سیاست های نگهداری پیشگیرانه در سطح قابلیت اطمینان بحرانی سیستم تحت فرسایش

Conventional preventive maintenance (PM) policies generally hold same time interval for PM actions and are often applied with known failure modes. The same time interval will give unavoidably decreasing reliabilities at the PM actions for degradation system with imperfect PM effect and the known failure modes may be inaccurate in practice. Therefore, field managers would prefer policy with an acceptable reliability level to keep system often at a good state. A PM policy with the critical reliability level is presented to address the preference of field managers. Through assuming that system after a PM action starts a new failure process, a parameter so-called degradation ratio is introduced to represent the imperfect effect. The policy holds a law that there is same number of failures in the time intervals of various PM cycles, and same degradation ratio for the system reliability or benefit parameters such as the optimal time intervals and the hazard rates between the neighboring PM cycles. This law is valid to any of the failure modes that could be appropriately referred as a ‘general isodegrading model’, and the degradation ratio as a ‘general isodegrading ratio’. In addition, life cycle availability and cost functions are derived for system with the policy. An analysis of the field data of a loading and unloading machine indicates that the reliability, availability and cost in life cycle might be well modeled by the present theory and approach.

Preventive maintenance (PM) is a necessary activity to restore or keep the function of a repairable system in good state. How to assess the effect of this activity and how to arrange it properly for addressing one's satisfactions have been long focused in practice since 1960s [1]. It has been revealed that maintenance effects can be subdivided into a perfect, a non-effect, and an imperfect [2] and [3]. A perfect effect restores the system to good-as-new, a non-effect to bad-as-old, and an imperfect effect to partly good. For system subjected to age/degradation, the imperfect should be generally for PM, and the perfect and non-effects are extreme. Present paper will discuss the imperfect. Conventional replacement/PM policies [5], [6], [7], [8], [9] and [10] are derived using non-decreasing hazard rate functions on a basis of the imperfect effect model by Brown and Proschan [4] or the consideration with minimal repair by Barlow and Hunter [1]. They hold generally a same time interval T for replacement/PM actions and are often applied with known failure modes. The age-T policy will give unavoidably decreasing reliabilities at the PM actions for degradation system with imperfect PM effect and the known failure modes may be inaccurate in practice for complicated system. An interesting phenomenon worthy to pay attention is that the field managers preferred the PM opportunities being derived from an acceptable specified reliability level [11], [12] and [13]. This phenomenon is known in our field investigation into the loading and unloading machinery used at Chinese railway terminals. Major causes may be included as: 1. Proper PM actions should be at best derived from the inspection in service. Under this case, unless there is a fine support system for decisions making, the known failure models can be seldom quantified accurately in practice for the complicated repairable system. 2. Following the management rules with T-age PM policy [14], the machinery exhibit increasing hazard rates at the PM actions due to the imperfect PM effect. Correspondingly, the reliabilities at the PM actions show a gradual decreasing. 3. To keep the machinery operated at a good technical state with reliability above an acceptable reliability level, the field managers took often unconsciously the measurement of shortening the T intervals as the PM action increasing. However, there is no effort addressing the preference of field managers. Key issues are how to quantify the imperfect effect and how to arrange PM actions properly to address one's satisfactions. These should be very difficult tasks. Some good efforts have made in the proportional hazard (PH) model [15], [16] and [17] and the work by Soares and Garbatov [18]. The PH model uses the proportional age reduction factor to the baseline of hazard rate or to the operation time. Considering the reliability of the ship hull girder with age failure mechanism after repair smaller than the initial value for new, Soares and Garbatov introduced a reduction factor for piece of time to represent the ‘recovered’ time of a repair action. Through assuming that system after PM action starts a new failure process, a parameter so-called degradation ratio should be introduced to represent the imperfect effect of PM on the system reliability, availability, or benefit between neighboring PM cycles. This paper tries to develop a PM policy with a critical reliability level to meet the preference of field managers. Aim is addressed on that the policy is valid to arbitrary failure process/mode. Relative approaches are also explored for predicting the life cycle availability and cost under this policy.

For a repairable system subject to an intrinsic degradation with T-age PM policy, it is unavoidable to exhibit a decreasing of reliabilities as the PM action increasing. Therefore, it is preferred by field managers to hold an acceptable reliability level to keep the system often at good technical state. A PM policy has been suggested for degradation system with an acceptable reliability level. It consists of the following considerations: 1. PM actions should be taken at an acceptable critical reliability level Rc. This level can keep the system operates at a good technical state with an acceptable operational cost per unit time, or an acceptable availability. 2. Optimal time interval in a PM cycle should be determined by a maximum availability, or a minimum cost per unit time. 3. Number of PM cycles for the system in life cycle should be derived from an acceptable level of the availability or the cost per unit time in life cycle. Through assuming that system after a PM action starts a new failure process, a parameter so-called degradation ratio is introduced to represent the imperfect effect. The policy holds a law of same number of failures for the time intervals of various PM cycle and a same degradation ratio for the system reliability or benefit parameters such as optimal time intervals and hazard rates between neighboring PM cycles. In addition, this law is valid to any of the failure modes. Therefore, the law should be appropriately referred as a general isodegrading model and the degradation ratio a general isodegrading ratio. Life cycle availability and cost functions have been deduced for the system with the PM policy. The number of PM cycles for the system in life cycle may be derived from an acceptable cumulative availability or an acceptable cumulative cost in life cycle. The analysis results of the field-test data of a loading and unloading machine indicates that the availability and cost of the machine with the suggested PM policy have been well predicted and therefore, the present theory and approach are available.