In rotating machinery, the fit accuracy between bearings and shafts directly affects the rotational accuracy, service life, and operational stability of the equipment. Excessive interference can cause bearing deformation or even damage, while excessive clearance may lead to bearing slippage and increased vibration, affecting performance. Therefore, in mechanical design, selecting appropriate tolerances for the fit between bearings and shafts is crucial for ensuring reliable operation of the equipment.

1. Basic fit relationship between bearing and shaft

The fit between the bearing and the shaft mainly involves the fit between the inner ring of the bearing and the shaft. The common types of fit are as follows:

How to choose the tolerance fit between bearing and shaft?

The choice of bearing fit mainly depends on the type of load, operating temperature, installation method and operation requirements. The following are recommended tolerance matches for different application scenarios:

1. The effect of load type on cooperation

Different types of loads have different requirements for bearing fit.

Rotating load (such as motor shaft, machine tool spindle)

The inner ring of the bearing rotates relative to the shaft, so it needs to be overfit to prevent slipping. K6, m5, n6 and other tolerance grades are recommended.

Static loads (e.g. press support bearings)

The inner ring of the bearing does not rotate relative to the shaft, and clearance fit or transition fit is allowed. Tolerances such as h5, h6 are recommended.

Swing load (such as engine rocker arm bearing)

The inner ring of the bearing has slight oscillation on the shaft, and excessive interference will lead to fatigue damage. Therefore, transitional fit (such as js5, k5) is recommended.

2. Influence of temperature change on tolerance

In high temperature environment, the expansion of the shaft may lead to an increase in the interference and even jam the bearing. Therefore:

Normal temperature operation (-20℃~100℃): can be directly selected according to the standard tolerance.

High temperature operation (> 100℃): the amount of interference should be reduced appropriately. It is recommended to use m5 or n5 instead of tighter fit.

Low temperature environment (-20℃): A slightly larger interference can be used to compensate for the low temperature shrinkage effect.

3. Shaft manufacturing and installation method

Grinding shaft (high precision shaft): small tolerance, can use more accurate fit, such as k5, n5.

Turning shaft (ordinary machining shaft): the tolerance range is large, it is recommended to use n6 and m6 to avoid assembly difficulties.

Cold assembly: h6/m6 is recommended to use the bearing's tiny interference to lock itself.

Hot assembly: optional n6, the bearing is heated to 120~150℃, and then assembled by thermal expansion and contraction to improve the assembly accuracy.

3. Problems that may be caused by incorrect tolerance matching

✅ Bearing and shaft fit selection should consider load type, temperature change, installation method and manufacturing accuracy.

✅ Overload rotation is recommended to use interference fit, such as k6, m5, n6, to prevent bearing slip.

✅ In high temperature environment, the amount of interference should be reduced, and in low temperature environment, the amount of interference should be increased to prevent loosening.

✅ The shaft with different manufacturing accuracy should be selected with appropriate tolerance range to ensure the reliability of assembly.