A lubricating grease is usually a mixture of 85 to 90 percent
mineral oil or synthetic oil together with a thickener. In a
majority of all greases, the thickener is a metallic soap. One
example is lithium stearate for lithium soap.
The function of the thickener, the metallic soap, is to hold
the lubricating oil in a semi-liquid state for easier handling.
When there is rise in temperature, the oil bleeds out from the
thickener and functions as a lubricating agent. When the
temperature drops again, the thickener soaks up the oil again
to become semi-solid once more.
The type of grease chosen for a particular bearing lubrication
application must therefore be chosen very carefully. High
temperature grease used in low temperature applications may
cause the bearings to seize due to lack of lubrication because
the oil does not bleed out. The common types of grease in use
for rolling contact bearings are the calcium, sodium and
lithium greases.
Calcium Soap Greases
These do not dissolve in water. They are recommended for
installations exposed to water at temperatures below 60 degree C.
They offer good protection against salt water in marine
environments.
Sodium Soap Greases
Also called soda greases, they may be utilized over a wide range
of temperatures up to 120 degree C. However, if too much water
penetrates into the bearings, there is a risk that the grease
will be washed out and the lubricating properties become
deteriorated.
Lithium Soap Greases
These have excellent resistance to high temperatures. They can
also be used over a wider range of temperatures from -50 to 150
degree C. They are not water soluble.
Additives are also added to some greases to improve their
properties. Some examples of these additives are anti-rust,
anti-oxidants, extreme pressure additives, and stabilizers.
Although it is very convenient to use grease for lubrication of
rolling contact bearings, where some bearings come pre-packed
with grease ready for use, grease lubrication becomes unsuitable
if the operating temperature becomes high.
The high temperature may be because of high ambient temperature
environment, or because the heat evolved in the bearing due to
friction from high speed or heavy loading. Sometimes the use of
oil becomes more logical if the lubricating intervals for grease
lubrication becomes too short, perhaps due to leakage from seals.
In oil lubrication, the heat generated from the bearings are
able to be transfered to a larger volume of oil which in turn
can be pumped through heat exchangers for cooling. In this way,
the oil functions both as a lubricating agent as well as a
cooling agent.
Oils can also have additives to improve their properties. Some
examples are anti-oxidants, corrosion protection additives,
anti-foaming additives, surface tension additives, wetting agents,
and extreme pressure additives. These additives are put in
according to the application of the oil.
Compared to greases, oils can enable bearings to be operated at
a wider range of temperatures. However, there are limits to this,
especially at higher temperatures. At high temperatures of 90
degree C and above, mineral oils oxidize rapidly and they lose
their properties to lubricate. Synthetic oils are increasingly
being used for higher temperature applications.
Which to use? Oil or grease? The choice is yours!
Until next time...
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