Page 188 - Rollingbearings

B.7 Bearing execution
Selecting preload If the shaft temperature in operation is Preloading with springs
higher than the housing temperature, the
Depending on the application, there may be preload, which was adjusted at ambient By preloading bearings it is possible to
a need to preload a bearing arrangement temperature during mounting, will change reduce the noise in, for example, small elec-
For example, if a high degree of stiffness or Since thermal growth of a shaft makes it tric motors or similar applications In this
positional control is required then preload larger both in the axial and in the radial example, the bearing arrangement com-
may be suitable Similarly, where there is a direction, the back-to-back arrangements prises a preloaded single row deep groove
very light or no external load on the bearing are less sensitive to thermal effects than the ball bearing at each end of the shaft (ig. 6)
in operation then preload may be required to face-to-face arrangements The simplest method of applying preload is
ensure a minimum load When adjusting preload in a bearing sys- to use a wave spring The spring acts on the
Applying the preload is typically done by tem, it is important that the established outer ring of one of the two bearings This
measuring a force, sometimes a displace- preload value is attained with the least pos- outer ring must be able to be axially
ment over a distance or path, or by measur- sible variation To reduce variation when displaced
ing the frictional torque during mounting mounting tapered roller bearings, the shaft The preload force remains practically con-
Empirical preload values can be obtained should be turned several times to ensure stant, even when there is axial displacement
from proven designs and can be applied to that the rollers are in correct contact with of the bearing as a result of thermal
similar designs For new designs, SKF rec- the guide lange of the inner ring elongation
ommends calculating the appropriate The requisite preload force can be esti-
preload range by using SKF SimPro Quick or mated using
SKF SimPro Expert and then checking it by
testing in the application The agreement F = k d
between the calculation and the actual
application depends on how closely the esti- where
mated operating temperature and elastic F = preload force [kN]
behaviour of the associated components k = a factor, described in the following text
– most importantly the housing – coincide d = bearing bore diameter [mm]
with the actual conditions in operation In
this context, the effects of start-up at low
ambient temperature must be included in Fig. 4 Fig. 5
Bearing execution Considerations for preload Back-to-back arrangement, tapered roller Face-to-face arrangement, tapered roller
the testing

bearings
bearings
Depending on the bearing type, preload may

cylindrical roller bearings, for example, can
B.7 be either radial or axial Super-precision L L
only be preloaded radially because of their
design, while angular contact ball bearings
or tapered roller bearings can only be
preloaded axially
Single tapered roller bearings or angular
contact ball bearings are generally mounted
together with a second bearing of the same
type and size in a back-to-back (load lines
diverge, ig. 4) or face-to-face (load lines Fig. 6
converge, ig. 5) arrangement The same is Preloading with springs
true for single row angular contact ball
bearings
The distance L between the pressure cen-
tres is longer when the bearings are
arranged back -to -back compared to bear-
ings that are arranged face -to-face The
back -to -back arrangement can accommo-
date larger tilting moments

186