This presents the challenge for a pulp and paper mill: two motor designs are required depending on the method of coupling to the load. Often these motors are mixed in spare inventory, resulting in improper use and early failure. A new toroidal roller bearing design does offer some flexibility. These bearings may be capable of operation under either a coupled or belted load, meet minimum load requirements, and provide adequate life, but they are not directly dimensionally interchangeable with traditional bearings used in motors.

Ball Bearing Configurations

The motor is built with the bearings mounted directly to the shaft of the motor with an interference fit. There is a shoulder machined on the shaft, and the inner race of the bearing is positioned directly against this shoulder. The bearings are mounted in an interference (or “press”) fit,

meaning that clearances are 0.0001–0.0015 in tight. The endplates of the motor have bearing bores machined to provide clearance or a “loose fit” with the bearing outside diameter. It is usually 0.0000–0.0023 in loose. Most often, the drive end bearing is held captive in the end- plate, and the opposite drive bearing is allowed some axial movement in that endplate to allow for thermal growth or the shaft and rotor assembly. Often a spring supplies an axial preload to the bearings to minimize noise and keep the balls loaded.

Since radial loads are relatively low on coupled loads, shaft material selection can be of normal strength steel such as AISI or SAE Grade 1137. Primarily torsional loads are present.

Roller Bearing Configurations

Like the motor with two ball bearings, the roller bear- ing motor is built with a ball bearing and a cylindrical roller or two self-aligning spherical roller bearings mounted directly to the shaft of the motor. There is a shoulder machined on the shaft, and the inner race of the bearing is positioned directly against this shoulder. The roller bearings are mounted in a “press fit,” mean- ing that clearances are 0.0005–0.0019 in tight. The endplates of the motor have bearing bores machined to provide clearance or “loose fits” with the outer ring of the bearing giving 0.0000–0.0023 in loose as well. Since roller bearings accommodate limited axial loads, the opposite bearing to the drive end bearing is held captive in the endplate, and the drive bearing provides some axial movement to allow for thermal growth or the shaft and rotor assembly. In the case of cylindrical roller bearings, this axial movement is accommodated within the bearing as opposed to the housing bores. Because roller bearings require higher minimum loads to func- tion than ball bearings, they are normally only used for belted or overhung loadings.

Motors requiring roller bearings with high overhung loads require higher-strength shaft steel. Depending on calculated radial shaft loads, the motor designer may select grades such as AISI/SAE 1045 or 4140.

Toroidal Roller Bearing Configuration

A toroidal roller bearing provides self-aligning capabili- ties and axial movement within the bearing, and it requires lower minimum loading than other roller bear- ings. The toroidal bearing mounts directly to the shaft with an interference or tight fit of 0.0005–0.0022 in. The outer ring is mounted to the endplate with a clear- ance or loose fit of 0.0000–0.0023 in. It can provide large axial movements within the bearing and, therefore, is a nonaxial load carrying bearing and must be mounted with a captive (or held) bearing capable of handling some thrust loading.

Motors using a toroidal roller bearing offer advantages in that they may be used on either a coupled or heavy belt- ed load. The shaft material selection must be for the worst-case condition of a heavy overhung load requiring higher-strength shaft steel. Depending on calculated radial shaft loads, the motor designer may select AISI or SAE grades such as 1045 or 4140.