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: hansen
Date posted: Thu Jan 16 2:44:19 US/Eastern 2003
could somebody explain me why when I put preventive maintenance on components, the subsytem (where the components are) doesn't show me expected NOF
thanks for your help
Subject: Failure Reduction
Reply Posted by: J. L. Romeu
Date Posted: Tue Jan 28 13:54:39 US/Eastern 2003
We have received an inquiry regarding "why, when a customer puts up a preventive maintenance (PM) schedule on components, the subsystem (where the components are) doesn't show Expected NOF (number of Failures) reduction".
Per the problem statement, we infer that the PM program has not succeeded in reducing the previous number of failures in the system. One reason for this to occur is as follows:
Assume the system has a complex failure mechanism, composed of "n" superimposed and independent failure modes (e.g. random problems, wear-out, etc.). Assume, for simplicity, that these n competing risks are all distributed Exponentially with rate .
Then, assume that the PM scheme has been placed considering only k< n of these failure modes -or that it has been placed based on other, non-stochastic, considerations.
In such a case, the real failure rate of the system is n > k which is the erroneous failure rate upon which the MP is based. If this is the case, then the probability of observing a failure before the time for a scheduled maintenance occurs is very high. Therefore, the number of failures occurring after the MP is in place is still high, leading to thinking that the MP "doesn't show the (reduction) in NOF", as would be expected.