OPTIMAL MAINTENANCE SCHEDULING FOR A SYSTEM WITH INDIRECT AND DIRECT PREVENTIVE MAINTENANCES

This paper investigates an optimal maintenance scheduling for a repairable system which is periodically maintained by two types of preventive maintenance(PM) activities; indirect PM and direct PM. The indirect PM is a passive and short-term PM activity reducing the stress of the operating environment, and the direct PM is an active and long-term PM activity prolonging the lifetime of system. In our proposed PM model, imperfect PMs are assumed and the system is minimally repaired at failures between PMs. The imperfect PM is formulated using the improvement factor which represents the degree of improvement by PM. A scheduling is defined to be optimum if it minimizes the expected cost rate comprised of the indirect PM cost, direct PM cost, repair cost and replacement cost. An application example is given to illustrate the implementation of the scheduling and investigations for various choices of maintenance variables are performed to provide insights for an effect of the variables and inter-relationship of them