Bearings are key components supporting the normal operation of motors . Besides manufacturing process control, the design and configuration of motor bearings are crucial. For example, vertical and horizontal motors should use different bearing configurations, and motors with different speed requirements should also use different bearings. In a conversation with a friend, he shared his unique insights on bearing selection, which Ms. Can has summarized and compiled to share with everyone.
Configuration of bearings for horizontal electric motors
Horizontal electric motors should ideally use one ball bearing and one cylindrical bearing at each end. Currently, most domestically produced small and medium-sized 2-pole motors use two ball bearings. Although some manufacturers have adjusted the tolerance zone of the bearing housing diameter towards a clearance fit to alleviate the problem of ball compression after bearing heating, the coefficient of thermal expansion of the bearing outer sleeve is larger than that of the cast iron parts supporting the bearing housing—the end cover or bearing sleeve. Furthermore, the temperature at the bearing outer sleeve is also higher. Therefore, while the bearing outer sleeve might slide slightly axially within the bearing housing at room temperature, it may not be able to move freely when the bearing is heated during operation. This is the main reason why motors using two ball bearings have short bearing life and are prone to overheating. In motors using two ball bearings, after a temperature rise test, the bearings sometimes burn out upon restarting the motor on the second attempt. This is because the clearance of the ball bearing cannot withstand the thermal expansion and contraction of the rotor, and the outer sleeve cannot creep within the bearing housing, causing the balls to bear a large axial force.
The above problems can be avoided by using one ball bearing and one cylindrical bearing. A reputable motor manufacturer in China changed the two-pole high-voltage motor in frame size H560 from two ball bearings to one ball bearing and one cylindrical bearing, and the subsequent operation was excellent. Apart from replacing the bearings during routine maintenance, no malfunctions occurred during normal operation. Moreover, the temperature measured at the bearing was only 20℃. This is the first successful case in China of changing the traditional two-ball bearing configuration of a two-pole motor to one ball bearing and one cylindrical bearing, and it is now gradually being promoted in explosion-proof motors.
To address the issues of overheating and noise at limiting speeds when replacing ball bearings with cylindrical bearings, a lightweight cylindrical bearing can be used and placed near the external fan end for better cooling. This also allows the bearing noise to be masked by the fan noise. If a high-power 2-pole motor requires a larger bearing size, and the lightweight cylindrical bearing is insufficient for the limiting speed requirements, the bearing lubricant can be replaced with thin oil.
Configuration of bearings for vertical electric motors
When a vertical electric motor is used to drive a belt pump in a petrochemical system, due to the instantaneous upward axial force during startup of the multi-stage belt pump, coupled with transportation requirements and the need to mitigate bearing damage caused by rotor thermal expansion, the upper bearing of the motor (usually 2-pole or 4-pole) should be two opposed single-row radial thrust ball bearings, and the lower bearing should be a cylindrical bearing, preferably a lightweight series. To prevent damage to the thrust ball bearings, a pre-applied axial force should be applied to the bearing outer sleeve of one of the two opposed bearings. The magnitude of this force should be sufficient to ensure that the bearing is not damaged during transportation and startup due to the reverse axial force from the shaft extension end to the unloaded end.
Selection of bearing specifications and models
For 2-pole motors, light-duty bearings should be used whenever possible; for 4-pole or higher motors that do not rotate in either direction, medium-duty bearings are preferred; for low-speed motors that rotate in either direction, heavy-duty bearings are preferred. Light, medium, and heavy-duty motors should all use one ball bearing and one spherical bearing, with the ball bearing placed at the shaft extension end of the 2-pole motor.
In recent years, some motor manufacturers have adopted large-clearance ball bearings in 2-pole motors that use two ball bearings. This is a measure to alleviate bearing damage caused by reduced clearance due to bearing heat during operation. However, the mechanical noise of large-clearance bearings also increases. If one ball bearing and one spherical bearing are used, the above-mentioned drawbacks can be eliminated if the bearing housing tolerances are selected appropriately.
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