Engineered Software

Example 3
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Life Data Analysis

Maintenance Optimization

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It is known that a sheet metal leveler fails due to a build-up of dirt. To prevent failures, the current policy is to clean the leveler after 40 million feet of metal have been leveled. Failure data for the sheet metal leveler has been collected and is contained in the computer file "LEVELER.DAT". Again check in the "Censored" box indicates the leveler was cleaned without failing.

A production manager familiar with Examples 1 and 2 has requested a Weibull analysis to determine what corrective action, if any, should be taken. A Weibull analysis was performed using the following steps.

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

The software returns an estimate of 4.595 for the shape parameter, with an 80% confidence interval (10% on each tail) of 3.744 to 5.64. The manager is instructed that the leveler data indicates "wear-out", and that the correct procedure is to conduct preventive maintenance - cleaning the leveler.

The manager states that the policy of cleaning the leveler after every 40 million feet of leveled metal was based on meeting production schedules, and wants to know if there is a procedure for determining the "correct" schedule for cleaning the leveler (Chapter 5 of the manual describes the theory behind optimizing preventive maintenance schedules).

To determine the optimum schedule for cleaning the leveler, more information is needed. The cost of cleaning the leveler is $180, and the cost of a failure is $1,220. This decision must be made based on costs. After all, if the cost of cleaning the leveler was greater than the cost of failure, the best policy is to run to failure. To determine the optimum schedule for cleaning the leveler

  1. Determine the parameters of the Weibull distribution (This has already been done).
  2. Select Maintenance.
  3. Select Optimum Schedule for Preventive Maintenance.
  4. Select the MLE parameter estimates from the table.
  5. Enter "180" for the cost of a PM and "1220" for the cost of a failure. 

The software will determine the optimum time between leveler cleanings; 29.28 million feet of leveled metal, and the optimum cost, $7.888 per million feet of leveled metal.

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