Research On The Optimization And Reliability Of Preventive Maintenance System

In industry and daily life, some equipment fault will seriously affect the production order and put the society and personal safety in endanger. In order to avoid the occurrence of failures, it is necessary to study the reliability of equipment, and to take appropriate preventive maintenance (PM) policy. Based on the delay time modeling technique, this thesis analyzed the practical scenarios of performing and planning PM policy in iron and steel industry, and improved the traditional PM policy. In the aspect of studying the reliability of redundant or standby systems, this thesis analyzed the influence of random environment on the lifetime of k-out-of-n systems and studied the reliability of a system with a warm standby spare part. The main contents and innovations of this thesis are listed as follows:1) Considering that the systems or units may experience some production wait time in practical production process, and this will also provide the opportunity for maintenance. Knowing that the hazard rate of the units is not constant and is related to the usage of the unit. In most cases, the hazard rate of units rises rapidly in the late period of a unit life. To reduce the hazard rate and improve the stability of the systems or units, it is necessary to consider whether for the unit to be repaired when it encounters a production wait in its late period of life. To our knowledge, this kind of PM policy has not been studied in previous age-based replacement literature. In this thesis, we considered the replacement of a unit that is based on age replacement with consideration of production wait. We formulated this joint age-based replacement limit and threshold optimization problem with the objective of minimizing the expected cost per unit time in the long run. A real-data example was presented to illustrate the applicability and effectiveness of our model.2) In practical cases, the hazard rate of the units is relatively stable in the early period of use and rises rapidly in the late period. The failure may not happen instantaneously and the sign of the earlier can sometime be observed. To avoid major failures and great economic losses and save unnecessary monitoring cost, it will be naturally to consider that what PM policy should be taken in during the period that the sing of earlier failure is present, this is referred to as the delay-time period of failure. So far, there is not much research in this area. Considering that the system has an increasing hazard rate function, this thesis studied an age-based replacement policy during the delay time with consideration of production wait time. The unit does not need to be replaced in its normal stage. In the delay time stage, the age-based replacement limit and threshold were optimized with the objective of minimizing the expected cost per unit time in the long run. A numerical example is given to demonstrate the model.3) When assessing the reliability of k-out-of-n redundant systems, it is usual in the literature to assume that the lifetimes of components of a system are mutually independent and identically distributed. In practice, however, there exists some form of dependence between their lifetimes. This thesis considered that the common environment acting on all components is a major factor inducing dependence, and studied the influence of random environment on the lifetime of k-out-of-n systems and on the joint distribution of n components.4) In the field of reliability engineering, standby redundancy is a technique that has been widely applied to improving system reliability in the stage of system design. In this thesis, we considered a general standby system which includes the cases of cold, hot and warm standby systems. First, we obtained an explicit expression for the reliability function of the system under the age replacement policy, and analyze the factors which affect the reliability of systems. Secondly, we consider a conditional k-out-of-n system with a warm standby part. We obtained an explicit expression for the reliability function of the system, and presented some comparison result of reliability and numerical illustrations.