Periodic Preventive Maintenance
From run-to-failure, we progress to periodic preventive maintenance, which is sometimes called “historical” maintenance. This is where the histories of each machine type are analyzed and periodic overhauls are scheduled to occur before the statistically expected problems occur. It has long been known that most groups of similar machines will exhibit failure rates that are somewhat predictable if averaged over a long time. This gives rise to the so-called “Bathtub Curve” which relates failure rate to operating time, as follows:
If this curve applied to all machines of the group, and if the shape of the curve is known, preventive maintenance could be used advantageously, but unfortunately, this is not the case in practice.
Preventive maintenance also includes such activities as changing lube oil and filters, periodic cleaning and inspecting, etc. Maintenance activity may be scheduled on the basis of calendar time, machine operating hours, number of parts produced, and so on. Preventive maintenance became very popular in the early 1980s when small computers began to be used for planning and tracking maintenance work.
In a famous study of preventive maintenance by United and American Airlines, it was found that for a large class of rotating machines, the failure rate greatly increased just after the periodic overhauls — in other words, the overhaul reduced the reliability of the machines. It is as if the machine reverts to the beginning of the bathtub curve after each overhaul.
From this study and subsequent observations, it was found that periodic overhauls result in 20 % to 25 % of startup failures. About ten percent of these can be attributed to defective new bearings.
It is obvious that preventive maintenance is an inefficient use of resources for most machines; however, there are cases where it can be used to good effect. Examples are machines which exhibit wear related to use such as rock and ore crushers, and machines that are subject to corrosion such as equipment handling caustic substances.
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