Selection of CNC machine tool turntable bearings
Commonly used turntables in CNC machine tools include indexing tables and CNC turntables.
When CNC machine tools process certain parts, they not only require linear feed motion on the X, y, and Z axes, but also require circular motion around the X, y, and Z axes, which are called a, B, and C axes respectively. The CNC turntable can be used to realize circular feed motion, and in addition, indexing motion can be completed. The function of the indexing table is only to change the surface of the workpiece, and with the automatic tool changer, the workpiece can be installed at one time and multiple processes can be completed on multiple surfaces. This greatly improves work efficiency. Although the appearance of the metal processing online CNC turntable is not much different from that of the indexing table, it has a series of structural features. Since the CNC turntable can realize feed motion, it has many similarities with the feed transmission mechanism of CNC machine tools in structure. The difference is that the transmission mechanism realizes linear feed motion, while the CNC turntable realizes circular feed motion.
Rotary worktables are widely used in various CNC milling machines, boring machines, various vertical lathes and vertical milling machines. In addition to asking for a U-turn
The workbench can bear the weight of the workpiece well, and its rotation accuracy under load also needs to be ensured. As the core component of the turntable, the turntable bearing must not only have a high load-bearing capacity, but also have high rotational accuracy, high overturning resistance and high rotational speed during the operation of the turntable.
1. Thrust ball bearings carry cylindrical rollers through the bearings
Thrust ball bearings can withstand a certain axial force, so the bearings are mainly used to bear the weight of the workpiece; cylindrical roller bearings are mainly used for radial positioning and bearing external radial forces (such as cutting forces, milling forces, etc.). ). This design is widely used and the cost is relatively low. Since thrust balls are point contact bearings, their axial load-bearing capacity is relatively limited and they are mainly used for small and medium-sized machine tool turntables. In addition, lubrication of the thrust ball is difficult.
2. Static pressure bearings precision cylindrical roller bearings
A hydrostatic bearing is a sliding bearing that relies on external pressure oil supply to establish a hydrostatic bearing oil film in the bearing to achieve lubrication. Hydrostatic bearings work in a lubricated state from start to stop, with no wear, long service life and low starting power. In addition, the bearing has the advantages of high rotation accuracy, large oil film stiffness, and suppression of oil film oscillation. Precision cylindrical roller bearings have good radial load-bearing capacity. Due to the use of precision bearings, the rotation accuracy of the turntable can also be guaranteed. The turntable designed with this design can withstand large axial forces. Some parts weigh more than 200t and the turntable diameter is greater than 10m. However, this design also has some shortcomings because the hydrostatic bearing must be attached with a dedicated oil supply system. To supply pressure oil, maintenance is more complicated and the cost is relatively high.
3. Crossed roller bearings
Crossed roller bearings are widely used in turntables. Crossed roller bearings have two raceways and two rows of cross-arranged rollers. Compared with the traditional thrust bearing combination, the crossed roller bearing has a compact structure and small size, which simplifies the design of the workbench, thereby reducing the cost of the turntable. Due to the optimized preload force, this bearing has a high stiffness, thus ensuring the stiffness and accuracy of the turntable. The design of two rows of crossed rollers greatly increases the effective span of the bearing and gives the bearing a high anti-overturning torque. In crossed roller bearings, they are divided into two types: the first is cylindrical crossed roller bearings, and the second is tapered crossed roller bearings. Generally speaking, cylindrical crossed roller bearings are cheaper than tapered crossed roller bearings and are suitable for relatively low rotational speeds in turntable applications, while tapered crossed roller bearings are purely rolling designs of tapered rollers that have high operating accuracy, speed capabilities, Advantages include reducing shaft length and processing costs. Crossed roller bearings are suitable for use in all types of vertical or horizontal boring machines, as well as in vertical grinding machines, vertical lathes and large gear milling machines.
1. The accuracy of bearings used on machine tool spindles must reach P5 level or above, while for turntable bearings of high-speed, high-precision machine tools such as CNC machine tools and machining centers, P4 level or above accuracy is required.
2. Spindle bearings include angular contact ball bearings, cylindrical and tapered roller bearings, crossed tapered roller bearings, and crossed cylindrical roller bearings. As far as the contact angle of angular contact ball bearings is concerned, the more popular contact angles are 15o and 25o; usually, the contact angle of 15o has relatively high rotational speed performance, while the contact angle of 25o has high axial load-bearing capacity.
Third, no contact angle is required for crossed roller bearings. A single set of bearings can meet the needs of axial and radial loads. If higher rigidity is required, transverse tapered roller bearings can be selected. Since the choice of preload has a great impact on the application of precision angular contact ball bearings, for example, in high load-bearing and high rigidity situations, medium or heavy bearing preloads are generally selected; and for some high-speed, high-precision applications , in the early selection of bearings, we need to pay attention to selecting the appropriate preload. Generally, light preload is more common. Preload is generally divided into three types: light, medium and heavy. Generally, light preload is more common. For example, crossed roller bearings and YRT bearings all choose high preload negative clearance design.
If metal dust in the processing area of the workbench enters the interior of the CNC turntable, it will be adsorbed to the rotor. The CNC turntable will affect the performance of the motor and even cause its failure. Therefore, usually in the design, it is necessary to adopt a combination of labyrinth structure and air sealing to continuously Blow in clean and dry gas to ensure safe and reliable operation of the motor. Full consideration must be given when sealing, as if the motor encounters coolant intrusion, its insulation will be reduced or degraded. In addition, the bearings and encoders used on the turntable are precision mechanical moving parts and detection system components, and the reliability of their sealing material structures must be ensured during use.
