A general rule of thumb for roller bearings places a minimum load equal to about 0.02 times the dynamic radial-load rating. For ball bearings, that number is 0.01. Maintaining (at least) minimum loads is especially important when bearings see high accelerations and speeds that are roughly 75% of recommended ratings.
Electric motors typically incorporate a locating and nonlocating bearing arrangement to support the rotor and locate it relative to the stator. Locating bearings position the shaft and support radial and axial loads, while nonlocating bearings handle radial loads and let shafts move axially to prevent overloading from thermal expansion.
The most common setup for smaller motors in horizontal machines consists of a pair of deep-groove ball bearings mounted on a short shaft in a cross-locating arrangement. Medium and large electric motors typically use deep-groove ball bearings for locating. The nonlocating bearing may be a ball bearing, cylindrical-roller bearing, or toroidal-roller bearing, depending on the loads, speeds, and operating environment.
Regardless of type, bearings need a minimum load so rolling elements rotate and form a lubricant film rather than skid. Skidding raises operating temperatures and degrades lubricants.
Motors for vertical machines typically incorporate deep-groove ball bearings, angular-contact ball bearings, or spherical-roller thrust bearings.