NASA Reliability Centred Maintenance Guide for Facilities and Collateral Equipment

3.2.2.1 PM Tasks
Although numerous ways have been proposed for determining the correct periodicity of preventive maintenance tasks, none are valid unless the in-service age-reliability characteristics of the system or equipment affected by the desired task are known. This information is not normally available and must always be collected for new systems and equipment. PT&I techniques should be used as an aid in determining equipment condition vs. age. Careful analysis of similar kinds of hardware in industry has shown that, overall, more than 90 of the hardware analyzed showed no adverse age-reliability relationship. This does not mean that individual parts do not wear; they do. It means that the ages at failure are distributed in such a way that there is no value in imposing a preventive maintenance task. In fact, in a large
number of cases, imposing an arbitrary preventive task increases the average failure rate through “infant mortality.”

The Mean Time Between Failures (MTBF) is often used as the initial basis for determining PM interval. This approach is incorrect in that it does not provide any information about the effect of increasing age on reliability. It provides only the average age at which failure occurs, not the most likely age. In many cases a Weibull distribution, as used by the bearing industry to specify bearing life, will provide more accurate information on the distribution of failures. The best thing that can be done if good information on the effect of age on reliability is not available, is to monitor the equipment condition.