Introduction of Spindle Motor
Spindle is for short of High Frequency Spindle, also referred to as "direct drive spindle" sometimes, which is part of built-in type motor. It shortens the length of main drive chain of machine tools into zero, to achieve "zero transmission" of machine tool, featured compact, high mechanical efficiency, which can perform excellent in high speed rotary, high precision of rotary, low noise, little vibration etc. Therefore, electric spindle have more and more wide application in modern CNC machine tools. In foreign countries, electric spindle has become a high-tech product of mechatronics, produced by professional factories with high technical level, such as Fischer in Switzerland, GMN in Germany, Precise in the United States,Gamfior in Italy, NSK in Japan etc.
1. The structure of Spindle
Spindle comprises motor without shell, main spindle, bearings, shell of spindle unit, drive module and cooling devices etc. The rotor of motor incorporate with main spindle by pressure-fit. The main spindle is supporting by bearings frond and behind. The stator is fit in the shell of main spindle unit through cooling coat. There is sensor of testing speed, angle at the back of spindle. The front cone hole and is installed knife.
2. The bearing of spindle
Bearing is the vital part influencing of spindle life and carrying capacity. The performance is extremely important for the using functions of electric spindle. At present, main bearings of spindle use ceramic ball bearings, Hydrostatic Fluid Bearings and active magnetic bearings.
Ceramic ball bearings are widely used economically. The ceramic ball have light weight, high hardness, which can greatly reduce centrifugal force of bearing and internal load, to reduce wear and tear, thereby enhancing bearing life. The GMN company from German and STEP-TEP company from Switzerland use ceramic ball bearings in spindle motor for machining center and milling machine.
Hydrostatic Fluid Bearings is non-direct contact bearings, featured low wear and tear, long life, high rotary angle and low vibration etc. The application in spindle can extend knife life and improve processing quality and efficiency. The Ingersoll company from USA use their patent technolgy of Hydrostatic Fluid Bearings in electric spindle unit.
Active Magnetic Bearing rely on several couple of radial suction (or push) around cycle with 180 degree angle. The spindle will suspend in the air, which make spindle neck no contact of bearings with radial clearance about 1mm. When bear load, the space position of spindle will change little. The control device can change magnetic pole suction (or push) values correspondingly in accordance with the location changes of spindle from position sensors. It can make spindle return to original position quickly so as to ensure the spindle rotating around its inertia shaft with high speed. So it have high speed, precision but costly prices, which have not widely used nowadays.
3. Cooling of Electric Spindle
As spindle motor integrated motor into the main spindle unit, and it has high speed, it will generate much heat in operation. The heat will rise caused the temperature of spindle. With high temperature, the thermal characteristics and dynamics of deterioration of spindle will be worse which affecting the normal work of Spindle. Therefore we must take certain measures to control the temperature of spindle lasting at a certain value. The present way to cool spindle rotor and bearings is by force circulation oil cooling, which is to force the cooling oil through oil cooling devices into circulation outside of rotor and bearings of spindle to take away the heat created by high speed rotating. In addition, in order to reduce the heating of spindle bearings, to lubricate bearings reasonably is also necessary. As for the ceramic ball bearings, we can use oil mist or gas lubrication.
4. The Drive of Spindle
At present, the spindle motors are used AC asynchronous induction motor. As it is used in high-speed machine tool, starting the machine need to activate it soon from static to hundreds even thousands of circles per minute. Starting torque is big, so the starting current need be 5-7 times than rated current of common motor. The way of driving have inverter drive and vector control drive. The drive control of inverter have speciality of constant torque drive, which torque and power output is proportional. The latest converter use the advanced transistors technology (ABB company produce SAMIGS series inverter), which can realize stepless sift gears. The drive control of vector control drive is constant torque-driven at low-speed, constant power drive at medium or high-speed end.
5.The basic technical data of electric spindle
Spindle includes basic parameters including diameter of sleeve, the maximum speed, power output, torque and tool interface, of which sleeve diameter is main parameters of Spindle.