Linear and nonlinear preventive maintenance 1 models 2

4 Preventive maintenance (PM) is a maintenance program with activities initiated at predetermined intervals, or 5 according to prescribed criteria, and intended to reduce the probability of failure, or the degradation of the functioning 6 of an item. In the literature, a number of PM models have been introduced to depict the effectiveness of PM. Based on 7 these models, approaches to scheduling PM policies have been considerably studied. This paper attempts to review 8 existing PM models, and investigate their inter-relationships. We then categorize these models into three classes: 9 linear, nonlinear, and a hybrid of both. These three PM model classes depict the relationships of the hazard functions 10 before, and after a PM. Possible extensions to these three PM models are discussed. The statistical properties for 11 models are derived, and approaches to optimizing the PM policy are given. 12