Bearing Selection Guide for Motor Design

Bearing Selection in Motor Design: Key Considerations for Engineers

For motor designers, selecting the right bearing is critical. One of the main tasks in motor structural design is calculating the bearing's rated and fatigue life to determine its proper size. Bearing selection involves more than just considering grease life, wear, and noise caused by lubrication aging. Depending on the motor's application, designers must also evaluate precision, fit, clearance, cage type, lubricant, sealing structure, installation, and other special requirements.

Control Factors of Motor Bearings in Design and Manufacturing

1. Mechanical Usage and Design Life
When selecting bearings, increasing the fatigue life factor may require choosing a larger bearing. However, this must be balanced with shaft strength, rigidity, and installation dimensions. Fatigue life is not always the only limiting factor. Bearings for different machines have reference design lives, often expressed using empirical fatigue life coefficients based on operating conditions.

2. Bearing Installation and Fit
The fit between the bearing inner diameter and shaft, and outer diameter and housing, is critical. Too loose a fit can cause relative sliding at the contact surfaces, leading to rapid wear, damage to the shaft or housing, and debris entering the bearing, increasing heat, vibration, and failure. Conversely, a too-tight or excessively interference fit can reduce internal bearing clearance and generate uneven deformation, increasing noise. Geometric accuracy of the shaft and housing also affects the bearing ring’s original precision, impacting performance.

3.Principles for Rolling Bearing Fits

●Ring Relative to Load: Rings rotating or oscillating under load should use an interference or transition fit; rings fixed relative to the load should use a clearance fit. For non-separable bearings serving as floating supports, the ring fixed relative to the load should be the floating ring with a clearance or transition fit.

●Load Type and Magnitude: Bearings under impact or heavy loads typically require tighter fits than those under light or normal loads. Radial load magnitude is expressed as the ratio of equivalent dynamic radial load to rated dynamic radial load; larger loads call for higher interference.

●Bearing Clearance: Interference fits reduce clearance. It is essential to verify post-installation clearance to ensure correct fit and performance.

●Other Factors: Shaft and housing materials, strength, thermal conductivity, external conditions, heat paths, and installation or adjustment methods also affect fit selection.

4.Factors Affecting Bearing Clearance

Bearing internal clearance is the movement of one ring relative to the other when one is fixed, measured radially, axially, or angularly. During operation, clearance decreases due to fit and temperature differences. Theoretically, a slightly negative running clearance maximizes bearing life, but maintaining this optimal clearance is difficult. Changes in operating conditions can increase negative clearance, significantly reducing bearing life or causing overheating. Therefore, initial clearance is usually set slightly above zero.

1）Reduction Due to Interference Fit: When a bearing is mounted with a static fit, the inner ring expands and the outer ring contracts, reducing internal clearance. The reduction varies with bearing type, shaft and housing design, dimensions, and materials—typically 70–90% of the interference.

2）Reduction Due to Temperature Difference: During operation, the outer ring is usually 5–10°C cooler than the inner ring or rolling elements. If the housing is heavily heated, the shaft is connected to a heat source, or heat-carrying fluid flows inside a hollow shaft, the temperature difference increases, further reducing clearance.