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What are the types of machine bearings and how to select them?
 


 

As the core part of machine tool spindle and turntable, bearing plays an important role in the performance of machine tool.This paper mainly expounds the selection of spindle bearing and the selection of rotary table bearing.

As a key part of the machine, the main shaft can directly affect the rotation precision, speed, rigidity, temperature rise and noise of the machine tool, and thus affect the machining quality of the workpiece.High performance bearings must be used in order to maintain good machining capacity.

Selection of spindle 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 include angular contact ball bearings, tapered roller bearings, and cylindrical roller bearings.

1.Precision angular contact ball bearing
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 ultra-high speed spindle applications, ceramic balls (usually si3N4 or Al203) are also used.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 contact Angle of angular contact ball bearing, the contact Angle of 15 and 25 is currently popular.Generally, the contact Angle of 15 has a high speed performance, while the contact Angle of 25 has a higher axial bearing capacity.Because the selection of preload is very important for the application of precision angular contact ball bearings, for example, medium or heavy bearing preload is usually used in high-load and high-rigid situations.For some high speed and high precision applications, it is necessary to choose the suitable preload in the early selection of bearing.The preload is generally divided into light, medium and heavy duty.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.This kind of bearing eliminates the preload adjustment of the customer, thus saving the installation time.

2.Precision cylindrical roller bearings
In the application of machine tool spindle, double row precision cylindrical roller bearings are also used, usually with precision angular contact ball bearings or thrust bearing assembly applications.This type of bearing can withstand a large radial load and allows for high speed.The two rows of roller in the bearing are arranged in a cross way, and the fluctuation frequency of rotation is greatly increased than that of single row bearing, and the amplitude is reduced by 60-70%.Such bearings usually have two forms: NN30, NN30K, two series bearing inner ring with baffle, outer ring can be separated;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).The cylindrical bore bearings are usually hot, and used to reduce the bearing clearance, or to pretighten the bearing.The inner ring can be separated from the NNU49 series bearings, and the main shaft is generally installed on the inner ring to improve the rotation precision of the spindle.

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.Because the roller bearing of tapered roller is the design of line contact, it can provide high rigidity and load for the main shaft.In addition, tapered roller bearing is a kind of pure rolling bearing design, which can reduce the torque and heat of the bearing very well, so as to ensure the speed and precision of the spindle.Since tapered roller bearings can adjust the axial preload (clearance) during the installation process, this enables the customer to better optimize the bearing clearance adjustment during the whole use cycle of the bearing.

Selection of rotary table bearing

The rotary table used in nc machine tools has a dividing table and a CNC 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.The nc rotary table can be used to realize circular movement. Besides, it can also finish the dividing motion.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.The shape of nc turntable is not very different from dividing table, but it has a series of characteristics in structure.Since the nc rotary table can be implemented into the movement, it has a lot in common with the feed drive mechanism of the nc machine tool.The difference is that the actuator realizes the linear feed motion, and the nc turntable realizes the circular movement.

Rotary table is widely used in all kinds of CNC milling machines, boring machines, all kinds of vertical cars and vertical milling machines.In addition to requiring the rotary table to bear the workpiece weight well, it also needs to ensure the rotating precision under load.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.Thrust ball bearing + cylindrical roller bearing
The thrust ball bearing can withstand certain axial force, so the bearing is mainly used to bear the weight of the workpiece.Cylindrical roller bearings are mainly used for radial orientation and external radial force (such as cutting force, milling force, etc.).This kind of design is widely used and relatively inexpensive.Since the thrust ball is a point contact bearing, its axial bearing capacity is relatively limited, mainly used in small or medium-sized machine tool rotary table.It is also difficult to lubricate the thrust ball.

2.Hydrostatic bearing + precision cylindrical roller bearing
Static pressure bearing is an external supply pressure oil, in the bearing internal static pressure bearing oil film for lubrication of the sliding bearing.Static pressure bearing from start to stop is always lubricated work, so no wear, long service life, low starting power;In addition, the bearing also has high rotation precision, large oil film rigidity, and can inhibit oil film oscillation and other advantages.Precision cylindrical roller bearing has good radial bearing capacity, and the rotary precision of rotary table can be guaranteed because of the precision bearing.The rotary table designed with this class can withstand high axial force, some workpieces weigh more than 200t, and the diameter of the turntable is over 10m.However, this kind of design also has some shortcomings, because the static pressure bearing must come with a special oil supply system to supply the pressure oil, the maintenance is complicated, and the cost is relatively high.

3.Cross roller bearings
Cross roller bearings are widely used in rotary table.Cross roller bearings are characterized by two raceways, two rows of rows of rollers.Compared with the traditional thrust bearing and radial centering bearing combination, the cross roller bearing is compact and compact, and simplifies the design of the workbench, thus reducing the cost of the turntable.Because of the optimized pretension force, this kind of bearing has high stiffness, so the stiffness and precision of the turntable are also guaranteed.Due to the design of two rows of cross roller, the effective span of the bearing can be significantly improved, so the bearing has a high anti-overturning moment.In cross roller bearings, there are two types: the first one is the cylindrical cross roller bearing and the second one is the cone cross roller bearing.Generally, the cylindrical cross roller bearing is lower than the cross roller bearing of the cone, which is suitable for the rotary table application with relatively low speed.And the cone cross roller bearing adopts the pure rolling design of tapered roller, which has high running precision and high speed, and reduces the axial length and machining cost.Cross roller bearings are suitable for various types of vertical or horizontal boring machines, as well as vertical mill, vertical car and large gear milling machine.

In conclusion, in order to select suitable size and type of bearings, We need to consider various operating conditions, such as operation speed, lubrication, installation type, spindle rigidity and precision, etc.As far as the bearing itself is concerned, it is necessary to fully understand its design characteristics and its advantages and disadvantages so as to give full play to its performance.As the core part of machine tool spindle and turntable, bearing plays an important role in the performance of machine tool.


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