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: Conversion Factor
Reply Posted by: BWD
Date Posted: Wed Sep 11 13:50:13 US/Eastern 2002
The conversion factors that RAC uses in the analysis of data for NPRD-95(Non-electronic part reliability database)is 28.8 miles per hour for wheel vehicles and 10.0 for track vehicles. These factors are used to convert the total miles to hours, then statictical analysis is used to determine the failure rate at some confidence level.
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.