To override the default occurrence mappings, simply create an occurrence map attribute. Like the severity map attribute described earlier, the occurrence map must be a checklist attribute named “OccurrenceMap” that is associated with the design itself (this attribute only needs to be created in the model you actually run the FMECA).
For example, say that you wished to override the default occurrence mapping using the following (adapted from B.S. Dhillon’s Engineering Maintainability, 1999):
Rank
Probability of
Occurrence
Q
1
1 in106
< 0.00005
2
1 in 20,000
0.00005
3
1 in 4,000
0.00025
4
1 in 1,000
0.001
5
1 in 400
0.0025
6
1 in 80
0.0125
7
1 in 40
0.025
8
1 in 20
0.05
9
1 in 8
0.125
10
1 in 2
0.5
You would then set up an occurrence map attribute as follows:
It must be named “OccurrenceMap”
· It must be of type Checklist
· It must be associated with the Design
· It must be “Mutually Exclusive”
· It must have values assigned
· It must have one entry for each Occurrence rating (on a 10-point scale) to which failure probs. will be mapped
· For each entry, assign the minimum likelihood of failure (Q number) that is to be mapped to that entry
Here is what our FMECA looks like when we map the occurrence ratings using the user-defined occurrence map attribute described above:
As you can see, this occurrence map groups the ratings a little closer together—the higher ratings are a little lower and the lower ratings are a little higher.
Now let’s look at what happens when we don’t use an occurrence time interval. Remember, when there is no time interval, the Q number (probability of occurrence) cannot be calculated. Instead, the failure rate of each failure is used in place of the Q number. This failure rate (which appears in its own column
in the example above) is calculated by multiplying the failure ratio by the part’s failure rate.
As you can see, the failure rates—which, unlike the probability of occurrence, do not range between zero and one—map to significantly higher occurrence ratings. One could infer from this that, when mapping failure rates to occurrence ratings (rather than the probabilities of failure), a user-defined occurrence map should be created specifically to accommodate the range of failure rates in the model.
