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Parameter design of electromagnetic bearing for high-speed grinding machine spindle system Liu Shuqin1, Zhan Dongan2, Zhai Lie 2, Xie Youbai (1. Shandong University of Science and Technology, Jinan, Shandong 2500. Xi'an Jiaotong University) points such as easy maintenance, high speed and high rigidity. Then, the layout design of the spindle system was carried out for the 150/s high-speed BN grinding machine. The structural parameters of the electromagnetic bearing were completed by calculating the parameters such as radial and axial bearing capacity.
Grinding is a kind of precision machining method. The traditional grinding technology has small machining allowance and low production efficiency. In recent years, with the development of technology closely related to grinding, some new concepts have emerged in the grinding technology. For example, the circumferential speed of the high-efficiency medium grinding wheel exceeds 100 / s, breaking through the traditional grinding speed limit, so that the grinding performance has occurred. The fundamental change has truly achieved both quality and efficiency. High-speed grinding has been rapidly developed and applied abroad, and has attracted great attention in China. High-speed grinding is based on the development of related technologies.
The related technology described here refers to the related technologies involved in high-speed grinding, such as the manufacture, use and dressing of high-speed spindle systems (diamond or BN grinding wheels), high rigidity of the high-speed spindle system and its slewing technology. Dynamic design with high vibration resistance, precision measuring system, liquid supply cooling system, etc. This paper conducts in-depth analysis and research on the application of BN grinding wheel in high-speed and high-efficiency grinding and the design of electromagnetic bearing for spindle support.
1 BN grinding wheel and its application of high-speed grinding mainly processing ferrous metal materials, the most suitable abrasive for this processing occasion is BN abrasive. This BN (ubi Boron Nitride) superhard material was first synthesized by General Electric Company in 1957. It is this appearance of BN abrasives that opens up new application prospects for grinding technology. BN abrasives have great differences compared with ordinary abrasives. The abrasives have the following properties: (1) high hardness, good wear resistance and long service life. The hardness is only twice that of the gold abrasive. The grinding ratio is AL(2), which has high thermal stability and good chemical inertness. The thermal stability can reach 1300 ° C ~ 1400 ° C, the heat resistance temperature is much higher than the diamond wheel and the iron group element is chemically inert, more resistant to the chemical attack of iron, cobalt and nickel than diamond.
(3) The grinding temperature is low. Good thermal conductivity, is the surface quality of AL abrasive (4) processing. The surface roughness of the grinding is low, and the surface is in a state of compressive stress, which avoids surface microcracks existing in ordinary grinding.
(5) The part processing process is short. In high-speed or powerful grinding, BN grinding can complete the traditional milling, grinding, polishing and other multi-process operations in the sub-adjustment, shortening the machining process.
BN grinding wheels are divided into two categories due to different manufacturing methods: bonding type and plating type. We use electroplated BN grinding wheels. The grinding wheel is thin and low in cost, and the bonding agent has strong holding force on the BN abrasive, and the grinding speed can reach above 250 / s, and can be used for metal materials such as tool steel, abrasive steel, stainless steel, heat resistant alloy, titanium alloy and the like. Composite materials for efficient grinding. Many manufacturers abroad have produced BN grinding machines, and the grinding speed of BN grinding wheels is developed from the normal speed of 40 / s to 60 / s to 100 / s. For example, the BN grinding machine produced by Guehring Autoation of Germany has a speed of up to 100/200/s. Aahen University of Engineering in Germany has adopted a BN grinding wheel with a speed of 508 / s. This speed has exceeded the working limit of machine tools and grinding wheels. At present, domestic units are carrying out research on BN grinding wheel and its application technology, carrying out BN grinding machine parts design example analysis and experience exchange "Mechanical Design" March 2000 No3 design, but the spindle system uses electromagnetic bearing technology. No reports have been reported.
2 Application of electromagnetic bearing technology For the BN grinding machine using ultra-high speed grinding, the design research of the supporting technology in the spindle system is a key technology. Only by increasing the grinding speed to 100 / s or more can the advantages of the BN grinding wheel be fully realized. Conventional ball bearings and plain bearings have been unable to adapt to the requirements of such ultra-high-speed grinding due to their high frictional resistance. The hydrostatic bearings integrate the advantages of hydrostatic bearings and dynamic pressure bearings. The bearing stiffness is high, but no load power loss. Larger, and increased as the spindle speed increases, the air bearing can be used for very high spindle speeds, but its stiffness is low. Ceramic ball bearings and electromagnetic bearings that have appeared in recent years are widely used in high-speed spindle systems. In foreign countries, electromagnetic bearings are high-tech products of electromechanical magnetization. With their unique advantages, they are becoming more and more popular in the design of high-speed spindle systems. It is favored by engineers and technicians, such as Soiete de in France and Kapp in Germany, which have successfully used electromagnetic bearings on the grinding wheel spindle system of their high-speed grinding machines.
2.1 Characteristics of electromagnetic bearings Electromagnetic bearings are new high-performance bearings that use electromagnetic force to suspend the rotor in space. Compared with ordinary bearings, electromagnetic bearings have the following characteristics: (1) no mechanical contact, no wear, low energy consumption , low noise and long life.
(2) It does not need lubrication and sealing, no pollution, and can work in vacuum or corrosive medium.
(3) Electronic circuit control, high reliability and easy maintenance. High stiffness and stiffness and damping are adjustable online. The dynamic control can resist the vibration caused by the rotor imbalance force and overcome the gyro effect.
(4) The circumferential speed of the rotor is high and the rotation precision is high, so the rotor can realize micro-feeding of radial and axial super-finishing.
(5) The requirements for sensors and control circuits are high, so the total manufacturing cost is high.
2.2 The application of electromagnetic bearings in the spindle system The design requirements of the grinding spindle system are limited to five degrees of freedom, leaving only one degree of freedom of rotation. In response to this feature, we carried out the structural design of the five-degree-of-freedom control of the spindle spindle electromagnetic bearing (Fig. 1). The spindle speed can reach 30 000 r/in and the peripheral speed of the grinding wheel can reach 150 / s or more.
3 Electromagnetic bearing design The in-depth study of the electromagnetic bearing theory shows that the correct design of the electromagnetic bearing is not only the premise of ensuring the bearing capacity and stable suspension. To improve the performance indexes of the electromagnetic bearing, the design parameters must be considered in the design stage. The impact of the indicator. Strictly speaking, the design of the electromagnetic bearing should be considered with the design of the entire electromechanical system. Due to the particularity of the electromagnetic bearing, the dynamic design of the system is more complicated than conventional oil-lubricated bearings. For oil-lubricated bearings, there is no need to worry about whether the lubricating medium can provide the expected stiffness and damping during the vibration of the rotor. However, for the electromagnetic bearing, it is entirely possible to make the electromagnetic due to magnetic saturation or power supply voltage and control current limitation. Bearings do not provide the corresponding stiffness and damping under fixed conditions. We have studied this issue in the literature. Another important problem is that magnetic saturation, the strong nonlinear relationship between magnetic field force and current, displacement, and the inductance of the coil windings that make up the electromagnetic bearing constitute multiple constraints and limitations of the electromagnetic bearing in the design process. The design of the electromagnetic bearing is complicated and requires repeated adjustments. It is often necessary to make coordinated selections in multiple sets of data.
The design parameter values ​​for electromagnetic bearings in this paper are based on the above considerations. In this paper, the special problems of BN grinding machine electromagnetic bearing design are analyzed in detail. Taking into account the requirements of high-speed grinding of BN grinding wheel, the following indexes are specified for the grinding machine spindle system: (1) radial bearing 1: static load F load F (2) thrust bearing: dynamic load F (3) single-sided working air gap δ(4) rotor maximum amplitude Δδ(5) ambient temperature 40°C, allow temperature rise 60°C1, 8. retain bearing 2, 7. radial sensor 3, 6. radial bearing 4. motor 5. thrust bearing 9. Grinding wheel 10. Axial sensor 3.1 Radial bearing design 3.1.1 Radial bearing bearing capacity design piece core. Each of the 4 magnetic poles (NSSN) constitutes an electromagnet. In order to reduce the volume of the electromagnetic bearing, high-quality silicon steel sheet is used, and the saturation magnetic induction intensity is 1.8 T. Considering that the regulation current of the high-speed magnetic bearing may be greater than the current value determined by the maximum bearing capacity and the influence of temperature rise, etc., to avoid entering the magnetic Saturated, core magnetic induction maximum B "Mechanical Design" March 2000 No3 Example Analysis and Experience Exchange Parts Design In the design stage of electromagnetic bearings, bearing capacity is the primary problem to be solved. The calculation can be performed using a simplified magnetic circuit (regardless of core magnetoresistance, magnetic flux leakage and eddy currents). In the case of uniform air gap flux, for the double air gap double excitation coil magnetic circuit, the air gap magnetic induction strength formula is obtained by the Ampere loop law: where: μ air permeability NI magnetomotive force (N is the number of turns of the coil, I is the excitation current) δ represents the unilateral working air gap.
When considering only the entire magnetic circuit cross-sectional area is the same, the suction generated by the uniform air gap flux (ignoring the edge effect) is calculated using Maxwell's formula: where: S effective magnetic pole area.
According to the structure shown in Fig. 2, the maximum suction force in the direction of the coordinate axis is: where: B magnetic induction maximum value h lamination thickness D rotor diameter due to the rotor structure and the size of the bearing magnet are interdependent, the bearing capacity can be Figure 3 is determined. It can be seen from Fig. 3 that the requirement of the rotor straight = 1 200 N is selected. However, this design has a limit on the length of the spindle. In order to design a spindle rotor with a large stiffness and a bending mode beyond the working speed, the shorter the rotor, the better. However, considering the factors such as permitting centrifugal load and selecting the rotor according to the system design requirements, the design steps of the other parameters of the rotor 3.1.2 radial bearing design are as follows: the static working point current and the number of winding turns are designed by calculating the magnetomotive force. . Calculate the inductance and resistance of the winding, and finally estimate the temperature rise.
The magnetomotive force can be determined by the following formula: =6 A. The radial bearing has no static load in the x direction, and the quiescent current of the left and right electromagnets is I 3 Ay. The static load (here the weight of the rotor) is assumed, so The quiescent operating point current of the lower electromagnet should be designed as follows: the inductance L of each N åŒ coil is: where: S The area of ​​a single magnetic pole.
Indicates the resistance of each N åŒ coil. Ï„ represents the actual temperature rise. It is worth noting that in the case of satisfying the bearing capacity, whether it is a large current, a low winding turns, or a small current, a high winding, and a number of excitations, it is necessary to consider the inductance, the power amplifier, and the coil temperature rise. Select Group 2 data as the final design data.
The number of coil turns N 3 .2 The design of the magnetic pole area of ​​the thrust bearing reduces the magnetomotive force. This reduces the number of turns in the coil. Moreover, the maximum value of the magnetic induction strength of the thrust disk and its core is not high, and a relatively inexpensive silicon steel sheet can be used. When a double magnetic pole single-beam N åŒ coil is excited, the electromagnetic suction is calculated by the following equation: where: S effective area of ​​the magnetic pole and S inner and outer ring magnetic pole area, then S is determined by equations (7) and (8) to determine the outer diameter d of the ring d Determine the design example and experience exchange of the component design by the window area. Mechanical Design, March 2000 No3 3.2 Dimensional Error Caused by the Wedge Angle α Dispersion Analysis 3 .2.1 Analysis of the Influence of the Dimensional Error on the Wedge Angle α In the case of the same dimensional error value, the variation of the wedge angle α caused by the eccentricity error of the arc of the working surface of the star wheel is far greater than the dimensional error of the eccentric arc R of the working surface and the dimensional error of the radius R of the outer ring hole. The amount of change in the wedge angle α. Therefore, strictly controlling the eccentricity error of the arc of the working surface of the star wheel is the fundamental measure to control the spread of the wedge angle α.
3.2.2 Influence of wear on the wedge angle α during the working process During the working process, the relative movement speeds of the roller and the working surface of the star wheel and the contact point of the outer ring hole are the same, and the force is similar. The wear of the working surface of the star wheel reduces R, and the wear of the jacket hole increases R. When the formula (3) is satisfied, it is known from the formulas (4) and (5) that the degree of influence of the ΔR wedge angle α is the same as the increase and decrease characteristic. Therefore, the influence of the wear of the star wheel working surface and the outer ring hole on the wedge angle α can compensate each other.
It can be seen from the above 2 that, in the case where the formula (3) is satisfied, the wear causes a change in the radius r of the roller, and the influence on the wedge angle α is minimized.
4 Measures to control the spread of the wedge angle α. In the analysis of 3.2 of this paper, the following measures can be taken to control the dispersion of the wedge angle α: =R 2 r for structural size design to ensure that the variation of the roller radius r has the least influence on the wedge angle α .
4.2 Key quality control of the eccentricity error of the working surface arc of the star wheel.
4.3 The surface characteristics of the working surface of the star wheel and the outer ring hole are such that the influence of their wear on the wedge angle α can cancel each other out.
4.4 Improve the wear resistance of the working surface of the star wheel and the surface of the outer ring hole to prevent excessive wear and cause changes in the radius of curvature and eccentricity of the working surface.
5 Problems in the traditional design Due to the lack of sufficient understanding of the dimensional error of the overrunning clutch of the eccentric arc, the structural tolerances provided by the traditional design data are not scientific enough. For example, the diameter of the roller and the inner diameter tolerance of the outer ring are too strict, and the eccentric tolerance of the arc of the working surface of the star wheel is too low.
Reference material machine design manual. Edited by the editorial board. Mechanical Industry Press.
Parameter design value Speed ​​radial bearing 1 Radial bearing 2 Total power of thrust bearing 4 Conclusion Magnetic bearing is used in the spindle system of BN grinding wheel grinding machine. Compared with traditional high-speed spindle supporting technology, it has easy maintenance, high speed and high rigidity. Unparalleled advantages. With digital control, it is also possible to develop process monitoring functions during operation. The BN grinding machine using this technology has a higher initial investment cost, but its superior performance makes it more economical. Therefore, it is gaining more and more applications in foreign countries. We hope that domestic research institutes and researchers of factory companies will actively work together to develop this application technology and apply electromagnetic bearing technology to various BN grinding machines as early as possible.
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Design of electromagnetic bearing parameters for high speed CBN grinding machine spindle system