The commonly used rotary tables in CNC machine tools include indexing tables and CNC rotary tables. When processing certain parts, CNC machine tools sometimes need circular motion around the three coordinate axes of X, Y, and Z in addition to linear feed motion of the three coordinate axes of X, Y, and Z, which we call A, B, and C axes respectively.
CNC rotary tables can be used to realize circular feed motion. In addition to circular feed motion, CNC rotary tables (referred to as CNC turntables) can also complete indexing motion. The function of the indexing table is only to transfer the workpiece to change the surface, and to cooperate with the automatic tool changer to achieve multiple processes of several surfaces in one installation of the workpiece, thus greatly improving the work efficiency. The appearance of the CNC turntable is not much different from that of the indexing table, but it has a series of characteristics in structure. Since the CNC turntable can realize feed motion, it has many similarities in structure with the feed drive mechanism of the CNC machine tool. The difference is that the drive mechanism realizes linear feed motion, while the CNC turntable realizes circular feed motion.
The rotary table is widely used in various CNC milling machines, boring machines, various vertical lathes, vertical milling machines and other machine tools. In addition to requiring the rotary table to be able to bear the weight of the workpiece well, it is also necessary to ensure its rotation accuracy under load. As the core component of the turntable, the turntable bearing must not only have a high load-bearing capacity, but also high rotation accuracy, high anti-overturning ability, and high speed capacity during the operation of the turntable. In the design of the rotary table, the more commonly used bearing types are roughly divided into the following types:
thrust ball bearings cylindrical roller bearings
Thrust ball bearings can withstand a certain axial force, so the bearing is mainly used to bear the weight of the workpiece; while cylindrical roller bearings are mainly used for radial positioning and bearing external radial forces (such as cutting force, milling force, etc.). This type of design is widely used and the cost is relatively low. Since the thrust ball is a point contact bearing, its axial load-bearing capacity is relatively limited, and it is mainly used in small or medium-sized machine tool rotary tables. In addition, the lubrication of the thrust ball is also difficult.
Hydrostatic bearings are sliding bearings that rely on external pressure oil supply to establish a hydrostatic oil film in the bearing to achieve liquid lubrication. Hydrostatic bearings work under liquid lubrication from start to stop, so there is no wear, long service life, and low starting power; in addition, this type of bearing has the advantages of high rotation accuracy, large oil film stiffness, and the ability to suppress oil film oscillation. Precision cylindrical roller bearings have good radial bearing capacity, and due to the use of precision-grade bearings, the rotation accuracy of the rotary table can also be well guaranteed. The rotary table using this type of design can withstand very high axial forces. Some workpieces weigh more than 200 tons and the turntable diameter exceeds 10 meters. However, this type of design also has some shortcomings. Since hydrostatic bearings must be equipped with a dedicated oil supply system to supply pressure oil, maintenance is more complicated and the cost is relatively high.
Crossed roller bearings
Crossed roller bearings are also widely used on turntables. The characteristic of a crossed roller bearing is that there are two raceways in the bearing and two rows of cross-arranged rollers. Compared with the traditional thrust bearing radial centering bearing combination, the crossed roller bearing has a compact structure, small size, and simplifies the worktable design, thereby reducing the cost of the turntable.
In addition, due to the use of optimized preload, this type of bearing has a high rigidity, so the rigidity and precision of the turntable are also guaranteed. Thanks to the design of two rows of crossed rollers, the effective span of the bearing can be significantly increased, so this type of bearing has a high anti-overturning moment. Among the crossed roller bearings, there are two types: the first is the cylindrical crossed roller bearing, and the second is the tapered crossed roller bearing. Generally, cylindrical crossed roller bearings are cheaper than tapered crossed roller bearings and are suitable for relatively low speed turntable applications; tapered crossed roller bearings use the pure rolling design of tapered rollers, so this type of bearing has the following characteristics:
• High operating accuracy
• High speed capability
• Reduced shaft length and processing costs, limited changes in geometric dimensions due to thermal expansion
• Nylon separators, low moment of inertia, low starting torque, easy to control angular indexing
• Optimized preload, high stiffness, small runout
• Line contact, high stiffness, high operating accuracy of guide rollers
•Carburized steel provides excellent impact resistance and surface wear resistance
•Simple but fully lubricated
When installing the bearing, customers only need to preload the cross roller bearing to the recommended value, without having to go through a complicated installation and adjustment process like hydrostatic bearings. Cross roller bearings are easy to install and easy to adjust the original installation form or maintenance method. Cross roller bearings are suitable for various types of vertical or horizontal boring machines, as well as vertical mills, vertical lathes and large gear milling machines.
In short, as the core component of the machine tool spindle and rotary table, the bearing plays a vital role in the performance of the machine tool. In order to select the right size and type of bearing, we need to comprehensively consider various working conditions, such as operating speed, lubrication, installation type, spindle stiffness, precision and other requirements. As for the bearing itself, only by fully understanding its design characteristics and the advantages and disadvantages brought about by it can we give full play to the best performance of the bearing.
