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. 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. In order to maintain excellent machine tool processing capability, high-performance bearings must be equipped.
The bearing accuracy used on the machine tool spindle shall be ISOP5 or above (P5 or P4 is the accuracy grade of ISO, usually from low to high (P0, P6, P5, P4, P2). For the spindle support of high-speed and high-precision machine tools such as CNC machine tools and machining centers, ISOP4 or above accuracy shall be selected. Spindle bearings include angular contact ball bearings, tapered roller bearings, and cylindrical roller bearings.
1. Precise angular contact ball bearing Precise angular contact ball bearing is widely used. The rolling body of angular contact ball bearing is a ball. Because it is a point contact (different from the linear contact of roller bearing), it can provide higher rotational speed, smaller heat output and higher rotational accuracy. In some ultra-high-speed spindle applications, hybrid bearings with ceramic balls (generally si3N4 or Al203) will also be used. The metalworking reporter believes that compared with the traditional fully hardened steel ball, the characteristics of the ceramic ball material itself endow the ceramic ball bearing with the characteristics of high rigidity, high speed, high temperature resistance and long service life, so as to meet the needs of high-end customers for machine tool bearing products. As far as the contact angle of angular contact ball bearing 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 bearing 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 generally selected; For some applications with high speed and high precision, it is necessary to select appropriate preload in the early selection of bearings. Metal processing experts said that 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, at present, several major bearing manufacturers in the world generally provide bearings with pre-grinding bearing end faces and preloading, which is commonly known as universal matching precision angular contact ball bearings. This type of bearing eliminates the preload adjustment of customers, thus saving installation time.
2. In the application of precision cylindrical roller bearings in machine tool spindles, double-row precision cylindrical roller bearings will also be used, usually in combination 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 metal processing line 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 detachable inner ring, the raceway is usually finished after the inner ring is installed with the main shaft to improve the rotation accuracy of the main shaft.
3. Precise tapered roller bearing is an ideal choice for machine tool applications with heavy load and certain speed requirements, such as rough grinding of forgings, threading machine of oil pipelines, heavy lathe and milling machine, etc. 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.