DFMEA – Probability Of Occurrence Ranking

One possible source of reference data is the reject and service history for similar components. A sample of a Probability of Occurrence ranking scale is shown below:

Probability Of Occurrence Ranking

Ranking Description
1 Remote possibility of happening. Where similar parts have been used for similar functions in previous design or processes, failures have been non-existent.
2-3 Low failure rate with similar parts having similar functions in previous designs or processes.
4-6 Moderate failure rate with similar parts having similar functions in previous designs or processes. It is generally a failure that had occurred occasionally in the past but not in major proportions
7-9 Frequent failure rate with similar parts having similar functions in previous designs or processes.
10 High probability of failure.

The company should adopt these sampling ratings to their industry and include specific probability values. This table of a Probability of Occurrence ranking scale provides guidance for assigning criteria values to the causes and their failure modes on a consistent basis. It is only a sample and should not be used without adaptation to the industry and company.
[ASLR2]
The project team should evaluate the likelihood of each cause and its associated failure mode and reach a consensus on their ranking values. The idea is to find comparative differences among the causes so they can later be prioritized (with the total RPN scores).
* The design verification steps that have been or are being used with the same or similar designs.

* List all current design controls which are intended to prevent the specific design-based causes(s) of the potential failure modes from occurring, or are intended to detect the design-based causes(s) of the potential failure or the resultant failure mode.

FMEA-Process-Flow-Design-Controls

Examples: design checklists, peer reviews, reliability analysis, variation stack-up analysis, FEA, rapid prototyping, hand off check lists, environmental stress analysis, etc. Design controls are intended to assure the design adequacy for the failure mode and/or cause under consideration.

A suggested technique when listing the Design Controls in their column is to add the prefix ‘p’ to DVs which are intended to prevent the failure mode cause or the prefix ‘d’ to DVs which are intended to detect the failure mode or its cause.  It is not uncommon for a company just beginning to use FMEA to find that it does not have many DVs.

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