Reliability HotWire

Reliability HotWire

Issue 134, April 2012

Tool Tips

*How can I use RGA 7 to create a reliability growth test plan?

RGA 7's Growth Planning Folio helps you to set an idealized growth curve and planned MTBF goal at each phase of a reliability growth testing program. When the testing begins, you can then track the actual results against the goals in order to obtain an early warning signal if changes are necessary to meet the final MTBF target.

To add a growth planning folio to your RGA project, choose Project > Add Growth Planning.

Specify the number of test phases in the control panel (this will determine the number of rows in the data sheet). Then enter the cumulative test time that will be accrued by the end of each phase. For example, if the test plan consists of three phases with 100 hours of test time in each phase, you would enter 100, 200 and 300.

Click the Calculate icon to display the Planning Calculations window. This tool considers six factors related to the reliability growth testing strategy. You can choose to calculate one of these factors based on the inputs you provide for the other five. For example, if you wish to calculate the Initial MTBF that should already be achieved before the reliability growth program begins, you will be required to make the following inputs to define the final MTBF goal and the planned test strategy:

  • Goal MTBF is the final MTBF target that you hope to achieve by the end of the test program.
  • GP Design Margin is a “safety factor” that can be adjusted to make sure the growth potential will be reached. The higher the margin, the smaller the risk that the reliability in the field will be lower than the requirement but, at the same time, the more rigorous the reliability growth program will be.
  • Avg EF is the average effectiveness factor for corrective actions (i.e., the percent reduction in the failure intensity after fixes have been implemented).
  • Management Strategy is the percentage of all unique failure modes discovered during testing that management plans to fix.
  • Discovery Beta is the beta parameter of the discovery function for the type B failure modes (i.e., the failure modes that will have fixes applied at the end of a test phase). The discovery function represents the rate at which new, distinct failure modes are encountered during testing.

After you have entered the information required, click Calculate and then click OK to close the window. In the control panel, click the Plot icon to create a plot of the idealized growth curve and the MTBF goals planned for each stage of testing.

For a more detailed discussion of the Growth Planning Folio only, see http://www.weibull.com/hotwire/issue119/hottopics119.htm. For an example that discusses all aspects of this process, including the analysis of the test data and the use of the MultiPhase Plot to track the actual results against the plan, see http://www.ReliaSoft.com/newsletter/v10i1/rgpanalysis.htm.

*How can I use Weibull++/ALTA to obtain the overall reliability of a product that comprises several different components?

You can easily perform this analysis in Weibull++ 8 or ALTA 8 by creating a reliability block diagram. First, create a standard folio for the product and then create separate data sheets in the folio for each component. In each data sheet, enter the data available, choose an appropriate distribution and calculate the parameters.

Once the parameters of all the components have been calculated, you will then create a diagram to represent the reliability-wise configuration of the product. To do this, choose Insert > Tools > Diagram.

When you add a block to the diagram, you will be asked to select the data sheet that the block will represent. The list of available data sheets includes any data sheet in any Weibull++ or ALTA standard folio, as long as it has been calculated and is not using one of the competing failure modes distributions.

The following example shows a diagram for a water pump, with three blocks representing the components connected in series. Each block represents the data sheet of that component.

The series configuration in this example indicates that the product will fail if any single one of its components fails. You can arrange the blocks in a series, parallel or combination of both configurations to represent how the components affect the overall reliability of the product.

Once the configuration is complete, click Calculate to analyze the diagram. You can then use the QCP tool to calculate the product’s overall reliability.