Double-row cylindrical roller bearings with tapered bores are linear contact bearings. They have higher load capacity and stiffness than point contact angular contact bearings. They are often used in spindle systems of medium, large and heavy machine tools with large loads, high stiffness requirements and relatively low speeds. Below, Zhongke Bearing Network (hereinafter referred to as Huazhou Network) will share with you the assembly and adjustment methods of double-row cylindrical roller bearings used in spindle systems.
1. Selection of spindle rolling bearings.
There are manufacturing errors in both the spindle and the bearing, which will inevitably affect the rotation accuracy of the spindle assembly. When assembling the spindle assembly, if the error effects of the two are partially offset, the rotation accuracy can be further improved. Since the inner ring of the rolling bearing rotates with the spindle, its radial runout (or runout) has the greatest impact on the rotation accuracy of the bearing. Therefore, generally only the relative position of the inner ring of the bearing and the spindle journal is adjusted.
(1) Front bearing selection: Since the radial runout of the spindle at the front support has the greatest impact on the rotation accuracy of its end, the bearing selection method should be used first to reduce the radial runout at the front support. The actual operation process is to measure the inner rings and spindle journals of a batch of rolling bearings and mark their high points before assembly (bearings in some countries have marks at the high points, but in recent years, as the bearing accuracy has improved, the marks have been removed); then group them according to the actual radial runout, select those with similar runouts for assembly, so that the high points of the two are in different directions, and the front bearing selection is completed.
(2) Rear bearing selection: After the front bearing is selected, the rear bearing can be selected to further improve the rotation accuracy of the spindle assembly. Since the spindle uses the same reference to finely grind each journal, the high points of the front and rear journals to the spindle axis are often in the same direction. Therefore, as long as the high points of the rear bearing and the front bearing are arranged in the same axial plane and on the same side of the axis, better results can often be achieved. If the high points of the radial jump of the outer rings of the front and rear bearings are installed in the same direction and in a different direction from the high points of the seat hole, the rotation accuracy of the spindle can be further improved.
2. The clearance (play) of the spindle rolling bearing and its adjustment.
The clearance (play) of the spindle rolling bearing has an important influence on the working performance of the spindle assembly and the life of the bearing. When the bearing works with a large clearance, it will cause the spindle position (radial or axial) to shift and directly affect the processing accuracy. At this time, the bearing load area is also small, and the load is concentrated on one or several rolling elements in the force direction, causing greater stress concentration, causing the bearing to mature and wear more aggravated and reducing the life.
In addition, the rigidity and vibration resistance of the spindle assembly are also greatly weakened. When the bearing is adjusted to zero clearance, the load area increases, the force condition of the rolling element tends to be uniform, and the rotation accuracy of the spindle is improved. When the bearing preload (preload) is adjusted to negative clearance, the rolling element produces elastic deformation, the contact area with the raceway increases, and the load area is expanded to 360°, then the rigidity and vibration resistance of the spindle assembly can be significantly improved. Therefore, maintaining a reasonable clearance of the rolling bearing is an important measure to improve the rotation accuracy, rigidity and vibration resistance of the spindle assembly.
The static stiffness of the spindle assembly increases with the decrease of the bearing clearance. The change of the stiffness of the bearing near the zero clearance (the slope of the curve) is the largest, and the stiffness increases slowly thereafter. However, the influence of the bearing clearance on the dynamic characteristics of the spindle assembly is more complicated. The damping value of the spindle assembly (the ratio of the actual damping value to the critical damping value is the damping ratio) first increases with the decrease of the bearing clearance, and the damping value at the zero clearance is the largest, and then the interference increases with the decrease of the clearance), and the damping value decreases instead.
This is because the bearing gradually tends to be "rigid", which reduces its structural damping and shows more friction damping characteristics within the material. Static stiffness and damping characteristics jointly affect the overall dynamic response of the spindle assembly. There is an optimal clearance position, when the resonance amplitude of the front end of the spindle is the smallest, the static stiffness and resonance frequency are high, and the roundness error and roughness of the machined surface are also small. When the clearance is too small, the increase in stiffness is no longer significant, but the wear and heat generation of the bearing are greatly increased, thereby reducing the bearing life and worsening the use conditions. It can be seen that the selection of the optimal bearing clearance should not only consider the static stiffness, but also the damping value, so as to obtain the minimum shaft end resonance amplitude, higher stiffness, resonance frequency and machining accuracy. According to experience, the bearing clearance of the spindle of medium and large machine tools should be adjusted to -0.005~-0.015mm.
3. Check and adjust the correct installation of the inner and outer rings of the bearing .
The ratio of the diameter to the width of double-row cylindrical roller bearings is generally between 4 and 7. Due to the large diameter-width ratio of double-row cylindrical roller bearings, the guiding performance when installed in the spindle box hole and on the spindle is poor. In addition, the large size and mass of large machine tool parts bring certain difficulties to the assembly work, which easily causes uneven force when installing bearings. The above reasons cause the installation position of the inner and outer rings of the spindle bearing to be offset, and the bearing rolling elements have poor contact with the inner and outer rings of the bearing, resulting in reduced spindle stiffness and rotation accuracy. The method to check whether the inner and outer rings of the spindle bearing are installed correctly is as follows: While pre-tightening the inner ring of the bearing, continuously rotate the spindle and observe the rotation of the bearing rolling elements.
If the main shaft rotates easily and the rolling elements of the bearing rotate with the main shaft, the inner and outer rings of the bearing should be installed correctly; if the main shaft has a certain amount of preload, there is a blockage when rotating, but the individual rolling elements of the bearing do not rotate with the main shaft, or the individual rolling elements are loose when moved by hand, it should be caused by the inner or outer ring of the bearing being installed crookedly.
The magnetic gauge seat can be installed on the spindle housing and the spindle respectively, rotate the spindle and observe the change of the dial indicator (see Figure 1, Figure 2), adjust the position of the inner and outer rings of the bearing to minimize the change of the dial indicator (generally less than 0.01 ~ 0.04mm, related to the size of the bearing). At this time, the inner and outer rings of the bearing have been installed correctly. Figure 1 Check whether the outer ring of the bearing is installed correctly Figure 2 Check whether the inner ring of the bearing is installed correctly If there is still a situation where individual rolling elements do not rotate with the rotation of the spindle, it means that the spindle housing hole or the spindle diameter is not round.
