Conical hole double row cylindrical roller bearings belong to linear contact bearings, which have a higher load-bearing capacity and stiffness than point contact angular contact bearings. They are commonly used in medium, large, and heavy-duty machine tool spindle systems with large loads, high stiffness requirements, and relatively low rotational speeds.
1. Selection of spindle rolling bearings.
There are manufacturing errors in both the spindle and bearings, which inevitably affects the rotational accuracy of the spindle components. When assembling the spindle assembly, if the error effects of the two are partially offset, it can further improve its rotational accuracy. Due to the fact that the inner ring of a rolling bearing rotates with the spindle, its radial runout (also known as oscillation) has a significant impact on the bearing's rotational accuracy. Therefore, generally only adjust the relative position of the bearing inner race and the main shaft journal.
(1) Front bearing selection: Due to the significant impact of the radial runout of the spindle at the front support on its end rotation accuracy, 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 main shaft journals of a batch of rolling bearings before assembly, and type markings at their respective high points. Then, group them according to the actual radial runout, and take the ones with similar runout for assembly, so that the high points of the two are opposite, completing the selection of the front bearing.
(2) Rear bearing selection: After selecting the front bearing, selecting the rear bearing can further improve the rotation accuracy of the spindle assembly. Due to the fact that the main shaft uses the same benchmark for precision grinding of each journal, the high points of the front and rear journals on the main shaft center runout 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, good results can often be achieved. If the radial jump points of the front and rear bearing outer rings are installed in the same direction and opposite to the high points of the seat hole, it can further improve the rotation accuracy of the spindle.
2. Clearance (clearance) and adjustment of spindle rolling bearings
The clearance amount (clearance amount) of the spindle rolling bearing has a significant impact on the working performance and bearing life of the spindle components. When the bearing operates with a large clearance, it will cause the displacement of the spindle position (radial or axial) and directly affect the machining accuracy. At this point, the bearing's load-bearing area is also small, and the load is concentrated on one or several rolling elements in the direction of force, causing significant stress concentration, causing the bearing to ripen and wear more severely, thereby reducing its lifespan. In addition, the stiffness and vibration resistance of the spindle components are also greatly weakened. When the bearing is adjusted to zero clearance, the load-bearing area increases, the force distribution of the rolling element tends to be uniform, and the rotation accuracy of the spindle is improved. When the bearing is pre tightened (pre cut) and adjusted to a negative clearance, the rolling element undergoes elastic deformation, increasing the contact area with the raceway, and expanding the bearing area to 360 °. As a result, the stiffness and vibration resistance of the spindle assembly can be significantly improved. Therefore, maintaining a reasonable amount of clearance in rolling bearings is an important measure to improve the rotational accuracy, stiffness, and vibration resistance of spindle components. The static stiffness of the spindle assembly increases with a decrease in bearing clearance. The stiffness change (curve slope) of the bearing near zero clearance is large, and the subsequent increase in stiffness is slower. However, the influence of bearing clearance on the dynamic characteristics of spindle components is relatively complex. The damping value of the spindle component (the ratio of the actual damping value to the critical damping value is that the damping ratio first increases with the decrease of bearing clearance, and the damping value at zero clearance is large, and then increases with the decrease of clearance interference), and the damping value also decreases.
This is because bearings gradually tend to become "rigid", resulting in a decrease in structural damping and a greater tendency to exhibit frictional damping characteristics within the material. The static stiffness and damping characteristics jointly affect the overall dynamic response of the spindle assembly, and there is a good gap position. At this time, the resonance amplitude at the front end of the spindle is small, while the static stiffness and resonance frequency are high. 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, while the wear and heat generation of the bearing greatly increase, thereby reducing the bearing life and deteriorating the service conditions. From this, it can be seen that the selection of optimal clearance for bearings should not only consider static stiffness, but also consider damping values to obtain small shaft end resonance amplitude, high stiffness, resonance frequency, and machining accuracy. Based on experience, the bearing clearance of the main shaft of general medium and large machine tools should be adjusted by -0.005~-0.015mm.
3. Inspection and adjustment of correct installation of inner and outer rings of bearings.
The ratio of diameter to width of double row cylindrical roller bearings is generally between 4 and 7. Due to the large diameter to width ratio of double row cylindrical roller bearings, the guiding performance is poor when installed in the spindle box hole and on the spindle. Furthermore, the large size and mass of parts in large machine tools bring certain difficulties to assembly work, which can easily cause uneven force when installing bearings. The above reasons have caused the installation position of the inner and outer rings of the spindle bearing to shift, resulting in poor contact between the bearing rolling element and the inner and outer rings of the bearing, resulting in a decrease in the spindle stiffness and rotation accuracy. The method to check the installation of the inner and outer rings of the spindle bearing is as follows: during the process of pre tightening the inner ring of the bearing, continuously rotate the spindle while observing the rotation of the bearing rolling element.
If the spindle rotates easily and each rolling element of the bearing rotates with the spindle, the inner and outer rings of the bearing should be installed properly; If the main shaft already has a certain amount of preload and feels blocked during rotation, and individual rolling elements of the bearing do not rotate with the main shaft, or if they are loose when moved by hand, it should be caused by the installation deviation of the inner or outer rings of the bearing.
