As the key component of the machine tool, the performance of the spindle will directly affect the parameters of the machine tool such as rotation accuracy, rotation speed, rigidity, temperature rise and noise, and then affect the processing quality of the workpiece, such as the dimensional accuracy of the part, surface roughness and other indicators. Therefore, in order to maintain the machining capacity of the machine tool, it is necessary to use high-performance bearings. The bearing accuracy used on the machine tool spindle should be ISO P5 or above (P5 or P4 is the accuracy of ISO, usually P0, P6, P5, P4, P2 from low to high), while for the spindle support of high-speed and high-precision machine tools such as CNC machine tools and machining centers, ISO P4 or above accuracy should be selected; In the selection guide of main shaft bearings, it is stated that main shaft bearings include angular contact ball bearings, tapered roller bearings, cylindrical roller bearings, deep groove ball bearings and other types.
1、 Precision angular contact ball bearing
Among the above-mentioned bearings, precision angular contact ball bearings are widely used. We all know that the rolling body of angular contact ball bearing is ball; Because it is a kind of point contact (different from the linear contact of roller bearings), it can provide higher rotational speed, smaller heat output and higher rotation accuracy. In some high-speed spindle applications, ceramic ball hybrid bearings will also be used. Compared with the traditional quenched steel ball, the ceramic ball material itself endows the ceramic ball bearing with high rigidity, high speed, high temperature resistance and long service life, so as to meet the needs of high customers for machine tool bearing products.
As far as the contact angle of angular contact ball bearings is concerned, 15 ° and 25 ° contact angles are popular at present; Generally, the contact angle of 15 ° has higher rotational speed performance, while the contact angle of 25 ° has higher axial load capacity. Because the selection of preload has a great impact on the application of precision angular contact ball bearings, for example, in the case of high load bearing and high rigidity, the medium or heavy bearing preload is usually selected; For applications with high speed and high precision, we need to pay attention to the selection of appropriate preload in the early selection of bearings. Generally, light preload is common. Preload is generally divided into light, medium and heavy; In order to facilitate the use of customers, at present, several major bearing manufacturers in the world generally provide bearings with pre-grinding bearing surfaces and preloading, which is the form of paired precision angular contact ball bearings we usually call. This type of bearing eliminates the preload adjustment of customers, thus saving installation time.
2、 Precision tapered roller bearing
In the application of machine tools with heavy loads and certain speed requirements, such as rough grinding of forgings, threading machines for oil pipelines, heavy lathes and milling machines, the selection of precision tapered roller bearings is an ideal solution. Because the roller of tapered roller bearing is designed with linear contact, it can provide high rigidity and load for the main shaft; In addition, the tapered roller bearing is a pure rolling bearing design, which can effectively reduce the running torque and heat of the bearing, thus ensuring 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.
For the accuracy selection of tapered roller bearings, ISO P5 bearings are mainly used in traditional lathes and milling machines; For machining center, grinding machine and other applications, precision bearings of ISO P4 or above are usually selected. In the spindle design, there are two common layouts of tapered roller bearings. Two tapered roller bearings are used at the front and rear respectively, and face to face installation is adopted; This type of design has compact structure, high rigidity and is easy to install and adjust. The second is to use two tapered roller bearings installed face to face as the front of the main shaft, and then use two bearings with double outer rings and two single inner rings (TDO) as floating; Due to its floating capacity, this kind of design can well withstand the axial thermal expansion of the main shaft, and its rigidity is very high, ensuring the accuracy of the machine tool.
3、 Precision cylindrical roller bearing
In the application of machine tool spindle, double-row precision cylindrical roller bearings are also used, usually combined with precision angular contact ball bearings or thrust bearings. This type of bearing can bear large radial load and allow high speed. The two rows of rollers in the bearing are arranged in a cross manner. The fluctuation frequency during rotation is significantly higher than that of the single row bearing, and the amplitude is reduced by 60%~70%. This type of bearing usually has two forms: NN30 and NN30K series bearing inner ring with retaining edge, and outer ring can be separated; The outer ring of NNU49 and NNU49K series bearings is provided with a retaining edge, and the inner ring can be separated. The inner ring of NN30K and NNU49K series is a tapered hole (taper 1:12), which is matched with the tapered journal of the main shaft. The inner ring can be expanded by axial movement of the inner ring, so that the bearing clearance can be reduced or even preloaded (negative clearance state). Cylindrical hole bearings are usually hot-loaded, using interference fit to reduce bearing clearance, or preloading bearings. For NNU49 series bearings with separable inner rings, the main shaft is usually installed on the inner ring before finishing the raceway to improve the rotation accuracy of the main shaft.
4、 Precision deep groove ball bearing
In the application of machine tool spindle, precision deep groove ball bearing will also be used. This type of bearing is generally only used to bear radial load. Because the clearance is not adjustable, it is often used in occasions where the accuracy is not high and preload is not required, such as the spindle of ordinary drilling machine. Angular contact ball bearings are usually used in pairs, which takes up a large amount of space. Deep groove ball bearings can achieve a small angle of angular contact through preloading. The preloaded bearings can eliminate axial and radial clearance at the same time, double the spindle accuracy and even further improve the bearing accuracy after running under disaster load. And the noise of the main shaft during operation is also improved a lot. Therefore, precision deep groove ball bearing can also be used as the main shaft bearing of ordinary drilling machine.