The selection of radial clearance for rolling bearings is crucial as it is one of the critical factors that determine whether the bearings can function properly.
Proper selection of radial clearance can ensure the reasonable distribution of loads between the rolling elements of the bearing. It can limit the axial and radial displacement of the shaft (or housing), guarantee the rotational accuracy of the shaft, and enable the bearing to operate under given temperatures while reducing vibration and noise. This is advantageous in improving the service life of bearings.
The difference between the theoretical clearance and the clearance generated by the interference fit of the housing or the shaft with the bearing after expansion or shrinkage of the collar is installed is referred to as “installation clearance”.
Upon adding or subtracting the accumulated dimensional changes due to thermal variations inside the bearing, it is referred to as “operational clearance”.
The operational clearance refers to the existing clearance when the bearing is mounted on a machine and undergoes loading and rotation. The effective clearance plus the elastic deformation generated by bearing loads is known as the “operational clearance”.
As shown in figure 2, the bearing has the longest fatigue life when the operational clearance is marginally negative. However, as the negative clearance increases, the fatigue life of the bearing decreases notably.
Therefore, when selecting the clearance for bearings, it is generally suitable to have a slightly positive or zero value for operational clearance.
When selecting radial clearance for bearings, the following factors should be taken into consideration:
- The working conditions of the bearings, such as loads, temperature, and speed.
- The requirements for bearing performance, such as rotational accuracy, friction torque, vibration, and noise.
- The interference fit between the bearing and the shaft or housing, which reduces the radial clearance of the bearing.
- The temperature difference between the inner and outer rings of the bearing during operation reduces the radial clearance.
- The difference in the coefficient of thermal expansion between the shaft and the housing can cause the radial clearance of the bearing to increase or decrease.
Based on experience, the optimal operational clearance for ball bearings is close to zero, while roller bearings should maintain a small amount of operational clearance.
In components that require good support rigidity, bearings can allow for a certain amount of preloading.
Under normal working conditions, it is recommended to choose the basic component first to achieve a suitable operational clearance for the bearing. If the basic component does not meet the requirements, then an auxiliary component should be chosen.
The large radial clearance auxiliary component is suitable for bearings with interference fit between the bearing and the shaft or housing. The small radial clearance auxiliary component is suitable for applications that require high rotational accuracy, strict control of the axial displacement of the housing, and noise and vibration reduction.
Furthermore, to improve the stiffness of the bearing or to reduce noise, the operational clearance should be further reduced, while to account for severe temperature increase of the bearing, the operational clearance should be further increased. Specific analysis should be conducted based on the usage conditions.