Bearing as the core component of machine tool spindle and turntable, played a significant role in the operation of the machine tool performance. This paper mainly illustrates the selection of main shaft bearing turntable bearing selection. 

 

Spindle as key components of the machine tool, the rotation of the machine performance will directly affect the precision and speed, rigidity, and noise parameters such as temperature, affecting the processing of the workpiece quality. In order to maintain good machine processing capacity, must be with high performance bearing. 

 

"In the selection of main shaft bearing" 

 

 

Should be used on the machine tool spindle bearing precision ISOP5 or above (P5 or P4 is the accuracy level of the ISO, usually from low to high as P0, P6, P5, P4, P2), and for CNC machine tools, machining center such as high speed, high precision machine tool spindle bearing, to choose ISOP4 or more accuracy. Spindle bearings including angular contact ball bearings, tapered roller bearings, and cylindrical roller bearings and other types. 

 

1. Precision angular contact ball bearing 

 

 

Of precision Angle contact ball bearing is widely used, angular contact ball bearing roller is the ball, because it is a kind of point contact (different from the line contact roller bearing), so can provide higher speed, less calorific power and higher rotation accuracy. In some of the ultra high speed spindle applications, will also adopt ceramic ball (generally for Si3N4 or Al203) hybrid bearings. Compared with the traditional full hardening steel ball, ceramic ball material itself gives the characteristics of ceramic ball bearings with high rigidity, high speed, high temperature resistant, long service life characteristics, to meet the needs of high-end clients of machine tool bearing products. 

 

In terms of the contact Angle of the angular contact ball bearings, the current relatively popular in the contact Angle is 15 and 25; Usually 15 contact Angle has a relatively high speed performance, and the contact Angle of 25 has high axial bearing capacity. Due to the choice of preloading for the influence of precision Angle contact ball bearing application is very large, as in the circumstance of high load, high rigidity, tend to choose medium or heavy bearing preloading; And for some applications, high speed, high precision in the early selection of bearings, need to pay attention to choose the appropriate preloading. Preloading generally divided into light, medium, heavy three, generally lighter preloading is common. In order to facilitate the use of customers, several major bearing manufacturers in the world is now generally provide advance grinding bearing end face and preloading bearings, also is what people usually call the universal form of matching precision angular contact ball bearing. Preloading of the bearing was relieved from the customers, so as to save the installation time. 

 

2. Precision cylindrical roller bearing 

 

 

In the application of machine tool spindle, double row precision cylindrical roller bearing can be used to, usually with precision angular contact ball bearing and thrust bearing composite applications. This kind of bearing can withstand the bigger radial load and high speed are allowed. Two columns of bearing roller with cross arranged, rotates sharply higher than single row bearing, the frequency of the wave amplitude is reduced by 60% ~ 70%. Such bearings usually has two forms: NN30, NN30K two series of bearing inner ring belt guard, outer separable; NNU49, NNU49K two series of bearing outer ring guard, inner ring can be separated, which NN30K and NNU49K series inner ring for the taper hole (taper 1:12), cooperate with the conical shaft neck of the spindle, the axial movement of inner ring, can make the inner ring swell, so bearing clearance can be reduced or even preloaded bearing (negative clearance status). 

 

Cylindrical hole bearing usually adopt hot charging, using the interference fit decrease the bearing clearance, or preloaded bearing. Inner circle of separable NNU49 series bearings, general inside is installed on the spindle to race again after finish machining, in order to improve the spindle rotation accuracy. 

 

3. Precision taper roller bearing 

 

 

In some overloaded and has certain requirements for speed of machine tool applications, such as forging snagging, pipeline car silk machine, heavy duty lathe and milling machine, etc., choose precision taper roller bearing is a kind of ideal solution. As a result of the tapered roller bearing roller is the design of the line contact, so it can offer high rigidity and bearing for the spindle; In addition, tapered roller bearing is a kind of pure rolling bearing is designed, it can reduce the bearing running torque and fever, to ensure that the spindle rotation speed and precision. Because of the tapered roller bearing to adjust axial preloading during the installation process (clearance), it can let the customer in the bearing throughout the life of the bearing clearance adjustment better optimization. 

 

【 】 in the selection of turntable bearing 

 

 

That is commonly used in CNC machine tool rotary table with indexing table and nc rotary table. Nc machine tool in machining some parts, in addition to the X, Y, Z three axes linear feed movement, sometimes also need to have around the X, Y, Z three axis circular motion, called A, B, C axis. Nc rotary table can be used to realize circular week for sports, in addition, also can complete degree movement. And the function of dividing workbench is the workpiece transposition in surface, and the automatic tool change device, on the surface of the workpiece installation can be completed once a few multiple working procedure, therefore, greatly improving the work efficiency. And appearance of the nc rotary table indexing table is not much difference, but has the characteristics of a series on the structure. Because of the nc rotary table to realize feed movement, so it is structurally and nc machine tool feed drive mechanism have a lot in common. Difference is driving mechanism of the implementation is linear feed movement, and the implementation of the nc rotary table is round week to exercise. 

 

Rotary table widely used in all kinds of CNC milling machine, boring machine, all kinds of vertical lathe, vertical milling machine tool. In addition to the rotary table can withstand the work piece weight, also need to ensure that its under the bearing rotation accuracy. Turntable bearings, as the core component of turntable, the turntable during operation, not only has the very high bearing capacity, should also have high rotation accuracy and high ability to overthrow, and high speed. 

 

1. + cylindrical roller bearing thrust ball bearing 

 

 

Thrust ball bearing can absorb a certain amount of axial force, so the bearing is mainly used to support the weight of the workpiece; And the radial cylindrical roller bearing is mainly used for positioning and withstand external radial force (such as cutting force, the milling force, etc.). The class design widely applied, and the cost is relatively low. Due to the thrust ball bearing is a kind of point contact, so its axial bearing capacity is relatively limited, mainly used for small or medium-sized machine tool rotary table. In addition to thrust ball lubrication is also more difficult. 

 

2. Hydrostatic bearing + precision cylindrical roller bearing 

 

 

Hydrostatic bearing is a kind of external oil supply pressure, static pressure within the bearing load so as to realize the sliding bearing lubrication oil film. Hydrostatic bearing from start to stop work under lubrication, so there is no wear and tear, long service life, low starting power; In addition, the bearing has high precision, rotating oil film stiffness big, inhibit oil film oscillation, etc. Precision cylindrical roller bearing has good radial bearing capacity, and the adoption of precision bearings, rotary table rotation accuracy can be guaranteed. Use the class design of the rotary table can withstand high axial force, some more than 200 tons, the weight of the workpiece turntable diameter of more than 10 m. But this kind of design also has some disadvantages, because of the hydrostatic bearing must comes with a set of special oil supply system to supply pressure oil, maintenance is more complex, and cost is high. 

 

3. Cross roller bearings 

 

 

The application of cross roller bearing on the turntable is common. Cross in the characteristic of roller bearing is a bearing has two race, two rows of roller of cross array. Combined with the traditional radial centering bearing thrust bearing + compared to cross roller bearing compact structure, small size, and simplify the table design, so as to reduce the cost of the turntable. Thanks to the optimization of pre-tightening force, this kind of bearing has the very high stiffness, and stiffness and precision of the turntable are secured. Thanks to two cross roller bearing design, the effective span can be significantly improved, so this kind of bearing has a high resistance to overturning moment. 

 

In the cross roller bearing, and divided into two types: the first is a cylindrical cross roller bearing, the second is tapered cross roller bearings. Usually, cylindrical cross roller bearing cross price cone roller bearing is low, suitable for application of rotary table speed is relatively low; Cross roller bearing and cone adopted pure rolling taper roller design, with high precision and speed capability, reduce the axial length and the advantages of the processing cost. Cross roller bearings are suitable for various types of vertical or horizontal boring machine, as well as the vertical mill, vertical lathe and large gear milling machine and other applications. 

 

To sum up, in order to choose appropriate size and type of bearings, need comprehensive consideration of various conditions, such as speed, lubrication, installation type, such as spindle rigidity, accuracy requirements. In terms of the bearing itself, only fully understand the design characteristics and the advantages and disadvantages of the resulting, in order to give full play to the performance of the bearing. Bearing as the core component of machine tool spindle and turntable, played a significant role in the operation of the machine tool performance.