Page 118 - Intro Predictive Maintenance

Predictive Maintenance Techniques 109

and types of lubricants required to maintain plant equipment. Elimination of unneces-
sary duplication can reduce required inventory levels and therefore maintenance costs.

As a predictive maintenance tool, lubricating oil analysis can be used to schedule oil
change intervals based on the actual condition of the oil. In midsize to large plants, a
reduction in the number of oil changes can amount to a considerable annual reduc-
tion in maintenance costs. Relatively inexpensive sampling and testing can show when
the oil in a machine has reached a point that warrants change.

6.3.2 Wear Particle Analysis
Wear particle analysis is related to oil analysis only in that the particles to be studied
are collected by drawing a sample of lubricating oil. Whereas lubricating oil analysis
determines the actual condition of the oil sample, wear particle analysis provides direct
information about the wearing condition of the machine-train. Particles in the lubri-
cant of a machine can provide significant information about the machine’s condition.
This information is derived from the study of particle shape, composition, size, and
quantity.

Analysis of Particulate Matter
Two methods are used to prepare samples of wear particles. The first method, called
spectroscopy or spectrographic analysis, uses graduated filters to separate solids into
sizes. Normal spectrographic analysis is limited to particulate contamination with a
size of 10 microns or less. Larger contaminants are ignored. This fact can limit the
benefits that can be derived from the technique. The second method, called ferro-
graphic analysis, separates wear particles using a magnet. Obviously, the limitation
to this approach is that only magnetic particles are removed for analysis. Nonmag-
netic materials, such as copper, aluminum, and so on that make up many of the wear
materials in typical machinery are therefore excluded from the sample.

Wear particle analysis is an excellent failure analysis tool and can be used to under-
stand the root-cause of catastrophic failures. The unique wear patterns observed on
failed parts, as well as those contained in the oil reservoir, provide a positive means
of isolating the failure mode.

6.3.3 Limitations of Tribology
Three major limitations are associated with using tribology analysis in a predictive
maintenance program: equipment costs, acquiring accurate oil samples, and interpre-
tation of data.

Capital Cost
The capital cost of spectrographic analysis instrumentation is normally too high to
justify in-plant testing. Typical cost for a microprocessor-based spectrographic system