Optimum Piggyback Preventive Maintenance Policies

The piggyback PM (preventive maintenance) policy has its root in the opportunistic replacement policy. It specifies a PM interval for a part in a system. And yet, when it is PM-due, such a part is never PM´ed until a UM (unscheduled maintenance) of another part occurs. The main differences between the piggyback PM policy and earlier opportunistic replacement policies are: 1) All PMs are done piggyback during a UM. 2) UM rate and service cost rate are separately treated because one or the other or both is important depending on the viewpoints. This paper treats three series models of piggyback situations. They are illustrated by two practical examples, and enhanced via simulation by the Xerox MERLIN. They all confirm the intuition that for an increasing-failure-rate part the more PM the less UM. Traditionally, at minimum service cost rate, it is implicit that the corresponding UM rate is reduced from the PM-less case. In this paper, however, by treating the UM and service cost rates separately, the decision can then be consciously made whether the PM interval should be taken below the minimum service cost rate point in order to reduce the UM rate further. This is a region for tradeoff between the UM and service cost rates. The piggyback effect is lost when the piggybacked part has a very low failure rate. But, it can provide a better alternative than the age or block replacement policy when the piggybacked part has a high enough failure rate.