Note: This article originally appeared in Quality Magazine under the title “Quality 101: Demystifying Design FMEAs”
Optimized Method For Establishing Design FMEA Ratings Part II
by Richard Harpster, President of Harpco Systems
Establishing the Design FMEA Occurrence Rating
The second rating that must be determined is the occurrence rating. The occurrence rating in the Design FMEA is the rating that corresponds to the probability that the failure mode will occur due to the cause. In the example provided, an occurrence rating must be established for the probability of the component failing because the component material has been specified too thin. An occurrence rating must also be established for the probability of the component failing because the component material type has been specified incorrectly. A typical occurrence rating table follows:
| Rating | Criteria: Probability of Occurrence |
| 10 | >/= 100 per thousand vehicles/items. |
| 9 | 50 per thousand vehicles/items. |
| 8 | 20 per thousand vehicles/items. |
| 7 | 10 per thousand vehicles/items. |
| 6 | 5 per thousand vehicles/items. |
| 5 | 2 per thousand vehicles/items. |
| 4 | 1 per thousand vehicles/items. |
| 3 | 0.5 per thousand vehicles/items. |
| 2 | Greater than 0 but less than 0.1 per thousand vehicles/items. |
| 1 | Failure is unlikely. |
In our example, we are doing a durability test to assess the adequacy of our material thickness and material type specifications. If this were a complete Design FMEA, we might have 20 or more specifications that control whether or not the component will structurally fail due to road impact forces. It is also quite possible that when we run the durability tests we may not even know where these specifications are when we run the tests. Consequently, if failures occur when the tests are run we will have a probability of component structural failure due to road load impacts but will not have what the Design FMEA is asking us for: the probability of the failure mode due to each of the causes. Unfortunately, finding oneself without good objective data to establish the occurrence ratings occurs more often than not when trying to establish occurrence ratings for a Design FMEA.
If you do not have good objective data to establish the probability of the failure mode due to the cause that you are trying to develop the rating for, the following methodology should be used:
- When trying to determine the occurrence rating, one should include several types of information including design control results and the performance of similar products in the field. Often times the information describes the probability of the failure mode occurring but is insufficient to develop an accurate rating for each failure mode/failure cause relationship.
- In these cases, the probability of the failure mode should be rated first and this rating used to set the maximum rating for any of the failure causes for the failure mode.
- Assign the failure mode probability rating to any cause that you believe is a primary contributor to the probability of the failure that you are experiencing.
- Assign the rating of 1 to all other causes linked to the failure mode.
In the example provided, the durability tests were conducted and found that component structural failure due to road load impacts could be expected at a probability of 1 per thousand with the current design. Based on experience, the designers had confidence in the thickness specification but believed the material type selection had property specifications that are too wide and that at the edges of the specifications may be prone to failure. Consequently, an occurrence of 1 was assigned to the material thickness specification cause and a rating of 4 to the material type specification cause.
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Note:
In Part I of this paper DFMEA Severity Ratings are explained:
Establishing Design FMEA Ratings Part I
In Part III, the limitations of RPN and SxO ratings are discussed:
Establishing Design FMEA Ratings Part III
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