The conductive shaft structure of large horizontal NC ECM machine tool is mainly composed of power introduction part, support part, rotary sealing part and other accessories. All parts cooperate with each other to complete the functions of conductive shaft, such as conduction, rotation, sealing, etc.
2.1 Overall structure of conductive shaft
The threaded hole was preset on the base plate, and the support assembly was fixed on the floor through the connection between the bolt and the threaded hole. During installation, the base plate passed through the conductive shaft body and adjusted the position to ensure that the support assembly can support the conductive shaft. Fix the support assembly on the floor through bolts to complete the overall installation of the conductive shaft. Finally, the elastic force of the carbon brush holder was debugged so that the shaft could be rotated while the carbon brush was pressing the axis of the conductive shaft. The overall scheme of the conductive shaft is shown in Fig.1.
2.2 The power transmission structure
The circuit designed in this paper is to control the rotation of one end and the static stable and reliable power supply at the other end. The carbon brush conduction is to press the carbon brush on the circumferential surface of the conductive shaft by using the elastic force. And the carbon brush is fixed on the base plate to realize the power supply between the fixed part and the relative rotating part. The schematic diagram of the carbon brush power supply scheme is shown in Fig.2.
Changing the number of carbon brushes or base plates can provide different sizes of current with high flexibility. When installing the carbon brush holder, the angle of the carbon brush holder can be adjusted to make it at a certain angle with the normal direction of the conductive shaft body, so that the force on the carbon brush is more uniform in the process of rotation and the whole rotation process is more stable. The specific way of using carbon brush to lead electricity is: draw the current from the negative pole of the power supply, enter the power leading device through the cable, connect the carbon brush and conductive shaft with the current by relying on the base plate. Finally, the current on the conductive shaft is guided to the cathode by the pull rod. Two placement schemes of single side copper bar and symmetrical copper bar were proposed. The carbon brush holder was simplified into a carbon brush. And the simplified three-dimensional model is shown in Fig.3.
The maximum working current of large horizontal NC ECM machine tool can reach 20 000 A. With the accumulation of time, the thermal effect of current will produce a lot of heat in the current flowing area. The increase of temperature is easy to cause thermal deformation of the conductive shaft, which will reduce the positioning accuracy of the conductive shaft. The conductive shaft will not work in severe cases. Import the three-dimensional model files of the above two schemes into the workbench as the electric field simulation model. Set the surface contact coefficient to simulate the conductive efficiency between the carbon brush holder and the substrate and between the carbon brush holder and the conductive shaft in the actual conductive process. The location where the conductive shaft often fails is in the contact area between the carbon brush and the conductive shaft. In order to accurately analyze the current at these locations during operation, the grids at these locations were encrypted, as shown in Fig.4.
When setting the boundary conditions, take the side of the lead copper bar as the position where the current is applied. The flow direction of the current is from the copper bar to the substrate, and the current flows into the shaft body of the conductive shaft by means of the carbon brush. It is necessary to set the front face of the conductive shaft to the 0 voltage level, so as to ensure the flow direction of the current. The electric field distribution of the two different lead schemes is shown in Fig.5. As shown in Fig.5 (a), the electric field of the unilateral power transmission scheme is the area with the highest current density. As can be seen from Fig.5 (b), the area of bilateral power transmission is also the area with the highest current density, which is different from the unilateral power transmission, The symmetrical copper bar scheme can make the electric field evenly distributed on both sides of the substrate and greatly reduce the maximum current density.
The maximum current of the symmetrical copper bar scheme is smaller, and the thermal deformation of the conductive shaft structure is significantly reduced. The temperature field distribution generated by the current can be homogenized to make the thermal expansion deformation uniform and reduce the thermal expansion deformation as a whole. Therefore, the symmetrical copper bar scheme was selected for research.
2.3 The supporting structure
In the conductive shaft device, the support part is mainly used to support the conductive shaft and ensure that the conductive shaft can complete the predetermined rotation function. The support assembly is used to support and rotate the conductive shaft body. The support assembly was composed of main shaft support, bearing, upper support cross bar and support pull rods on both sides, and was fixed with nut main shaft support. The structure can realize the accurate positioning of the conductive shaft body, adjust the position of the conductive shaft body by relying on the bolts on the support, and facilitate maintenance and disassembly while completing the support and rotation function of the conductive shaft.
2.4 The rotary seal structure
The sealing structure of the conductive shaft adopted the combination of mechanical seal and packing seal to ensure the reliable sealing of the conductive shaft in the working process. The sealing structure is shown in Fig.6. The primary seal of the sealing device was mechanical seal, which adopted the combination of moving ring and static ring. The moving ring was pressed on the bushing according to the elastic force and rotates with the conductive shaft. The static ring was in close contact with the outer cavity shell of the rotary sealing structure by means of tension, and remained relatively stationary. The contact surface of the two will have a layer of dense liquid film, which reduced the friction between the dynamic ring and the static ring, realized the sealing and completes the preset rotation function at the same time. The secondary packing seal of the rotary sealing device was the packing seal, and the filler was organic synthetic fiber and carbon fiber, which was conducive to the stable operation of the rotary sealing device.