The Periodic Preventive Maintenance Policy for Deteriorating Systems by Using Improvement Factor Model

Many researchers have shown that the imperfect preventive maintenance (PM) can reduce the wear out and aging effects of deteriorating systems (or machines) to a certain level between the conditions of as good as new and of as bad as old. The concept of the improvement factor is used to measure the extent of the restoration for a deteriorating system in this paper. The proposed improvement factor is considered as a variable depending upon the system's age (or the operating time), the number of PM performed in the specified finite time span, and the cost ratio of each PM to the replacement. By applying Lie and Chun's model, the proposed improve- ment factor model consists of three different functions to measure various age restoration situa- tions. By minimizing the expected cost rate per unit time over a finite time span, an optimal preven- tive maintenance policy for a deteriorating system is proposed in this paper. It is assumed that the periodic PM is performed for the deteriorating system with a minimal repair at each failure. In this paper, it is considered that a deteriorating system undergoes N times of periodic PM with a minimal repair at each failure during the specified finite time span (T) and is replaced at T. The expression to compute the expected cost rate per unit time is derived and the optimal num- ber of PM is also obtained for the Weibull failure case.