Reliability HotWire

Reliability HotWire

Issue 110, April 2010

Tool Tips

*How can I use ALTA to make warranty predictions based on data from a quantitative accelerated life test?

If you have data from a quantitative accelerated life test, you can use ReliaSoft’s ALTA software to analyze the data in order to make estimates about the performance of the product under normal use conditions. If you have information about the projected sales for the product, you can take that analysis a step further and forecast the total expected returns for future warranty periods.

  • First, use one of the Standard Folios in ALTA to enter the accelerated test data and fit a lifetime distribution and life-stress relationship model.
  • Next, choose Project > Add Report to open the Report Wizard. Select your Standard Folio in the Default Data Source field and then choose Based on an existing Template and select the Warranty Forecasts template that is shipped with the software, as shown next.
  • Click OK to add the report to your project. In this worksheet, you can set the value in the Time Increment field to determine the length of the forecasted warranty periods. You also can enter the expected sales/shipment quantity for each period. When you have entered these values, the worksheet will automatically calculate the expected returns in each warranty period. For example, in the following worksheet, the time increment is set to 720 hours (i.e. approximately 3 months if the units operate 8 hours per day). Therefore, the forecasted warranty periods are 720, 1440, 2160, 2880 and 3600 hours. The anticipated sales have been entered in the area on the left side of the worksheet (i.e. 10,000 new units enter the population in each period). The area on the right side of the worksheet presents the forecasted returns from each batch of sales and the row at the bottom of the worksheet presents the total expected returns in each warranty period. For example, in Period 3, the model projects ~27.4 returns from the units that were shipped in the first sales period and ~2.2 returns from the units that were shipped in the second sales period, for a total of ~29.6 returns in Period 3.

*How can I quickly apply the same changes to the subordinate items of a block in Lambda Predict 3?

The application properties of blocks in Lambda Predict 3, such as temperature and environment, can affect the properties of items below them in the system hierarchy. Depending on the block’s settings, these particular properties of the items contained in the block can change when the block’s application parameter values are changed. This means that you can ensure that changes made to the application parameters of, for instance, a subsystem will be applied to all sub-subsystems and components within that subsystem.

As shown below, the Update Modes node in the Item Properties area contains the settings that determine how the updates are applied.

The setting of the Update Children field determines what happens when you change the block’s properties:

  • Always: All subordinate blocks and components will assume the application parameters of that block. For MIL-HDBK-217F blocks, this also applies the physical parameters of the block (i.e. its quality settings) to any of its children that have the given quality type (e.g. a change to the Quality, Capacitors parameter will affect capacitors below the block, but will not affect coils below the block because they do not use the Quality, Capacitors parameter).
  • Never: Changes made to the parameters will not be applied to any subordinate blocks or components.
  • On Matching: Changes made to a given parameter of the block will be applied to subordinate items only if the current value of the parameter is the same for the block and the subordinate item. For example, consider a block with an Environment parameter of "Ground, fixed" and two subordinate items: component A (Environment: "Ground, mobile") and component B (Environment: "Ground, fixed"). If you change the block's Environment parameter to "Ground, benign," component B's Environment parameter will also change to "Ground, benign," but component A's Environment parameter will remain unchanged.

For temperature changes, the setting of the Update Temperature Mode field determines what happens when the block’s temperature value changes:

  • Absolute: The temperatures of the subordinate blocks or components will be replaced with the specified value.
  • Increment: The specified value will be added to the current temperature values for the subordinate blocks or components. For example, if component A has a temperature of 50 and component B has a temperature of 60 and you enter an incremental addition of 10, then the temperatures will change to 60 and 70, respectively.