Page 333 - Analysis and Design of Machine Elements

11.4.3 Design Procedures and Guidelines Rolling Contact Bearings 311
Bearings are normally selected after the minimum diameter of mating shaft has been
determined from stress or rigidity analyses. Also, the radial and axial forces acting on
the power transmission elements, along with the location of bearings with respect to
other elements have been decided.
The procedures for making appropriate selections and design of bearing system are:
1. Tentatively select the type of bearing according to the specific bearing application;
2. Decide the bore size of bearing according to the diameter of mating shaft;
3. Specify radial load R and axial load A on each bearing, determine the equivalent load
by P = XR + YA;
4. Specify the life expectancy L and reliability appropriate for the application;
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5. Calculate the required basic dynamic load rating C;
6. Select the smallest bearing with the required basic dynamic load rating C and a bore
that closely matches the shaft diameter by consulting bearing catalogues;
7. Calculate the equivalent static load by P = X R + Y A if needed;
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8. Determine mounting conditions, such as the diameter and tolerance of shaft seat
and housing bore, means of axially locating bearings, and other special needs, such
as seals or shields, to ensure bearings can be installed conveniently in a machine.
Designers should consult manufacturers’ catalogues for more details.

11.4.4 Practical Considerations in the Application of Bearings

After bearing type, bore size and load carrying capacity have been decided, some prac-
tical considerations including assembly and disassembly, positioning, retaining, adjust-
ment, tolerance, preloading, lubrication and sealing become crucial in determining suc-
cessful performance of a bearing.

11.4.4.1 Assembly and Disassembly
Bearings are installed with a light interference fit between the bore of bearing and shaft.
Therefore, a rather heavy load is required to apply axially to the bearing inner ring. To
reduce extremely high installation load in large bearing assemblies, it may be necessary
either to thermally expand the inner ring or to contract the shaft in a mixture of dry
ice and alcohol. Heating a bearing should not damage bearing materials or any prein-
stalled lubricant. Removal of bearings can be realized by bearing pullers, as shown in
Figure 11.12.

11.4.4.2 Axial Positioning
A shaft usually requires two supports. Each support may be composed of one or more
bearings. Radially, bearings are confined by a housing. Axially, bearings are held in
position by either a shaft shoulder, a bolted cover or other retaining elements. When
installing bearings, it is important to prevent both axial movement and over constrain.
If both bearings are held tightly, any dimensional changes due to thermal expansion
could lead to dangerous unexpected loads on bearings.
Three methods are usually employed for axially positioning bearings. Figure 11.13
shows a common practice of retaining two bearings each in one direction. When using
deep-groove ball bearings in Figure 11.13a, the right bearing has its outer ring fit loosely