Quality control procedures are designed to track characteristics of quality and to take action to maintain quality within limits. In some instances, the action called for may be equipment maintenance. The maintenance function then acts in a supporting role to keep equipment operating effectively to maintain quality standards as well as to maintain the quantitative and cost standards of output.
There are alternative policies that may be appropriate, depending on the situation and the relative costs. First, is routine preventive maintenance economical, or will it be less costly to wait for breakdowns to occur and then repair the equipment? Are there guidelines that may indicate when preventive maintenance is likely to be economical? What service level for repair is appropriate when breakdowns do occur? How large should maintenance crews be to balance the costs of downtime versus the crew costs? Should a larger crew be employed in order to give more rapid service at higher cost? In addition, there are longer range decisions regarding the possible overhaul or replacement of a machine.
The decision concerning the appropriate level of preventive maintenance rests on the balance of costs, as indicated. Managers will want to select a policy that minimizes the sum of preventive maintenance costs plus repair, downtime, and quality related costs.
A Curve (a) can be drawn representing the increase in costs that results from higher levels of preventive maintenance. These costs increase because higher levels of preventive maintenance mean that we replace parts before they fail more often, we replace more components when preventive maintenance is performed, and we perform preventive maintenance more frequently. Curve (b) drawn represents the decline in breakdown and repair, downtime, and quality related costs as the level of preventive maintenance increase. The quality related costs are too often ignored but they can be of great significance in their impact on product quality directly, management and worker attitudes, and customer reactions. With higher levels of preventive maintenance, we should experience fewer actual breakdowns, and therefore positive impacts on other costs.
The total incremental cost curve is the sum of curves (a) and (b). The optimal policy regarding the level of preventive maintenance is defined by the minimum of that curve.
There is a combination of costs that leads to the decision not to use preventive maintenance. Suppose that the costs referred to did not decline as the level of preventive maintenance increased or declined more slowly than preventive maintenance costs increased. Then preventive maintenance would not be justified because the minimum total cost would occur with no preventive maintenance. The optimal policy would then be simply to repair the machine when breakdowns occur.
Preventive Maintenance (PM):
Assume a preventive maintenance (PM) policy for a single machine that provides for an inspection and perhaps the replacement of parts after the machine has been running for a fixed time. This is PM cycle. It requires an average cost, CPM, to accomplish the PM. A certain proportion of breakdowns will occur before the fixed cycle has been completed. For these cases, the maintenance crew will repair the machine, requiring an average cost, CR, for the repair. This is the repair cycle. The probability of the occurrence of the two different cycles depends on the specific probability distribution of the time between machine breakdowns and the length of the standard PM cycle. If the distribution of machine breakdown time has a low variability and the standard PM cycle is perhaps only 80 percent of the average run time without breakdowns, actual breakdown would occur rather infrequently and most cycles would be PM cycles. If the distribution were more variable for the same standard PM period, more actual breakdowns would occur before the end of the standard period. Shortening the standard period would result in fewer actual breakdowns, and lengthening it would have the opposite effect for any distribution.