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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: Anil Kumar (anilkumar_sng@yahoo.com )
Organization:Centre For Reliability, Chennai, India
Date posted: Thu Dec 27 1:39:01 US/Eastern 2001
Subject: Reliability Centered Maintenance (RCM)
Message:
How does the RPN numbers from FMEA are linked to the frequency of maintenance in RCM.


Reply:

Subject: FMEA-RCM Link
Reply Posted by: Patrick Hetherington (phetherington@alionscience.com )
Organization: RAC
Date Posted: Thu Dec 27 7:39:54 US/Eastern 2001
Message:
The RPN (Risk Priority Number) from an FMEA performed in similar fashion to the Automotive Industry Action Group (AIAG) methodology is comprised of 3 elements. These are severity ranking, detection ranking and occurrence ranking. Severity ranking is on a scale of 1-10 where 10 is most severe and 1 is very benign. Detection relates to the design controls that are in place such as finite element analysis, derating or life testing which have the ability to detect or anticipate a failure prior to customer use. Detection ranking is also on a scale from 1-10 where 10 represents minimal or no design controls and 1 represents design controls with almost absolute certainty. Occurrence ranking relates to how frequently the failure can be expected to occur in service. Occurrence ranking is also on a scale of 1-10 where 10 represent a very high. RPN=SxDxO. Scales can be arbitrary and ranking is subjective. The relationship of RPN to maintenance intervals in RCM is two fold. RCM is the process of matching preventive maintenance actions and intervals to an item failure characteristic in light of the consequences of failure. Therefore the severity ranking relates to the consequences of failure and occurrence ranking relates to the frequency of failure and therefore the frequency of preventive maintenance required to achieve some level of availability. However, understanding how frequently an item fails does not tell the entire picture required to determine effective preventive maintenance. One must understand the failure modes, pattern of failure (wearout, infant mortality, and constant) and cost of a preventive maintenance action in comparison to an unscheduled repair. There is no direct link from the RPN of an FMEA to the maintenance interval, however, the FMEA is an essential prerequisite of successful RCM analysis.


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