Machine tool bearing selection analysis

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 article 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, speed, rigidity, temperature rise and noise of the machine tool, which affects the machining quality of the workpiece. In order to maintain excellent machine tool processing capabilities, high performance bearings must be used.

1 spindle bearing selection

The accuracy of the bearing used on the machine tool spindle should be ISOP5 or above (P5 or P4 is the ISO accuracy level, usually from low to high (P0, P6, P5, P4, P2), and for high speed CNC machine tools, machining centers, etc. For high-precision machine tool spindle bearings, ISOP4 or higher accuracy is required. Spindle bearings include angular contact ball bearings, tapered roller bearings, and cylindrical roller bearings.

1.1 precision angular contact ball bearings

Precision angular contact ball bearings are widely used. The rolling element of an angular contact ball bearing is a ball because it is a point contact (different from the line contact of the roller bearing), so it can provide higher rotation speed and smaller heat generation. And higher rotation accuracy. In some ultra-high-speed spindle applications, hybrid bearings with ceramic balls (usually si3N4 or Al203) 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 load carrying capacity. Since 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, care must be taken to select the appropriate preload. The preload is generally divided into three types: light, medium and heavy. Generally, light preload is more common. In order to facilitate the use of customers, the world's major bearing manufacturers generally provide pre-grinded bearing end faces and preloaded bearings, which is commonly referred to as the universal pairing precision angular contact ball bearing form. This type of bearing eliminates customer preload regulation and saves installation time.

1.2 precision cylindrical roller bearing

In machine tool spindle applications, double row precision cylindrical roller bearings are also used, usually in combination with precision angular contact ball bearings or thrust bearings. These bearings can withstand large radial loads and allow for higher speeds. The two rows of rollers in the bearing are arranged in a crosswise manner. The fluctuation frequency during rotation is greatly improved compared to the single row bearing, and the amplitude is reduced by 60%-70%. These bearings usually come in two forms: NN30, NN30K two series bearing inner ring with ribs, outer ring can be separated; NNU49, NNU49K two series bearing outer ring with ribs, inner ring can be separated, of which NN30K and NNU49K series The inner ring is a tapered hole (taper 1:12), which cooperates with the tapered journal of the main shaft, and moves the inner ring axially to make the inner ring expand, so that the bearing clearance can be reduced or even pre-tensioned (negative clearance) status). Cylindrical bore bearings are usually hot-fitted, with an interference fit to reduce bearing clearance or preload the bearing. For NNU49 series bearings with separable inner ring, the main shaft is usually mounted on the inner ring and then the raceway is finished to improve the spindle rotation accuracy.

1.3 precision tapered roller bearings

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 tapered roller bearings are capable of adjusting the axial preload (play) during installation, this allows the customer to better optimize bearing clearance throughout the life of the bearing.

2 rotary table bearing selection

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 to have circular motion around the three coordinate axes X, Y and Z, respectively A, B, C axis. The CNC rotary table can be used to achieve the circumferential feed motion, in addition to the indexing movement. The function of the indexing table is only to change the surface of the workpiece, and it can be used together with the automatic tool changing device 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 there are a series of features in the structure. 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 vehicles 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 rotary table 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.

2.1 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 for small or medium-sized machine tool rotary table. In addition, the lubrication of the thrust ball is also difficult.

2.2 hydrostatic bearing + precision cylindrical roller bearing

The hydrostatic bearing is a sliding bearing that supplies pressure oil externally and establishes a static pressure bearing oil film in the bearing to achieve lubrication. The hydrostatic bearing always works under lubrication from start to stop, so there is no wear, long service life and low starting power. In addition, the 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 the precision of the rotary table can be guaranteed due to the use of precision bearings. The rotary table designed with this type can withstand high axial forces. Some workpieces weigh more than 200t and the turntable has a diameter of more than 10m. However, this type of design also has some shortcomings. Since the hydrostatic bearing must be supplied with a special oil supply system to supply the pressure oil, the maintenance is complicated and the cost is relatively high.

2.3 crossed roller bearing

Cross roller bearings are commonly used on turntables. 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 and compact, and simplifies the workbench design, thus reducing the cost of the turntable. Thanks to the optimized preload, the bearings are highly rigid and the rigidity and precision of the turret are guaranteed. Thanks to the design of the two-row cross-roller, the effective span of the bearing can be significantly improved, so this type of bearing has a high anti-overturning moment. In the cross-roller bearing, it is divided into two types: the first is a cylindrical cross-roller bearing, and the second is a tapered cross-roller bearing. In general, cylindrical cross-roller bearings are less expensive than conical crossed roller bearings and are suitable for use in turntable applications where the rotational speed is relatively low. Conical crossed roller bearings use a purely rolling design of tapered rollers for high running accuracy and speed capability. Strong, reducing the advantages of shaft length and processing cost. Crossed roller bearings are suitable for all types of vertical or horizontal boring machines, as well as vertical grinding, vertical turning and large gear milling machines.

In summary, in order to be able to select the appropriate size and type of bearing, it is necessary to comprehensively consider various working conditions, such as operating speed, lubrication, installation type, spindle stiffness, accuracy and other requirements. As far as the bearing itself is concerned, the bearing performance can be fully exploited only by fully understanding its design features and the advantages and disadvantages. As the core component of the machine tool spindle and turntable, the bearing plays a decisive role in the performance of the machine tool.