Engineered Software

Example 2


Life Data Analysis

Maintenance Optimization

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Failure data for a bearing has been collected and is contained in the computer file "BEARING2.DAT".  A check mark in the "Censored" box indicates the bearing was removed from service without failing. A production manager familiar with Example 1, immediately began training the mechanics responsible for installation of the bearings. After several weeks passed with no change in failure rates, this manager asked for help in explaining why there was no improvement.

A Weibull analysis was performed using the following steps.

  1. Select File from the main menu.
  2. Select Open and select the file "BEARING2.DAT".
  3. Select Parameter Estimation from the menu.
  4. Select Weibull.
  5. Select Maximum Likelihood Estimation

The software returns an estimate of 1.094 for the shape parameter. The lower 90% confidence limit for the estimated shape parameter is 0.9088, and the upper 90% limit is 1.317. This means that given the failure data, we are 80% confident (10% on each tail) that the true value of the shape parameter is between 0.9088 and 1.317. In this case, it should be assumed that the failure rate is constant (the shape parameter is equal to 1.0). There is no way to improve performance other than redesign. Training and preventive maintenance increase costs and have no effect on reliability.