When discussing the axial preloading of motor bearings, several key factors need to be considered: the operating conditions of the motor, the type of load (static or dynamic), changes in ambient temperature, and the expected service life.
Load type:
For light load applications, it is usually not necessary to apply axial preload.
In applications with heavy loads or high dynamic performance requirements, appropriate axial preloading can improve the stiffness and stability of bearings, reduce vibration, and help extend their lifespan. However, excessive axial preloading may lead to increased friction, decreased efficiency, and potential overheating issues.
Environmental temperature changes: Temperature changes can affect the clearance inside bearings, especially for precision equipment. When operating under extreme temperature conditions, appropriate axial preload can help maintain the correct clearance and contact pressure, ensuring stable bearing performance.
Motor type and application:
Servo motors typically require more precise control and may require the use of preloaded bearings to reduce rotational errors.
In high dynamic applications such as wind turbines and industrial drives, appropriate axial preload can help improve system responsiveness and stability.
Recommended value: Bearing manufacturers typically provide specific guidance on how to apply axial preload. For most applications, the recommended axial preload is approximately 20% to 30% of the bearing’s rated load, but this needs to be adjusted according to specific motor and load conditions. The subsequent content of this article will also discuss the calculation of data bodies.
Measurement method: Special tools such as axial load testers are usually used to accurately apply and measure axial preload. Ensure proper inspection of the bearings after installation to confirm whether the preload meets the design requirements.
Maintenance and monitoring: Regular inspection and adjustment of axial preloading are necessary, especially when operating conditions change or the expected lifespan ends. The use of vibration analysis, temperature monitoring, and other methods can help identify possible abnormal situations and adjust the preloading state in a timely manner.
For motors that require preloading, this article specifically introduces the calculation method.
Since the axial preload of bearings is usually applied to deep groove ball bearings in motors, we use deep groove ball bearings for illustration.
Deep groove ball bearings have a residual clearance after leaving the factory. For general horizontal internal rotating motors, the inner ring and shaft of the motor bearing are tightly matched, while the outer ring and shaft are loosely matched. Therefore, after the installation of the bearing, the inner ring of the bearing will experience radial expansion due to fitting reasons, while the outer ring will hardly change. Therefore, such changes will cause the internal clearance of the bearing to decrease. Similarly, when the motor is running, the temperature of the shaft is higher than that of the bearing chamber, which causes the thermal expansion of the inner ring of the bearing to be relatively larger than that of the outer ring, thus further reducing the remaining clearance inside the bearing. The remaining clearance in the end is what we call the working clearance of the bearing.
Generally speaking, when selecting motor bearings, a value larger than 0 is chosen to ensure the best performance of the bearings and the safety of the bearing clearance.
It is precisely the existence of this residual clearance that creates space for the bearing rolling elements to collide with the inner and outer race raceways in the non load zone. This collision caused noise from the motor bearings. Especially for some noise sensitive situations, such as air conditioning motors, eliminating this noise is of great significance.
Based on the above reasons, we usually use axial preload to eliminate the axial clearance inside the motor bearings. We know that for deep groove ball bearings, the elimination of axial clearance also means the elimination of radial clearance. Therefore, there is no space for collision and vibration of the rolling elements inside the raceway. So the noise that comes from this is also eliminated.
However, from the clearance curve, it can be seen that if the axial load is too large, excessive negative clearance inside the bearing will have a significant impact on the service life of the motor bearing. Therefore, in order to balance the two, it is necessary to limit the axial load.
The preload value of motor bearings based on noise considerations is:
F=kd
Among them, F is the preload, N
K is the coefficient
D is the inner diameter of the bearing, mm
Based on the consideration of noise elimination, the k value is generally taken as 5-10. And such preload has little impact on the service life of motor bearings, effectively reducing the noise of motor bearings.
On the other hand, if the motor is affected by factors such as vibration during storage and transportation, it is prone to false Brinell indentation. Pseudo Brinell indentation is a metal surface damage caused by the reciprocating rolling and sliding of motor rolling elements at a certain position on the raceway.
To avoid this situation, the k value is usually set to 10-20.
Readers will easily notice that the above range of values is very broad. This is mainly due to the accumulation of axial dimensional tolerances in the motor structure. If the scope is too narrow, it will increase the difficulty of implementation in engineering.
Generally, based on my personal experience, I would recommend a k value of around 10. This approach not only ensures that the stress value is centered, which is beneficial for engineering implementation, but also helps to balance the effects of both aspects.
