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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

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Posted by: John ( )
Date posted: Sun Sep 9 21:40:53 US/Eastern 2001
Subject: Plotting Reliability
When plotting cumulative MTBF vs. Time for multiple units, (sample size is 12, with 2000 hours on each unit)what rules of thumb should be used for selecting the time interval to calculate the MTBF. The shape of the curve will change depending on this time interval (t = 100, 500, 1000, ... vs. t = 50, 100, 150, 200, ...)? t = 100 means each of the 12 units has 100 hours on it, so the total hours is 1200 hours. I have seen the term instantaneous MTBF, what is this used for and can this be calculated from my data? Thank you. John


Subject: Reliability Growth Plotting
Reply Posted by: Bruce Dudley ( )
Organization: Reliability Analysis Center
Date Posted: Mon Sep 10 14:59:49 US/Eastern 2001
The general rule of thumb for determining the time intervals to be used in the computation of cumulative Mean-Time-Between-Failure (MTBF) is the number and length of the intervals should be chosen small enough to reflect trends, but long enough to smooth the data. This advice is from MIL-HDBK-189 “Reliability Growth Management”. So in your case where only a few failure events may be encountered, long time intervals of one to two thousand cumulative hours should be considered. As for instantaneous MTBF, the term is usually calculated by dividing the cumulative value by one minus the growth factor (alpha). This means the instantaneous MTBF could be as great as twice the cumulative for a high growth program or the same as the cumulative for a zero growth program.

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