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Deep groove ball bearing base hole system, why is there a negative tolerance?

Speaking of Deep Groove Ball Bearings because I bought several bearings 6204 these days. There is a radial bearing below the rolling bearing branch. The radial bearing includes a deep groove ball bearing or a radial ball bearing. The inner hole size of the base hole system is a negative tolerance.

In order to prevent undue relative sliding of the bearing inner ring and shaft, outer ring and housing hole when the machine is running, the correct fit must be selected. Generally, a proper tight fit between the shaft and the inner ring is the simplest and most effective method to prevent the shaft and the inner ring from sliding relative to each other. Especially for the thin-walled bearing ring, the proper tight fit can make the bearing ring receive uniform force during the movement, so that the bearing capacity of the bearing can be fully exerted. However, the fit of the bearing should not be too tight. Because of the elastic expansion of the inner ring and the contraction of the outer ring, the radial clearance of the bearing is reduced or completely eliminated, which affects normal operation.

The inner diameter d of the bearing and the shaft are matched by the base hole system, but the tolerance zone is located below the zero line, that is, the upper deviation is 0. Compared with the general base hole system, it is easier to obtain a tighter fit under the same name. The matching of the outer diameter D and the housing hole takes the base shaft system, and its tolerance zone is the same as the general base shaft system, located below the zero line, and the upper deviation is 0, but compared with the general tolerance system, the tolerance zone is not exactly the same. The fit between the bearing and the hole is generally looser than that of the shaft.

Blackened is a complete sentence. It can be considered that the negative tolerance of the bearing base hole system is to obtain a tighter fit under the same name fit, and this fit will not be too tight to affect the normal operation of the bearing. So that in the clearance fit of the same name, the clearance can be reduced or the transition fit can be achieved, and the interference fit can be achieved in the transition fit of the same name, and the interference will not be too large to affect the operation of the bearing.

Similarly, the tolerance zone of the radial ball bearing is also different from the general base hole system, the purpose is also to fit tightly and not too tight. Ordinary Class 0 bearings are generally not marked on the bearings.