1、 Bearing fit is generally a transitional fit, but in special circumstances, interference fit can be selected, but it is rare. Because the fit between the bearing and the shaft is based on the inner ring of the bearing and the shaft, using a base hole system, the bearing should be completely aligned with zero. In practical use, we can also consider it this way. However, in order to prevent inner ring rolling and damage to the surface of the shaft when the minimum limit size of the bearing inner ring is matched with the shaft, our bearing inner rings have a lower tolerance of 0 to a few micrometers to ensure that the inner ring does not rotate. Therefore, bearings generally choose a transition fit, and even if a transition fit is chosen, it cannot exceed a 3-wire interference fit.

Generally, a precision level of 6 is chosen for coordination, and sometimes it also depends on the material and processing technology. In theory, a precision level of 7 is a bit too low, while a precision level of 5 requires grinding.
The general selection is: the inner ring of the bearing is matched with the shaft, and the shaft is selected as K6. The outer ring of the bearing is matched with the hole, and the hole is selected as K6 or K7.
2、 Tolerance standards for the fit between bearings and shafts
1. When the tolerance zone of the bearing inner diameter forms a fit with the tolerance zone of the shaft, the tolerance code that originally belonged to the transitional fit in the general base hole system will become an over fit, such as k5, k6, m5, m6, n6, etc., but the over fit amount is not large; When the inner diameter tolerance of the bearing is used to form a fit with h5, h6, g5, g6, etc., it is no longer a clearance but an over fit.
2. The outer diameter tolerance zone of bearings is a special tolerance zone due to its different tolerance values from general reference shafts. In most cases, the outer ring is fixed when installed in the housing hole, and some bearing components require structural adjustment. Its fit should not be too tight and is often matched with H6, H7, J6, J7, Js6, Js7, etc.
3、 Give examples to illustrate
In general, the shaft is marked with 0 to+0.005. If it is not frequently disassembled, an interference fit of+0.005 to+0.01 is sufficient. If frequent disassembly is required, a transitional fit is sufficient. We also need to consider the thermal expansion of the shaft material itself during rotation, so the larger the bearing, the better the clearance fit is -0.005~0, and the maximum clearance fit should not exceed 0.01. Another issue is dynamic interference and static clearance.
4、 Match tolerance
The sum of the tolerances of the holes and shafts that make up the fit. It is the allowable variation in gap or interference. The size and position of tolerance zones for holes and shafts constitute the fitting tolerance. The size of the tolerance between the hole and the shaft indicates the accuracy of the fit between the hole and the shaft. The size and position of the tolerance zone for hole and shaft fit indicate the accuracy and nature of the fit between the hole and shaft. The size of the fitting tolerance is equal to the size of the tolerance zone; Fit tolerance zone size and position=fit properties.
5、 Selection of tolerance level
The tolerance level of the shaft or bearing seat hole that matches the bearing is related to the accuracy of the bearing. The tolerance level of the shaft matched with P0 level precision bearings is generally IT6, and the bearing seat hole is generally IT7. In situations where high requirements are placed on rotational accuracy and smooth operation, such as electric motors, the shaft should be selected as IT5 and the bearing seat hole as IT6.
6、 Selection of tolerance zone
The equivalent radial load P is divided into several cases, including "light", "normal", and "heavy" loads, and its relationship with the rated dynamic load C of the bearing is: light load P ≤ 0.06C, normal load 0.06C ＜ P ≤ 0.12C, and heavy load 0.12C ＜ P.
1. Axis tolerance zone
The tolerance zone for installing radial bearings and angular contact bearings on the shaft shall refer to the corresponding tolerance zone table. In most cases, when the shaft rotates and the radial load direction remains unchanged, that is, when the inner ring of the bearing rotates relative to the load direction, a transition or interference fit should generally be selected. When the shaft is stationary and the radial load direction remains unchanged, that is, the inner ring of the bearing is stationary relative to the load direction, a transition or small clearance fit can be selected (too large a clearance is not allowed).
2. Shell hole tolerance zone
Refer to the corresponding tolerance zone table for the tolerance zone of the housing holes for installing radial bearings and angular contact bearings. When selecting, attention should be paid to the outer ring that swings or rotates in the direction of the load, and clearance fit should be avoided. The magnitude of the equivalent radial load also affects the selection of the outer ring fit.
3. Selection of bearing seat structure form
Unless there is a special need, the bearing seat of rolling bearings generally adopts an integral structure. Split type bearing seats are only used when there are difficulties in assembly or when the advantage of convenience in assembly becomes the main consideration. However, they cannot be applied to tight fit or more precise fit, such as K7 and tighter fit than K7, or seat holes with tolerance level IT6 or more precision, which cannot use split type bearing seats.

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