China is a big country in machine tool demand and production, but it only fully meets the needs of domestic industry in the middle and low-end sectors, and the high-end sector has been less involved.With the development of the industry and the upgrading of competition, the best choice for the long-term development of an enterprise is to improve the technical content of its products and have independent patents and designs.Therefore, as far as numerical control vertical lathe is concerned, how to realize the product from low-end to high-end, from primary processing to high precision manufacturing is the direction that enterprises should think about and strive for in the tide of transformation.

In vertical turning equipment, rotary table is one of the core structures reflecting the overall performance of the machine tool and realizing the machining accuracy of the workpiece.We require it to operate at high speed and bear heavy parts at the same time, with accurate operation precision and very high anti-overturning torque capacity, and the bearing supporting the table is the key to complete this important mission.Before most of the domestic work table choose thrust bearing and radial bearing with the use, this layout form of complex structure, more materials, especially in the placement of large box parts, the whole machine tool space volume is more huge.Bearing installation and preload adjustment is also difficult, more difficult to ensure the alignment, the overall accuracy of the table is difficult to improve.Therefore, now more design began to choose a more compact structure of the cross roller bearing, not only to save material costs, but also to simplify the design scheme, higher limit speed, better running accuracy and stability, bearing capacity and rigidity is stronger.This article is to introduce how to use cross roller bearing in vertical lathe to achieve perfect engineering performance.

One, jumping accuracy

Bearing runout can be divided into synchronous runout and asynchronous runout, the impact of synchronous runout on the overall runout of the table can be reduced by grinding the working table and the maximum extent, but the impact of asynchronous runout in this link can not be eliminated, it is mainly decided by the roller diameter tolerance and roundness.Therefore, the bearing asynchronous runout control of the better, the final stage of the radial and axial runout is also smaller, that is, the higher the running accuracy.In the choice of bearing brand and accuracy level, it is recommended not only to pay attention to the bearing assembly runout, but also to have a deep understanding of the precision standard affecting the asynchronous runout of the bearing.

Bearing fit

When selecting the bearing type, it is suggested that the inner ring and shaft, outer ring and tooth ring should be closely matched.Since the outer ring is a rotating part, the fitting amount should be slightly larger;The inner ring will move down under the pressure of the end cap and adjust to a certain pre-tightening amount, so the tightening amount should be slightly smaller.However, if the inner ring is considered to be a static part, and the inner ring and shaft are designed to be loose fit (radial clearance exists between the inner ring and the shaft), then it is possible to install the inner ring, lock the inner ring or when the bearing is under load, the inner ring deflection occurs, that is, eccentricity.This eccentricity will result in stress concentration at the contact zone boundary between the roller and the raceway, causing serious scratches, pressure pits and spalling, and prematurely ending the bearing life.

Shape and position tolerance

The mounting surface of shaft and gear ring should have the same flatness, verticality, roundness and cylindricity as precision bearing.Controlling these form and position tolerances can not only obtain better assembly precision, but also avoid stress concentration caused by excessive eccentricity of inner and outer rings of the bearing and extend the service life of the bearing.As for the machining standard of shape and position tolerance, it is recommended to consult the bearing supplier and match the bearing precision.

Axial preload

In the cutting test of vertical turning, the roughness of end face and outer circle of workpiece is paid much attention to.System rigidity includes the rigidity of frame structure, the rigidity of support bearing and so on.

In practical application, bearing roller A mainly bears overturning torque, while roller C mainly bears axial load.When bearing the outer ring of the bearing, the roller in row A tends to be detached from the bearing ring, that is, the axial clearance increases, while the roller in row C is compressed, that is, the axial clearance decreases.If the bearing axial preload is insufficient, after bearing, the roller in row A not only loses the preload, but also has A clearance between the roller and the raceway, so the system rigidity drops rapidly. When the cutting force produces 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 roller and the raceway may exceed the allowable limit because the roller in row C has to bear both the workpiece weight and the internal preload of the bearing, greatly reducing the service life of the bearing.Therefore, considering the influence of temperature and load, it is very important to set the preload of bearing.

Notes for bearing installation

This section does not introduce the installation steps of bearings in detail, but focuses on the operation details that need extra attention in the installation process:

1. When the lower inner ring of the bearing is loaded into the shaft seat and returned to the room temperature, the clearance between the inner ring and the baffle shoulder shall be checked with a 0.025mm feeler 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, strike the end face with a steel bar of medium hardness to make it in place, so as to avoid the occurrence of eccentricity of the inner and outer ring due to improper installation and damage to the bearing.

　2. After the installation of the bearing lower inner ring, outer ring and roller, adjust the parallelism between the end face of gear ring and the end face of the spindle (the plane shown by the red line in the figure) to make the lower inner ring parallel to the outer ring;After the installation of the upper inner ring is completed, the positive and negative rotating bearings are used to position the rollers and spacers correctly, and then the parallelism between the end face of the gear ring and the end face of the upper inner ring is adjusted to ensure the evenness of the three bearing rings, so as to avoid eccentricity between the bearing rings, stress concentration and bearing damage.

　　3. After the installation of the upper inner ring, the positive and negative rotating bearings must be installed to make the rollers and spacers automatically align, and then add the gasket and end cover to lock.If the rollers are arranged aslant and locked in the raceway without rotating bearings, serious heating, raceway burns, scratches and precision loss will occur during the operation of the bearings.

4. Tighten the end cover bolts one by one according to the diagonal sequence (as shown in FIG. 6). The locking torque can be increased for 3 to 4 times to the final locking amount, so as to avoid the end cover being pressed in.At the same time, the levelness of the end cover can be checked again after each round of locking to eliminate eccentricity as far as possible.

Other questions

If the roughness of the workpiece found in the cutting test is not good, in addition to the bearing rigidity problem, there may be other components of the equipment vibration is larger, transferred to the worktable and workpiece, affecting the cutting quality.Therefore, vibration detection and analysis device is needed to find the vibration source, so as to solve the cutting quality problem caused by vibration more quickly and accurately.

If the rotary table is driven by the meshing of the gear ring, attention should be paid to adjust the proper meshing position during installation to avoid the eccentric of the gear ring and the premature abandonment of the bearing due to the concentration of internal stress.

In the field of nc vertical lathe design, the choice of cross roller bearing to support the rotary table has become the mainstream.However, how to give full play to the advantages of this type of bearing at the beginning of application and the stage of installation and debugging, so that it can better cooperate with other structural components to display the excellent overall performance of the machine tool, is a problem that the bearing suppliers need to discuss in depth with the machine tool manufacturers in the future.