Page 134 - Rollingbearings

B.5 Operating temperature and speed
Dissipated heat • Zone 1 – Boundary lubrication condi- SKF model of bearing
tion, in which only the asperities carry the
The heat dissipated is the sum of: load, and so friction between the moving friction
surfaces is high.
• heat dissipated by the shaft, housing and • Zone 2 – Mixed lubrication condition, in In the SKF model for calculating bearing
ambient airlow, e.g. cooling effects in arc- which a separating oil ilm carries part of friction, the total frictional moment, M, is
tic conditions the load, with fewer asperities in contact, derived from four sources:
• heat dissipated via the lubricant or lubri- and so friction decreases.
cation system • Zone 3 – Full ilm lubrication condition, M = M + M + M seal + M drag
rr
sl
in which the lubricant ilm carries the load,
but with increased viscous losses, and so
friction increases. where
Bearing friction, • Zone 4 – Full ilm lubrication with ther- M = the rolling frictional moment, and
rr
includes effects of lubricant starva-
mal and starvation effects, in which the
power loss and inlet shear heating and kinematic replen- M = the sliding frictional moment, and
tion and inlet shear heating [Nmm]
ishment reduction factors compensate
sl
starting torque partially for the viscous losses, and so fric- includes the effects of the quality of
lubrication conditions [Nmm]
tion evens off.
M seal = the frictional moment from integral
Bearing friction is not constant and depends seals [Nmm]
on certain tribological phenomena that Where bearings are itted with con-
occur in the lubricant ilm between the roll- tact seals, the frictional losses from
the seals may exceed those gener-
ing elements, raceways and cages. M drag = the frictional moment from drag
Operating temperature and speed Calculating values for these four sources of
Friction changes as a function of speed, in
ated in the bearing.
a bearing with a given lubricant, are shown
in diagram 3. Four zones are distinguishable:
losses, churning, splashing, etc., in
an oil bath [Nmm]

friction is complex. Therefore, we recom-
mend using the SKF Bearing Calculator
(skf.com/bearingcalculator).
For detailed information on the calcula-

the frictional moment
(skf.com/go/17000-B5).
When the total frictional moment, M, of
B.5 Bearing frictional moment as a function of speed Diagram 3 tions, refer to The SKF model for calculating
the bearing is known, you can calculate the
Frictional moment
bearing frictional power loss using
P loss = 1,05 × 10 –4 M n

where
P loss = bearing frictional power loss [W]
M = total frictional moment [Nmm]
n = rotational speed [r/min]

1 2 3 4
Speed

Boundary Mixed Full ilm Full ilm lubrication
lubrication lubrication lubrication with thermal and
starvation effects
= Load
= Load share carried by asperities
= Load share carried by lubricant ilm

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