Page 55 - Intro Predictive Maintenance

46 An Introduction to Predictive Maintenance

MAINTENANCE

IMPROVEMENT PREVENTIVE CORRECTIVE
(MI) (PM) (CM)

Reliability-driven Equipment-driven Predictive Time-driven Event-driven
Modification Self-scheduled Statistical analysis Periodic Breakdowns
Trends
Emergency
Retrofit Machine-cued Vibration monitoring Fixed intervals Remedial
Redesign Control limits Tribology Hard time limits
Change order When deficient Thermography Specific time Repairs
As requred Rebuilds
Ultrasonics
Other NDT
Figure 3–1 Structure of maintenance.
lubrication could be reduced by using permanently lubricated, long-life bearings. If
that is not practical, at least an automatic oiler could be installed. A major selling point
of new automobiles is the elimination of ignition points that require replacement and
adjustment, introduction of self-adjusting brake shoes and clutches, and extension of
oil-change intervals.

Corrective Maintenance
The little finger in our analogy to a human hand represents corrective maintenance
(i.e., emergency, repair, remedial, unscheduled). At present, most maintenance is cor-
rective. Repairs will always be needed. Better maintenance improvement and pre-
ventive maintenance, however, can reduce the need for emergency corrections. A shaft
that is obviously broken into pieces is relatively easy to maintain because little human
decision is involved. Troubleshooting and diagnostic fault detection and isolation
are major time consumers in maintenance. When the problem is obvious, it can
usually be corrected easily. Intermittent failures and hidden defects are more time-
consuming, but with diagnostics, the causes can be isolated and then corrected.
From a preventive maintenance perspective, the problems and causes that result in
failures provide the targets for elimination by viable preventive maintenance. The
challenge is to detect incipient problems before they lead to total failures and to
correct the defects at the lowest possible cost. That leads us to the middle three
fingers—the branches of preventive maintenance.

Preventive Maintenance
As the name implies, preventive maintenance tasks are intended to prevent unsched-
uled downtime and premature equipment damage that would result in corrective or