Abstract:Based on the theory of the power law process model for repairable systems, an aperiodic imperfect preventive maintenance method for numerical control (NC) machine tools in wear-out stage was proposed. A hybrid model of failure intensity function, in which the factors that a preventive maintenance reduced the effective age of machine tools and failure intensity increased as the number of preventive maintenance increased were considered, was presented. Taking mean cost as a minimal objective function, the maintenance intervals and the number of optimum maintenance were given.Finally, a discussion for a real numerical example was presented to show how the optimal results depended on the different cost parameters. The results show that for the machine tools with a large shape parameter of power law process model, they should be repaired with a lager number of preventive maintenance, but for those that have no obvious preventive maintenance effect or have a serious deterioration, they all should be replaced as soon as possible; in addition, the more expensive the machine tools are, the more preventive maintenance should be paid attention to.