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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: Kenneth
Date posted: Sat Dec 7 21:56:04 US/Eastern 2002
Subject: MTBF useless??
Message:
Since there're so many doubts pertaining to MTBF. Can i say that MTBF is useless when describing mechanical parts since wearout predominates after certain amount of time. Just like someone point out the example of a motor having wear life of 5 yrs and MTBF of 150,000hrs. In this case there's no need to specify MTBF because it serves no purpose at all right? Even in our spares provisioning, we should not use MTBF of 150,000hrs since in real life it's not going to survive close to that value. Can i suggest that using wear life in our reliabiliy or spares provisioning analysis is more appropriate when comes to mechanical parts?


Reply:

Subject: MTBF Useful
Reply Posted by: BWD (bdudley@alionscience.com )
Organization: RAC
Date Posted: Thu Dec 12 15:53:03 US/Eastern 2002
Message:
After reviewing your concern with MTBF and wearout for motors, I would say that you need to consider both reliability features. For example, suppose that you have 1000 motors with the 150,000 MTBF and life wearout of 5 years or roughly 44,000 hours. After operating the units for the five years you should expect 292 random failures [defect related] and you would have 708 wearout failures plus those random failures that were replaced early in the 5 year operation and wore out. So, the spare provisioning would include the effects from both situations resulting in a need that is greater than the total number of units. Therefore, my conclusion is MTBF requirements are a necessary item in specifying the overall motor needs.


Reply:

Subject: Credible Reliability Prediction
Reply Posted by: Larry George (pstlarry@attbi.com )
Organization: Problem Solving Tools
Date Posted: Wed Jan 1 19:22:46 US/Eastern 2003
Message:
Tradition often requires MTBF prediction, for the sake of comparing stories. It is convenient to use comparable constant failure rates for comparable parts, even though we know parts wear out. You asked, "Can i suggest that using wear life in our reliabiliy or spares provisioning analysis is more appropriate when comes to mechanical parts?" Use age-specific failure rates to make actuarial forecasts of service requirements, Sum[n(k-i)*a(i); i = 0,1,2,...,k] where n(k-i) is the installed base of age k and a(i) is the failure rate of a product or part of age i. Actuarial forecasts are more accurate and precise than (installed base)*(1/MTBF). Please contact me if you would like more information about credible MTBF, reliability, and failure rate predictions and about actuarial forecasts.


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