How to choose precision machine tool spindle bearings
Rolling bearings are mainly used in the following three parts of machine tools: spindles, ball screws and general transmission shafts. 1. Rotation accuracy of precision machine tool spindle system The bearing accuracy of rolling bearings used on precision machine tool spindles should be P5 and above. For spindle supports of high-speed and high-precision machine tools such as CNC machine tools and machining centers, super-precision bearings of P4 and above should be selected. As the basic accessory of machine tools, the performance of spindle bearings directly affects the machine tool’s speed, rotation accuracy, rigidity, anti-chatter cutting performance, noise, temperature rise and thermal deformation, etc., which in turn affects the accuracy and surface quality of machined parts. Therefore, high-performance machine tools must be equipped with high-performance bearings. The rotation accuracy of the spindle system refers to the accuracy of the radial runout, cross-sectional runout and axial movement measured on the reference plane of the front end of the spindle when the machine tool is rotating manually or motorized at low speed without load. The accuracy of the spindle system is mainly affected by the following factors: 1) The radial runout of the groove of the bearing ring will cause the radial runout of the spindle axis of the spindle system, thus reflecting these errors on the surface to be processed. 2) Inconsistent diameters and shape errors of the bearing rolling elements will cause regular errors in the spindle. 3) The side swing of the groove on the end face will cause the axial movement of the spindle. The axial movement of the spindle has a particularly significant impact on precision grinders, especially bearing grinders. If the method of branch groove grinding is used in the process, it will cause The scrap rate will increase significantly, and the noise will also increase significantly. 4) The size and shape errors of the bearing installation working surface will cause corresponding deformation of the bearing raceway, causing the inner and outer rings of the bearing to tilt, making the stiffness of the bearing inconsistent in all directions, which will reduce the rotation accuracy of the spindle system. The end faces of the nuts, spacers, and washers used to adjust the clearance need to be ground and processed, and the perpendicularity to the rotation axis of the shaft system must correspond to the accuracy of the installed bearing, otherwise the working accuracy of the bearing will be reduced. 2. Precision machine tool spindle lubrication and sealing We mentioned earlier that lubricant is an important part of the bearing configuration. Using different lubrication methods, the data of the bearing’s limit speed are different. The lubricant must be replenished in time according to the design requirements (the grease filling amount of high-speed precision spindle system is generally 10%~20% of the bearing space). Reasonable addition of lubricant can reduce the friction and wear of the shaft system and extend the fatigue life of the bearing. At the same time, it can discharge the friction heat of the bearing system and play the role of cooling the bearing system; in addition to grease, the lubrication of the bearing system also includes liquid oil lubrication, oil mist lubrication and oil-gas lubrication. Oil-gas lubrication is widely used in foreign high-speed spindle systems. Already widely used, oil-gas lubrication is fuel-saving and pollution-free, and can significantly increase the DN value of the spindle system, and can intelligently control the required lubricant supply amount of the shaft system. The design of the spindle structure, lubrication and sealing must adopt different forms according to different situations. If the seal is worn, it must be replaced in time. If the contact point between the sealing lip and the rotating body exceeds 15m/s, a non-contact sealing structure must be used. Most high-speed precision machine tools use labyrinth seals, and oil-draining grooves need to be designed at appropriate positions, because the contact type Seals not only cause friction loss and overheating at the seal contact, but also affect the rotation accuracy of the bearing system. 3. As a precise, efficient and sensitive transmission element, the ball screw should not only use high-precision screws, nuts and balls, but also pay attention to the selection of bearings with high axial stiffness, small friction torque and high operating accuracy. Ball screw supports used to be commonly used bidirectional thrust angular contact ball bearings, tapered roller bearings, needle roller and thrust roller combination bearings, deep groove ball bearings and thrust ball bearings, etc. At present, the single row thrust angular contact ball bearing with a 60° contact angle is most commonly used for ball screw support. Moreover, the accuracy level is mainly P4 and above.
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