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Forum: Reliability & Maintainability Questions and Answers

Topic: Reliability & Maintainability Questions and Answers

Topic Posted by: Reliability & Maintainability Forum (src_forum@alionscience.com )
Organization: System Reliability Center
Date Posted: Mon Aug 31 12:47:36 US/Eastern 1998

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Original Message:

Posted by: Gary
Date posted: Tue Sep 10 16:27:57 US/Eastern 2002
Subject: Conversion from kilometers to hours
Message:
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)? Thanks!!


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Reply:

Subject: Converting MTBF units
Reply Posted by: Larry George (pstlarry@attbi.com )
Organization: Problem Solving Tools
Date Posted: Fri Sep 13 17:33:39 US/Eastern 2002
Message:
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 pstlarry@attbi.com and ask for the paper. Free samples are available. Send MPH and MBF data, and Larry will send back the MTBF, free.


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