Regarding the analysis of the causes of shaft breakage, we have discussed this issue extensively using case studies. Reducing and eliminating stress points and removing external force factors are the principles for preventing shaft breakage. Analyzing the structural characteristics of the shaft, we should strive to avoid abrupt diameter changes and slender shafts with large-diameter rotors during the design phase. During the manufacturing process, necessary measures should be taken to relieve stress.
(1) Control of machining stress. For electric motor products, most are machined from cylindrical blanks. Due to the structural characteristics of the shaft, it is basically a stepped structure, thicker in the middle and thinner at both ends. The locations where the shaft diameter changes are where stress is concentrated. Therefore, it is necessary to reduce the chance of stress concentration by machining in multiple processes. Machining transition fillets at these locations to reduce machining stress is essential. In actual machining, transition fillets may be understood as a superficial quality control measure, but their most substantial function is one of the measures to avoid stress.
(2) Relief of welding stress. Welding stress only applies to shafts with flanges, i.e., flanges of a certain height are welded onto a cylindrical blank to achieve a fit with the rotor core. This type of shaft is more prone to fracture.
According to the welding process, the stress is divided into instantaneous stress and residual stress. The welding stress of the shaft should be controlled from the welding process and the finished product stage. In order to reduce the instantaneous stress of welding, the preheating of the shaft before welding is particularly important. In order to reduce the residual stress of welding, the shaft should be subjected to stress relief treatment after welding. The treatment can be carried out by ultrasonic vibration or by heating.
Cracks in welded shafts generally occur at the joint end between the web plate and the main shaft. Some are longitudinal cracks that extend inward along the direction of the weld. The early signs of this type of problem are severe motor noise and stator rubbing. It is difficult to inspect even if the cracks are not severe. It is necessary to analyze and judge by the radial runout detection method.
(3) Motor installation and transmission control. For motors with relatively high power, belt pulley transmission is not recommended because this transmission method causes the motor shaft to be subjected to bending moment, which may lead to shaft breakage in severe cases.
These requirements are specified in the product technical specifications of motors and are also outlined in the user manuals of reputable motor manufacturers. However, these requirements may not always be implemented in actual use. For motors that must use belt pulley drives, especially larger motors, it is recommended to use cylindrical roller bearings on the drive end.
(4) Reliability of motor mounting reference. In actual use, it can be found that some motor mounting references are not in direct contact with the ground, but are raised in height by means of a frame. In this case, special attention should be paid to the reliability of the reference. If the reference is unbalanced, it is very easy to cause shaft breakage due to the positional relationship between the motor and the driven equipment.
(5) Daily monitoring and measurement during motor operation. During motor operation, the placement of the motor itself and the relative position of the equipment have a significant impact on the operating quality. Periodic inspections and daily monitoring should be conducted to identify and correct problems, and prevent quality accidents from occurring.
