There are two types of axial preload for bearings, one is constant pressure preload and the other is positioning preload. Under constant pressure preload, the radial stiffness of the bearing increases slightly as the rotational speed increases, while the axial and angular stiffness decrease rapidly. Under positioning preload, the radial, axial and angular stiffness of the bearing increase rapidly with the increase of rotation speed, but the increase of axial and angular stiffness is relatively gentle.
1. Effect of preload. As preload increases, the radial, axial, and angular stiffnesses of the bearing increase slightly, but the effect is small. Compared with fixed-position preload, this effect is more pronounced with constant-pressure preload. Increased preload raises both the inner- and outer-ring contact angles and the contact load, thereby increasing radial, axial, and angular stiffness. However, the changes in contact load and contact angle caused by preload are smaller than those caused by rotational speed and component displacement, so their effect on bearing stiffness is limited.
This is also the reason why the change under positioning preload is smaller than that under constant pressure preload.
2, influence of channel curvature radius As the channel curvature radius of the inner and outer rings increases, the radial, axial and angular stiffness decreases, but this effect is very small. Only the change in stiffness under positioning preload is slightly more obvious. This is because the increase in the channel curvature radius increases the contact deformation. Therefore, its effect on stiffness can generally be ignored when selecting the channel curvature radius.
3, the influence of the number of balls Under positioning preload, the increase in the number of balls slightly increases the radial, axial and angular stiffness. The increase in the number of balls will increase the stiffness, but under the same preload load, the increase in the number of balls will reduce the contact load. Although their combined effect can increase the stiffness of the bearing, it is less. Under constant pressure preload, the increase in the number of balls causes the radial stiffness to increase significantly, while when the rotational speed increases to a certain value, the axial and angular stiffness decrease, but the change is very small.
This is because under constant pressure preload, the increase in the number of balls reduces the contact load of the inner ring, but also reduces the contact angle of the inner ring. Their combined effect significantly increases the radial stiffness of the bearing, while the axial and angular stiffness slightly decreases. Therefore, when the number of balls increases, the preload load should be increased accordingly. Only when the contact load is the same, increasing the number of balls can increase the bearing stiffness.
4. Effect of ball diameter Under fixed-position preload, increasing ball diameter slightly increases radial, axial and angular stiffness. Larger balls experience greater centrifugal force, reducing the outer-ring contact angle and increasing the inner-ring contact angle. At the same time, the contact loads on both rings increase. The combined result is greater bearing stiffness. Since changes in centrifugal force have relatively little effect on contact load under fixed-position preload, changes in ball diameter have only a very small effect on stiffness. Under constant-pressure preload, increasing ball diameter increases radial stiffness but reduces axial and angular stiffness, although the effect is small.
This is because as the ball diameter increases, the centrifugal force of the ball increases, the contact angle of the inner and outer rings decreases, and the contact load of the outer ring increases, while the contact load of the inner ring remains basically unchanged. Therefore, the radial stiffness increases, while the axial and angular stiffness decrease slightly. Therefore, reducing the ball diameter not only improves speed performance, but does not reduce stiffness performance. This also proves theoretically that reducing the diameter of the ball is one of the current development trends of spindle bearings.
5. Effect of the initial contact angle Under fixed-position preload, increasing the initial contact angle significantly reduces radial stiffness and markedly increases axial and angular stiffness. This is because the radial component of contact stiffness decreases and the axial component increases as the initial contact angle increases. At the same preload, the contact load also decreases. Under constant-pressure preload, increasing the initial contact angle significantly reduces radial stiffness. At low speeds, axial and angular stiffness increase, while at high speeds they remain essentially unchanged. This is because constant-pressure preload permits axial displacement of the inner and outer rings. To maintain force equilibrium, the outer-ring contact angle approaches 0, and the initial contact angle has virtually no effect on the outer-ring contact angle.
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 improve the axial and angular stiffness, while increasing the initial contact angle under constant pressure preload not only cannot improve the axial and angular stiffness, but also reduces the radial stiffness.
(Source: Rolling Mill Bearings)