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What is the reason for the bending or breaking of the motor shaft?

Date:2022-05-31   Author:XINDA MOTOR

The motor shaft is a key component that directly bears the mechanical load. It must be able to withstand various alternating stresses without causing residual deformation and fatigue damage. For this reason, the shaft should have sufficient strength and rigidity. In general, the maximum deflection of the shaft cannot exceed 5% of the air gap, and the critical speed of bending and the critical speed of torsion should be 1.3 times the maximum speed of the motor.

For small motors, the rotating shaft is relatively thin. When the shaft extension is impacted, or it is not aligned with the driven equipment, or the belt tension is too large (pulley drive motor), the shaft extension of the rotating shaft has a higher probability of bending. Therefore, in the motor During storage and transport, the necessary precautions should be taken.

For relatively large motors, the diameter of the shaft extension is larger, the shaft extension is less subject to force and bending, and the broken shaft occurs more in the weak links of the shaft, that is, the root of the shaft extension, the root of the bearing, etc.

Asynchronous motor shaft breaking accidents only occasionally occur in occasions such as hoisting machinery, reciprocating compressors, punching machines, ball mills and other occasions where frequent starting and forward and reverse rotation of alternating heavy loads are dragged. The breaking position is generally at the first step of the shaft extension. , that is, the root of the shaft extension. This fracture section generally presents a wave-shaped convex and concave shape, which is a typical fatigue fracture. In order to prevent the shaft breaking problem, the machining transition fillet at the step position, that is, the diameter change position, should not be too small, and the undercut should not be too deep and too narrow to prevent excessive stress concentration.

For large motors, in order to ensure that the strength and stiffness of the shaft meet the requirements, sometimes a forged shaft is used to improve the strength, toughness and hardness of the shaft, reduce the amount of processing, and effectively eliminate processing stress. However, it is specially reminded that in order to prevent defects caused by poor process control, ultrasonic flaw detection should be carried out after rough machining for large high-speed motor shafts.


For high-voltage motors, there are many cases where welded shafts are used, that is, 4 or 6 ribs are welded on a suitable main shaft to meet the diameter matching requirements of the rotor punching pieces. In view of the particularity of the welding process, such shafts will have large welding stress and processing stress, so necessary stress relief measures must be taken, especially the end position of the connection between the rib and the main shaft, and the longitudinal welding of the rib and the main shaft. The seam positions are the parts that are more prone to cracks.