SKF - Rolling Bearings

Lubrication
Lubricating greases
Lubricating greases consist of a mineral or
synthetic oil combined with a thickener. The
thickeners are usually metallic soaps. However,
other thickeners like polyurea are also
available, and can be used in applications
where there are, for example, high temperatures.
Additives are also included to enhance
certain properties of the grease. The consistency
of the grease depends largely on the type
and concentration of the thickener, and on the
operating temperature of the application.
When selecting a grease, the most important
factors to be considered are:
• temperature range
• consistency
• base oil viscosity
• protection against corrosion
• load carrying ability
• miscibility
Temperature range – the SKF traffic
light concept
The temperature range over which a grease
can be used depends largely on the type of
base oil and thickener as well as the additives.
The relevant temperatures are schematically
illustrated in diagram 1 in the form of a
“double traffic light”.
The extreme temperature limits, low and
high, are well-defined.
• The low temperature limit (LTL), the lowest
temperature at which a grease enables the
bearing to be started up without difficulty,
is largely determined by the base oil and its
viscosity.
• The high temperature limit (HTL) is determined
by the type of thickener and its
dropping point. The dropping point is the
temperature at which a grease loses its
consistency and becomes a fluid.
As indicated in diagram 1 by the red zones,
SKF does not recommend using a grease
above or below its temperature limits to lubricate
bearings. Although grease suppliers indicate
the specific values for the low and high
temperature limits in their product literature,
the temperatures for reliable operation are
indicated by the SKF values for the following
limits:
• low temperature performance limit (LTPL)
• high temperature performance limit (HTPL)
Within these two limits, which is indicated by
the green zone in diagram 1, the grease fulfils
its function reliably and the relubrication interval
or grease life can be determined accurately.
Since the definition of the high temperature
performance limit is not standardized internationally,
care must be taken when interpreting
supplier data.
At temperatures above the high temperature
performance limit (HTPL), grease degrades
with increasing rapidity and the byproducts
of oxidation have a detrimental
effect on the lubricant. Therefore, temperatures
in the amber zone, between the high
temperature performance limit (HTPL) and
the high temperature limit (HTL), should only
be allowed to occur for very short periods and
not longer than a few hours.
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