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The function of a lubricant in a rolling
element bearing is to prevent metal to metal contact between components,
prevent wear and protect against corrosion. Two methods of lubrication
are normally employed grease and oil. In the case of SRB Split Bearings
grease lubrication is most often employed.
Grease Lubrication:
Greases can be used to lubricate SRB
split roller bearings under most normal conditions. Grease is the preferred
method of lubrication because it can be more easily retained within the
bearing enclosure and housing, the latter simplifying sealing arrangements.
Greases are essentially oils thickened usually with a metal soap, a third
ingredient are additives such as rust inhibitors, or extra pressure additives.
The oils employed may be mineral or synthetic depending upon the application.
SRB bearings are heat treated to retain
dimensional stability up to 140ºC. At temperatures up to 100ºC,
standard high quality greases may be used. We suggest good quality lithium
soap or complex based greases having extra pressure additives and a penetration
number of 3. It is important to note that all values given in this catalogue
for axial capacity assume the use of a grease with extra pressure (EP)
additives. If EP additives are not present then axial capacity is reduced
by 50%
At temperatures exceeding 100ºC
care must be taken to ensure that the correct thickener and viscosity
of base oil are selected. The performance of grease at such temperatures
is dependent on a stable thickener and the temperature/viscosity ratio
of the base oil. A stable base oil and soap thickener are important as
is the ability of the oil to offer adequate viscosity at an elevated temperature.
In cases of water splash, calcium soap
based greases may be used, these are particularly resistant to water wash
out.
Care should be taken when mixing greases
with different soap thickeners and base oil types. Please contact SRB
Technical Services for further advice.
Re-lubrication:
The re-lubrication intervals will be
dependent on the prevailing operating conditions.
Greases age and oxidise due to a number
of considerations these include load, speed, temperature, cleanliness,
presence of water and even airflow through the bearing.
For retained type bearings, initial re-lubrication
intervals for guidance purposes would be 2 – 4 weeks with 3 –
6 mls added. For expansion type bearings, initial re-lubrication intervals
would be 3 – 4 months with 3 – 6 mls added. More accurate intervals
and quantities should be established from observations taken during bearing
operation. If re-lubrication can be carried out whilst the bearing is
in operation, this will allow for even distribution of the grease. This
means of re-lubrication should only be undertaken if it is safe to do
so.
Bearing Lubrication
SRB split roller bearings are rarely
lubricated with oil, in cases where oil is selected as a means of lubrication,
then special consideration must be given to the bearing housing design
and sealing.
Oil Sump: The oil sits in the bearing
housing at a level approximately ½ way up the bottom dead centre
rolling element. Oil circulation around the bearing is then provided via
the bearing rotation agitating the oil sump. It is very important to provide
a sufficiently dimensioned oil sump as too small a volume will result
in increased frequency of oil change and elevated operating temperatures.
Oil Mist: An oil/air mist is injected
into the bearing via nozzles, normally a total oil loss system, this provides
extremely high speed capability at high cost. Oil Circulation: Oil is
circulated into the bearing housing assembly from an external oil sump.
This allows the oil to be cooled and filtered, additionally an external
oil sump normally allows for a higher volume of oil. Whilst being a more
optimum solution, specialist housing designs must be provided. There is
also a cost and space requirement to this system.
For further advice on oil selection and
oil lubrication systems please consult SRB Technical Services.
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