Page 216 - Intro Predictive Maintenance

Tribology 207

9.1.3 Ferrography

This technique is similar to spectrography, but there are two major exceptions. First,
ferrography separates particulate contamination by using a magnetic field rather than
by burning a sample as in spectrographic analysis. Because a magnetic field is used
to separate contaminants, this technique is primarily limited to ferrous or magnetic
particles.

The second difference is that particulate contamination larger than 10 microns can be
separated and analyzed. Normal ferrographic analysis will capture particles up to 100
microns in size and provides a better representation of the total oil contamination than
spectrographic techniques.

9.1.4 Oil Analysis Costs and Uses
There are three major limitations with using tribology analysis in a predictive main-
tenance program: equipment costs, acquiring accurate oil samples, and interpretation
of data.

The capital cost of spectrographic analysis instrumentation is normally too high to
justify in-plant testing. The typical cost for a microprocessor-based spectrographic
system is between $30,000 and $60,000; therefore, most predictive maintenance pro-
grams rely on third-party analysis of oil samples.

Simple lubricating oil analysis by a testing laboratory will range from about $20 to
$50 per sample. Standard analysis normally includes viscosity, flash point, total in-
solubles, total acid number (TAN), total base number (TBN), fuel content, and water
content. More detailed analysis, using spectrographic or ferrographic techniques, that
includes metal scans, particle distribution (size), and other data can cost more than
$150 per sample.

A more severe limiting factor with any method of oil analysis is acquiring accurate
samples of the true lubricating oil inventory in a machine. Sampling is not a matter
of opening a port somewhere in the oil line and catching a pint sample. Extreme care
must be taken to acquire samples that truly represent the lubricant that will pass
through the machine’s bearings. One recent example is an attempt to acquire oil
samples from a bullgear compressor. The lubricating oil filter had a sample port on
the clean (i.e., downstream) side; however, comparison of samples taken at this point
and one taken directly from the compressor’s oil reservoir indicated that more conta-
minants existed downstream from the filter than in the reservoir. Which location actu-
ally represented the oil’s condition? Neither sample was truly representative. The oil
filter had removed most of the suspended solids (i.e., metals and other insolubles) and
was therefore not representative of the actual condition. The reservoir sample was not
representative because most of the suspended solids had settled out in the sump.

Proper methods and frequency of sampling lubricating oil are critical to all predictive
maintenance techniques that use lubricant samples. Sample points that are consistent