Example 1
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Example 1 |
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Failure data for a bearing has been collected and is
contained in the computer file "BEARING.DAT". A check in the
"Censored" box indicates the bearing was removed from service without failing.
This is called censored data, and cannot be ignored without creating severe errors. This data can be entered into the software by opening the "BEARING.DAT" file. To analyze the data, selecting Parameter Estimation, Weibull, and then Maximum Likelihood Estimation menu. A shape parameter of 0.524 is calculated. Ninety percent confidence limits for this value are 0.4346 and 0.6317 (Confidence limits are important to distinguish values from 1.0. Is 0.97 equal to 1.0? What about 0.91 or 1.03? If the confidence interval includes 1.0, then the parameter should be considered equal to 1.0). This information tells the maintenance engineer or technician that the bearings are not being properly installed, or the bearing manufacturer is shipping defective bearings. This analysis was used on a coating line, and it was found that by using laser alignment to install bearings, the bearing life was extended by over a factor of 10. The MLE 3- Parameter routine also calculates a location parameter. In some cases, there is an extremely low probability of failing for some period of time. A location parameter is used to model this. The location parameter can also be negative. This means there is a probability of failure before the item was put into use. This is used to model failures caused by transportation and shelf-life failures. The probability plotting and hazard plotting routines do not give confidence limits, but allow the user to visually determine how well the proposed distribution fits the data. |
