.lastname@example.org and ask for the paper. Free samples are available. Send MPH and MBF data, and Larry will send back the MTBF, free.