in some way, but in each case, the effects are different.
They may affect operations. They may also affect product
quality, customer service, safety or the environment. They
will all take time and cost money to repair.
It is these consequences which most strongly influence
the extent to which we try to prevent each failure. If a failure mode has serious consequences, we are likely to go to
great lengths to try to prevent it. If it has little or no effect,
then we may decide to undertake no preventive action.
In other words, the consequences of failures are far more
important than their technical characteristics.
For example, one failure which could affect the pump
shown in Figure 1 is "bearing seizes due to normal wear
and tear". Assuming that it takes 4 hours to replace a failed
bearing, and that the unanticipated failure of the bearing
only comes to the attention of the operators when the level
in the tank drops to the low level switch, the tank only contains a 2.5 hour supply of water, so it would run dry and
remain empty for 1.5 hours while the bearing is repaired.
One condition-based task which could apply to this
bearing is to monitor vibration levels using a vibration
analyser. If incipient failure is detected, the first priority of
the operators would be to fill the tank before the bearing
seizes, thus giving themselves 5 hours to do a 4 hour job.
This in turn enables them to avoid the consequences of an
empty tank (and also avoid possible secondary damage to
the pump). The task does not "save" the bearing – that is
doomed whatever happens.
This example demonstrates that the main reason for
doing any kind of proactive maintenance is to avoid, reduce
or eliminate the consequences of failure. A formal review
of failure consequences focuses attention on maintenance
tasks that have most effect on the performance of the organ-
isation, and diverts energy away from those which have
little or no effect. This helps ensure that whatever is spent
on maintenance is spent where it will do the most good.