1. Introduction
In every manufacturing system, breakdowns or variability in the operation times of a machine can interrupt the smoothness of the entire system, resulting in economic loss. To avoid breakdowns of the system, management should give attention to regular preventive maintenance immediately after regular production so that the manufacturing system can produce without any breakdowns in the next production cycle. The maintenance period may take various amounts of time depending on the condition of the machinery in the system. Long maintenance times may cause shortages. Generally speaking, the just-in-time production system considerably reduces the cost of material inventories. This reduction captured the imagination of some to such an extent that they adopted the notion that inventories have no value and should be totally eliminated. However, a well-designed just-in-time production system requires the allocation of inventory buffers for efficient operation.