SKF - Rolling Bearings

Lubrication
Grease lubrication
The majority of rolling bearings are grease
lubricated. Compared to oil, the advantage
is that grease is more easily retained in the
bearing arrangement, particularly where
shafts are inclined or vertical. Grease also
contributes to sealing the arrangement
against solid and liquid contaminants as well
as moisture.
The terms used to categorize speed ranges,
temperatures and loads for grease lubrication
can differ from those used for bearings. Terms
typically used for grease lubricated bearings
are defined in the following tables:
• table 1: speed ranges for grease lubricated
radial bearings
• table 2: grease temperature
• table 3: load ranges for greases
The quantity of grease applied to a bearing
depends on the application. Too little grease
leads to metal-to-metal contact and premature
bearing failure. Excessive amounts of
grease cause the operating temperature within
the bearing to rise rapidly, particularly when
running at high speeds. Bearings with seals or
shields (capped bearings) are filled by SKF with
a sufficient amount of grease to provide long
bearing service life.
Depending on the speed range († table 1),
SKF recommends the following grease fill
percentages for bearings:
• 100% for slow speeds
• 30–50% for medium to high speeds
The free volume in the housing should be
partly filled with grease. Before operating at
full speed, the excess grease in the bearing
must be given time to settle or escape during
a running-in period. At the end of the runningin
period, the operating temperature drops
considerably, indicating that the grease has
been distributed in the bearing arrangement.
In applications where bearings operate at
very slow speeds and good protection against
contaminants and corrosion is required, SKF
recommends filling the housing up to 90% with
grease.
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