Reliability HotWire

Reliability HotWire

Issue 132, February 2012

Tool Tips

*What tools do Xfmea and RCM++ provide to assess risk other than Risk Priority Numbers?

Traditionally, the RPN is defined by obtaining the product of the three ratings: Severity x Occurrence x Detection. The FMEA team then uses the RPN values to rank the issues, assess the relative risk and determine which corrective actions to take.

However, it is important to remember that the RPN is the product of three subjective ratings and different circumstances may produce similar or identical RPNs. For example, an RPN of 100 could mean:

  • Severity = 10, Occurrence = 10, Detection = 1
  • Severity = 2, Occurrence = 5, Detection = 10
  • Severity = 1, Occurrence = 10, Detection = 10

In accordance with some common industry practices, Xfmea/RCM++ support multiple additional ways that FMEA practitioners may wish to use the severity, occurrence and detection ratings for evaluating risk and prioritizing corrective actions. This includes:

  • SxO: This metric is calculated based on the product of the severity and occurrence ratings only.
  • SOD: This metric displays the severity, occurrence and detection ratings together, but it is not a calculated value. For example, if the severity is 8, the occurrence is 4 and the detection is 7, then the resulting SOD metric will be 847. When the issues are sorted in descending order, they will be prioritized first by severity, then by occurrence and then by detection.
  • SD: This metric displays the severity and detection ratings together and is similar to the SOD value except that it is based on the severity and detection ratings only.

Within the software, you can display the initial and revised values of any metric or rating in the FMEA Hierarchy. And you can use the Priority Highlights feature to use red, yellow and green highlight colors to identify high, medium and low priority issues within the analysis.

For even more control over how the software prioritizes and highlights failure causes, you can use the Risk Ranking feature to customize the number of priority levels, as well as their associated names and colors. For information on how to do so, refer to the Tool Tip in Issue 128 of the Reliability HotWire.

You can also base your decision on which actions to take by ranking the issues by a single rating. For example, you can use the FMEA filtered view to see a list of causes and then sort by the desired rating; you can run a query on the causes with the highest occurrence ratings; you can generate an "Effects Ranked by Initial Severity" plot, etc.

To see a graphical chart that displays the issues based on their severity and occurrence ratings you can use the Plot Viewer to create an Occurrence / Severity Matrix chart. The matrix includes thresholds for high, medium and low priority issues.

Occurrence/Severity Matrix Plot

*Is there a fast way to calculate multiple metrics for any analyzed data sheet?

When you analyze a data sheet in Weibull++, ALTA or RGA, it is easy to use the Quick Calculation Pad (QCP) to calculate any particular metric of interest whenever you need it. But did you know that you can also access these calculations (and many others) by using the Function Wizard in conjunction with a spreadsheet-based report?

These customizable spreadsheets can save time and increase your analysis capabilities. For example, you can use them to:

  • Calculate and display multiple different metrics simultaneously.
  • Compare metrics from different data sets side-by-side.
  • Integrate a metric into another calculation (e.g., multiply the BX% life by the population size in order to estimate the number of expected failures).
  • Generate a standard report template with the typical results that are of interest for any analyzed data set.
  • And much more.

To add a report to a project in Weibull++ 7, ALTA 7 or RGA 7, choose Project > Add Report or right-click the Reports node in the Project Explorer.

The Report Wizard will allow you to select the default data source for any function that is based on an analyzed data sheet (e.g., reliability, BX% life, mean life, failure rate, etc.). This is not required to create the report and you can change it at any time from within the report window’s control panel.

When you use the Function Wizard to define a metric, you can choose to base the calculation on whatever data sheet is currently defined as the default data source for the report (e.g., if you want the report to function as a template) or specify a particular data sheet for each individual function (e.g., if you want to compare results from two different analyses).

In the simple Weibull++ report shown next, all of the functions are based on a default data source that could be changed at any time if you want to obtain the same results for a different analysis. In this example, the user can enter any time into cell C15, and any 1-sided confidence levels into cells C16 and C17. The functions in row 21 have been configured to automatically calculate the reliability based on the current inputs (shown in blue text) and whatever data sheet is currently associated with the report.

Weibull++ Report and Function Wizard

More information is available in the user documentation for Weibull++, ALTA and RGA. Each user’s guide includes a detailed appendix of the available functions with information about any inputs that are required.