Timken Super Precision Bearings for Machine Tool Applications

BALL BEARINGS
Preloading of precision ball bearings to a predetermined axial load
for “universal” mounting is accomplished by grinding a certain
amount of stock off faces of the inner and outer rings so that before
mounting the bearing, faces on the abutting side are offset an
amount equal to the deflection under “preload.” When mounted,
these faces are clamped together, the opposite bearing faces
become flush and the bearing parts are subjected to compressive
forces, bringing the balls into contact with their respective raceways
to take up the initial clearances of the bearings. Thus, the preload
built into the bearings is automatically obtained. The condition of a
preloaded ball bearing is similar to that of one in operation under
axial load. This initial axial load serves to decrease the axial and
radial deflections when subsequent operational loads are imposed
on the bearing assembly.
Bearings are preloaded no more than necessary. Excessive preload
adds little to the rigidity of the spindle, but appreciably reduces the
range of operating speeds by causing bearings to run hot at higher
speeds. To meet conditions of speed, mounting arrangement and
maximum rigidity consistent with low operating temperatures,
Timken precision ball bearings are designed and produced with
preloads varying from light to heavy and, in some instances, with
negative preload.
In many cases, the amount of bearing preload is a trade-off between
having the desired degree of rigidity and reducing any adverse effect
preloading has on the equipment. If the operating speed is high, a
heavy preload can lead to excessively high operating temperatures,
resulting in shortened bearing life. For these reasons, three classes
of ball bearing preloads are most commonly used – light, medium and
heavy. In certain applications, such as high-speed motorized router
spindles, specially preloaded, super precision ball bearings are
required. Such bearings are “zero” preloaded – that is, the faces of
the inner and outer rings are ground flush under negligible load.
The light, medium and heavy standard preload values for Timken
super precision angular contact ball bearings and for both high
and low contact angles are located with the dimension tables in
Section C.
Axial deflection curves of various preload conditions for duplex
pairs of 15 degree contact angle super precision ball bearings are
shown in Figure 96 and the radial deflection curves for the same
bearings are shown in Figure 97.
Increasing Axial Deflection
Increasing Radial Deflection
A No Preload
B Light Preload
C Medium Preload
D Heavy Preload
Axial Load
Increasing Axial Load
ENGINEERING
Fig. 96. Effect of most commonly used preloads on axial deflection.
Radial Load
Increasing Radial Load
A No Preload
B Light Preload
C Medium Preload
D Heavy Preload
Fig. 97. Effect of most commonly used preloads on radial deflection.
A
B
C
D
A
B
C
D
A
Timken application engineering expertise is only a phone call away. Highly skilled technical assistance can be
reached by calling (US and Canada): 1-800-223-1954. Outside this region, call 330-438-3000 or visit our website at:
www.timken.com
TIMKEN MACHINE TOOL CATALOG 103