Kodo Ito

Optimal preventive maintenance policies for a shock model with given damage level

Purpose – This paper considers and discusses analytically the optimal preventive maintenance (PM) policies of aged plants such as fossil‐fired power plants.Design/methodology/approach – Shocks are assumed to occur at a nonhomogeneous Poisson process and the total damage due to each shock is additive. The system undergoes the PM at a certain time or the total damage exceeds a managerial level. The expected cost rate until PM is derived and optimal policies which minimizes it are discussed.Findings – There exists a unique optimal time (T*) or managerial level (k*) which minimizes the expected cost rate. But there does not exist a positive pair (T*, k*), simultaneously.Research limitations/implications – The damage occurrence distribution is assumed to be nonhomogeneous Poisson one.Practical implications – Useful methods to consider the optimal PM policies for power plant engineers.Originality/value – This paper contributes to users of aged power plants economically and practically.

An optimal inspection policy for a storage system with high reliability

Abstract A system such as missiles and spare parts of aircrafts has to perform a normal operation at any time when it is used. However, a system is in storage for a long time from the transportation to the usage and its reliability goes down with time. Such a system should be inspected and maintained at periodic times to hold a higher reliability than a prespecified value q . This paper suggests a periodic inspection of a storage system with two kinds of units where unit 1 is inspected and maintained at each inspection, however, unit 2 is not done. The system is replaced at detection of failure or at time when the reliability is below q . The total expected cost until replacement is derived and an optimal inspection time which minimizes it is discussed. Numerical examples are given when failure time distributions are exponential and Weibull ones.

Optimal inspection policies for a system in storage

Abstract A system such as missiles and spare parts of aircrafts has to perform a normal operation in a severe environment at any time when it is used. However, missiles are in storage for a long time from the delivery to the usage and its reliability goes down with time. Thus, a system in storage should be inspected and maintained at periodic times NT (N=1,2,…) to hold a higher reliability than a prespecified value q. The following inspection model is considered: A system has two types of units, where unit 1 is maintained and unit 2 is not done. A system is also overhauled if its reliability becomes equal to or lower than q. The number N ∗ of inspections and time ( N ∗ T+t 0 ) until overhaul are derived. Using these results, the average cost C(T) is obtained and an optimal inspection time to minimize C(T) is discussed.

Optimal periodic testing policy for circuit with self-testing

This paper considers periodic testing policies for a system with self-testing. The system can detect its failure by either self-testing or periodic inspection. If the system fails then its failure is detected by self-testing while it is on-line, or otherwise, it is detected at the next periodic test. Introducing the loss cost elapsed between a failure and its detection, the expected costs are obtained. Optimal intervals of periodic testing which minimize the expected costs are analytically derived. Numerical examples are given when both times of failure and its detection by self-testing are exponential distributions.

OPTIMAL MAINTENANCE OF AIRFRAME CRACKS

As an airframe has finite lifetime and has to be designed lightweight, the maintenance of airframe is indispensable to operate aircraft without any serious troubles. After an airframe begins to operate, it suffers stresses and the stress causes the damage such as cracks of the airframe. Cracks grow with operation time and cause catastrophic phenomenon such as the mid-air disintegration when they become greater than a critical size. So, the managerial crack size is prespecified and Preventive Maintenance (PM) undergoes when the inspected crack size exceeds it. In this paper, optimal PM policies of airframe crack failure are discussed. Airframe states are represented as the Markov renewal process, and one-step transition probabilities are discussed. The total expected cost from the start of operation to the end by failure is defined and the optimal PM policies which minimize it is discussed.

Applied Maintenance Models

In the advanced nations, comfortable lives of citizens depend on a wide variety of social infrastructures such as electricity, gas, waterworks, sewerage, traffic, information networks, and so on. For the steady operation of these infrastructures without any serious troubles such as emergency stop of operation, the steady maintenance is indispensable and the maintenance budget becomes extremely expensive in the most advanced nations because of the high personnel costs. In the twenty first century, such conflicts between the needs of utmost variety of infrastructures and the demands of least maintenance budgets becomes a serious social and industrial issue in these nations. Cost-effective maintenance has become an important key technology to resolve the inherent conflict.

Optimal Times of Burn-In Tests for Multilevel Assembly Lines

Producers make burn-in tests of items before delivery to eliminate initial failures. However, if the testing takes too long, it causes great delay in production schedules, and sometimes may raise the production cost. Therefore, we have to design optimal burn-in tests by comparing the influence on production activities with the possible damage due to initial failures. This paper considers optimal burn-in policies for multilevel assembly lines, introducing various kinds of cost factors. When the failure times are exponential, optimal policies which minimize the expected cost rate are discussed. Numerical examples are finally given when the failure time is exponential.