Related Topics:

Failure Modes and Reliability Analysis (FMRA)

Causes

Quantitative Criticality Analysis

Using Rating Scales in the FMRA

Xfmea allows you to use qualitative ratings (e.g., "Level B - Reasonably Probable," "Level E - Extremely Unlikely," etc.) to rate causes and failure modes based on their relative probabilities of occurrence. In addition to being used in RPN calculations and qualitative criticality analysis, these ratings can also be used to assign quantitative models to cause and failure records in the FMRA (in the absence of more specific data).

By using rating scales to assign quantitative models to FMRA records, you can:

To use a rating scale in the FMRA, an authorized user must first configure the scale by associating a quantitative model (e.g., a constant failure probability of 5%) with each qualitative rating (e.g., "Level C - Occasional"). Then any user can choose to base an FMRA record's reliability policy on the record's assigned rating. The following sections explain how to edit the rating scales and how to base a policy on a rating scale.

Editing Rating Scales

Authorized users can edit the occurrence scale in the Rating Scale window for the current project or the library.

The failure probability scale can only be edited for the current project. To do this, choose Project > Management > Configurable Settings > Probability of Failure.

In the Rating Scale window (regardless of whether it is used to edit the occurrence scale or the failure probability scale), the Quantitative Value will be treated as either a fixed probability (that is the same for all times) or a probability at a specified time (assuming an exponential distribution). You select how the value is treated in the Treat Quantitative Values As area.

Note: When you convert an existing library or project from Xfmea 5, the software will set the quantitative values of these scales to match those in the "RS DFR Focus" profile that ships with Xfmea 8. Thus, after converting data, you may have to re-assign or manually adjust the associated profiles. See Convert from Version 5.

Note: If you are using the Rating Scale window to edit the occurrence scale in an enterprise library, the time specified for this option will always be entered in base units. This is because the same standard library might be used by different databases.

Note: To solve for the parameter of the exponential distribution given the unreliability point, the software uses the same calculation method used in the Quick Parameter Estimator. For example, if the given unreliability point is 1% at 1,000 hours, then the model is an exponential distribution with lambda = 0.00001. (You can choose to use mean time for exponential distributions on the Settings page of the Application Setup.)

Basing a Reliability Policy on a Rating Scale

When a user chooses to set the reliability policy of a failure or cause record based on a qualitative rating, the software will use the rating scales defined for the project to create a probability model for that record.

Tip: You can also view and select a rating scale by clicking the Select Existing icon that appears when you click inside the Initial/Revised Occurrence or Failure Probability field on the Properties tab for the selected failure/cause record.

If you later change the rating that the reliability policy is based on, or if you change the qualitative value associated with the rating in the rating scale, the software will automatically update the associated model.

 

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