As a key component of machine tools, the performance of the spindle will directly affect the parameters of the machine tool's rotation accuracy, speed, rigidity, temperature rise and noise, which will in turn affect the processing quality of the workpiece, such as the dimensional accuracy of the parts, surface roughness and other indicators. Therefore, in order to maintain excellent machine tool processing capabilities, high-performance bearings must be used. The precision of the bearings used on the machine tool spindle should be ISO P5 or above (P5 or P4 is the ISO precision level, usually from low to high P0, P6, P5, P4, P2), and for the spindle support of high-speed and high-precision machine tools such as CNC machine tools and machining centers, ISO P4 or above precision should be selected; spindle bearings include angular contact ball bearings, tapered roller bearings, and cylindrical roller bearings.
Among the above-mentioned bearings, precision angular contact ball bearings are the most widely used. We all know that the rolling element of angular contact ball bearings is a ball; because it is a point contact (different from the line contact of roller bearings), it can provide higher speed, less heat generation and higher rotation accuracy. In some ultra-high-speed spindle applications, hybrid bearings with ceramic balls (generally Si3N4 or Al2O3) are also used. Compared with traditional fully hardened steel balls, the characteristics of ceramic ball materials themselves give ceramic ball bearings the characteristics of high rigidity, high speed, high temperature resistance and long life, thus meeting the needs of high-end customers for machine tool bearing products.
As for the contact angle of angular contact ball bearings, the more popular ones are 15° and 25° contact angles; usually the contact angle of 15° has a relatively high speed performance, while the contact angle of 25° has a higher axial load capacity. Since the choice of preload has a great influence on the application of precision angular contact ball bearings, for example, in high-load and high-rigidity occasions, medium or heavy bearing preloads are generally selected; and for some high-speed and high-precision applications, we need to pay attention to the selection of appropriate preloads in the early selection of bearings, and generally light preloads are more common. Preload is generally divided into three types: light, medium and heavy. In order to facilitate customers' use, several major bearing manufacturers in the world currently generally provide bearings with pre-ground bearing end faces and preloaded, which is what we usually call universal matching precision angular contact ball bearings. This type of bearing eliminates the need for customers to adjust the preload, thus saving installation time.
In some heavy-duty machine tool applications with certain speed requirements, such as rough grinding of forgings, threading machines for oil pipelines, heavy lathes and milling machines, choosing precision tapered roller bearings is an ideal solution. Since the rollers of tapered roller bearings are designed with line contact, they can provide high rigidity and load for the spindle; in addition, tapered roller bearings are a pure rolling bearing design, which can well reduce the torque and heat of the bearing operation, thereby ensuring the speed and accuracy of the spindle. Since tapered roller bearings can adjust the axial preload (clearance) during installation, this allows customers to better optimize the bearing clearance adjustment throughout the entire service life of the bearing. In addition
, in some high-speed applications where the inner ring rib line 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 ribs of TSMA bearings have multiple axial lubrication holes, which can be lubricated with circulating oil or oil mist. Centrifugal force can distribute the oil to the contact area between the rollers and the ribs, so that the bearings can be more fully lubricated and the bearing speed can be increased. 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 bearings with fixed inner ring flanges; and the floating flange is positioned by the "pressure" system, which can ensure that the bearing can perform at its best under various working conditions, and has been successfully used by domestic customers.
For the precision selection of tapered roller bearings, ISO P5 grade bearings are mainly used in traditional lathes and milling machines; if it is used in machining centers, grinders and other applications, ISO P4 or above precision bearings are usually selected. In spindle design, there are two common tapered roller bearing layouts. The first is to use a tapered roller bearing at the front and rear ends, and use a face-to-face installation method; this type of design has a compact structure, high rigidity and is easy to install and adjust.
The second type is to use two tapered roller bearings installed face to face as the front end of the spindle, and the back end uses a double outer ring and two single inner ring bearings (TDO) as the floating end; due to its floating ability, this type of design can well withstand the axial thermal expansion of the spindle, and has high rigidity, ensuring the accuracy of the machine tool
In the application of machine tool spindles, double-row precision cylindrical roller bearings are also used, usually in combination with precision angular contact ball bearings or thrust bearings. This type of bearing can withstand large radial loads and allow high speeds. The two rows of rollers in the bearing are arranged in a cross manner, and the fluctuation frequency during rotation is greatly increased compared to single-row bearings, and the amplitude is reduced by 60%~70%. This type of bearing usually has two forms: the inner ring of the two series NN30 and NN30K bearings has a rib and the outer ring is separable; the outer ring of the two series NNU49 and NNU49K bearings has a rib and the inner ring is separable. The inner ring of the NN30K and NNU49K series is a tapered hole (taper 1:12), which cooperates with the tapered journal of the spindle. The inner ring can be moved axially to expand the inner ring, so that the bearing clearance can be reduced or even preloaded (negative clearance state). Cylindrical bore bearings are usually hot-mounted, using interference fit to reduce bearing clearance or preload bearings. For NNU49 series bearings with separable inner rings, the raceway is usually finished after the inner ring is installed on the spindle to improve the spindle rotation accuracy.
