■ boundary dimensions

■ magnitude and direction of loads

■ speed: fixed, variable, or high

■ required precision

■ shaft and housing material

■ coupling, belt, or gear drive

■ horizontal or vertical mounting

■ environment

■ vibration level

■ noise level

■ temperature

■ required bearing life

■ lubrication: grease versus oil, integral seals.

A general overview of the standard bearing types, their design characteristics, and their suitability for the demands in motor applications is shown in the matrix in Table 2.

Other important bearing-specific criteria must be con- sidered when designing a bearing arrangement, including load carrying capacity and life, friction, permissible

speeds, bearing internal clearance or preload, lubrication, and sealing.

The magnitude of the load is one of the factors that usually determines the size of the bearing to be used. General- ly, roller bearings are able to support heavier loads than similar-sized ball bearings. Ball bearings are mostly used where loads are light or moderate. For heavy loads and where shaft diameters are large, roller bearings are usually the more appropriate choice.

Cylindrical roller bearings, needle roller bearings, and toroidal roller bear- ings can only support pure radial loads, while other radial bearings can accom- modate some axial loads in addition to radial loads.

Angular contact ball bearings can support moderate axial loads at rela- tively high speeds. For moderate and heavy axial loads acting in one direc- tion, spherical roller thrust bearings can be used.

A combined load comprises a radial and an axial load acting simultaneously. The ability of a bearing to carry an axial load is determined by the angle of contact or load action internal to  the

bearing—the greater the angle, the more suitable the bearing for axial loads. For combined loads, single- and double-row angular contact ball bearings and single-row taper roller bearings are most commonly used, although deep groove ball bearings and spherical roller bearings may also be suitable depending on the ratio of axial to radial loading.

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Single-row angular contact ball bearings and tapered roller bearings can only accommodate axial loads acting in one direction. For axial loads of alter- nating direction, these bearings must be combined with a second bearing.

When a load acts eccentrically on a bearing, such as from an overhung sheave or pulley, a tilting moment will occur. Double-row bearings (e.g., deep groove or angular contact ball bear- ings) can accommodate tilting moments, but paired single-row angu- lar contact ball bearings or taper roller bearings are more suitable.

Angular misalignments may occur between the shaft and housing. Exam- ples are when the shaft bends (flexes)

under the operating load, when the bearing seating in the housing is not machined to the same height, or when shafts are supported by bearings in separate housings that are too far apart.

Rigid bearings (i.e., deep groove ball bearings and cylin- drical roller bearings) cannot accommodate any misalign- ment or can only accommodate very minor misalignments, unless by force. Self-aligning bearings (i.e., spherical roller bearings, toroidal roller bearings, and spherical roller thrust bearings) can accommodate misalignment produced under operating loads and also compensate for initial errors of mis- alignment resulting from machining or mounting errors.