Overview

The maintenance of an operating unit after failure is costly, and sometimes, it requires a long time to repair failed units. It would be an important problem to determine when to maintain preventively the unit before it fails. However, it would be not wise to maintain the unit too often. From this viewpoint, commonly considered maintenance policies are preventive replacement for units with no repair as described in Chapters 3 through 5 and preventive maintenance for units with repair discussed in Chapter 6. It may be wise to maintain units to prevent failures when their failure rates increase with age.

The usual preventive maintenance (PM) of the unit is done before failure at a specified time T after its installation. The mean time to failure (MTTF), the availability, and the expected cost are derived as the reliability measures for maintained units. Optimum PM policies that maximize or minimize these measures have been summarized in Chapter 6. All models have assumed that the unit after PM becomes as good as new. Actually, this assumption might not be true. The unit after PM usually might be younger at PM, and occasionally, it might be worse than before PM because of faulty procedures, e.g., wrong adjustments, bad parts, and damage done during PM. Generally, the improvement of the unit by PM would depend on the resources spent for PM.

It was first assumed in ^[1] that the inspection to detect failures may not be perfect. Similar models such...