A common error in FMEAs (failure mode effect analysis) is having a statement as a “potential failure mode” instead of a “potential cause of failure” or vice versa.
This error often leads to FMEAs that seem to be all over the place without a clear and consistent approach and mostly stems from:
An incorrect process step description
A process step description that is too broad, i.e. trying to use a process step statement to cover multiple sub-processes
Lack of clarity of the requirements of the process step
Look at the example below of the section of a PFMEA. The process involves oven-curing the glue on a part.
“Oven temperature set too low” has appeared both as a potential failure mode and as a potential cause of failure. So, which is correct, or are they both? The same would apply for “oven temperature set too high”.
An easy way to check this, is to ask the question – what are the requirements of the process step? In this case it could be - parts fully cured at 130 degrees for 60 minutes. The things that can therefore go wrong (potential failure mode) are:
part heated to less than 130 degrees
part heated to more than 130 degrees
part heated for less than 60 minutes
part heated for more than 60 minutes
Clearly, “oven temperature set too low” (or “oven temperature set too high”) is NOT a potential failure mode when we are guided by the process requirements, but it could be a potential cause of failure.
When examined closely, it would seem that the process step statement – “cure parts in oven” has been assumed to also cover the setup of the oven. If the oven set up is part of the process and its FMEA is not covered elsewhere, it would be right to have it as a separate process step.
Regardless, listing out the requirements of a process step will help to keep focus on what should be its failure modes and a reminder of the objective of the process step. Most PFMEA templates have a “requirements” column that helps to fulfill this function and it usually sits between the process step and potential failure mode columns (see below). The requirements column also serves as a good guidance for creating concise and specific descriptions of potential failure modes.
While we have used a PFMEA in this example, the same principle applies to DFMEAs and other types of FMEAs.
Stay cool and have fun!
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