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China is a big country in machine tool demand and production, but it is only in the low and middle end fields that it can fully meet the needs of domestic industry, while the high-end fields have been less involved. With the development of the industry and the upgrading of the competition, improving the technical content of products, owning independent patents and designs is the best choice for the long-term development of enterprises. Therefore, as far as CNC vertical lathe is concerned, how to realize the product from low-end to high-end and from primary processing to high-precision manufacturing is the direction that should be considered and worked hard in the tide of enterprise transformation.
On the vertical lathe equipment, the rotary table is one of the core structures that reflect the overall performance of the machine tool and achieve the machining accuracy of the workpiece. We require it to have accurate operation accuracy and very high anti overturning torque capability while running at high speed and bearing heavy workpieces. The bearing supporting the workbench is the key to completing this important mission. Previously, most domestic workbenches used thrust bearings and radial bearings together. This layout has a complex structure and uses more materials. Especially when large box parts are placed, the space volume of the whole machine tool is larger. It is also difficult to install and pre tighten the bearing, ensure the alignment, and improve the overall accuracy of the workbench. Therefore, more designs are now beginning to select cross roller bearings with more compact structure, which not only saves material costs, but also simplifies the design scheme, has higher limit speed, better operation accuracy and stability, and has stronger bearing capacity and rigidity. This article introduces how to apply cross roller bearing on vertical lathe to realize its perfect engineering performance.
1 Runout accuracy
Bearing runout can be divided into synchronous runout and asynchronous runout. The impact of synchronous runout on the overall runout of the workbench can be minimized by grinding the workbench, but the impact of asynchronous runout cannot be eliminated in this link, which is mainly determined by the outer diameter tolerance and roundness of the roller. Therefore, the better the asynchronous runout control of the bearing, the smaller the final radial and axial runout of the workbench, that is, the higher the operating accuracy. When selecting the bearing brand and accuracy grade, it is recommended not only to focus on the bearing assembly runout, but also to deeply understand the accuracy standards that affect the asynchronous runout of the bearing.
2 Bearing fit
It is recommended that the inner ring and shaft, and the outer ring and gear ring should be tight fit when selecting the bearing. As the outer ring is a rotating part, the fitting amount should be slightly larger; The inner ring will move downward under the pressure of the end cap and be adjusted to a certain pre tightening amount, so the tightening amount should be slightly smaller. However, if the inner race is considered as a static part and the inner race and shaft are designed as loose fit (there is a radial clearance between the inner race and shaft), it is possible that the inner race will deflect when installing the inner race, locking the inner race or bearing is loaded, that is, eccentricity will occur. This eccentricity will cause stress concentration at the boundary of the contact area between the roller and the raceway, causing serious scratches, pits and peeling, and ending the life of the bearing earlier.
3 Geometric tolerance
The mounting surface of shaft and gear ring shall have flatness, perpendicularity, roundness and cylindricity consistent with that of precision bearing. Controlling these geometrical tolerances can not only obtain better assembly accuracy, but also avoid excessive eccentricity of the inner and outer rings of the bearing resulting in stress concentration and prolong the service life of the bearing. It is recommended to consult the bearing supplier about the processing standard of form and position tolerance to match the bearing accuracy.
4 Axial preload
In the cutting test of vertical lathe, much attention is paid to the roughness of the end face and the outer circle of the workpiece, and the rigidity of the system is one of the keys to determine its advantages and disadvantages. The rigidity of the system includes the rigidity of the frame structure, the rigidity of the supporting bearing, etc. The rigidity of the bearing often depends on its axial preload.
In practical application, row A rollers of the bearing mainly bear overturning torque, while row C rollers mainly bear axial load. When the bearing outer ring is loaded, row A rollers tend to disengage from the bearing ring, that is, the axial clearance increases, while row C rollers are compressed, that is, the axial clearance decreases. If the axial preload of the bearing is insufficient, after bearing, the A-row rollers not only lose preload, but also have clearance with the raceway, so the system rigidity decreases rapidly. Then when the cutting force generates overturning torque, the radial deformation of the workpiece will be greatly increased, and the surface roughness obtained is also very poor; If the axial preload is too large, the contact stress between the C-row roller and the raceway may exceed the allowable limit because the C-row roller must bear both the weight of the workpiece and the preload force inside the bearing, greatly reducing the service life of the bearing. Therefore, it is particularly important to reasonably set the axial preload of the bearing by fully considering the influence of temperature and load.
Figure 1
Figure 2
5 Precautions for bearing installation
This section does not introduce the installation steps of the bearing in detail, but focuses on the operation details that need additional attention during the installation process:
(1) When the lower inner ring of the bearing is installed into the shaft seat and returns to the room temperature, the clearance between the inner ring and the shoulder should be checked with a 0.025mm feeler gauge to ensure that it has been installed in place; The same is true for the outer ring. If the outer ring is not in place, knock the end face with a steel bar of medium hardness to make it in place, so as to avoid eccentricity of the inner and outer ring due to improper installation and damage to the bearing.
Figure 3
(2) After the installation of the lower inner ring, outer ring and roller of the bearing is completed, adjust the end face of the gear ring to the "0" position? Parallelism of shaft end face (the plane shown by the red line in the figure), making the lower inner ring parallel to the outer ring; After the installation of the upper inner ring is completed, rotate the bearing forward and backward to make the rollers and spacers correctly positioned, and then adjust the parallelism between the end face of the gear ring and the end face of the upper inner ring to finally ensure that the three bearing rings are horizontal, so as to avoid eccentricity between the bearing rings, causing stress concentration and damage to the bearing.
Figure 4
(3) After the installation of the upper inner ring is completed, the bearing must be rotated forward and backward to make the roller and spacer automatically align, and then add shims and end cap to lock. If the rollers are arranged on the raceway askew and locked without rotating the bearing, serious heating, raceway burns, scratches and loss of accuracy will occur when the bearing is running.
Figure 5
(4) Lock the end cover bolts one by one in diagonal sequence (as shown in Figure 6), and the locking torque can be increased to the final locking amount in 3 to 4 times to prevent the inner ring from eccentric due to uneven or excessive force on both sides when the end cover is pressed in. At the same time, the levelness of the end cover can also be checked again after each round of locking to eliminate eccentricity as much as possible.
Figure 6
6 Other issues
If the roughness of the machined workpiece is not good during the cutting test, in addition to the rigidity of the bearing, it may also be that the vibration of other components in the equipment is large, which is transmitted to the workbench and workpiece and affects the cutting quality. Therefore, a vibration detection and analysis device is needed to find the vibration source, so as to solve the cutting quality problems caused by vibration more quickly and accurately.
If the rotary table is driven by gear ring meshing, attention should be paid to adjusting the appropriate meshing position during installation to avoid eccentric ring gear, which will cause stress concentration in the bearing and premature scrapping.
In the design field of CNC vertical lathe, the use of cross roller bearings to support the rotary table has become the mainstream. However, how to maximize the advantages of such bearings at the beginning of application and during the installation and commissioning stage, so that they can better cooperate with other structural components to show the overall excellent performance of the machine tool, is a problem that bearing suppliers need to discuss with machine tool manufacturers in the future.
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