Effect of Preloading and Rotating Speed on Bearing Stiffness

Time：2020.11.05  Source：Roller Bearings Suppliers

Bearing as an important part of mechanical rotation, and the stiffness and temperature of the bearing will directly affect the efficiency of the bearing to 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, the pre-tightening rolling bearing is often used, such as: the spindle bearing of the machine tool.

The so-called pretension, is in the installation of a certain method in the bearing and maintain an axial force, in order to eliminate the axial clearance in the bearing, and in the rolling body and inner and outer ring contact deformation.

Due to the effect of prestressing force, rolling element and the inner and outer contact will produce elastic deformation, and the increase of contact area, increase in the number of rolling body is involved in tolerance, also could be in the range of more than 180 degrees in roller bearing, and sometimes may also be within the scope of the 360 degrees all rolling element bearing, and in so doing, certainly more than a handful of rolling element bearing condition is better, but also to the load.From the above discussion, it can be seen that the contact deformation of the preloaded bearing is certainly smaller than that of the unpreloaded bearing when it is working under the same load. 

Therefore, the bearing's bearing stiffness can be improved and the wear of the bearing in use can be compensated at the same time.

When the bearing is preloaded, the relative displacement of the inner and outer ring in radial and axial direction will be greatly reduced compared with the bearing without preloaded.The positioning of pre-tightened tapered roller bearing reduces the pre-tightened amount due to the running close of the side and the roller end face, so the temperature of the bearing during the running period decreases correspondingly.The higher the preload is, the more the temperature drop is caused by roller and side running.The rougher the surface roughness, the more preload reduction caused by running andclosing

During constant pressure preloading, the bearing clearance (preloading) and the actual level of axial load do not change even if run-on occurs. Therefore, the bearing temperature does not change.

Bearing rigidity of machine tool spindle is an important performance index.Stiffness is related not only to load and rotational speed, but also to friction heat and preloading mode.Stiffness calculation is also the foundation of dynamic characteristic analysis of spindle unit.Need ZZ 22320 E,click here to learn more.

Firstly, the influence of pre-tightening mode and speed

Under constant pressure preloading, the radial stiffness of the bearing increases slightly with the increase of rotating speed, while the axial and angular stiffness decreases rapidly.Under pre-positioning, the bearing's radial, axial and angular stiffness increase rapidly with the increase of rotating speed, but the axial and angular stiffness increase gently.The stiffness variation of ceramic ball bearings is similar to that of all steel bearings, but the variation is relatively gentle.Under the positioning pre-tightening, the centrifugal force of the inner ring and the ball, as well as the frictional heat, increase the contact load of the inner ring, while the contact Angle of the outer ring decreases, and the contact Angle of the inner ring increases, thus increasing the contact stiffness. 

However, the reduction of the contact Angle of the outer ring slows down the increase of axial and angular stiffness.

1. Influence of preload

With the increase of preload, the radial, axial and angular stiffness of the bearing increases slightly, but the influence is very small.Compared with positioning preloading, this effect is more significant for constant pressure preloading.This is because the increase of the pre-load increases the contact Angle of the inner and outer rings, and also increases the contact load, thus increasing the radial, axial and angular stiffness.

However, the change of contact load and contact Angle caused by preload is smaller than that caused by rotation speed and part displacement, so the influence on bearing stiffness is limited.This is also the reason why the change under positioning preloading is less than that under constant pressure preloading.

2. Influence of the radius of channel curvature

With the increase of the radius of curvature of the inner and outer ring of the channel, the radial, axial and angular stiffness decreases. However, this effect is small, and only the change of stiffness is slightly more obvious under positioning preloading. This is because the increase of the radius of curvature of the channel increases the amount of contact deformation.Therefore, the influence of the radius of curvature on the stiffness can be ignored when selecting the radius of curvature of the channel.

3. Influence of ball count

The radial, axial and angular stiffness increases slightly when the number of balls increases.The increase in the number of balls increases the stiffness, but under the same pre-load, the increase in the number of balls will reduce the contact load. The result of their joint action can increase the stiffness of the bearing, but less.

Under constant pressure preloading, the radial stiffness increases obviously when the number of balls increases, while the axial and angular stiffness decreases when the rotating speed increases to a certain value, but the change is very 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 increases the radial stiffness of the bearing significantly, while the axial and angular stiffness slightly decrease.

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

4. the ball diameter of the shadow

Under the positioning preload, the ball diameter increases, and the radial, axial and angular stiffness increases slightly.The increase of ball diameter increases the centrifugal force of the ball, decreases the contact Angle of the outer ring and increases the contact Angle of the inner ring, but at the same time increases the contact load of the inner ring. The result of their combined action increases the bearing stiffness.

The influence of centrifugal force variation on contact load is small under the pre-tightening of orientation, so the influence of ball diameter variation on stiffness is small.Click here to learn more about ZZ 22320-E1A-MA-T41A.

Under constant pressure preloading, the radial stiffness increases with the increase of ball diameter, while the axial and angular stiffness decreases on the contrary, but the influence is small.This is because the increase of ball diameter increases the centrifugal force of the ball, reduces the contact Angle of the inner and outer rings, and increases the contact load of the outer ring, while the contact load of the inner ring remains basically unchanged. Therefore, the radial stiffness increases, while the axial and angular stiffness slightly decreases.Therefore, reducing the ball diameter not only improves the speed performance, but also does not reduce the stiffness performance.This also proves from theory that reducing small diameter ball is one of the development trends of spindle bearing at present.

Secondly,the relationship between bearing rigidity and preload

It is an important problem to determine the bearing axial preload of high speed motorized spindle unit with angular contact ceramic ball bearing.The increase of bearing axial preload can improve the bearing in high speed operation caused by centrifugal force and gyro torque to reduce the roll ratio, and can also improve the rigidity of the spindle.Because the rigidity of motorized spindle generally refers to the radial rigidity, so the bearing from the bearing preload on the radial rigidity research and analysis.