Analysis of the causes of frequent motor shaft fractures
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This post was last posted by I Believe in LEE on 2019-3-6 10:48 The editor has a customer with a water ring vacuum pump and a 132KW pulley drive motor. Two motors were damaged for two consecutive days. The shafts were broken at the same position. The pump was disassembled and inspected to find any abnormality or jamming in the rotor distribution plate. The process pipeline also said that no abnormalities were found. The motor was replaced with a new one. The reason is still being found. The current of the currently running 132KW motor has been around 230A and the rated current is 240A. It feels risky. Could you please help me analyze what might be the cause?
Figure 2. The broken section of the shaft is relatively smooth, and the remaining 1/3 section is rough. Based on the preliminary judgment of the appearance of the section, it is due to processing. There is stress concentration in the fracture section. During operation, micro cracks are generated due to stress concentration. The cracks gradually deepen and form a smooth surface that accounts for about 2/3 of the shaft section. The rough surface is caused by the instant tearing and destruction of the remaining area after the crack expands to about 2/3 of the section and the load exceeds the yield limit of the material.
How does the motor shaft diameter compare with the pump shaft diameter? Why is the motor shaft broken but the pump shaft is not broken? Personally think: 1. From the cross-section, the tempering of the quenching and tempering heat treatment of the motor shaft is insufficient, resulting in excessive hardness but insufficient toughness of the motor shaft. ； 2. The belt drive will generate a large radial force. When the shaft rotates, the fatigue strength of the shaft against repeated bending (tension and compression) is assessed, and the fracture location is at the empty slot position at the root of the shaft extension. ； 3. I don’t know whether it is a flat belt, a V-belt or a gear belt transmission. Flat belts require a lot of pulling force to obtain sufficient friction, while V-belts and gear belts have much smaller pulling forces due to their structure. 4. You can try to use a larger motor (the shaft is stronger), and the power consumption is about the same. 5. If possible, you can use a reducer (box) for transmission.
Belt too tight? If the tightness remains unchanged, move the motor toward the axial direction of the pulley and install the pulley deeper. This can reduce the bearing capacity of the section
Agree with this friend's method, shorten the cantilever length of the motor pulley, and make the motor pulley as close as possible to the motor bearing to reduce the force acting on the shaft and bearings.
It is recommended to send a panoramic video so that you can see the entire equipment. Is your equipment modified or assembled by yourself, or is it a manufacturer's brand product? Is it a problem with the inner belt or a problem that arises the day after tomorrow? Does the shaft power of the load match the power of the motor? The more comprehensive the problem reflected, the more conducive it is to analyzing and solving the problem. Only sending one broken shaft is too limiting.
When it broke for the first time, I suspected it was a problem with the processing materials, etc., but it broke twice within a few hours, and the current of the currently running motor is still running at full load, :Q
The motor shaft diameter is 80 and the pump is 120 V belt drive. The reason has not yet been found. I will not consider increasing the motor power for the time being. It used to be a reducer, and later it was changed to a pulley.
This equipment is from Guangdong Foshan Water Pump Factory. It was changed from the previous reducer to a pulley drive, but it has been running for several years. The continuous shaft breakage has only recently appeared. The load motor was 110KW at the factory, but it was changed to 132KW two years ago.
Modification is possible, but it requires design standards for inspection. For example, the factor of the continuous short axis is that there is a margin in the design. The amount of this margin is directly related to the length of the pulley and the change in tension and tightness of the belt. Try loosening the belt a little. In addition, whether the connection between the motor and the foundation is prestressed enough is often overlooked. For example, although the anchor bolts and screws are tightened, they are not necessarily in place to provide the necessary prestress. This force can ensure that the motor runs smoothly with minimal vibration. This point should be paid attention to.
When was the belt drive changed? Did you do anything else before the break?