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Selection Skills of Bearings in the Design of CNC Machine Tools

by:JNSN     2021-11-03

China CNC Machine Tool Network News: Bearings are an important part of the spindle assembly. Its type, structure, configuration, accuracy, installation, adjustment, lubrication and cooling all directly affect the performance of the spindle assembly. The rotation accuracy of the spindle is largely determined by the bearing.

     1. Spindle rolling bearing      (1) The type of rolling bearing     The friction resistance of the rolling bearing is small, it can be preloaded, the lubrication and maintenance are simple, and it can work stably in a certain speed range and load fluctuation range. Rolling bearings are produced by specialized factories and are easy to purchase and maintain. But compared with sliding bearings, rolling bearings are very noisy, the number of rolling elements is limited, the stiffness is variable, the vibration resistance is slightly worse, and the speed is very limited. Rolling bearings are classified into ball bearings and cylindrical roller bearings according to the structure of the rolling elements. Ceramic bearing means that the rolling elements of the bearing are made of ceramic materials, while the inner and outer rings are still made of bearing steel. The reason why the rolling element is selected is mainly because it has the following characteristics: one is light weight, the other is the thermal expansion coefficient is small, and the third is the elastic modulus. Therefore, it has the characteristics of low centrifugal force, low dynamic friction, stable pre-tightening force, small elastic deformation and high rigidity.     (2) Reasonable arrangement of spindle bearings      1) Selection and configuration of spindle bearings      The machine tool spindle has two front and rear supports and three front, middle and rear supports. The former is more common. The configuration form of the two-support spindle bearing, including the selection, combination and arrangement of the spindle bearing, is mainly based on the requirements for the speed, load-bearing capacity, rigidity and accuracy of the spindle components involved.    The following general principles should be followed when determining the configuration of the two-support spindle bearing:     ① Adapt to the requirements of stiffness and load capacity: The choice of the spindle bearing configuration should first meet the required stiffness and load capacity.    ②Adapt to speed requirements: the maximum speed allowed by bearings of different models, specifications and accuracy levels is different. Under the same conditions, the point contact is higher than the line contact; the cylindrical roller is higher than the tapered roller.    ③Adapt to the requirements of accuracy: The configuration of the thrust bearing of the spindle assembly to bear the axial force directly affects the axial position accuracy of the spindle. 2) Three-support spindle assembly Some machine tools have a longer headstock length due to structural design reasons, and the support span between the two supports of the spindle is much larger than the optimal span. At this time, additional intermediate supports should be considered. Improve the rigidity and vibration resistance of the spindle assembly.    3) Application design of main shaft rolling bearing     ①The stiffness and load-carrying capacity of main shaft rolling bearing     The stiffness of rolling bearing mainly depends on the contact stiffness between rolling element and raceway. The stiffness of the motion bearing is related to the bearing type, load and preload condition, and exhibits nonlinear characteristics as the load increases.    ②The lubrication of the spindle bearing      lubrication has a great influence on the working performance of the spindle assembly and the life of the bearing. Unreasonable lubrication may cause heat increase, reduce spindle working accuracy, and accelerate bearing wear. Lubricants used in rolling bearings can be divided into two categories, namely grease lubricants and liquid lubricants. ③The sealed rolling bearing sealing device of the main shaft rolling bearing prevents the coolant, chips, dust, debris, etc. from entering the bearing, and keeps the lubricant in the bearing without leakage, so as to give full play to the performance of the bearing and ensure the service life of the bearing . The requirements for the sealing device of the main shaft assembly are: suitable for the rotation speed of the main shaft, adapt to the working environment, and have a compact structure. The seal of the spindle bearing is divided into contact type and non-contact type. The former can be divided into radial sealing ring sealing box felt ring sealing, generally suitable for low-speed spindles. Non-contact seals can be divided into gap seals, curved seal box gasket seals. In order to ensure the sealing effect, the radial gap between the rotating part and the fixed part should be less than 0.2-0.3mm, and there should be oil return control at the seal. To prevent oil leakage.    2. Spindle sliding bearing      sliding bearing has the characteristics of good vibration resistance, high rotation accuracy, smooth movement, etc., it is applied to high-speed or low-speed precision, high-precision CNC machine tools.     Main shaft sliding bearings can be divided into two categories: dynamic pressure bearings and hydrostatic bearings according to the way of producing oil film. According to different fluid media, it can be divided into liquid sliding bearing and pneumatic sliding bearing.     (1) Hydrodynamic bearing      When the main shaft rotates at a certain speed, the hydrodynamic bearing will bring the lubricating oil to flow from the large gap to the small gap, forming a pressure oil film to float the main shaft and create a load.     (2) Hydrostatic bearing      Hydrostatic bearing is composed of a set of special oil supply system, throttle and bearing. Compared with hydrodynamic bearings, hydrostatic bearings have the following advantages: high load carrying capacity, high rotation accuracy, oil film has the effect of homogenizing errors, can improve processing accuracy, good vibration resistance, stable operation, and can work at extremely low speeds. It can also work at extremely high speeds, with low friction and long bearing life.     (3) The static pressure bearing that uses air as the medium is called aerostatic bearing, also called air bearing or air bearing, and its working principle is the same as that of a hydrostatic shaft.     (4) Magnetic bearing is a new type of high-performance bearing, with special performance that various traditional bearings cannot compare.

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