The adjustment and preloading of rolling bearing clearance are crucial steps in the assembly of rolling bearings. Mastering the technical concepts of clearance adjustment and preloading accurately and applying this technical method correctly in assembly work are the keys to ensuring the quality of bearing assembly. The clearance of a rolling bearing refers to the maximum displacement that one bearing ring can produce in the radial or axial direction when the other ring is fixed. Accordingly, the clearance is divided into two types: radial clearance and axial clearance.

During the assembly of rolling bearings, the clearance should be neither too large nor too small. Excessively large clearance will reduce the number of rolling elements bearing the load at the same time, and the load on a single rolling element will increase accordingly, which in turn lowers the rotational accuracy of the bearing and shortens its service life. Excessively small clearance will increase friction, generate more heat, and aggravate wear, also shortening the bearing's service life. Therefore, the clearance of many bearings needs to be strictly controlled and adjusted during assembly.

Preloading refers to the operation of applying an axial force to the inner or outer ring of the bearing during assembly to eliminate the bearing clearance and cause initial deformation at the contact parts between the rolling elements and the inner and outer rings. Preloading can improve the rigidity and rotational accuracy of the bearing under working conditions. Most bearings that bear large loads and have high requirements for rotational accuracy need to work in a state of zero clearance or even a small amount of interference, and in such cases, the bearing must be preloaded during assembly. Usually, the adjustment and preloading of clearance can be achieved by axially moving the inner ring of the bearing relative to the outer ring.

It can be easily seen from the above technical concepts that adjusting the clearance of rolling bearings can improve the bearing capacity and rotational accuracy of the bearings and extend their service life, but it will also intensify bearing friction and increase heat generation. Therefore, good lubrication and heat dissipation conditions must be ensured when adjusting or preloading the clearance. Improper adjustment or poor lubrication will instead aggravate bearing wear and shorten its service life. Therefore, the following issues should also be noted when correctly carrying out the clearance adjustment and preloading of rolling bearings:

I. Assembly technical requirements are the fundamental basis for selecting assembly process methods

Adjusting the clearance of rolling bearings can effectively improve their rotational accuracy and bearing capacity, extend service life, and reduce vibration and noise, but not all rolling bearings require clearance adjustment during assembly. Although preloading can improve the rigidity and rotational accuracy of bearings, it will intensify friction, damage the lubricating oil film, and generate a large amount of heat at the same time. Therefore, preloaded bearings must adopt forced lubrication and cooling measures. This technical method is only used in scenarios with extremely high requirements for bearing rigidity and rotational accuracy, and is a relatively special technical means. In production practice, it is only applied to the assembly of machine tool spindles and is almost never used in the bearing assembly of other transmission mechanisms.

Whether clearance adjustment and preloading are required during the assembly of rolling bearings shall be determined in accordance with the assembly technical requirements specified in technical documents. Specifically, assembly technical requirements usually stipulate that bearings for high-speed, heavy-load or high rotational accuracy applications need clearance adjustment or preloading; otherwise, the original clearance of the bearing shall be retained, and only axial fixing is required during assembly. In terms of bearing types, tapered roller bearings, angular contact bearings and thrust bearings require clearance adjustment; for general low-speed, light-load radial ball bearings, clearance adjustment is not required in most cases, and only axial fixing is needed.

II. Clearance adjustment and preloading must meet the requirements under thermal equilibrium conditions

The actual ideal working clearance of a rolling bearing is the clearance adjusted after the bearing temperature rise stabilizes. Therefore, the adjustment of bearing clearance should be carried out in two stages: first, adjust the bearing clearance at room temperature in accordance with relevant operating procedures and technical requirements until the clearance is appropriate, and the bearing should rotate flexibly without jamming when turned by hand; then, loosen the adjustment mechanism appropriately (to prevent sudden seizure of the bearing due to temperature rise during testing) and conduct an idle running test. The idle running time from low speed to high speed should not exceed 2 hours, and the idle running time at the maximum speed should not be less than 30 minutes. During the test, the bearing should run flexibly with low noise, and the working temperature should not exceed 50℃; finally, reset and lock the adjustment mechanism.

III. Maintain a good lubrication state

Good lubrication can not only reduce friction but also cool the bearing and shaft components. After the clearance of the rolling bearing is adjusted, friction will intensify, and the generated heat will increase the temperature of the entire transmission system. If the heat cannot be dissipated in a timely manner, the dimensions of the transmission components will change, which in turn affects the stability of the rolling bearing clearance and generates more heat, forming a vicious circle. Therefore, for rolling bearings with adjusted clearance, it is not only necessary to maintain good lubrication to reduce friction, but more importantly, continuous circulating lubricating oil must be used to take away a large amount of heat, control temperature rise, and achieve thermal equilibrium of the transmission system.

Before conducting the rotation test, it is necessary to first check whether the oil supply at all parts of the lubrication system is normal, especially for the preloaded bearing parts, special attention should be paid to whether the lubricating oil supply is sufficient and the working state is good.

To sum up, correctly assembling and rationally using rolling bearings in accordance with process requirements, and earnestly implementing the clearance adjustment and preloading processes of rolling bearings are effective means to improve the rotational accuracy and bearing capacity of bearings and reduce the vibration and noise of the transmission system. In addition to meeting the general technical requirements for rolling bearing assembly, the impact of bearing temperature rise and lubrication on the adjustment work must be a key consideration during operation. After completing the idle running test, a detailed inspection and secondary adjustment should be carried out to ensure patient and meticulous operation.