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 Mathis ( )
Date posted: Wed Jul 28 12:18:14 US/Eastern 1999
Subject: Weibull vs Exponential
I have some questions concerning Wiebull vs Exponential failure rate grading. According to MIL-PRF-55365, paragraph 1.1, table III and table IV a Wiebull failure rate level B (0.1% failures / 1000hours at a 90% confidence level) graded part is substitutable for a Exponential 'S' (0.001% failures / 1000hours at a 60% confidence level) part. What is the technical rational behind this? If the reliability of a wiebull 0.1% is equal to or better than an exponential 0.001% does the system level prediction or FMECA reflect the difference? Is the difference due to the confidence level of 90% vs 60%? If a system had all "S" (0.001%) parts and you substituted all of them with "B" (0.1%) parts wouldn't the system reliability be considerably worse? Can anyone explain this!


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