FMEA - Determine Causes of failure and Rank their likely occurrence
Having identified the failure modes and the effects of failure, we now need to determine the possible causes of failure. In this article we will be looking at establishing those causes and the processes used to ensure all causes are considered and their likely occurrence determined by the FMEA team.
Introduction
At this point we need to understand that the effect of the failure mode and the cause are not linked, other than through the failure mode itself. In other words, we are looking for the cause/s of the failure mode. The failure mode has effects that are experienced (sometimes called symptoms) as a result of that failure mode.
This is described in the sentencing model opposite.
To ensure that the best result is achieved the following should be noted: -
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Ensure that brainstorming is managed effectively (this a creative process and should have the basic rules applied)
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Do not constrain thoughts around current detection methods, this comes later
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Keep the boundary diagram and other visual aids in view to focus the brainstorming effort
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One cause can be the cause of more than one failure mode
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All failure modes are captured and recorded for discussion and analysis after the brainstorming session
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Causes are limited to those of the specific failure modes and cannot include errors outside of the boundary diagram
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Potential mistakes that could be made during manufacture of the design can only be considered if they are as a direct result of poor design (the principles of design for manufacture).
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A causal theory should be developed to root cause level for all failure modes with a severity of 9 or 10.
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Robustness linkages should be considered if they are within the agreed scope of the FMEA.
The team should brainstorm all the possible causes of failure. Failure of the design to perform as intended.
Once the causes of failure have been established, they are arranged in order against the failure mode (a continuation of the flow established during the last process step (see diagram below).
Using the Design FMEA Ranking tables, each potential cause is ranked as to the likelihood of occurrence. The ranking is recorded alongside the cause in question.
Because the use of ranking tables is not an “exact science”, the introduction of past experiences and other data is useful.
The FMEA flow is imprtant. Ensure that the causes are arranged against each failure mode. You will now have an FMEA report that has the effects of the failure mode and the causes of the failure mode aligned to the individual failure modes (see graphic above left). This is critical at the next stage because we will want to be looking at how to reduce the likelyhood of failure and this is around the failure mechanism for each failure mode.
Assessing Questions
On completion of this and every stage of the FMEA, assessing questions need to be addressed. This will include: -
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Have all failure modes been considered fully?
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Have cause theories been developed for all causes (these must be what could actually happen)?
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Have causes met the correct sentencing criteria (see above figure)?
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Has the scope of the FMEA been adhered to for each possible cause?
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Has the FMEA report structure been completed correctly (this is important when calculating RPN)?
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Have all previous questions been closed out and any new ones entered on the question log?
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Has the FMEA report been communicated to all interested parties?
Next Time
In the next article I will explain how to assess current preventative and detection controls and their effectiveness. I will also describe how to calculate RPN as a method of establishing action priorities.
Graham Cripps
Results Consortium Ltd