تولید بهنگام بهینه دستگاه پرداخت موجودی برای تعمیر و نگهداری پیشگیرانه منظم

Most manufacturing organizations, if not all, are trying to implement some of the ideas adopted by the just-in-time (JIT) philosophy, a philosophy that promises a continuous improvement in total productivity. Preventive maintenance, an essential element of the just-in-time structure, induced the idea of this paper. Performing regular preventive maintenance results in a shutdown of the production unit for a period of time to enhance the condition of the production unit to an acceptable level. During such interruption, a just-in-time buffer is needed so that normal operations will not be interrupted. The optimum just-in-time buffer level is determined by trading off the holding cost per unit per unit of time and the shortage cost per unit of time such that their sum is minimum.

The success of the just-in-time production system lies in the considerable reduction in material inventories. This reduction captured the greatest public attention and to an extreme, some people led to adopt the mistaken notion that inventories are of no value and should be totally eliminated. However, a well designed just-in-time production system requires some inventory to operate efficiently as illustrated herewith. In this paper, we assume that the production unit runs for a maximum scheduled period T units of time before preventive maintenance interruption occurs. Before the end of period T, the just-in-time buffer S is built at a finite rate of α units per unit time. This finite replenishment starts (S/α) units of time before the end of period T, so that before the end of period T, a buffer of level S is accumulated. At the end of period T, the regular preventive maintenance starts; and lasts for a period of t units of time. This time t is a random variable that depends on how long it takes to enhance the condition of the production unit to an acceptable level that prevents any random failures, during T, as well as, maintains the same quality of output as before. During t, the demand is met from the just-in-time buffer, at the rate of β units per unit of time. If the buffer supply time is greater than t, then the number of units short will be zero; otherwise the number of units short will be β(t−S/β)=βt−S. After the production unit is preventively maintained during t, the production process resumes for another period T.

Regular preventive maintenance may encounter in addition to the routine checkup, some minor or major repairs resulting in a shutdown of the production unit for a period of time to enhance the condition of the production unit to an acceptable level. This time depends on the nature of the required maintenance actions. Such situations call for a just-in-time buffer to efficiently withstand the randomness in the duration of the outage time. In this paper, the optimum just-in-time inventory buffer level is found by trading off the holding cost and the shortage cost such that their sum is minimum. This research can be extended to include a safety stock level in addition to the just-in-time buffer and withstand the possible breakdown during the machine running time.