High performance bearings for the hottest machine

2022-08-13
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As a key component of machine tool, the performance of high-performance bearing

spindle used in machine tool spindle and turntable will directly affect the rotation accuracy, speed, rigidity, temperature rise, noise and other parameters of machine tool, and then affect the processing quality of workpiece. In order to maintain excellent machining capacity of machine tools, high-performance bearings must be used

reasonable selection of spindle bearings

the bearing accuracy used on the spindle of the machine tool should be ISO P5 or above (P5 or P4 is the accuracy grade of ISO, usually P0, P6, P5, P4, P2 from low to high). For the spindle support of high-speed and high-precision machine tools such as CNC machine tools and machining centers, the accuracy of ISO P4 or above should be selected. Spindle bearings include angular contact ball bearings, tapered roller bearings, and cylindrical roller bearings

1. Precision angular contact ball bearings

among the above bearings, precision angular contact ball bearings (Figure 1) are the most widely used. The rolling element of angular contact ball bearing is a ball, because it is a kind of point contact (different from the line contact of roller bearing), so it can provide higher speed, smaller heating capacity and higher rotation accuracy. In some ultra-high speed spindle applications, hybrid bearings with ceramic balls (generally Si3N4 or Al2O3) will also be used. Compared with the traditional fully quenched steel ball, the characteristics of ceramic ball material endow ceramic ball bearings with the characteristics of high stiffness, high speed, high temperature resistance and long service life, so as to meet the needs of high-end customers for machine tool bearing products

in terms of the contact angle of angular contact ball bearings, at present, the more popular contact angles are 15 and 25; Generally, the contact angle of 15 has relatively high speed performance, while the contact angle of 25 has high axial bearing capacity. Because the choice of preload has a great impact on the application of precision angular contact ball bearings, for example, in the occasions of high load-bearing and high rigidity, medium-sized or heavy-duty bearing preload will generally be selected; (4) the sample specification is for some applications with high speed and high precision. In the early selection of bearings, attention should be paid to the selection of appropriate preloads. Preload is generally divided into light, medium and heavy, and generally light preload is common. In order to facilitate the use of customers, at present, several major bearing manufacturers in the world generally provide bearings with pre ground bearing end faces and preloaded bearings, which is commonly referred to as universal matched precision angular contact ball bearings. The results of the 5th National desertification and desertification land monitoring show that the bearing eliminates the preload adjustment of the customer, thus saving the installation time

2. Precision tapered roller bearing

in some machine tool applications with heavy load and certain requirements for speed, such as raw grinding of forgings, threading machines for oil pipelines, heavy-duty lathes and milling machines, choosing precision tapered roller bearings is an ideal scheme. Because the roller of tapered roller bearing is designed in line contact, it can provide high rigidity and load for the spindle; In addition, tapered roller bearing is a pure rolling bearing design, which can well reduce the running torque and heat of the bearing, so as to ensure the speed and accuracy of the main shaft. Because tapered roller bearings can adjust the axial preload (clearance) during installation, this allows customers to better optimize the adjustment of bearing clearance throughout the life cycle of the bearing

in addition, in some high-speed applications with inner ring flange line speed greater than 30m/s, some specially designed tapered roller bearings can also meet the requirements, such as TSMA bearings or Hydra rib hydraulic floating flange bearings (Figure 2). The flange of TSMA bearing has multiple lubricating oil holes in axial direction, which can be lubricated by circulating oil or oil mist. Centrifugal force can distribute the oil to the contact area between the roller and the flange, so as to more fully lubricate the bearing and improve the bearing speed. The Hydra rib hydraulic floating flange bearing is designed to optimize the preload of the spindle system. The bearing has a floating flange that contacts the big end of the roller, unlike the common bearing with a fixed inner ring flange; And the floating flange is positioned by the "pressure" system, so that the bearing can play the best performance under all working conditions, and has been successfully used in domestic customers

for the accuracy selection of tapered roller bearings, ISO P5 grade bearings are mainly used in traditional lathes and milling machines; For machining centers, grinders and other applications, we usually choose ISO P4 or precision bearings above different experimental machines with different experimental methods. In the design of spindle, there are two common layout of tapered roller bearings. The first is that a tapered roller bearing is used at the front end and the rear end respectively, and the face-to-face installation method is adopted. This kind of design has compact structure, high stiffness and is easy to install and adjust

the second is to use two tapered roller bearings installed face to face as the front end of the main shaft, while the rear end uses a double outer ring and two single inner ring bearings (TDO) as the floating end; Due to its floating capacity, this kind of design can well withstand the axial thermal expansion of the spindle, and the rigidity is very high, ensuring the accuracy of the machine tool

3. Precision cylindrical roller bearings

in the application of machine tool spindles, double row precision cylindrical roller bearings will also be used, usually combined with precision angular contact ball bearings or thrust bearings. This kind of bearing can bear large radial load and allow higher speed. The two rows of rollers in the bearing are arranged in a cross manner. When rotating, the fluctuation frequency is significantly higher than that of a single row bearing, and the amplitude is reduced by 60% - 70%. This kind of bearing usually has two forms: NN30 and nn30k series, the inner ring of the bearing has a flange, and the outer ring can be separated; The outer rings of nnu49 and nnu49k series bearings have ribs, and the inner rings can be separated. Among them, the inner rings of nn30k and nnu49k series are tapered holes (taper 1:12), which cooperate with the tapered Journal of the main shaft. Moving the inner ring axially can expand the inner ring, so that the bearing clearance can be reduced or even pre tighten the bearing (negative clearance state). Cylindrical hole bearings are usually hot installed, using interference fit to reduce bearing clearance, or preloading bearings. For nnu49 series bearings with separable inner rings, the raceway is generally finished after the main shaft is installed on the inner ring to improve the rotation accuracy of the main shaft

reasonable selection of turntable bearings

rotary worktables commonly used in CNC machine tools include indexing worktables and CNC rotary worktables. When NC machine tools process some parts, in addition to the linear feed motion of the X, y and Z coordinate axes, sometimes they also need circular motion around the X, y and Z coordinate axes, which are called axes a, B and C respectively

the CNC rotary table can be used to realize the circular feeding movement. In addition, it can also complete the indexing movement. The function of the indexing worktable is only to rotate the workpiece and change the surface, which is used in conjunction with the automatic tool change device to realize that the workpiece can complete multiple processes on several surfaces at one time. Therefore, the work efficiency is greatly improved. The shape of NC turntable is not much different from that of indexing worktable, but it has a series of characteristics in structure. Because NC turntable can realize feed movement, it has many similarities with the feed drive mechanism of NC machine tools in structure. The difference is that the driving mechanism realizes linear feed motion, while the CNC turntable realizes circular feed motion

rotary table is widely used in all kinds of CNC milling machines, boring machines, all kinds of vertical lathes and vertical milling machines. Due to the small risk of establishing a recycled plastic particle factory. In addition to the requirement that the rotary table can well bear the weight of the workpiece, it is also necessary to ensure its rotation accuracy under the load. As the core component of the turntable, the turntable bearing should not only have high bearing capacity, but also have high rotation accuracy, high overturning resistance, and high speed capacity during the operation of the turntable

1. Thrust ball bearing + cylindrical roller bearing

thrust ball bearing can bear certain axial force, so it is mainly used to bear the weight of workpiece; Cylindrical roller bearings are mainly used for radial positioning and bearing external radial forces (such as cutting force, milling force, etc.). This kind of design is widely used, and the cost is relatively low. Because the thrust ball is a kind of point contact bearing, its axial bearing capacity is relatively limited. It is mainly used for small or medium-sized machine tool rotary worktables. In addition, it is difficult to lubricate the thrust ball

2. Hydrostatic bearing + precision cylindrical roller bearing

hydrostatic bearing is a kind of sliding bearing that relies on external supply of pressure oil and establishes hydrostatic bearing oil film in the bearing to realize liquid lubrication. Hydrostatic bearing always works under liquid lubrication from start to stop, so it has no wear, long service life and low starting power; In addition, this bearing also has the advantages of high rotation accuracy, large oil film stiffness, and can inhibit oil film oscillation. Precision cylindrical roller bearings have good radial bearing capacity, and due to the use of precision bearings, the rotation accuracy of the rotary table can also be guaranteed. The rotary table designed with this type can bear high axial force. The weight of some workpieces is more than 200t, and the diameter of the turntable is more than 10m. However, this kind of design also has some shortcomings. Because the hydrostatic bearing must be equipped with a special oil supply system to supply pressure oil, the maintenance is more complex and the cost is relatively high

3. Cross roller bearings

cross roller bearings (Fig. 3) are widely used on turntables. The characteristic of crossed roller bearing is that there are two raceways and two rows of crossed rollers in the bearing. Compared with the traditional thrust bearing + radial centering bearing combination (Fig. 4), the cross roller bearing has compact structure and small volume, and simplifies the design of the workbench, thus reducing the cost of the turntable

in addition, due to the use of optimized preload, this kind of bearing has high stiffness, so the stiffness and accuracy of the turntable are also guaranteed. Thanks to the design of two rows of cross rollers, the effective span of the bearing can be significantly improved, so this kind of bearing has a high overturning moment. Among the cross roller bearings, there are two types: the first is cylindrical cross roller bearing, and the second is conical cross roller bearing. Generally, the price of cylindrical cross roller bearing is lower than that of conical cross roller bearing, which is suitable for turntable applications with relatively low speed; The tapered cross roller bearing adopts the pure rolling design of tapered roller, which has the advantages of high operation accuracy, strong speed ability, reduced shaft length and processing cost. Cross roller bearings are suitable for various types of vertical or horizontal boring machines, as well as vertical grinding, vertical turning and large gear milling machines

in a word, as the core component of the machine tool spindle and turntable, the bearing plays a decisive role in the operation performance of the machine tool. In order to select the appropriate size and type of bearing, we need to comprehensively consider various working conditions, such as operating speed, lubrication, installation type, spindle stiffness, accuracy and other requirements. As for the bearing itself, only by fully understanding its design characteristics and the resulting advantages and disadvantages, can the best performance of the bearing be brought into play. (end)

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