Page 424 - Intro Predictive Maintenance

World-Class Maintenance 415

• Repair defects.
• If it works, don’t fix it.

Condition Monitoring
Statistics and probability theory are the basis for condition-monitoring maintenance.
Trend detection through data analysis often rewards the analyst with insight into the
causes of failure and preventive actions that will help avoid future failures. For
example, stadium lights burn out within a narrow time range. If 10 percent of the lights
have burned out, it may be accurately assumed that the rest will fail soon and should,
most effectively, be replaced as a group rather than individually.

Scheduled
Scheduled, fixed-interval preventive maintenance tasks should generally be used only
if there is opportunity for reducing failures that cannot be detected in advance, or if
dictated by production requirements. The distinction should be drawn between fixed-
interval maintenance and fixed-interval inspection that may detect a threshold condi-
tion and initiate condition-monitoring tasks. Examples of fixed-interval tasks include
3,000-mile oil changes and 48,000-mile spark plug changes on a car, whether it needs
the changes or not. This approach may be wasteful because all equipment and their
operating environments are not alike. What is right for one situation may not be right
for another.

The five-finger approach to maintenance emphasizes eliminating and reducing main-
tenance need wherever possible, inspecting and detecting pending failures before they
happen, repairing defects, monitoring performance conditions and failure causes, and
accessing equipment on a fixed-interval basis only if no better means exist.

18.9.3 Maintenance Improvement
Picture these divisions as the five fingers on your hand. Maintenance improvement
efforts to reduce or eliminate the need for maintenance are like the thumb, the first
and most valuable digit. We are often so involved in maintaining that we forget to
plan and eliminate the need at its source. Reliability engineering efforts should empha-
size elimination of failures that require maintenance. This is an opportunity to pre-act
instead of react.

For example, many equipment failures occur at inboard bearings that are located in
dark, dirty, inaccessible locations. The oiler does not lubricate inaccessible bearings
as often as he or she lubricates those that are easy to reach. This is a natural tendency.
One can consider reducing the need for lubrication by using permanently lubricated,
long-life bearings. If that is not practical, at least an automatic oiler could be installed.