Gaps and preload in rolling bearings
By radial or axial clearance, we mean a complete radial or, respectively, axial movement in both directions of one ring relative to the other.
Optimal values of radial and axial clearances for these bearing conditions allow to ensure rational distribution of load between rolling elements, necessary shaft and body displacement in radial and axial directions, as well as improve and improve stability of vibro-acoustic characteristics, reduce friction losses.
In bearings of unregulated types, there are three types of radial clearances:
The clearance clearance is always less than the initial clearance due to the deformation of the rings in the radial direction after the bearing is installed in the workplace.
When the bearing unit is operating and the temperature regime is stable, a working gap is formed which can be more or less than the landing, depending on the bearing installation, the perceived load and the temperature difference between the shaft and the housing.
GOST 24810-81 establishes conventions for groups of gaps and numerical knowledge of radial and axial clearances in the state of delivery for rolling bearings. GOST 24810-81 does not apply to linear angular contact ball bearings.
Unregulated bearing types are manufactured with relatively small gaps; after installation on the shaft and in the housing they can work without additional adjustment.
Recommended clearance dimensions for rolling bearings are given in the special reference literature.
11 bearings designed for normal operating conditions, when the temperature difference between the outer inner rings is negligible (5-10 ° C), should have a gap corresponding to the main one - the normal group (GOST 24810-81).
Radial single-row ball bearings designed to absorb only axial forces, should have an increased radial clearance, which allows increasing the angle of contact in the bearing, t. increase its axial load capacity. Radial non-self-aligning bearings with increased radial clearance are used for deviations from alignment of seats.
The axial and radial clearances of the adjustable bearings can be set within certain limits only when mounted in the machine assembly. The required axial clearance in thrust bearings is also established during installation. Depending on the design of the unit, the axial clearance is controlled by the outer or inner ring of the bearing.
The optimal gap value is set experimentally for each particular node.
If the bearings are assembled with a large clearance, then only one or two balls or rolls perceive the entire load (Figure 4.32, a). Bearing conditions at such large gaps are unfavorable, and therefore such gaps are unacceptable.
Reducing the gaps results in a more even distribution of the load between the rolling elements, reduces vibration, increases rigidity of the support. The presence of some axial gaps positively affects the decrease in the moment of resistance to rotation. Conventional angular contact bearings are adjusted so that the axial clearance at a steady temperature regime would be close to zero. In this case, under the action of the radial load, there are about half the rolling bodies (Figure 4.32, b).
In some nodes, to increase the stiffness of the supports, the accuracy of rotation of the shaft and improve the vibroacoustic characteristics of the assembly, a bearing assembly with preload is used. In this case, more than half or all of the rolling elements of the bearing are under load (Figure 4.32, c). The essence of the preload is that the pair of bearings is preloaded with axial force, which eliminates the axial play in the set, creating an initial elastic deformation in the places of contact of the working surfaces of the rings with the rolling elements. If, then, the working axial load is applied to the bearing, the relative movement of its rings due to additional deformation of the working surfaces will be considerably less than before the pre-tensioning. Preloading causes the same deformation in both bearings. Such bearings operate under more severe conditions, since the load on the rolling elements, the moment of resistance to rotation and wear, and the life of the bearing are increased.
With preload, ball bearings are installed radial, radial-thrust, roller conical bearings, as well as bearings with short cylindrical rollers, mounted on the tapered neck of the shaft with interference, which can cause expansion of the inner ring and completely eliminate the radial clearance in the bearing. To reduce the gap, axial movement of the inner ring with an aperture having a conicity relative to the neck of the shaft or sleeve can also be performed.
Recommended axial clearance values for bearings of various types are given in the rolling bearings reference books.
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