Page 336 - Analysis and Design of Machine Elements

Analysis and Design of Machine Elements
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11.4.4.4 Axial Adjustment
Bearings require axial adjustment to obtain necessary running clearance without
impairing design life. The amount of clearance to be maintained is determined by
experience. In most cases, a small axial clearance must be left to allow shaft expansion
during operation.
Figure 11.13a illustrates typical axial adjustment methods by shims between bearing
cover and the housing. Usually stainless-steel shims are preferred, as brass shims wear
more easily and abrasive particles produced may contaminate bearings. Shims should
be used only against a nonrotating ring. Figure 11.14a illustrates axial adjustment by a
locknut against a rotating ring. When it is required to fine-tune the position of power
transmission elements on a shaft, the shims between the shaft cartridge and housing
can be adjusted, as shown in Figure 11.14b.

11.4.4.5 Rolling Bearing Fits
Tolerances and fits of shafts and housings are critical for rolling bearing operation. Since
bearings are manufactured either by national standards, such as the ANSI/ABMA in
USA, DIN in Germany, JIS in Japan, GB in China, or by the international standard ISO
[3], proper fits are obtained only by selecting proper tolerances for the mating shaft and
housing bore. A suitable fit must be carefully selected as too loose a fit may abrade shafts
and/or housing bores, while too tight a fit can cause internal interference and shorten
bearing life [2].
In normal practice, bearings are usually mounted on the rotating ring with a light
interference fit to prevent relative motion during operation, whether it is an inner ring or
outer ring [14]. The stationary ring is mounted with a close clearance fit to accommodate
axial sliding and to avoid undesirable thermally induced thrust loads. To ensure proper
operation and bearing life, mounting dimensions must be controlled to a total tolerance
of only a few thousandths of a millimetre [1]. Recommended fits depend on bearing
type, size, tolerance grade, as well as loads, speeds and applications. Since bearings are
standard products, shafts usually adopt the basic hole system and the tolerance of shaft
diameter selects k6, m6, n6 and so on. Tight interference fits are for high speed, heavy
load, high temperature and vibration applications. Housing bores take up the basic shaft
system and the tolerance of housing bore selects H7, Js7 and so on [17]. Most catalogues
specify limit dimensions for both shaft diameters and housing bore diameters.

11.4.4.6 Preloading
When high rotational precision, long life, small deflection and vibration, are required for
a bearing system, it is common practice to preload bearings to remove excessive internal
radial and axial clearances. Preloading are realized by producing an axial displacement of
one race relative to the other either by reducing the width of inner ring or outer ring, or
by inserting a shim between them [13]. For example, the outer rings in Figure 11.16a are
ground to provide the required gap, the bearings are preloaded automatically when the
pair of bearings is tightly clamped together. In Figure 11.16b, a pair of angular contact
ball bearings is preloaded by inserting a shim between inner rings and clamping the
inner ring with a nut on the shaft, and the outer ring with bolts in the housing.
11.4.4.7 Lubrication
Lubrication is important for proper functioning of bearings, especially in high speed
applications. Lubrication not only provides a low frictional film between contact
surfaces of rolling elements and raceways, but also helps dissipate heat and dispel