Tolerance selection of bearing and shaft fit

Bearing is an important component in contemporary mechanical equipment. Its main function is to support the mechanical rotating body, reduce the friction coefficient during its movement, and ensure its rotation accuracy.

First, the bearing fit is generally a transition fit, but in special cases, an interference fit is optional, but it is rare. Because the match between the bearing and the shaft is the match between the inner ring of the bearing and the shaft, the base hole system is used. Originally, the bearing should be completely zero. We can also think so in actual use. When the minimum limit size is matched, the inner ring rolls and damages the surface of the shaft. Therefore, our bearing inner ring has a lower deviation tolerance of 0 to several μ to ensure that the inner ring does not rotate. Therefore, the bearing generally chooses transition fit, even if Even if the transition fit is selected, the interference should not exceed 3 wires.

Generally, level 6 is selected for the matching accuracy level, and sometimes it depends on the material and processing technology. In theory, level 7 is a bit low, and if it is matched with level 5, grinding is required.

The general selection is: the inner ring of the bearing and the shaft are matched with the shaft to choose k6, the outer ring of the bearing and the hole to be matched with the hole to choose K6 or K7.

Second, the matching tolerance standard of the bearing and the shaft

1. When the inner diameter of the bearing When the tolerance zone and the shaft tolerance zone form a fit, the tolerance code that was originally a transition fit in the general base hole system will become an over-win fit, such as k5, k6, m5, m6, n6, etc., but the over-win amount is not large; when the bearing When the inner diameter tolerance is matched with h5, h6, g5, g6, etc., it is not a clearance but a win-win fit.

2. The bearing outer diameter tolerance zone is also a special tolerance zone because the tolerance value is different from the general reference shaft. In most cases, the outer ring is fixed in the housing hole, and some bearing components require structural requirements. Adjustment, its coordination should not be too tight, often with H6, H7, J6, J7, Js6, Js7 and so on.

3. Example

In general, the shaft is generally marked with 0~+0.005. If it is not often removed, it is +0.005~+0.01 interference fit. The disassembly is a transition fit. We also need to consider the thermal expansion of the shaft material itself during rotation, so the larger the bearing is, the better the clearance fit is -0.005 to 0, and the maximum clearance fit should not exceed 0.01. Another one is the interference of the moving coil and the clearance of the static ring.

Fourth, fit tolerance (fit tolerance)

refers to the sum of the hole and shaft tolerances that make up the fit. It is the amount of variation that allows for clearance or interference. The size of the tolerance zone and the position of the tolerance zone for the hole and shaft make up the fit tolerance. The size of the hole and shaft fit tolerance indicates the fit accuracy of the hole and the shaft. The size and position of the hole and shaft fit tolerance zone indicate the fit accuracy and fit nature of the hole and shaft. The size of the fit tolerance u003d the size of the tolerance zone; the size and position of the fit tolerance zone u003d the nature of the fit.

5. Selection of tolerance grades

The tolerance grades of the shaft or bearing seat hole matched with the bearing are related to the bearing accuracy. For the shaft matched with the P0 grade precision bearing, the tolerance level is generally IT6, and the bearing seat hole is generally IT7. For occasions with higher requirements on rotation accuracy and running stability (such as motors, etc.), the shaft should be IT5 and the bearing seat hole should be IT6.

6. Selection of tolerance zone

The equivalent radial load P is divided into 'lightLoad P≤0.06C Normal load 0.06C ＜P≤0.12C Heavy load 0.12C＜P

1、Shaft tolerance zone

For the tolerance zone of the shaft where the radial bearing and angular contact bearing are installed, refer to the corresponding tolerance zone surface. For most occasions, when the shaft rotates and the radial load direction does not change, that is, when the bearing inner ring 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 is unchanged, that is, when the inner ring of the bearing is static relative to the load direction, transition or small clearance fit can be selected (too much clearance is not allowed).

2. Tolerance zone of housing hole

For the housing hole tolerance zone of radial bearing and angular contact bearing, please refer to the corresponding tolerance zone table. When selecting, pay attention to avoid clearance fits for outer rings that oscillate or rotate in the direction of load. The size of the equivalent radial load also affects the fit selection of the outer ring.

3. Selection of bearing housing structure

Unless there are special needs, the bearing housing of the rolling bearing generally adopts an integral structure. The split bearing housing is only difficult to assemble, or it is convenient to assemble. It is only used when the advantages are the main consideration, but it cannot be used for tight or more precise fits, such as K7 and tighter fits than K7, and for seat holes with a tolerance class of IT6 or more. type bearing seat.