In general, power output governs shaft size and bearing bore diameter. Of course, load magnitude and direction also determines bearing size and type. Designers sizing motor bearings should consider additional forces such as magnetic pull from unsymmetrical air gaps, out-of-balance forces, pitch errors in gears, and thrust loads.
The calculation of loads on a single bearing models the bearing shaft as a beam resting on rigid, moment-free supports. Assuming the resulting load is constant in magnitude and direction, the equivalent dynamic bearing load comes from the general ABMA and ISO equation:
The type of connector or coupling between the drive and driven unit also determines loads on motor bearings. A belt or gear drive, for example, applies greater radial loads than a coupling drive. Here, cylindrical roller bearings at the drive end handle the higher loads. Spherical-roller and toroidal bearings are a good choice in applications with a combination of heavy loads, misalignment, and shaft deflection.
In coupling drives, proper alignment is important because misalignment may introduce additional forces and vibration that shorten bearing and motor life. Rigid coupled machines typically use three bearings on a shaft: two in the motor and a third in the coupled device. Accurately aligned rigid couplings tend to un-load the drive-end bearing. In these cases a deep-groove ball bearing for the drive end is a good choice.