As the core component of the machine tool spindle and turntable, the bearing plays a decisive role in the performance of the machine tool. This paper mainly describes the selection of spindle bearings and the selection of rotary table bearings.
As a key component of the machine tool, the spindle directly affects the parameters such as the rotation accuracy, rotation speed, rigidity, temperature rise and noise of the machine tool, which in turn affects the machining quality of the workpiece, such as the dimensional accuracy of the part and the surface roughness.
Therefore, in order to maintain excellent machine tool processing capabilities, high-performance bearings must be used. The accuracy of the bearings used on the machine tool spindle should be ISO P5 or higher (P5 or P4 is the ISO accuracy level, usually from low to high is P0, P6, P5, P4, P2), and for high-speed CNC machine tools, machining centers, etc. For high-precision machine tool spindle bearings, ISO P4 or higher accuracy is required; spindle bearings include angular contact ball bearings, tapered roller bearings, and cylindrical roller bearings.
Among the above types of bearings, precision angular contact ball bearings are the most widely used. We all know that the rolling element of an angular contact ball bearing is a ball; because it is a point contact (distinguished from the line contact of the roller bearing), it can provide higher speed, smaller heat generation and higher rotation. Precision. In some ultra-high-speed spindle applications, hybrid bearings with ceramic balls (typically Si3N4 or Al2O3) are also used. Compared with the traditional fully hardened steel ball, the ceramic ball material itself gives the ceramic ball bearing the characteristics of high rigidity, high rotation speed, high temperature resistance and long service life, so as to meet the demand of high-end customers for machine tool bearing products.
In terms of the contact angle of the angular contact ball bearing, the contact angles of 15° and 25° are currently popular; usually the contact angle of 15° has a relatively high rotational speed performance, and the contact angle of 25° has a high axial direction. Carrying capacity. Because the choice of preloading has a great influence on the application of precision angular contact ball bearings, for example, in high load bearing and high rigidity, medium or heavy bearing preloading is generally used; for some high speed and high precision applications. In the early selection of bearings, we need to pay attention to choose the appropriate preload, generally light preload is more common. Preloading is generally divided into three types: light, medium and heavy. In order to facilitate the use of customers, several major bearing manufacturers in the world generally provide pre-grinded bearing end faces and preloaded bearings, which is what we usually call universal matching precision. Angular contact ball bearing form. This type of bearing eliminates customer preload regulation and saves installation time.
Precision tapered roller bearing
In some machine applications with heavy loads and speed requirements – such as the forging of forgings, the wire machine of oil pipelines, heavy duty lathes and milling machines, the selection of precision tapered roller bearings is an ideal solution. . Because the roller of the tapered roller bearing is designed for line contact, it can provide high rigidity and load to the main shaft. In addition, the tapered roller bearing is a pure rolling bearing design, which can reduce the bearing operation. Torque and heat to ensure spindle speed and accuracy. Since the tapered roller bearing can adjust the axial preload (play) during installation, this allows the customer to better optimize the bearing clearance adjustment throughout the life of the bearing.
In addition, in some high-speed applications where the inner ring rim speed is greater than 30 m/s, some specially designed tapered roller bearings can also meet the requirements, such as TSMA bearings or Hydra-Rib hydraulic floating rib bearings. The rib of the TSMA bearing has a plurality of lubricating oil holes in the axial direction, which can be lubricated by circulating oil or oil mist. The centrifugal force can distribute the oil to the contact area between the roller and the rib, so that the bearing can be more fully lubricated and the bearing can be improved. Rotating speed. The Hydra-Rib hydraulic floating rib bearing is designed to optimize the preload of the spindle system. The bearing has a floating rib that is in contact with the large end of the roller, unlike the common bearing with a fixed inner ring rib; The floating rib is positioned by the “pressure” system, which ensures that the bearing will perform at its best under all conditions and has been successfully used in domestic customers.
The rotary table commonly used in CNC machine tools has an indexing table and a CNC rotary table. When machining certain parts of CNC machine tools, in addition to the linear feed motion of the three coordinate axes X, Y, and Z, sometimes there is a need for circular motion around the three axes of X, Y, and Z. For A, B, C axis
The CNC rotary table can be used to achieve a circular feed motion. In addition to the circular feed motion, the CNC rotary table (referred to as the CNC rotary table) can also complete the indexing movement. The function of the indexing table is to transfer the workpiece to the surface, and the automatic tool changing device can be used together to realize the multiple installations of several surfaces in one installation of the workpiece, thus greatly improving the working efficiency. The shape of the CNC turntable and the indexing table are not much different, but the structure has a series of features. Since the CNC rotary table can realize the feed motion, it has much in common with the feed drive mechanism of the CNC machine. The difference is that the drive mechanism realizes the linear feed motion, and the CNC rotary table realizes the circular feed motion.
The rotary table is widely used in various CNC milling machines, boring machines, various vertical cranes, and vertical milling machines. In addition to requiring the rotary table to withstand the weight of the workpiece well, it is also necessary to ensure its rotation accuracy under load. The turntable bearing, as the core component of the turntable, not only has a high load carrying capacity during the operation of the turntable, but also needs to have high rotation precision, high anti-overturning capability, and high rotational speed capability. In the design of the rotary table, the types of bearings used are roughly divided into several types:
Thrust ball bearing + cylindrical roller bearing
Thrust ball bearings can withstand certain axial forces, so the bearings are mainly used to bear the weight of the workpiece; cylindrical roller bearings are mainly used for radial positioning and external radial forces (such as cutting force, milling force, etc.) . This type of design is widely used and relatively inexpensive. Since the thrust ball is a point contact bearing, its axial bearing capacity is relatively limited, and it is mainly used in small or medium-sized machine tool rotary table. In addition, the lubrication of the thrust ball is also difficult.
Hydrostatic bearing + precision cylindrical roller bearing
A hydrostatic bearing is a plain bearing that supplies pressure oil externally and establishes a static pressure bearing oil film in the bearing to achieve liquid lubrication. Hydrostatic bearing always works under liquid lubrication from start to stop, so there is no wear, long service life and low starting power. In addition, this bearing also has the advantages of high rotation precision, large oil film stiffness, and can suppress oil film oscillation. Precision cylindrical roller bearings have good radial bearing capacity, and thanks to the use of precision bearings, the rotary table’s rotation accuracy is also well guaranteed.
The rotary table designed with this type can withstand high axial forces. Some workpieces weigh more than 200 tons and the turntable has a diameter of more than 10 meters. However, this type of design also has some shortcomings. Because the hydrostatic bearing must be equipped with a special oil supply system to supply the pressure oil, the maintenance is complicated and the cost is relatively high.
Crossed roller bearing
The application of crossed roller bearings on the turntable is also relatively common. Crossed roller bearings are characterized by two raceways in the bearing and two rows of rollers arranged in a row. Compared with the traditional thrust bearing + radial centering bearing combination, the cross roller bearing is compact, compact and simplifies the table design, thus reducing the cost of the turntable.