Application of FAG high speed angular contact ball bearings
The high-speed spindle system of the FAG bearing CNC cutting machine, as its name implies, has a high speed of rotation, but this has no strict limits. For the machining center and CNC milling machine represented by high-speed cutting machine, it generally refers to the spindle system with the maximum speed ≥ 10,000r/min, and correspondingly has a high angular addition (decrease) speed to realize the instantaneous speed increase and start of the spindle. stop. In order to meet the ever-increasing requirements of the manufacturing industry for machine tool machining accuracy, the high-speed cutting spindle should also have a high degree of rotation accuracy. Usually, the radial runout of the spindle is required to be less than 1 or 2 μm, and the axial yaw is less than 1 μm. In addition, the spindle must have sufficient static and dynamic stiffness to withstand certain cutting loads and maintain high rotational accuracy.
NTN Bearing The traditional spindle system consists of intermediate transmission links such as belts and gears. It not only has a large moment of inertia, it is difficult to achieve high angular addition (decrease) speed, and it is difficult to solve the vibration and noise of the intermediate transmission link at high speed. Therefore, the current high-speed spindle system is mostly an electric spindle unit that "binds the motor" to the main shaft and shortens the transmission chain to zero. The structure basically places the main motor between the front and rear bearings of the main shaft. Such an electric spindle has a short axial dimension, a compact structure, a high spindle rigidity, and a large output. This type of structure is used by GMN's electric spindles for machining centers and CNC milling machines. Its bearings are precision angular contact ball bearings suitable for high speed operation (generally equivalent to international standards P2 and P4). All key parts must be precision machined. And precision assembly, while cooling the stator and front and rear bearings of the main motor with constant temperature cooling water (GMN 23 ± 0.7 ° C or even ± 0.3 ° C). GMN has more than 20 types of electric spindles for machining centers and CNC milling machines. The maximum speed ranges from 8000r/min to 60,000r/min, the power ranges from 5.5kW to 76kW, and the torque ranges from 0.9Nm to 306Nm.
There are also very few high-speed light-cut milling machines that use an electric spindle that places the main motor behind the rear bearing of the spindle. The electric spindle of this structure has a long axial dimension, but can improve heat dissipation conditions. In addition, some machining centers and milling machines with a maximum spindle speed of 10,000 r/min to 15000 r/min use a coupling such as a diaphragm instead of an electric spindle to directly connect the main motor to the spindle, thereby eliminating spindle cooling. system. However, this method not only increases the moment of inertia, but also affects the angle plus (decrease) speed, and the non-linearity after the direct connection between the two causes vibration (the higher the speed, the higher the alignment requirement for linearity), so it is not suitable. For machines with higher spindle speeds.
The current high-speed electric spindle NACHI bearings are almost all built-in asynchronous AC induction motors. The advantage of the asynchronous type electric spindle is that the structure is relatively simple, the manufacturing process is relatively mature and the installation is convenient, and in particular, the magnetic field can be weakened to a greater extent, and the speed is easily realized. Such spindle motors are also being further improved. For example, Mitsubishi Electric Corporation of Japan has recently developed a high-speed, high-efficiency built-in induction spindle motor that reduces electrical losses and shortens the length of the stator.
Recently, foreign countries are discussing the problem of incorporating an AC permanent magnet synchronous motor in an electric spindle. Compared with the asynchronous induction motor, the permanent magnet synchronous motor has the following advantages: the rotor made of the permanent magnet material does not generate heat during the working process, and solves the problem that the built-in asynchronous induction motor rotor generates heat and is difficult to be sufficiently cooled; the power density is large. High work efficiency, can get larger power and torque with smaller size; low speed performance, easy to achieve precise control. However, the permanent magnet synchronous motor also has its weakness, that is, the power capacity is limited, and the weak magnetic field is difficult, which is not conducive to achieving high speed. So far, only a few machine tool companies have built-in permanent magnet synchronous motor electric spindles at the show, and the professional electric spindle manufacturer has not sold such products. Many people believe that it is more suitable to drive an asynchronous induction motor for a high-speed electric spindle that operates in a weak magnetic field.
At present, the PWM frequency conversion speed regulation technology is mainly used to realize the high speed of the electric spindle. Both the ordinary frequency converter and the vector control driver are implemented to perform this task. The former is a constant torque drive, and the output power is proportional to the rotational speed; the latter is a constant torque drive at the low speed end (below the rated speed) and a constant power drive at the middle and high speed ends. The electric spindles of high-speed machining centers and CNC milling machines basically use the latter (ie vector control drives). The vector control driver is divided into an open-loop type and a closed-loop type. The closed-loop type is equipped with a high-performance coding tool as an inspection component on the spindle to achieve position and speed feedback for better dynamic performance and orientation of the spindle. Stop and C axis function.
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