Page 120 - Intro Predictive Maintenance

Predictive Maintenance Techniques 111

sampling is machines with extreme loads. In this instance, weekly sampling is
recommended.

Understanding Results
Understanding the meaning of analysis results is perhaps the most serious limiting
factor. Results are usually expressed in terms that are totally alien to plant engineers
or technicians. Therefore, it is difficult for them to understand the true meaning, in
terms of oil or machine condition. A good background in quantitative and qualitative
chemistry is beneficial. At a minimum, plant staff will require training in basic chem-
istry and specific instruction on interpreting tribology results.

6.4 VISUAL INSPECTIONS
Visual inspection was the first method used for predictive maintenance. Almost from
the beginning of the Industrial Revolution, maintenance technicians performed daily
“walkdowns” of critical production and manufacturing systems in an attempt to iden-
tify potential failures or maintenance-related problems that could impact reliability,
product quality, and production costs. A visual inspection is still a viable predictive
maintenance tool and should be included in all total-plant maintenance management
programs.

6.5 ULTRASONICS
Ultrasonics, like vibration analysis, is a subset of noise analysis. The only difference
in the two techniques is the frequency band they monitor. In the case of vibration
analysis, the monitored range is between 1 Hertz (Hz) and 30,000Hz; ultrasonics mon-
itors noise frequencies above 30,000Hz. These higher frequencies are useful for select
applications, such as detecting leaks that generally create high-frequency noise caused
by the expansion or compression of air, gases, or liquids as they flow through the
orifice, or a leak in either pressure or vacuum vessels. These higher frequencies are
also useful in measuring the ambient noise levels in various areas of the plant.

As it is being applied as part of a predictive maintenance program, many companies
are attempting to replace what is perceived as an expensive tool (i.e., vibration analy-
sis) with ultrasonics. For example, many plants are using ultrasonic meters to monitor
the health of rolling-element bearings in the belief that this technology will provide
accurate results. Unfortunately, this perception is invalid. Because this technology is
limited to a broadband (i.e., 30kHz to 1MHz), ultrasonics does not provide the ability
to diagnosis incipient bearing or machine problems. It certainly cannot define the root-
cause of abnormal noise levels generated by either bearings or other machine-train
components.

As part of a comprehensive predictive maintenance program, ultrasonics should be
limited to the detection of abnormally high ambient noise levels and leaks. Attempt-
ing to replace vibration monitoring with ultrasonics simply will not work.