Machine tool bearing type
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.
First, the selection of spindle bearings
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.
