There are two ways of axial preload of bearings, one is fixed pressure preload and the other is positioning preload. Under constant pressure preload, the radial stiffness of the bearing increases slightly with the increase of speed, while the axial and angular stiffness decrease rapidly. Positioning preload, bearing radial, axial and angular stiffness are rapidly increased with the increase of speed, but the increase of axial and angular stiffness is relatively flat. 1、The effect of preload With the increase of preload, the radial, axial and angular stiffness of the bearing increases. Axial and angular stiffness increase slightly, but the effect is very small. Compared with the positioning preload, this effect is more significant on the fixed pressure preload. This is because the preload increases the contact angle of the inner and outer ring, and also makes the contact load increase, so that the radial, axial and angular stiffness have increased. However, the contact load and contact angle changes caused by preload are small compared to those caused by speed and part displacement, so the effect on bearing stiffness is limited. This is also the reason why the change under positioning preload is smaller than that of fixed pressure preload. 2, the influence of the radius of curvature of the groove As the radius of curvature of the inner and outer ring groove increases, radial, axial and angular stiffness decreases, but this effect is very small, only the change in stiffness under positioning preload is slightly more obvious, which is due to the increase in the radius of curvature of the groove contact deformation increases. Therefore, the general choice of the radius of curvature of the channel can not consider its influence on the stiffness. 3、The influence of the number of balls Under positioning preload, the ball number increases to make the radial, axial and angular stiffness slightly increase. The number of balls increases the stiffness, but the same preload, the number of balls will be increased to reduce the contact load, the result of their joint action, although the bearing stiffness increased, but less. Under the fixed pressure preload, the number of balls increases the radial stiffness, and when the speed increases to a certain value of axial and angular stiffness, but the change is very small. This is due to the fixed pressure preload, the ball number increases although the inner ring contact load is reduced, but at the same time make the inner ring contact angle reduced, their joint action makes the bearing radial stiffness increased significantly, while the axial and angular stiffness slightly reduced. Therefore, the number of balls should be increased accordingly to improve the preload, only when the contact load is the same one, increase the number of balls to make the bearing stiffness increase. 4、The influence of the ball diameter Positioning preload, ball diameter increases, radial, axial and angular stiffness increased slightly. Ball diameter increases the ball centrifugal force increases, the outer ring contact angle decreases, the inner ring contact angle increases, but at the same time make the inner and outer ring contact load increases, their combined effect of the results of the bearing stiffness increases. Since the centrifugal force change under positioning preload has less effect on the contact load, so the ball diameter change has little effect on the stiffness. Under the fixed pressure preload, the ball diameter increases radial stiffness increases, while the axial and angular stiffness is reduced, but the impact is small. This is because the ball diameter increases the centrifugal force of the ball, the inner and outer ring contact angle decreases, the outer ring contact load increases, while the inner ring contact load basically remains the same, so the radial stiffness increases, while the axial and angular stiffness is slightly reduced. Therefore, reducing the ball diameter not only improves the speed performance, but also does not reduce the stiffness performance. This also proves from theoretical divisions that reducing the diameter of the ball is one of the current development trends of spindle bearings. 5、The influence of the initial contact angle Positioning preload, the initial contact angle increases so that the radial stiffness is significantly reduced, axial and angular stiffness increased significantly. This is because the initial contact angle increases, the radial component of contact stiffness decreases and the axial component increases, meanwhile, the contact load decreases under the same preload. Under constant pressure preload, the radial stiffness is significantly reduced by increasing the initial contact angle; at low speed, the axial and angular stiffness increases, and at high speed, there is basically no change. This is due to the fact that under constant pressure preload, the inner and outer rings allow axial displacement, and in order to maintain the force balance, the outer ring contact angle is almost close to 0. The initial contact angle size has basically no effect on the outer ring contact angle. Similarly, the initial contact angle increases, the contact load decreases under the same preload. Therefore, increasing the initial contact angle under positioning preload can improve the axial and angular stiffness, while increasing the initial contact angle under constant preload not only cannot improve the axial and angular stiffness, but also reduces the radial stiffness.

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