Bearing assembly theory error analysis

1. The inner and outer rings are not in the same rotation center, resulting in deflection of the shaft deflection. This is a common phenomenon in actual running bearings. The deflection of the shaft line is one of the main causes of the failure of the thrust bearing. Causes of misalignment of bearings and shafts are:

(1) axis tilt;
(2) Rotor installation error;
(3) Rotor manufacturing error;
(4) Thermoelastic deformation of shafts, rotors, and bearings during operation. Axial deflection causes the oil film thickness, pressure distribution, and tile surface temperature of each tile to be different, which has a great influence on the oil film thickness and pressure distribution, and has a minor effect on the temperature distribution of the tile surface; the axial deflection causes unevenness in the bearing. The hydrodynamic pressure of the shaft causes the oil film force of the shaft to fluctuate. With the increase of the deflection degree, the fluctuation of the oil film force is more serious; under the full oil lubrication state, the slight deviation of the deflection angle will cause the minimum oil film thickness and the maximum pressure to change significantly. However, the maximum temperature of the tile surface changes very little. In summary, the shaft deflection causes the oil film thickness in the bearing to be uneven, which may cause the oil film to break and cause the bearing to fail. When mounting, in order to maintain as much concentricity as possible, the best approach is to use a monolithic housing and pull the two holes of the mounting bearing at one time. If bearings of different sizes are installed on one shaft, the bearing holes on the housing should still be bored out at a time. In this case, the liner can be used to install the bearings with smaller dimensions. When the two bearing bores are split into two shells, the two shells should be grouped together for boring.

2, the installation is not in place, there is deviation or not installed to the bearing position, resulting in the bearing clearance is too small, the shaft temperature is generally higher than the outer ring temperature, axial and radial thermal expansion of the shaft will be greater than the thermal expansion of the shell, if Bearing clearance is too small and may cause jamming.

3. Over-tight fit of rotating and non-moving rings The selection of bearing fittings should be based on the type and size of the bearing, the size and direction of the load, and the nature of the load. The correct choice of bearing coordination should ensure the normal operation of the bearing and prevent relative rotation between the inner ring and the shaft, outer ring and housing hole during operation. In general, when the direction of the working load is constant, the rotating ring has a tighter fit than the non-moving ring because the rotating ring bears the rotating load while the moving ring bears the local load. The higher the rotational speed, the greater the load and the stronger the vibration, the tighter fit should be used. When the bearing is mounted on a thin-walled shell or hollow shaft, a tight fit should also be used. However, over-tight fit is unfavorable. At this time, the internal clearance of the bearing may be reduced or even completely disappeared due to the elastic expansion of the inner and outer rings and the contraction of the outer ring. It may also be due to irregularities in the surface of the mated shaft and seat hole. The shape or uneven rigidity causes irregular deformation of the inner and outer rings of the bearing, which will destroy the normal operation of the bearing. Over-tightening can also make assembly and disassembly difficult, especially for heavy machinery.

4. During the installation process, the pre-tightening force is too large The pre-tightening force is to use the external force applied during the assembly process to give the bearing a proper preload. If the preload is too small, there will be gaps when the bearing is working, which will result in lower support stiffness and rotation accuracy, causing vibration and noise. If the preload is too large, the friction between the rolling elements and the inner and outer ring raceways will increase, and the temperature will increase too quickly during operation, which will reduce the transmission efficiency and reduce the bearing life. Only by applying a suitable pre-tightening force can eliminate the axial gap, reduce the reverse error, improve the accuracy of shaft rotation, reduce vibration and noise. Give proper bearing preload, even if the bearing after a certain period of time after the operation, the relative position of the bearing is slightly changed, the preload can still make the bearings inside and outside the ring position remains stable, so the installation of preload control is very important.