As shown in the illustration, the cost of failure is inversely related to frequency of maintenance, whereas the cost of maintenance is directly related to frequency of maintenance. In this particular illustrated example, what frequency of maintenance would be the most efficient in controlling the total cost? See illustration EL-0224.
• Total cost is the sum of cost of failure and cost of maintenance • On a graph, the most efficient point is usually where the total cost curve reaches its lowest point (minimum) • Maintenance done too rarely increases failure cost; done too often increases maintenance cost
• Look at where the total cost curve (not the individual cost curves) is at its lowest point on the horizontal axis marked with the different frequencies • Compare that lowest point to the labeled positions: Biannually, Annually, Semi-annually, Quarterly • Ask yourself: at which labeled frequency does moving left or right from that point make the total cost curve rise?
• Be sure you are following the total cost curve, not the cost of failure or the cost of maintenance alone • Visually identify the minimum point of the total cost curve and then match it to the closest labeled frequency on the x‑axis • Confirm that on both sides of that point (less and more frequent maintenance) the total cost line is higher than at the point you chose
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