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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: Ananda Perera (Ananda.Perera@alliedsignal.com )
Organization:AlliedSignal Aerospace Canada
Date posted: Mon Jul 26 17:08:38 US/Eastern 1999
Subject: ESS Vibration Exposure
Message:
Environmental Stress Screening Guidelines for Assemblies 1990 published by the Institute of Environmental Sciences indicate that; (1)After subsequent repairs and/or modifications, repeat screen at -3dB level (70% RMS g) for 50% duration; (2) Total number of vibration exposures should not exceed five. ##### It is not clear whether these values were based on the Random Vibration Spectrum shown in Page 15 which gives a 6 gRMS Vibration Level. QUESTION: The above parameters, can it be used for any Vibration Level ? Is there any other method to determine the maximum vibration exposure that will not use excessive amount of useful life ?


Reply:

Subject: ESS Vibration Exposure
Reply Posted by: Seymour Morris (smorris@alionscience.com )
Organization: Reliability Analysis Center
Date Posted: Thu Jul 29 8:12:32 US/Eastern 1999
Message:
I am not certain whether or not the 5X retest limitation applies only to the vibration levels referenced. However, in order to ensure that you are not using up an excessive amount of life through vibration screening you must first determine, either from historical experience on assemblies with similar mechanical characteristics or through life testing, what the product end of life is at various vibration levels. After establishing this relationship one can estimate what lifetime will be used up for a given vibration screening level. Rules of thumb such as those referenced can provide a false sense of security.


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