Writer: Eric Bearing Limited
Double row cylindrical roller bearings with tapered bore are linear contact bearings. Their load-carrying capacity and rigidity are higher than point contact angular contact bearings. They are often used for medium and large bearings with larger loads and higher rigidity, but the speed is relatively low. , Heavy-duty machine tool spindle system. Next, Eric Bearing Limited will share with you the assembly and adjustment methods of double row cylindrical roller bearings used in the spindle system.
1. Selection of spindle NSK rolling bearings
Both the spindle and NSK bearings have manufacturing errors, which will inevitably affect the rotation accuracy of the spindle assembly. When assembling the main shaft assembly, if the effects of the errors of the two are partially offset each other, the rotation accuracy can be further improved. Since the inner ring of the rolling bearing rotates with the main shaft, its radial runout (or runout) has the greatest impact on the bearing rotation accuracy. Therefore, generally only adjust the relative position of the bearing inner ring and the spindle journal.
(1) Front bearing selection: Since the radial runout of the main shaft at the front support has the greatest impact on the rotation accuracy of its end, the bearing selection method should first be used to reduce the radial runout at the front support. The actual operation process is to measure the inner ring and main shaft journal of a batch of rolling bearings separately before assembling, and type marks at their respective high points (bearings in some countries, there are already marks at the high points. In order to improve the accuracy of the bearing, the mark was removed); then grouped according to the actual radial runout, and assembled with the one with similar runout, so that the high points of the two are opposite to each other to complete the selection of the front bearing. NSK 7006CTRDULP3 bearings online , pls click here :
(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. Because the main shaft uses the same datum to grind each journal, the high points of the front and rear journals beating against the main shaft 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 obtained. If the radial jump points of the front and rear bearing outer rings are installed in the same direction and opposite to the high point of the seat hole, the rotation accuracy of the spindle can be further improved.
2. The clearance (clearance) of the NSK rolling bearing of the spindle and its adjustment
The amount of clearance (clearance) of the spindle NSK rolling bearing has an important influence on the performance of the spindle assembly and the life of the bearing. When the bearing works under a large gap, it will cause the deviation of the spindle position (radial or axial) and directly affect the machining accuracy. At this time, the load-bearing area of the NSK bearing is also small, and the load is concentrated on one or several rolling elements in the force direction, resulting in a larger stress concentration, causing the NSK bearing to mature and increase wear and reduce its life.
In addition, the rigidity and vibration resistance of the spindle assembly are also greatly weakened. When the NSK bearing is adjusted to zero clearance, the load-bearing area increases, the force of the rolling elements tends to be uniform, and the rotation accuracy of the main shaft is improved. When the NSK bearing preload (pre-cut load) is adjusted to a negative gap, the rolling elements are elastically deformed, and the contact area with the raceway is enlarged, so that the load-bearing area is expanded to 360°, and the rigidity and vibration resistance of the spindle assembly are both Can be significantly improved. Therefore, NSK rolling bearings maintain a reasonable amount of clearance, which is an important measure to improve the rotation accuracy, stiffness and vibration resistance of the spindle assembly.
The static stiffness of the main shaft assembly increases as the amount of bearing clearance decreases. The stiffness change (curve slope) of the bearing near the zero gap is the largest, and the stiffness increases slowly thereafter. However, the impact of NSK bearing clearance on the dynamic characteristics of the spindle assembly is more complicated. The damping value of the main shaft assembly (the ratio of the actual damping value to the critical damping value is that the damping ratio first increases with the reduction of the bearing clearance, the damping value at the zero clearance is the largest, and then the interference increases as the clearance decreases. Large), the damping value will decrease instead.
This is because NSK bearings are gradually becoming "rigid", which reduces the structural damping, and more of the internal friction damping characteristics of the material. The static stiffness and damping characteristics affect the overall dynamic response of the spindle assembly. There is an optimal gap position. At this time, the resonance amplitude of the front end of the spindle is the smallest, the static stiffness and resonance frequency are both high, and the roundness error and roughness of the machined surface Also smaller. When the amount of clearance is too small, the increase in stiffness is not significant, but the wear and heat generation of the bearing is greatly increased, which reduces the life of the bearing and deteriorates the service conditions. It can be seen that the selection of the optimal clearance for NSK bearings should not simply consider the static stiffness, but also consider the damping value at the same time, so as to obtain the smallest shaft end co-amplitude value, higher stiffness, resonance frequency and machining accuracy. According to experience, the bearing clearance of the spindle of general medium and large machine tools should be adjusted from -0.005 to -0.015mm.
3. Inspection and adjustment of the installation correctness of the inner and outer rings of NSK bearings
The ratio of diameter to width of double-row cylindrical roller bearings is generally between 4 and 7. Due to the relatively large diameter and width of double-row cylindrical roller bearings, the guiding performance when installed in the spindle box hole and on the spindle is poor. Furthermore, the size and quality of large-scale machine parts are relatively large, which brings certain difficulties to the assembly work, and easily causes uneven force when installing NSK bearings. The above-mentioned various reasons result in the deviation of the installation position of the inner and outer rings of the spindle bearing, and the poor contact between the bearing rolling elements and the inner and outer rings of the bearing, which reduces the rigidity and rotation accuracy of the spindle. The method to verify the installation correctness of the inner and outer rings of the spindle bearing is as follows: During the process of pre-tightening the inner ring of the NSK bearing, continuously rotate the spindle while observing the rotation of the bearing rolling elements.
If the spindle rotates easily, and the rolling elements of the NSK bearing rotate with the rotation of the spindle, the inner and outer rings of the NSK bearing should be installed correctly; if the spindle has a certain amount of preload, it feels blocked when rotating, and the individual rolling elements of the bearing If the rolling elements do not rotate with the rotation of the main shaft, or the individual rolling elements are loosened by hand, it should be caused by the deviation of the inner ring or outer ring of the bearing.
The magnetic meter holder can be installed on the main shaft box and the main shaft respectively. Rotate the main shaft and observe the change of the indicator value of the dial indicator. Adjust the position of the inner ring and outer ring of the bearing to minimize the change of the indicator value of the dial indicator (generally less than 0.01 ~ 0.04mm, related to the size of the bearing). At this time, the inner ring and outer ring of the NSK bearing have been installed and corrected. First, check whether the outer ring of the bearing is installed correctly, and secondly, check whether the inner ring of the NSK bearing is installed correctly. If it still exists, the individual rolling elements do not rotate with the spindle and the spindle is in front of the spindle. If the bearing, spindle box, dial indicator, and magnetic meter seat move, it means that the spindle box hole or spindle diameter is not round.