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introduction to the machine. While in the machine,
you must use effective seals to keep out contaminants
or utilize a strategy of grease purging to keep the
contaminants at bay. Of course, this only works if the
grease you use to purge is confirmed to be clean.
In addition to particles and the damage they
can do, you must consider moisture in greases as
well. When high moisture is present in oils, phase
separation or cloudiness allows you to see these
very high levels. However, the same clarity seen in
dry oils is not present in greases. As moisture levels
increase in greases, these high levels are not always
visually obvious.
As Jim Fitch pointed out in a recent Machinery
Lubrication article, "… it takes only a small amount
of water (less than 500 parts per million) to substan-
tially shorten the service life of rolling-element
bearings." Using this as a guide, we find that out of
22 greases tested in this same cleanliness study, only
one met this criteria for dryness. Most greases showed
moisture levels higher than 1,000 parts per million
(ppm), with the wettest greases in the study having
more than 10,000 ppm.
Not all greases are equally affected by moisture,
and the grease types that showed the highest level
in this study have been cited to be less sensitive to
the effects of higher moisture levels. However, it is
interesting to note that one grease saw its moisture
concentration cut in half (from nearly 15,000 ppm
to almost 9,000 ppm) after being placed in service
within the machine.
Even so, it may not be a good strategy to use your
machines to dry out greases. Perhaps the first steps
would be understanding the moisture levels your new
and stored greases are starting with, then working
with your suppliers and improving your storage
methods to reduce the initial moisture levels as low
as possible. By doing so, you stand to gain substantial
increases in bearing life and grease performance.
Perhaps it's time to take the focus on lubricant
cleanliness, which has been applied so effectively
to oil, and expand it to include grease. Many of the
tools that work for oil, such as filtration and mois-
ture removal technologies, aren't readily applicable
to grease. However, there are several things you can
do to improve grease cleanliness, starting with taking
the required measurements to determine just how
clean (or dirty) your current grease supply is.
While a continued focus on oil cleanliness
remains important, the next logical step in making
substantial gains in equipment reliability and life
extension may be achieved by taking a closer look at
the contaminants hiding in your greases.
ML
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the United States and other countries.
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