Page 270 - Intro Predictive Maintenance

Visual Inspection 261

“Check V-belt tension and correct as necessary.” How should the technician check
tension? Where should he or she measure? What tension levels are acceptable?

Effective visual inspection must be quantifiable, and all personnel must universally
apply the methods used. The specific methods will vary from simple visual inspec-
tions, such as looking for leaks or reading a gauge, to requiring test instruments, such
as vacuum gauges, dial indicators, and so on. In all cases, the methods used must
clearly define exactly how the inspection is to be performed, the exact location that
measurements or inspection is to be made, criteria for evaluation, and the acceptable
range of performance.
Generally, visual inspection can be broken into two major classifications: those that
can be conducted using only human senses and those that require the use of sensors
or instrumentation.

12.1.1 Human Senses
Humans have a great capability for sensing unusual sights, sounds, smells, tastes,
vibrations, and touches. Every maintenance manager should make a concerted effort
to increase the sensitivity of his or her own and that of the personnel’s human senses.
Experience is generally the best teacher. Often, however, we experience things without
knowing what we are experiencing. A few hours of training in what to look for could
have high payoff.
Human senses are able to detect large differences but are generally not sensitive to
small changes. Time tends to have a dulling effect. Have you ever tried to determine
if one color was the same as another without having a sample of each to compare side
by side? If you have, you will understand the need for standards. A standard is any
example that can be compared to the existing situation as a measurement. Quantita-
tive specifications, photographs, recordings, and actual samples should be provided.
The critical parameters should be clearly marked on the samples with display as to
what is good and what is bad. It is best if judgments can be reduced to “go/no-go.”
Figure 12–1 shows such a standard.

As the reliability-based preventive maintenance program develops, samples should be
collected to help pinpoint with maximum accuracy how much wear can take place
before problems will occur. A display where craftspeople gather can be effective. A
framed four-foot by four-foot pegboard works well because shafts, bearings, gears,
and other components can be easily wired to it or hung on hooks for display. An effec-
tive, but little used, display area where notices can be posted is above the urinal or on
the inside of the toilet stall door. Those are frequently viewed locations and allow
people to make dual use of their time.

12.1.2 Sensors
Because humans are not continually alert or sensitive to small changes and cannot get
inside small spaces, especially when operating, it is necessary to use sensors that