Page 215 - An Introduction To Predictive Maintenance

Tribology 205

Particle Count

Particle count tests are important to anticipating potential system or machine prob-
lems. This is especially true in hydraulic systems. The particle count analysis made
as a part of a normal lube oil analysis is different from wear particle analysis. In this
test, high particle counts indicate that machinery may be wearing abnormally or that
failures may occur because of temporarily or permanently blocked orifices. No attempt
is made to determine the wear patterns, size, and other factors that would identify the
failure mode within the machine.

Spectrographic Analysis
Spectrographic analysis allows accurate, rapid measurements of many of the
elements present in lubricating oil. These elements are generally classified as wear
metals, contaminants, or additives. Some elements can be listed in more than
one of these classifications. Standard lubricating oil analysis does not attempt to deter-
mine the specific failure modes of developing machine-train problems. Therefore,
additional techniques must be used as part of a comprehensive predictive maintenance
program.

9.1.2 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. Parti-
cles in the lubricant 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. Wear particle analysis is normally conducted in two
stages.

The first method used for wear particle analysis is routine monitoring and trending of
the solids content of machine lubricant. In simple terms, the quantity, composition,
and size of particulate matter in the lubricating oil indicates the machine’s mechani-
cal condition. A normal machine will contain low levels of solids with a size less than
10 microns. As the machine’s condition degrades, the number and size of particulate
matter increases. The second wear particle method involves analysis of the particu-
late matter in each lubricating oil sample.

Types of Wear
Five basic types of wear can be identified according to the classification of particles:
rubbing wear, cutting wear, rolling fatigue wear, combined rolling and sliding wear,
and severe sliding wear. Only rubbing wear and early rolling fatigue mechanisms gen-
erate particles that are predominantly less than 15 microns in size.