SKF - Rolling Bearings

Lubrication
Temperature zones
Temperature zones differ from grease to
grease and can only be determined by functional
bearing testing.
Typical temperature zones for commonly
available NLGI 2 greases without EP additives,
which are normally used for rolling bearings,
are shown in diagram 2. Since the data for each
grease type is a summary of many greases of
more or less similar composition, the transitions
for each zone are not distinct, but fall
within a certain range.
Temperature zones for SKF greases are
shown in diagram 3. These temperature
zones are based on extensive tests conducted
in SKF laboratories.
The temperatures shown in diagrams 2
and 3 relate to the measured temperature
of the non-rotating ring in operation.
Consistency
Greases are divided into various consistency
classes in accordance with the National
Lubricating Grease Institute (NLGI). The
consistency of lubricating grease should not
change drastically when operated within its
specified temperature range. Greases that
soften at elevated temperatures may leak
from the bearing arrangement. Those that
stiffen at low temperatures may restrict
bearing rotation or limit oil bleeding.
Greases with a metallic soap thickener and
a consistency of 1, 2 or 3 on the NLGI scale are
typically used for rolling bearings. The most
common greases have a consistency of 2.
Lower consistency greases are preferred for
low temperature applications or to improve
pumpability. Greases with a consistency of 3
are recommended for bearing arrangements
with a vertical shaft where a baffle plate is
arranged beneath the bearing to retain the
grease in the bearing.
In applications subjected to vibration, the
grease is heavily worked as it is continuously
thrown back into the bearing by vibration. For
these applications, higher consistency greases
that do not change their consistency during
operation may help.
Greases thickened with polyurea can soften
or harden depending on the shear rate in the
application. Therefore, in applications with
vertical shafts, there is a danger that under
certain conditions, polyurea grease leaks,
irrespective of the operating temperature.
Base oil viscosity
The base oil viscosity for greases typically
used to lubricate rolling bearings lies between
15 and 500 mm 2 /s at 40 °C (105 °F). Greases
based on oils having higher viscosities than
500 mm 2 /s at 40 °C (105 °F) bleed oil so slowly
that the bearing may not be adequately lubricated.
Therefore, if a viscosity higher than
500 mm 2 /s at 40 °C (105 °F) is required because
of slow speeds, and for bearings operating
at extremely high or low temperatures,
extreme care must be taken. Under these conditions,
contact the SKF application engineering
service.
Base oil viscosity also governs the max imum
recommended speed for a bearing. The permissible
speed for grease is also influenced
by the shear strength of the grease which is
determined by the thickener. To indicate the
speed capability, grease manufacturers often
quote a speed factor, which can be calculated
using
A = n d m
where
A = speed factor [mm/min]
d m = bearing mean diameter [mm]
= 0,5 (d + D)
n = rotational speed [r/min]
If the speed factor A exceeds 70% of the recommended
limits († table 5, page 257), verify
the influence of the selected lubricant on
the speed limits that are provided under
Speeds († page 117) and check whether the
rotational speed is within these limits.
For extreme high-speed applications, contact
the SKF application engineering service.
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