As an important part of mechanical equipment industry, machine tool industry plays an important role in the development of national economy.According to statistics, in general machine manufacturing, machine tools bear the processing workload accounted for 40% ~ 60% of the total manufacturing workload, so the level of development of machine tools industry is usually a sign of the level of industrial development of a country.With the development of modern manufacturing industry, customers have higher and higher requirements for industrial mother-machine tools.At present, the constant emergence of new materials, new technology and new equipment has brought many challenges to the machining ability of machine tools. In order to meet the processing of the above-mentioned structural parts, modern machine tools are developing towards the direction of "high speed, precision, compound and intelligent".

With the deepening of China's industrial structure upgrade, the machine tool industry also showed the trend of the industrial structure upgrade, the proportion of nc machine tools is gradually increased, in the field of numerical control, high-end CNC machine tools (such as heavy machine tools, intelligent machine tools, etc.) the proportion is also rising, heavy machine tool products primarily for the national energy (thermal power, hydropower, nuclear power, wind power), ship manufacturing, engineering machinery, metallurgy, aerospace, military industry, transportation, railway, car) and other major industrial pillar industries as well as national key project services, national key project for heavy duty machine tools provide a broad market.

This paper mainly introduces how to choose the type of bearing used in the machine tool for spindle bearing and turntable bearing.

Main shaft bearing

Spindle (see figure 1) as a key component of the machine tool, its performance will directly affect the machine tool's rotation accuracy, speed, rigidity, temperature rise and noise and other parameters, and then affect the workpiece processing quality, such as the size accuracy of parts, surface roughness and other indicators.Therefore, in order to maintain excellent machine tool processing ability, high performance bearings must be used.The bearing precision used on the spindle of machine tools should be ISO P5 or above (P5 or P4 is the accuracy level of ISO, usually P0, P6, P5, P4, P2 from low to high), while the precision of ISO P4 or above should be selected for the spindle support of high-speed and high-precision machine tools such as CNC machine tools and machining centers.Spindle bearings include angular contact ball bearings, tapered roller bearings, and cylindrical roller bearings.

FIG. 1. Spindle of machine tool

Precision angular contact ball bearings

Precision angular contact ball bearings (see figure 2) are the most widely used of the above types of bearings.We all know that the rolling body of angular contact ball bearings is a ball;Because it is a point contact (distinct from the line contact of roller bearings), it provides higher speed, less heat and higher rotation accuracy.Ceramic ball bearings (usually Si3N4 or Al2O3) are also used for ultra-high speed spindle applications.Compared with the traditional fully quenched steel ball, the ceramic ball material itself has the characteristics of high stiffness, high speed, high temperature resistance and long life, so as to meet the demand 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 Angle of;Usually 15?The contact Angle of the 25?The contact Angle has high axial bearing capacity.Because the choice of preloading has a great impact on the application of precision angular contact ball bearing, for example, in the case of high load, high rigidity, generally choose medium or heavy bearing preloading;For some high speed, high precision applications, we need to pay attention to the early selection of bearing, the appropriate preload, generally light preload is more common.Preloading is generally divided into light, medium and heavy three kinds;In order to facilitate the use of customers, several major bearing manufacturers in the world are generally provided with pre-abrasive bearing end face and preloaded bearing, which is commonly known as the universal matching precision angular contact ball bearing form.This type of bearing eliminates the need for customer preload adjustment, thus saving installation time.

Figure 2. Precision angular contact ball bearing

Precision tapered roller bearings

Precision taper roller bearing is an ideal choice in some heavy-duty machine tool applications that require certain speed, such as rough grinding of forgings, wire-turning machines for petroleum pipelines, heavy-duty lathes and milling machines.Since the roller of tapered roller bearing is designed for line contact, it can provide high rigidity and bearing capacity for the spindle.In addition, the tapered roller bearing is a pure rolling bearing design, which can well reduce the running torque and heat of the bearing, thus ensuring the spindle speed and accuracy.Since tapered roller bearings can adjust axial preloading (clearance) during installation, this allows customers to better optimize clearance adjustment throughout the life of the bearing.

In addition, some specially designed tapered roller bearings, such as TSMA bearings or hydra-rib fluid floating roller bearings, can also meet the requirements in high speed applications where the inner rim liner velocity is greater than 30 m/s (see figure 3).TSMA bearing flange has a number of axial direction of lubricating oil holes, can be lubricated by circulating oil or oil mist lubrication, centrifugal force can be distributed to the roller and flange contact area, thus to more fully lubricate the bearing, increase the bearing speed.Hydra-rib fluid floating flange bearing is designed to optimize the pre-loading of the main shaft system. The bearing has floating flange, which contacts with the big end of roller, unlike the common bearing with fixed inner ring flange.The floating flange is positioned by the "pressure" system, which ensures the best performance of the bearing under all working conditions and has been successfully used by domestic customers.

FIG. 3. Spindle design with hydraulic floating flange bearing

For precision selection of tapered roller bearings, ISO P5 class bearings are mainly used in traditional lathes and milling machines.If it is a machining center, grinding machines and other applications, usually choose ISO P4 or above precision bearings.In the design of spindle, there are two kinds of conical roller bearing layouts.The first is a tapered roller bearing for the front end and the rear end, which is installed face to face (see figure 4).This kind of design structure is compact, high rigidity and easy to install and adjust.

FIG. 4. Single row tapered roller bearings mounted in pairs

The second is to install two tapered roller bearings face to face as the front end of the spindle, and use a double outer ring and two single inner ring bearings (TDO) as the floating end (see figure 5).Due to the floating ability, this kind of design can well withstand the axial thermal expansion of the spindle, and the stiffness is very high, to ensure the precision of the machine tool.

FIG. 5. Front single row tapered roller bearings mounted in pairs, rear end TDO

Precision cylindrical roller bearing

Double row precision cylindrical roller bearings are also used in machine tool spindle applications, usually in combination with precision angular contact ball bearings or thrust bearings.Such bearings can withstand large radial loads and allow for a higher speed.The two rows of rollers in the bearing are arranged in a cross way. When rotating, the fluctuation frequency is greatly increased and the amplitude is reduced by 60%~ 70%.This kind of bearing usually has two forms: NN30, NN30K two series bearing inner ring with guard edge, outer ring can be separated;NNU49, NNU49K two series bearing outer ring with flange, inner ring can be separated, among which NN30K and NNU49K series inner ring for cone hole (taper 1:12), with the conical shaft neck of the spindle, axial movement of the inner ring, can make the inner ring swelling, so that bearing clearance can be reduced or even preloaded bearing (negative clearance state).Cylindrical bore bearings are usually hot-loaded, using interference fit to reduce the bearing clearance, or preloaded bearings.NNU49 series bearings, which can be separated from the inner ring, generally install the spindle on the inner ring and then finish the raceway to improve the spindle rotation accuracy.

Attached table 1: performance comparison of various spindle bearings

* single-row bearings only bear axial forces in a single direction.

Table 2: bearing accuracy class

Turntable bearing

CNC machine tools commonly used rotary table has dividing table and CNC rotary table.In the processing of some parts of CNC machine tools, in addition to the linear feed motion of X,Y and Z coordinate axes, circular motion around X,Y and Z coordinate axes is sometimes required, which are called as A,B and C axes respectively (see figure 6).

FIG. 6. Axis of machine tool

CNC rotary table can be used to realize circular feed motion.Nc rotary table (nc turntable) can not only realize circular feeding movement, but also complete indexing movement.And the function of the dividing table is only the workpiece transposing surface, and the use of automatic tool changing device, the workpiece can be installed to complete a number of processes, therefore, greatly improve the work efficiency.The appearance of CNC turntable and dividing table have no great difference, but it has a series of characteristics in the structure.Because the CNC turntable can realize feed motion, it has many common points with the feed drive mechanism of CNC machine tools.The difference is that the drive mechanism realizes linear feed motion, while the CNC turntable realizes circular feed motion.

Rotary table is widely used in various CNC milling machine, boring machine, vertical turning, vertical milling and other machine tools.In addition to the requirement that the rotary table can well bear the weight of the workpiece, it also needs to ensure its rotary accuracy under the load.As the core component of the turntable, the bearing of the turntable should not only have a high bearing capacity, but also have high rotary accuracy, high anti-overturning ability and high rotation speed.In the design of rotary table, the types of bearings used are generally divided into the following categories:

Thrust ball bearing + cylindrical roller bearing

Thrust ball bearing can bear a certain axial force, so the bearing is mainly used to bear the weight of the workpiece;Cylindrical roller bearings are mainly used for radial positioning and bearing external radial force (such as cutting force, milling force, etc.).This kind of design is widely used and relatively cheap.Because the thrust ball is a point contact bearing, its axial bearing capacity is relatively limited, mainly used for small or medium-sized machine tool rotary bench.In addition, the lubrication of thrust ball is also difficult.

Hydrostatic bearing + precision cylindrical roller bearing

Hydrostatic bearing is a kind of sliding bearing which relies on external pressure oil supply and establishes hydrostatic bearing oil film in the bearing to realize liquid lubrication.Hydrostatic bearing always works under liquid lubrication from start to stop, so it has no wear, long service life and low starting power.In addition, this kind of bearing also has the advantages of high rotation accuracy, large oil film stiffness, and can suppress oil film oscillation.Precision cylindrical roller bearings have good radial bearing capacity, and because of the use of precision grade bearings, the rotary accuracy of the rotary table can be well guaranteed.The rotary table designed with this kind of design can withstand very high axial force, some workpiece weight more than 200 tons, the rotary table diameter more than 10 meters.However, this kind of design also has some shortcomings, because static pressure bearing must be attached with a special oil supply system to supply pressure oil, maintenance is complex, and the cost is high.

Cross roller bearings

FIG. 7. Cross roller bearings

Cross roller bearings (see figure 7) are also widely used on turntables.Cross roller bearings are characterized by two raceways and two rows of cross rollers.Compared with the traditional combination of thrust bearing and radial centering bearing (see figure 8), cross roller bearing is compact in structure, small in size, and simplifies the design of work table, thus reducing the cost of turntable.

FIG. 8. Comparison of turntable design between cross roller bearing and traditional bearing

In addition, this kind of bearing has high stiffness due to the use of optimized preloading force, so the stiffness and accuracy of the turntable are also guaranteed.Due to the design of two row cross rollers, the effective span of the bearing can be significantly improved, so the bearing has a high anti-overturning moment.In the cross roller bearing, it is divided into two types: the first is cylindrical cross roller bearing, the second is tapered cross roller bearing.In general, cylindrical cross roller bearings are lower than tapered cross roller bearings and are suitable for rotary table applications with relatively low rotating speed.The tapered cross roller bearing adopts the pure rolling design of tapered roller, so this kind of bearing has

· high operating accuracy

· high speed capability

· reduced shaft length and machining costs, limited geometric dimension changes due to thermal expansion

· nylon separator, low moment of inertia, low starting torque, easy to control Angle indexing

· optimized preloading force, high stiffness and low runout

· line contact, high rigidity, high precision of guiding roller operation

· carburized steel provides excellent impact and surface abrasion resistance

· simple but well lubricated

In bearing installation, the customer only needs to preload the cross roller bearing to the recommended value, instead of having to go through a complicated installation and adjustment process like static pressure bearing.Cross roller bearings are easy to install and easy to adjust the original installation form or maintenance method.Cross roller bearings are suitable for various types of vertical or horizontal boring machines, as well as vertical grinding, vertical turning and large gear milling machines.

In a word, bearing, as the core component of spindle and turntable, plays a decisive role in the performance of machine tools.In order to select the right size and type of bearing, we need to comprehensively consider various working conditions, such as running speed, lubrication, installation type, spindle stiffness, accuracy and other requirements.As for the bearing itself, only by fully understanding its design characteristics and the advantages and disadvantages brought about by it, can we give full play to the best performance of the bearing.