Reliability engineers and their associated job functions often try hard to make their responsible supply-chain staff or purchasing departments understand that it is impossible for the maintenance department to plan, or accurately predict, spare parts demand. The reason is that a major proportion of equipment and systems fail randomly. A good example would be rolling element bearings in all industries worldwide.According to bearing manufacturers' statistics, 91% fail before they reach the end of their conservatively estimated design lives, and only 9% reach the end of design life. The millions that prematurely fail every year can all be slotted in one or more of the seven basic failure categories: 1) operator error, 2) design oversights, 3) maintenance mistake, 4) fabrication/processing defect, 5) assembly/installation error, 6)material defects and 7) operation under unintended conditions.
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