Deriving Failure Rates in a Hierarchy of FMEAs

Paper #:
  • 2017-01-0322

Published:
  • 2017-03-28
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Abstract:
When analyzing the failure rate (or occurrence) of a system failure cause, the typical approach is to obtain an occurrence rating from the results of testing. In many cases, the occurrence of a system failure cause can be derived from a combination of occurrences of failure causes of the element (of the system) failure mode coinciding with the system failure cause being assessed. One approach is to simply pass the maximum occurrence of all potential element failure causes up to the system failure cause. While this approach may work for failure causes that all must occur simultaneously to produce the failure mode, it ignores the more prevalent case where most failures occur independent of each other. In order to approach such a case, a logarithmic sum of the occurrences proves more accurate. Another method for achieving a more fine-tuned logarithmic sum of occurrences is to sum actual failure rates of component failure causes and then convert to an equivalent occurrence. This approach covers the majority of cases but there are special cases that would still need to be addressed such as the case where multiple failure causes must occur simultaneously to produce the failure mode, the case where one failure increases the occurrence of another, or any other complex relationship of failure causes. To address such special cases, a “complex analysis” worksheet, where the logical combination of occurrences and failure rates is custom defined by the engineer, is utilized.
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