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The purpose of the PFMEA (process failure modes and effects analysis) is to thoroughly critique the risks associated with your process. To do this it is necessary to identify each step of your process in quantifiable terms. These need to be precise, and to consider what you should have successfully completed at the end of each step of your process. These are typically defined on the process flow diagram. Each of these is then carried forward to the PFMEA form to be critiqued. Detailed below is the approach that should be adopted to compile a PFMEA.
Failure Modes
The failure modes are where you consider what can go wrong. A failure mode represents the inverse of the process description to a greater or lesser degree.
Effects
The effects are the consequences of failure. It is recommended that an effects matrix is used to ensure all of the effects are identified and for consistency of severity rankings. The effects matrix has a series of columns with the following heading:
Operator
Downstream processes
Part
Sub-system
System
Other related systems
Vehicle (or end product)
Customer plant
End user
Government Regulations
The failure modes are placed down the left hand side of the matrix. The effects are placed in the relevant columns with their respective severity rankings to the right of each effect. The severity rankings are on a scale of 1 – 10 depending on how serious each effect is. These are then cascaded back to the PFMEA form effects column. The highest severity being placed in the severity column on the PFMEA form, appropriate software can automate this process.
Causes
The causes are the reasons why the failure modes occur. Typically there is more than one cause for the majority of the failure modes that you will identify. It is recommended that a fishbone diagram is used to identify primary, secondary and tertiary causes. It should be noted that only primary causes can create the failure mode. The reason why it is important to use fishbone diagrams in the FMEA at this stage is because of its close association with problem solving. If you experience a problem logically the process FMEA and its associated fishbone diagrams should be one of the first places you look.
Prevention controls
These are existing activities which are in place to lower the likelihood of a cause from occurring. These should be considered for each cause.
Occurrence
This is an assessment of how often you believe this cause will create the designated failure mode, on a scale of 1 – 10.
Detection
These are current activities which are in place to detect either the cause or the failure mode. Their purpose is, to tell you when something has gone wrong.
Detection ranking
This is an assessment on a scale of 1 – 10 as to how effective the detection methods are at alerting you to the fact that something is wrong.
Classifications
This column is reserved for special symbols or letters which denote when a line in your FMEA is important, significant or critical. Guidance should be sort from your customer with regards to their particular requirements as they vary both in terms of characters used and numeric criteria to invoke them.
RPN
RPN or risk priority number is the multiplication of severity, occurrence and detection. Its original intent was to identify priority although today an increasing number of organisations are using alternate approaches to establish priority.
Recommended actions
What is important here is to identify effective changes which reduce the risk that you are exposed to. Instrumental to the recommended actions is when in time you choose to do the process FMEA. If it is undertaken at the earliest possible time, when the process design is still on paper, you have maximum freedom of choice at minimum cost.
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