The Influence of Preload and Speed on INA Bearing Stiffness

Writer: Eric Bearing Limited

INA bearings are important parts of mechanical rotation, and the stiffness and temperature of INA bearings will directly affect the efficiency of the bearing for 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, Preloaded rolling bearings are often used, 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 INA 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 is elastically deformed, and the contact area is increased, and the number of rolling elements participating in the bearing force increases, and it is possible to roll in a range greater than 180 degrees. The body participates in the force, and sometimes even all the rolling elements are subjected to force within a 360-degree range. 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 is definitely smaller than that of the 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 INA 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 larger 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 (preload) and axial load does not change. Therefore, the temperature of the INA bearing does not change.

What effect do 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 frictional heat and preloading methods. Stiffness calculation is also the basis for the analysis of the dynamic characteristics of the spindle unit.  INA  NKI65/35-XL bearings online , pls click here :

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.

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, while 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) 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 the 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 stiffness. 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 the radius of curvature of the channel

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 pronounced. This is due to the channel curvature. The increase in radius increases the amount of contact deformation. Therefore, when selecting the radius of curvature of the channel, its effect on stiffness can be ignored.

(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 rigidity, 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 rigidity 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 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.

(4) The shadow of the ball diameter

Under positioning preload, the diameter of the ball increases, and the radial, axial and angular stiffness increase slightly. The increase of the ball diameter increases the centrifugal force of the ball, reduces 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 and outer rings. As a result of their combined action, the stiffness of the bearing increases. Since the change of centrifugal force under positioning preload has little influence on the contact load, the change of ball diameter has little influence on the stiffness.

Under constant pressure preload, the radial stiffness of the ball diameter increases, while the axial and angular stiffness decrease, but the effect is small. This is because the increase of the ball diameter increases the centrifugal force of the ball, the contact angle of the inner and outer rings decreases, the contact load of the outer ring increases, and the contact load of the inner ring remains basically unchanged, so the radial stiffness increases, while the axial and angular stiffness decrease slightly. . Therefore, reducing the ball diameter not only improves the speed performance, but also does not reduce the stiffness performance. This also proved theoretically that reducing the diameter ball is one of the current development trends of spindle bearings.

(5) The influence of initial contact angle

Under positioning preload, the increase of the initial contact angle significantly reduces the radial stiffness, and the axial and angular stiffness increase significantly. This is because the initial contact angle increases, the radial component of the contact stiffness decreases, and the axial component increases. At the same time, the contact load decreases under the same preload.

Under constant pressure preloading, the increase of the initial contact angle significantly reduces the radial stiffness; at low speed, the axial and angular stiffness increase, and at high speed, there is basically no change. This is because the inner and outer rings allow axial displacement under constant pressure preload. In order to maintain the balance of force, the contact angle of the outer ring is almost close to 0, and the initial contact angle has little effect on the contact angle of the outer ring. Similarly, the initial contact angle increases, and the contact load decreases under the same preload.

Therefore, increasing the initial contact angle of the bearing under positioning preload can increase the axial and angular stiffness, while increasing the initial contact angle under constant pressure preloading not only fails to increase the axial and angular stiffness, but reduces the radial stiffness.

2. The relationship between bearing stiffness and preload

The determination of the axial preload force of the high-speed electric spindle unit using angular contact ceramic ball bearings is an important issue. The increase in the axial preload of the bearing can improve the adverse effects caused by the centrifugal force and the gyro torque when the bearing is running at high speed, reduce the spin-roll ratio, and increase the rigidity of the spindle. Because the stiffness of the electric spindle generally refers to the radial stiffness, the radial stiffness of the bearing is studied and analyzed from the bearing preload.

INA bearing stiffness trend As the bearing preload increases, the radial stiffness of the bearing becomes larger, which significantly improves the machining accuracy and work efficiency of the spindle system and improves the working performance of the spindle. Therefore, in the actual industry and mine, it is of great practical engineering significance to increase the preload within the allowable range. However, with the increase of the pre-tightening force, the bearing temperature increases, and the bearing heat generation also increases, which in turn increases the temperature of the spindle system, which seriously affects the working life of the bearing and the working performance of the spindle. Therefore, under the condition that the temperature rise allows, increasing the preload as much as possible is an important factor that needs to be considered when involving the spindle drive system.