The added width of a toroidal roller bearing may pre- sent challenges to the motor designer attempting to uti- lize existing endplate tooling to prevent the bearing from protruding inside the motor where it is unsupported. The loads from the bearing need to be directed to the strongest part of the endplate.

Also, due to the additional heat produced when used with heavy belted loads, a cooling fan mounted on the shaft may be used. This may result in a non-NEMA “BA” dimension (shaft shoulder to foot mount holes).

Table 1 illustrates a comparison of ISO radial capacity, required minimum loads, misalignment capabilities, and boundary dimensions for commonly used motor bearings.

Bearing Loads

The type of coupling connector used between the drive and driven unit will influence the loads on the motor bearings. There are two kinds of coupling drives: flexible and rigid. Good alignment is important in both cases; otherwise, additional forces may be induced into the bearing system to reduce service life.

Proper alignment is particularly important with a rigid coupling, where there are typically three bearings on a shaft. When rigid couplings are aligned

very accurately, by using laser aligning equipment, the drive end bearing might become relatively unloaded, the load being taken by the bearings on the non- drive end and the coupling shaft. In this case, a deep groove ball bearing is nor- mally recommended at the drive end.

A belt or gear drive will often load the motor bearings more heavily than a cou- pling drive. Belt and gear drives therefore often use cylindrical roller bearings at the drive end. In applications where there are heavy loads and a possibility of misalign- ment and/or shaft deflection, a self-align- ing roller bearing should be considered. Any time a belted load is used on a large motor, the pulley data should be analyzed to determine the appropriate bearing

selection. Most motor manufacturers can supply a work- sheet that may be used to define the data required for the analysis.

Bearing Selection

The purpose of using rolling bearings in electric machines is to support and locate the rotor, to keep the air gap small and consistent, and to transfer loads from the shaft to the motor frame. The bearings should enable high- and low-speed operation, minimize fric- tion, and save power. The designer must consider many different parameters when selecting the bearing type and arrangement to meet the requirements of any par- ticular motor application. The orientation of the motor and the drive coupling can have a significant impact on the final selection.

Electric motors and generators use a wide variety of bearing types, including deep groove ball bearings, angu- lar contact ball bearings, cylindrical roller bearings, taper roller bearings, spherical roller bearings, toroidal roller bearings, and spherical roller thrust bearings. The differ- ent bearing types are shown in Figure 1. In small horizon- tal machines, the most common arrangement consists of

Deep Groove Ball Bearing Angular Contact Ball Bearing Cylindrical Roller Bearing

Taper Roller Bearing

Spherical Roller Bearing

Toroidal Roller Bearing Spherical Roller Thrust Bearing

Bearing types.

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two deep groove ball bearings. In larg- er or heavier loaded machines, roller bearings are typically used. In vertical machines, deep groove ball bearings, angular contact ball bearings, or spher- ical roller thrust bearings are typically used, depending on the loads, speeds, temperature, and environment of the application.

In many cases, however, several fac- tors must be considered and weighed against each other when selecting a bearing type, so that no general rules can be given. The most important fac- tors to be considered when selecting a standard bearing type and an appropri- ate choice are: