Related Topics and Links:

RPNs and Related Metrics

Qualitative Criticality Analysis

MIL-1629A Criticality Analysis


 Criticality Analysis

Quantitative Criticality Analysis

In the Synthesis version, Xfmea/RCM++/RBI offer a methodology for quantitative criticality analysis that is patterned after MIL-STD-1629A but uses a more general approach that overcomes several inherent limitations and simplifications present in the military standard (including the assumption of a constant failure rate). Furthermore, and instead of dealing with an arbitrary number, this methodology presents the computed criticality number in terms of the probability "that a failure will be caused by the current mode," which is significantly more meaningful and powerful. For specific details on this approach, see http://www.reliawiki.org/index.php/Criticality_Analysis.

This analysis is performed directly in the FMRA with report output generated from the Reports window. If your organization is obligated to use the original method instead, see MIL-1629A Criticality Analysis.

Steps to Calculate Criticality in the FMRA

  1. Make sure that the applicable criticality analysis fields have been enabled for the project (on the FMEA > Criticality page of the interface style).

  2. Define the item(s) that will be included in the analysis.

  3. For each item, create an FMEA and define the functions, failure modes, effects and causes.

  4. If the FMRA is not already displayed in the System panel, choose View > FMRA > Show FMRA to make it visible.

  1. If the criticality analysis columns are not already displayed in the FMRA hierarchy, right-click any of the column headers, then click Customize Columns. Select the four columns shown next and click OK to save the changes.

  1. Click the first failure mode in the FMRA hierarchy to display its reliability-related properties in the Analysis panel. The following fields may be applicable for quantitative criticality analysis:

  1. Select the top-level item in the FMRA hierarchy and specify the relevant Operation properties.

Note: The operating time can be defined only for top-level (system) items in the hierarchy, and this time applies to all of the system's dependents. Thus, if your criticality analysis considers the failure modes associated with only one system, and if you want to compare the failure modes, then you only need to define the operating time for that system. However, if your criticality analysis considers multiple systems that you want to compare, then, to make a valid comparison, you need to enter the same operating time for each system.

  1. Finally, to perform the calculations, choose FMRA > Calculations > Calculate (Reliability).

The Calculation Options window provides the following options:

When you click OK, the software will calculate the probability of failure for each failure mode and automatically determine the Mode Ratios based on all of the failure modes that have been defined for each item. (To give a very simple example, if the item has two failure modes that both have the same probability of failure, then they will both contribute equally to the item’s probability of failure, and therefore they both have Mode Ratio = 0.5.)

Then the software will calculate the mode criticality and item criticality, and the results will be displayed in the Criticality column of the FMRA hierarchy.

Tip: To specify the displayed math precision for the calculated values, choose File > Application Setup then define your preferences on the Settings page.

  1. To generate a report, choose Home > Reporting > Reports to open the Reports window and select all of the items you wish to include in the analysis. Then select the desired criticality reports, which are located under the Criticality heading, as shown next.

The following reports are applicable for quantitative criticality analysis:

 

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