Causes Of Axial Float in Electric Motors

Axial float always exists to a greater or lesser extent during motor operation and cannot be completely eliminated. When the float reaches a certain magnitude, both the motor and mating equipment may suffer severe damage, and quite a number of motor failures stem from this issue. Past analyses on batches of motor axial-shift failures show that most failed motors with shaft float have certain design shortcomings. This indicates that axial float or shaft shifting shall be anticipated at the design stage to minimize its occurrence probability.

One bearing end is axially fixed while the other serves as the floating end. Small-frame motors rely merely on bearing assembly interference for positioning. This is especially true for motors fitted with sealed bearings without bearing end covers. Consequently, common failures occur: the supposedly fixed end is not fully restrained, transmitting unexpected axial float to mating equipment; excessive clearance between shaft and bearing in small-size motors allows axial displacement of bearings under vibration force.

- Excessive fitting clearance between shaft and rotor causes relative movement between rotor and shaft, misaligning stator and rotor cores and further aggravating axial displacement of the rotor and shaft relative to the stator.

- Wave spring washers shall be fitted at both motor bearing ends. Missing installation or poor-quality spring washers will trigger axial float.

- Axial force generated by the rotating fan, namely the aerodynamic axial force acting on fan blades. This effect is mild for low-speed motors, yet pronounced for higher-speed motors with relatively loose radial fits.

- Electromagnetically induced axial force arising from misalignment between stator and rotor. This effect exists in all motors, as no motor manufacturer can achieve perfect stator-rotor alignment. Misalignment of varying degrees may result from manufacturing defects such as stator-rotor skewing and spring-open deformation, pot-bottom distortion of cast-aluminium rotors, and warping of radial ventilation ducts.

- For sleeve-bearing motors, misalignment of the magnetic centre gives rise to shaft axial float.

- Excessive fitting clearances between bearing and shaft, as well as between bearing and end-cover housing, bring about axial float, usually accompanied by bearing creep.

For axial float mitigation, large-frame motors adopt end-cover positioning at one bearing end. Wave spring washers can be inserted axially between the bearing face and end cover for adjustment. For motors without inner bearing covers, retaining-ring grooves can be machined inside the end-cover bearing housing, with bearing retaining rings fitted during assembly. For horizontally mounted motors, the optimal solution is to guarantee good stator-rotor alignment. For vertically mounted motors, the influence of rotor self-weight on axial float must also be taken into consideration. Many manufacturers pay insufficient attention to the dimensional accuracy of bearing covers, leading to frequent such failures. High-quality manufacturers enforce strict component tolerance control, which differentiates overall motor performance.