What types of bearings are used in machine tool spindles?
What types of bearings are used in machine tool spindles?

Source: ZYS    Published:2020-04-14    

The selection of spindle bearings

The accuracy grade of the bearings used on the spindle of the machine tool should be ISOP5 or above, and for the spindle support of high-speed and high-precision machine tools such as CNC machine tools and machining centers, the accuracy of ISOP4 or above should be selected. Spindle bearings include angular contact ball bearings, tapered roller bearings, and cylindrical roller bearings.

1. Precision angular contact ball bearings

Precision angular contact ball bearings are widely used. The rolling elements of angular contact ball bearings are balls because it is a point contact (different from the linear contact of roller bearings), so it can provide higher speed, higher rotation accuracy and less heat. In some ultra-high-speed spindle applications, hybrid bearings with ceramic balls (usually si3N4 or Al203) will also be used. Compared with traditional fully hardened steel balls, the characteristics of ceramic ball materials endow ceramic ball bearings with high rigidity, high speed, high temperature resistance and long life, so as to meet the needs of high-end customers for machine tool bearing products.

For 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 speed performance, while the contact angle of 25 has high axial load carrying capacity. The choice of preload has a great influence on the application of precision angular contact ball bearings, such as for the applications of high load and high rigidity, medium or heavy bearing preload is generally selected; and for some high speed and high precision applications, it is necessary to pay attention to choose the appropriate preload. Preload is generally divided into three types: light, medium and heavy. Light preload is more common. In order to facilitate the use of customers, several major bearing manufacturers in the world generally provide pre-grinded bearing end faces and preloaded bearings, which is commonly known as universal matching precision angular contact ball bearings. This type of bearing eliminates the customer's preload adjustment, thereby saving installation time.

2 . Precision cylindrical roller bearings

In the application of machine tool spindles, double-row precision cylindrical roller bearings will also be used, usually combined with precision angular contact ball bearings or thrust bearings. Such bearings can withstand larger radial loads and allow higher speeds. The two rows of rollers in the bearing are arranged in a crossed manner, and the frequency of fluctuation during rotation is greatly increased compared with the single row bearing, and the amplitude is reduced by 60% -70%. Such bearings usually have two forms: NN30 and NN30K series bearings with ribs on the inner ring, the outer ring can be separated; NNU49, NNU49K series bearings with ribs on the outer ring, the inner ring can be separated. Among them, the inner ring of NN30K and NNU49K series is a tapered hole (taper 1:12), which cooperates with the tapered journal of the spindle to move the inner ring axially to expand the inner ring, so that the bearing clearance can be reduced or even preload the bearing (negative clearance status). Cylindrical bore bearings are usually hot fitted, using interference fit to reduce bearing clearance, or preload bearings. For the NNU49 series bearings with separable inner ring, the raceway is generally finished after the inner ring is mounted on the spindle to improve the rotation accuracy of spindle.

3. Precision tapered roller bearings

In some heavy-duty machine tool applications that have certain speed requirements, such as grinding of forging parts, turning machines for oil pipelines, heavy lathes and milling machines, it is an ideal solution to choose precision tapered roller bearings. Because the rollers of tapered roller bearings are designed with linear contact, it can provide high rigidity and load for the spindle; in addition, tapered roller bearings have pure rolling bearing design, which can reduce the torque and heat in the bearing operation to ensure 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 bearing clearance adjustment throughout the life of the bearing.

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