Page 51 - Intro Predictive Maintenance

42 An Introduction to Predictive Maintenance

It should be obvious that you must keep cost data for all maintenance efforts in order
to evaluate financially the cost and benefits of preventive versus corrective mainte-
nance and revenues. A computerized maintenance information system is best, but data
can be maintained by hand for smaller organizations. One should not expect imme-
diate results and should anticipate some initial variation. This delay could be caused
by the momentum and resistance to change that is inherent in every electromechani-
cal system, by delays in implementation through training and getting the word out to
all personnel, by some personnel who continue to do things the old way, by statisti-
cal variations within any equipment and facility, and by data accuracy.

If you operate electromechanical equipment and presently do not have a preventive
maintenance program, you are well advised to invest at least half of your maintenance
budget for the next three months in preventive maintenance tasks. You are probably
thinking: “How do I put money into preventive and still do the corrective mainte-
nance?” The answer is that you can’t spend the same money twice. At some point,
you have to stand back and decide to invest in preventive maintenance that will stop
the large number of failures and redirect attention toward doing the job right once.
This will probably cost more money initially as the investment is made. Like any other
investment, the return is expected to be much greater than the initial cost.

One other point: it is useless to develop a good inspection and preventive task sched-
ule if you don’t have the people to carry out that maintenance when required. Careful
attention should be paid to the Mean Time to Preventive Maintenance (MTPM). Many
people are familiar with Mean Time to Repair (MTTR), which is also the Mean Cor-
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rective Time (M ct). It is interesting that the term MTPM is not found in any text-
books the author has seen, or even in the author’s own previous writings, although
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the term M pt is in use. It is easier simply to use Mean Corrective Time (M ct) and
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Mean Preventive Time (M pt).
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PM Time/Number of preventive maintenance events calculates M pt. That equation
may be expressed in words as the sum of all preventive maintenance time divided by
the number of preventive activities done during that time. If, for example, five oil
changes and lube jobs on earthmovers took 1.5, 1, 1.5, 2, and 1.5 hours, the total is
7.5 hours, which divided by the five events equals an average of 1.5 hours each. A
few main points, however, should be emphasized here:

1. Mean Time Between Maintenance (MTBM) includes preventive and cor-
rective maintenance tasks.
2. Mean Maintenance Time is the weighted average of preventive and cor-
rective tasks and any other maintenance actions, including modifications
and performance improvements.
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3. Inherent Availability (A i ) considers only failure and M ct. Achieved avail-
ability (A a ) adds in PM, although in a perfect support environment. Oper-
ational Availability (A 0 ) includes all actions in a realistic environment.