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Bearing selection for machine tool spindle and turntable

Time:2018-08-13Click:325Size:BMS

The bearing accuracy on the machine tool spindle should be ISOP5 or higher (P5 or P4 is the ISO accuracy level, usually from low to high is P0, P6, P5, P4, P2), and for CNC machine tools, machining centers For spindle support of high-speed, high-precision machine tools, ISOP4 or higher accuracy is required. Spindle bearings include angular contact ball bearings, tapered roller bearings, and cylindrical roller bearings.

Among the above types of bearings, precision angular contact ball bearings are the most widely used. The rolling element of an angular contact ball bearing is a ball because it is a point contact (distinguished from the line contact of the roller bearing), so it can provide higher rotation speed, smaller heat generation and higher rotation precision. In some ultra-high-speed spindle applications, hybrid bearings with ceramic balls (typically Si3N4 or Al2O3) 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 angular contact ball bearings, the contact angles of 15 and 25 are currently popular; usually the contact angle of 15 has a relatively high rotational speed performance, while the contact angle of 25 has a higher 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.

In some machine applications with heavy loads and speed requirements, such as the forging of forgings, wire machines for oil pipelines, heavy duty lathes and milling machines, etc., the choice of precision tapered roller bearings is one. 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.

In addition, in some high-speed applications where the inner ring rim 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 gears. The rib of the edge bearing TSMA bearing has a plurality of lubricating oil holes in the axial direction, which can be lubricated by circulating oil or oil mist. The centrifugal force can distribute the oil to the contact area between the roller and the rib, so that the bearing can be more fully lubricated. Increase bearing speed. The Hydra-Rib hydraulic floating rib bearing is designed to optimize the preload of the spindle system. The bearing has a floating rib that is in contact with the large end of the roller, unlike the common bearing with a fixed inner ring rib; The floating rib is positioned by the "pressure" system, which ensures that the bearing will perform at its best under all conditions and has been successfully used in domestic customers.

For precision selection of tapered roller bearings, ISOP5 bearings are mainly used in conventional lathes and milling machines. For machining centers, grinding machines, etc., precision bearings with ISOP4 or higher are usually selected. . In the spindle design, there are two common tapered roller bearing layouts. The first is a tapered roller bearing with a front and rear end and a face-to-face mounting that is compact, rigid and easy to install and adjust.

The second is to use two tapered roller bearings to 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; Floating capacity, this type of design can withstand the axial thermal expansion of the spindle well, and the rigidity is high, which ensures the accuracy of the machine.

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. These bearings can withstand large radial loads and allow for higher speeds. The two rows of rollers in the bearing are arranged in an intersecting manner. The fluctuation frequency during rotation is greatly improved compared with the single-row bearing, and the amplitude is reduced by 60% to 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.


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