01 Definition of bearing clearance.
Definition of bearing clearance:When one ring of a bearing is fixed and the other ring is not loaded, the amount of movement from one limit position to another in the radial or axial direction δ. Depending on the direction of its movement, the former is called the radial clearance, and the latter is called the axial clearance. Figure 1 shows a schematic diagram of the radial and axial clearances of a radial ball bearing.
Fig.1. Clearance of radial ball bearings.
For rolling bearings that can bear radial loads, the definition of radial clearance in the non-preloaded state is: the arithmetic mean of the radial distance of one ring from one radial eccentric limit position to the opposite limit position relative to another ring in different angular directions without external load. This average includes the relative displacement of the ferrule at different angular positions and the displacement of the rolling element group relative to the ferrule at different angular positions. In the case of radial ball bearings, the theoretical radial clearance is the diameter of the outer raceway minus the diameter of the inner raceway and twice the diameter of the rolling elements.
For individual tapered roller bearings and thrust ball bearings where the inner and outer rings can be separated, the concept of clearance is not possible. However, after installing it on the shaft and bearing housing, a certain radial and axial clearance can be obtained by adjusting the axial relative position of the two rings.
When not specified, it is generally referred to as radial clearance. There are 7 groups of clearance sizes, of which the last 6 groups are represented by C1, C2, C3, C4, C5 and C9 respectively in the bearing number (see Table 1). There are 4 groups of clearances commonly used in rolling bearings, of which 0 groups are called "standard clearances" or "basic groups".
Table 1 Clearance code
Note: When the tolerance grade code and the clearance code are expressed at the same time, it can be simplified and the combination of the tolerance grade code and the clearance group number (0 groups are not indicated). For example, a bearing with a tolerance class of P6 and a radial clearance of C3 can be simplified to "P63"; A bearing has a tolerance class of P5 and a radial clearance of C2, which can be simplified to "P52".
02 The importance of bearing clearance and the principle of selection.
Clearance is an important indicator of rolling bearings and an important parameter in bearing applications. Different bearing clearances will have different effects on the operation of machinery, the most obvious of which is vibration and noise, and the other is bearing heating and abnormal damage. Therefore, it should be selected according to the regulations, and it should not be used wrongly.
In the commonly used rolling bearing clearance, the "basic group" is suitable for general working conditions, and the radial clearance of the group should be preferentially selected is larger than that of the "basic group", which is suitable for the bearing and the shaft (or bearing seat) to adopt a tight interference fit, or the temperature difference between the inner and outer rings is large, or the friction torque is required to be low; The radial clearance of the two groups is smaller than that of the basic group, which is suitable for occasions where the rotation accuracy of the shaft is required to be high and the axial displacement needs to be strictly limited.
03 Radial clearance of deep groove ball bearings and cylindrical roller bearings and increment after load application.
When a certain load is applied, the radial clearance of the deep groove ball bearing will increase due to the extrusion between the ball and the groove, and the increment of the deep groove ball bearing under the specified radial load is shown in Table 2.
Table 2 Radial clearance increase of deep groove ball bearings under measured loads
(gb/t 4604—2006)
1) When the measured load < 50N, the clearance increase < 2 m.
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