1. Under normal circumstances, a transition fit is adopted for bearing matching, but an interference fit can be selected in special cases, though such cases are rare. Since the fit between the bearing and the shaft is that of the bearing inner ring and the shaft and adopts the hole-based system, the bearing must be perfectly aligned with the zero datum (referring to the alignment of tolerance zone datums), which can also be understood in actual use as such. However, to prevent the inner ring from rotating and abrading the shaft surface when the bearing inner ring is mated with the shaft at the minimum limit dimension, a slight interference of 0 to several micrometers (μm) is reserved for the bearing inner ring to ensure the inner ring does not rotate. Therefore, a transition fit is generally sufficient for bearings. Even if a transition fit is selected, the interference shall not exceed 3 tenths of a millimeter (1 tenth = 0.01 mm).

The fit accuracy grade is usually Grade 6, and may sometimes be determined according to materials and processing techniques. Theoretically, Grade 7 accuracy is slightly lower, while a Grade 5 fit requires grinding processing to achieve.

For the fit between the bearing inner ring and the shaft, the shaft is generally selected with K6 or K7 tolerance zones.

2. Tolerance Standards for the Fit Between Bearing and Shaft

1. When the bearing inner diameter tolerance zone is mated with the shaft tolerance zone, the tolerance codes of original transition fits in the general hole-based system (such as k5, k6, m5, m6, n6, etc.) will become interference fits with a small interference; when the bearing inner diameter is mated with shaft tolerance zones such as h5, h6, g5, g6, it is no longer a clearance fit but an interference fit.

2. The bearing outer diameter tolerance zone is also a special tolerance zone due to its different tolerance values from the general shaft-based datum. In most cases, the bearing outer ring is fixed in the housing bore, and some bearing components need to be adjusted according to structural requirements. Therefore, its fit should not be too tight, and the commonly used tolerance zones are H6, H7, J6, J7, Js6, Js7, etc.

3. Examples

The shaft tolerance is usually marked as 0~+0.005 millimeters (mm). If the disassembly is infrequent, an interference fit of +0.005~+0.01 mm can be adopted; if frequent disassembly is required, a transition fit is used. Meanwhile, the thermal expansion of the shaft's own material during rotation must be considered. Therefore, the larger the bearing specification, the more suitable a clearance fit of -0.005~0 mm, and the maximum clearance fit shall not exceed 0.01 mm. In addition, a distinction must be made between the rotating ring interference (interference fit between the rotating component and the shaft) and the stationary ring clearance (clearance fit between the fixed component and the housing).

4. Fit Tolerance

Fit tolerance refers to the sum of the tolerances of the hole and the shaft that form a fit, and is the allowable variation of clearance or interference. The size and position of the tolerance zones of the hole and the shaft together constitute the fit tolerance: the size of the hole-shaft fit tolerance reflects the fit accuracy of the hole and the shaft; the size and position of the hole-shaft fit tolerance zone reflect both the fit accuracy and the fit nature (clearance, transition or interference). The specific relationship is: Fit tolerance size = Tolerance zone size; Fit tolerance zone size and position = Fit nature.

5. Selection of Tolerance Grade

The tolerance grade of the shaft or bearing housing bore matched with the bearing must be matched with the bearing accuracy. For shafts mated with P0 grade precision bearings, the tolerance grade is usually IT6, and the bearing housing bore is usually IT7; for occasions with high requirements for rotational accuracy and operational stability (such as motors), the shaft should be selected with IT5 tolerance grade, and the bearing housing bore with IT6.

6. Selection of Tolerance Zone

According to the magnitude of the equivalent radial load P, it can be divided into three categories: "light", "normal" and "heavy" loads. Their relationship with the basic dynamic load rating C of the bearing is: Light load P≤0.06C, Normal load 0.06C＜P≤0.12C, Heavy load 0.12C＜P.

1. Shaft Tolerance Zone

For shafts fitted with radial bearings and angular contact bearings, the tolerance zones shall refer to the corresponding tolerance zone tables. In most cases, when the shaft rotates and the radial load direction is fixed (i.e., the bearing inner ring rotates relative to the load direction), a transition fit or interference fit should generally be selected; when the shaft is stationary and the radial load direction is fixed (i.e., the bearing inner ring is stationary relative to the load direction), a transition fit or a small clearance fit (excessively large clearance is not allowed) can be selected.

2. Housing Bore Tolerance Zone

For housing bores of radial bearings and angular contact bearings, the tolerance zones shall refer to the corresponding tolerance zone tables. Attention should be paid to the selection: a clearance fit should be avoided for outer rings that oscillate or rotate in the load direction; the magnitude of the equivalent radial load also affects the selection of the outer ring fit.

3. Selection of Bearing Housing Structure

Unless there are special requirements, the bearing housing of rolling bearings generally adopts an integral structure. A split bearing housing can only be used when assembly is difficult or "easy assembly" becomes the main consideration, and it is not suitable for excessively tight or high-precision fits (such as K7 and fits tighter than K7); for example, a split bearing housing shall not be used for housing bores with IT6 tolerance grade or higher precision.