The influence of preload and speed on bearing stiffness and

Writer: Eric Bearing Limited

Bearings are important parts of mechanical rotation, and the stiffness and temperature of the bearing will directly affect the efficiency of the bearing on the mechanical operation. In order to provide the rotation accuracy of the bearing, increase the rigidity of the bearing device, and reduce the vibration of the shaft when the machine is working, it is often used Preloaded rolling bearings, such as machine tool spindle bearings. What is preload?

The so-called pre-tightening is to generate and maintain an axial force in the bearing in a certain way during installation to eliminate the axial clearance in the bearing and produce deformation at the contact between the rolling elements and the inner and outer rings.

Due to the preload, the contact area between the rolling elements and the inner and outer rings will be elastically deformed, and the contact area will increase. The number of rolling elements involved in bearing the force will increase, and it is possible to roll in a range greater than 180 degrees. The body participates in the force, and sometimes it is even possible that all the rolling elements are subjected to force within the range of 360 degrees. This is definitely better than the case where a few rolling elements are subjected to force, and it can bear more load. From the above discussion, it can be seen that when the preloaded bearing is under the same load when it is working, its contact deformation must be smaller than that of an unpreloaded bearing. Therefore, the support rigidity of the bearing can be improved, and the bearing can also be compensated for in use. A certain amount of wear.

When the preloaded bearing is subjected to a working load, the relative displacement of the inner and outer rings in the radial and axial directions is greatly reduced than that of the unpreloaded bearing. For tapered roller bearings with positioning preload, the amount of preload is reduced due to the running-in between the ribs and the roller end surface, so the temperature of the bearing running-in for a period of time also decreases accordingly. The greater the preload, the more significant the temperature drop caused by the running-in of the roller and the rib. The rougher the surface roughness, the more the preload reduction caused by running-in. During constant pressure preloading, even if running-in occurs, the actual level of bearing clearance (preloading) and axial load does not change. Therefore, the bearing temperature does not change.

What effect does preload and speed have on bearing stiffness?

The stiffness of machine tool spindle bearing is an important performance index. The stiffness is not only related to the load and speed, but also related to the frictional heat and preload mode. Stiffness calculation is also the basis for the analysis of the dynamic characteristics of the spindle unit.

1. The influence of preload mode and speed

Under constant pressure preload, the radial stiffness of the bearing increases slightly with the increase of speed, but the axial and angular stiffness decrease rapidly. Under positioning preload, the radial, axial and angular stiffness of the bearing all increase rapidly with the increase of speed, but the increase in axial and angular stiffness is relatively gentle. The stiffness change rule of ceramic ball bearings is similar to that of all-steel bearings, but the changes are relatively gentle. Under positioning preload, the centrifugal force of the inner ring and the ball, as well as the frictional heat, increase the contact load of the inner and outer rings. At the same time, the contact angle of the outer ring decreases and the contact angle of the inner ring increases, thereby increasing the contact stiffness, but the outer ring contacts The decrease in angle slows down the increase in axial and angular stiffness.     ERIC BEARING LTD can supply SKF 6334 M/C3  bearings , rich stock with very good price , if you are interested , pls do free to contact us . 

Under constant pressure preload, the centrifugal force of the ball increases, so that the contact load of the outer ring increases, while the contact angle decreases. Since the inner and outer rings allow axial displacement, the contact load of the inner ring remains basically unchanged, but the contact angle increases. Thermal displacement and centrifugal displacement have almost no effect on the contact load and contact angle of the inner and outer rings. Although the normal contact stiffness of the outer ring increases, the normal contact stiffness of the inner ring remains basically unchanged. The result of the series action increases the radial stiffness, but not much. The decrease in the contact angle of the outer ring makes the axial and angular stiffness Significantly reduced. Under positioning preload, the stiffness of ceramic ball bearings is lower than that of all-steel bearings, while under constant pressure preloading, the stiffness of ceramic ball bearings is greater than that of all-steel bearings. Under positioning preload, the contact load of all-steel bearings is more than twice that of ceramic ball bearings. Although the elastic modulus of ceramic balls is high, the stiffness of all-steel bearings is greater than that of ceramic ball bearings. Under constant pressure preload, the contact load of the inner ring does not change much, and the high elastic modulus of the ceramic ball makes the stiffness of the ceramic ball bearing greater than that of the all-steel bearing.

1. The influence of preload

With the increase of the preload, the radial, axial and angular stiffness of the bearing increase slightly, but the effect is small. Compared with positioning preload, this influence is more significant for constant pressure preload. This is because the increase in pre-tightening load increases the contact angle of the inner and outer rings, and at the same time increases the contact load, thereby increasing the radial, axial and angular rigidity. However, the change in contact load and contact angle caused by the preload load is smaller than the change caused by the speed and displacement of the parts. Therefore, the impact on the stiffness of the bearing is limited. This is also the reason why the change under positioning preload is smaller than that under constant pressure preload.

2. The influence of channel curvature radius

As the radius of curvature of the inner and outer ring channels increases, the radial, axial and angular stiffnesses decrease, but this effect is small. Only the stiffness changes under positioning preload are slightly more obvious, which is due to the curvature of the channel. The increase in radius increases the amount of contact deformation. Therefore, it is generally not necessary to consider its influence on the stiffness when selecting the channel curvature radius.

3. The influence of the number of balls

Under positioning preload, the increase in the number of balls will slightly increase the radial, axial and angular stiffness. The increase in the number of balls increases the stiffness, but under the same preload, the increase in the number of balls will reduce the contact load. The result of their combined effect can increase the stiffness of the bearing, but it is less.

Under constant pressure preloading, the increase in the number of balls causes the radial stiffness to increase significantly, while the axial and angular stiffness decreases when the speed increases to a certain value, but the change is small. This is because under constant pressure preloading, the increase in the number of balls reduces the contact load of the inner ring, but at the same time reduces the contact angle of the inner ring. Their combined effect makes the radial stiffness of the bearing increase significantly, while the axial and angular stiffness are slightly increased. There is a reduction.

Therefore, when the number of balls increases, the preload load should be increased accordingly. Only when the contact load is the same, the increase in the number of balls can increase the stiffness of the bearing.