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Chapter 19 Risk management 589
Condition-based
maintenance
Condition-based maintenance attempts to perform maintenance only when the facilities
require it. For example, continuous process equipment, such as that used in coating photographic
paper, is run for long periods in order to achieve the high utilization necessary for
cost-effective production. Stopping the machine to change, say, a bearing when it is not strictly
necessary to do so would take it out of action for long periods and reduce its utilization.
Here condition-based maintenance might involve continuously monitoring the vibrations,
for example, or some other characteristic of the line. The results of this monitoring would
then be used to decide whether the line should be stopped and the bearings replaced.
Mixed maintenance strategies
Each approach to maintaining facilities is appropriate for different circumstances. RTB is
often used where repair is relatively straightforward (so the consequence of failure is small),
where regular maintenance is very costly, or where failure is not at all predictable (failure is
just as likely to occur after repair as before). PM is used where the cost of unplanned failure
is high and where failure is not totally random. CBM is used where the maintenance activity
is expensive, either because of the cost of providing the maintenance itself, or because of the
disruption which the maintenance activity causes to the operation. Most operations adopt a
mixture of these approaches. Even an automobile uses all three approaches (see Fig. 19.8).
Light bulbs and fuses are normally replaced only when they fail. Engine oil is subject to
preventive maintenance at a regular service. Finally, most drivers also monitor the condition
of the auto, for example by measuring the amount of tread on the tyre.
Breakdown versus preventive maintenance
The balance between preventive and breakdown maintenance is usually set to minimize
the total cost of breakdown. Infrequent preventive maintenance will cost little to provide but
will result in a high likelihood (and therefore cost) of breakdown maintenance. Conversely,
very frequent preventive maintenance will be expensive to provide but will reduce the cost
of having to provide breakdown maintenance (see Fig. 19.9a). The total cost of maintenance
appears to minimize at an ‘optimum’ level of preventive maintenance. However, the cost
of providing preventive maintenance may not increase quite so steeply as indicated in
Figure 19.9(a). The curve assumes that it is carried out by a separate set of people (skilled
maintenance staff ) from the ‘operators’ of the facilities. Furthermore, every time preventive
maintenance takes place, the facilities cannot be used productively. This is why the slope
of the curve increases, because the maintenance episodes start to interfere with the normal
working of the operation. But in many operations some preventive maintenance can be
performed by the operators themselves (which reduces the cost of providing it) and at times
Figure 19.8 A mixture of maintenance approaches is often used – in a car, for example