Page 119 - Intro Predictive Maintenance

110 An Introduction to Predictive Maintenance

is between $30,000 and $60,000. Because of this, most predictive maintenance pro-
grams rely on third-party analysis of oil samples.

Recurring Cost
In addition to the labor cost associated with regular gathering of oil and grease
samples, simple lubricating oil analysis by a testing laboratory will range from about
$20 to $50 per sample. Standard analysis will normally include viscosity, flash point,
total insolubles, total acid number (TAN), total base number (TBN), fuel content, and
water content. More detailed analysis, using spectrographic, ferrographic, or wear par-
ticle techniques that include metal scans, particle distribution (size), and other data
can cost more than $150 per sample.

Accurate Samples
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 of the
oil’s condition. 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 also 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
with the objective of detecting large particles should be chosen. In a recirculating
system, samples should be drawn as the lubricant returns to the reservoir and before
any filtration occurs. Do not draw oil from the bottom of a sump where large quanti-
ties of material build up over time. Return lines are preferable to reservoir as the
sample source, but good reservoir samples can be obtained if careful, consistent prac-
tices are used. Even equipment with high levels of filtration can be effectively mon-
itored as long as samples are drawn before oil enters the filters. Sampling techniques
involve taking samples under uniform operating conditions. Samples should not be
taken more than 30 minutes after the equipment has been shut down.

Sample frequency is a function of the mean-time-to-failure (MTTF) from the onset of
an abnormal wear mode to catastrophic failure. For machines in critical service, sam-
pling every 25 hours of operation is appropriate. For most industrial equipment in con-
tinuous service, however, monthly sampling is adequate. The exception to monthly