Views: 141 Author: Site Editor Publish Time: 2022-09-03 Origin: Site
The spindle and turntable of the machine tool play a pivotal role in the performance of the machine tool. So, what types of bearings are used in machine tools? The following summarizes and analyzes
As a key component of the machine tool, the performance of the spindle will directly affect the parameters such as the rotation accuracy, speed, rigidity, temperature rise and noise of the machine tool, which in turn will affect the processing quality of the workpiece, such as the dimensional accuracy of the parts, surface roughness and other indicators.
Therefore, in order to maintain excellent machine tool processing capability, high-performance bearings must be used. The bearing accuracy used on the machine tool spindle should be ISO P5 or above (P5 or P4 is the ISO accuracy grade, usually P0, P6, P5, P4, P2 from low to high), while for CNC machine tools, machining centers and other high-speed,
For the spindle support of high-precision machine tools, the accuracy of ISO P4 or above must be selected; the spindle bearings include angular contact ball bearings, tapered roller bearings, and cylindrical roller bearings.
Among the above-mentioned bearings, angular contact ball bearings are the most widely used. We all know that the rolling elements of angular contact ball bearings are balls; because it is a point contact (different from the linear contact of a roller bearing),
it can provide higher rotational speed, lower heat generation and higher rotational accuracy.
In some ultra-high-speed spindle applications, hybrid bearings with ceramic balls are also used.
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 life, to meet the needs of high-end customers for machine tool bearing products.
As far as the contact angle of angular contact ball bearings is concerned, the contact angles of 15° and 25° are currently more popular; usually, the contact angle of 15° has a relatively high speed performance, while the contact angle of 25° has a higher axial Carrying capacity.
Because the choice of preload has a great influence on the application of precision angular contact ball bearings, for example, in high-load and high-rigidity occasions, medium or heavy bearing preload is generally selected; for some high-speed, high-precision applications,
In the early selection of bearings, we need to pay attention to selecting the appropriate preload, generally light preload is more common.
Preload is generally divided into three types: light, medium and heavy; in order to facilitate the use of customers,
Several major bearing manufacturers in the world generally provide bearings with pre-ground bearing end faces and preloaded bearings, which is what we usually call universal matching precision. Angular contact ball bearing form.
This type of bearing eliminates the need for customer preload adjustment, thus saving installation time.
In some machine tool applications with heavy loads and certain speed requirements, such as forgings, threading machines for oil pipelines, heavy-duty lathes and milling machines, it is an ideal solution to choose precision tapered roller bearings. .
Since the rollers of the tapered roller bearing are designed in line contact, it can provide high rigidity and load capacity for the main shaft; in addition, the tapered roller bearing is a pure rolling bearing design, which can reduce the bearing operation well.
Torque and heat, thus ensuring the speed and accuracy of the spindle. Since tapered roller bearings can adjust axial preload (clearance) during installation, this allows customers to better optimize bearing clearance adjustment throughout the bearing's life cycle.
In addition, in some high-speed applications where the linear speed of the inner ring rib 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 rib bearings.
The rib of TSMA bearing has multiple lubricating oil holes in the axial direction, which can be lubricated by circulating oil or oil mist. Rotating speed.
The Hydra-Rib hydraulic floating rib bearing is specially designed to optimize the preload of the spindle system.
The bearing has a floating rib, which is in contact with the big end of the roller, unlike the common bearing with a fixed inner ring rib; and The floating rib is positioned by a "pressure" system, which ensures that the bearing can exert the best performance under various working conditions, and has been successfully used by domestic customers.
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. Such bearings can withstand larger radial loads and allow higher rotational speeds.
The two rows of rollers in the bearing are arranged in a cross manner, and the fluctuation frequency during rotation is greatly increased compared with that of a single row bearing, and the amplitude is reduced by 60% to 70%.
This type of bearing usually has two forms: NN30, NN30K two series bearings have ribs on the inner ring, and the outer ring can be separated; NNU49, NNU49K two series bearings have ribs on the outer ring, and the 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 expand the inner ring, so that the bearing clearance can be reduced or even preloaded (negative clearance State).
Cylindrical bore bearings are usually shrink-fit, using interference fit to reduce bearing clearance, or preloading the bearing. For NNU49 series bearings with separable inner ring, the raceway is generally finished after the main shaft is installed on the inner ring to improve the rotation accuracy of the main shaft.
The commonly used rotary worktables in CNC machine tools include indexing worktables and CNC rotary worktables. When CNC machine tools process certain parts, in addition to the linear feed motion of the three coordinate axes of X, Y, and Z, sometimes a circular motion around the three coordinate axes of X, Y, and Z is required. For the A, B, and C axes.
The CNC rotary table can be used to realize circular feed motion. The CNC rotary table (referred to as CNC turntable) can not only realize the circular feed movement, but also complete the indexing movement.
The function of the indexing table is only to index the workpiece and change the surface, and it is used in conjunction with the automatic tool changer to realize that the workpiece can be installed in one time and can complete various processes on several surfaces.
Therefore, the work efficiency is greatly improved. The shape of the CNC turntable is not much different from that of the indexing table, but it has a series of characteristics in structure. Since the CNC turntable can realize the feed movement, it has many similarities with the feed drive mechanism of the CNC machine tool in structure. The difference is that the drive mechanism realizes the linear feed motion, while the CNC turntable realizes the circular feed motion.
Rotary table is widely used in various CNC milling machines, boring machines, various vertical lathes, vertical milling and other machine tools. In addition to the requirement that the rotary table can bear the weight of the workpiece well, it is also necessary to ensure its rotation accuracy under load.
The turntable bearing, as the core component of the turntable, must not only have a high bearing capacity during the operation of the turntable, but also have high rotation accuracy, high anti-overturning ability, and high rotational speed capability. In the design of the rotary table, the more commonly used bearing types are roughly divided into the following types:
Thrust ball bearings can withstand a certain axial force, so the bearing is mainly used to bear the weight of the workpiece; while cylindrical roller bearings are mainly used for radial positioning and bearing external radial forces (such as cutting force, milling force, etc.) .
This type of design is widely used, and the cost is relatively low. Since the thrust ball is a point-contact bearing, its axial bearing capacity is relatively limited, and it is mainly used in small or medium-sized machine tool rotary tables. In addition, the lubrication of the thrust ball is also more difficult.
A hydrostatic bearing is a sliding bearing that relies on external supply of pressure oil to establish a hydrostatic bearing oil film in the bearing to achieve liquid lubrication.
The hydrostatic bearing always works under liquid lubrication from start to stop, so there is no wear, long service life, and low starting power; in addition,
this kind of bearing also has the advantages of high rotation accuracy, high oil film stiffness, and can suppress oil film oscillation.
Precision cylindrical roller bearings have good radial bearing capacity, and due to the use of precision-grade bearings, the rotation accuracy of the rotary table can also be well guaranteed. Rotary tables using this type of design can withstand high axial forces, some workpieces weigh more than 200 tons, and the diameter of the turntable exceeds 10 meters.
However, this type of design also has some shortcomings.
Since the hydrostatic bearing must be equipped with a dedicated oil supply system to supply the pressure oil, the maintenance is relatively complicated, and the cost is relatively high.
The application of crossed roller bearings on turntables is also relatively common. The characteristic of the crossed roller bearing is that there are two raceways in the bearing and two rows of rollers arranged in a cross.
Compared with the traditional thrust bearing + radial centering bearing combination, the crossed roller bearing is compact in structure, small, and simplifies the table design, thereby reducing the cost of the turntable.
