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Application of high speed angular touch ball bearings
Source: Bearing network time: 2018-03-17
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The traditional spindle system includes intermediate transmission links such as belts and gears; not only the inertia of the rotation is large; it is difficult to complete the high angular addition (decrease) speed; and it is difficult to handle the oscillation and noise presented by the intermediate transmission link at high speed. Thus; today's high-speed spindle Most of the systems are electric spindle units that combine the motor and the main shaft and shorten the transmission chain to zero. The plan is basically to place the main motor between the front and rear bearings of the main shaft. Such an electric spindle has a short axial dimension. Compact planning; high spindle stiffness; large output. The electric spindle used by GMN in Germany for machining intermediate and CNC milling machines is such a plan; its bearings are fine angle touch ball bearings suitable for high speed operation (usually equivalent to international specifications P2 and P4 grade); all critical parts must be finely machined and finely equipped; together with the constant temperature cooling water (GMN 23 ± 0. 7 ° C or even ± 0.3 ° ° C) to cool the stator and front and rear bearings of the main motor. GMN has more than 20 types of electric spindles for machining intermediate and CNC milling machines; the maximum speed is from 8000r/min to 60,000r/min; the power is from 5. 5kW to 76kW; the torque is from 0.9Nm to 3 06Nm.
Today, there are also very few high-speed light-cut milling machines; the use of the main motor behind the spindle rear bearing of the electric spindle. This planned electric spindle has a long axial dimension; but can improve the heat dissipation conditions. In addition; some spindles have a maximum speed of 10000r Machining intermediate and milling machine from /min to 15000r/min; instead of using an electric spindle but passing through a coupling such as a film type; directly connecting the main motor to the main shaft; then eliminating the spindle cooling system. However, this method adds Moment of inertia; affecting the angle plus (decrease) speed; and the non-linearity after the direct connection of the two will cause oscillation (the higher the speed, the higher the alignment requirement for linearity); therefore, it is not suitable for the higher spindle speed. machine tool.
At that time, the high-speed electric spindle; it is simply built-in asynchronous communication induction motor. The advantage of the asynchronous type electric spindle is that the planning is relatively simple; the manufacturing process is relatively sophisticated and the device is convenient; in particular, it can weaken the magnetic force with greater limit. Such a spindle motor is also being further improved; for example, Mitsubishi Electric Corporation of Japan has announced a high-speed, high-efficiency built-in induction spindle motor that can reduce electrical losses and shorten the stator length.
More than abroad; foreign countries are discussing the problem of built-in communication permanent magnet synchronous motor in the electric spindle. Compared with asynchronous induction motor; permanent magnet synchronous motor has the following advantages: the rotor made of permanent magnet data does not generate heat during operation; Built-in asynchronous induction motor rotor is hot and difficult to fully cool, high power density; high operating efficiency; can get larger power and torque with smaller scale, good low speed function; easy to complete fine control, etc. Synchronous motors also have their shortcomings; that is, the power capacity is limited; the weak magnetic field is difficult; it is not conducive to the completion of high speed. So far; as long as a single machine tool company has a permanent magnet synchronous motor with an electric spindle at the show; and a professional electric spindle There is no such product for sale in the manufacturer. Many people think that it is more suitable for high-speed electric spindles that operate in weak magnetic fields; it seems to be driven by asynchronous induction motors.
At present, the primary use of PWM variable frequency speed control technology to complete the high speed of the electric spindle; to fulfill this mission, there are both the usual frequency converter; and the vector control drive. The former is constant torque drive; the output power is proportional to the speed, after At the low speed end (below the extra speed), it is a constant torque drive; at the middle and high speed ends, it is a constant power drive. The high speed machining intermediate and the electric machine spindle of the CNC milling machine; basically the latter is used (ie vector control drive). Vector control The drive has an open-loop type and a closed-loop type; the closed-loop type is equipped with a high-function encoder as a detecting component on the spindle; to complete the azimuth and speed. The maids are used to change the hoops. Ji: C axis function.
The function of the high-speed spindle for the bearing CNC cutting machine high-speed spindle; to a considerable extent depends on the spindle bearing and its smooth. Rolling bearing because of good rigidity, precision can be made higher, bearing capacity and planning is relatively simple; not only the usual The first spindle of the cutting machine tool, the color reef å’šå¾ å’šå¾ éª° 骰 å›— å›— å›— å›— 从 从 从 从 从 从 从 从 从 从 从 从 从 从 从 从 从 从 从 从 从 从 从 从 从 从 从 从 从 从 从 从 从 从 从.
The ball of the angular touch ball bearing (ie, the ball) revolves and rotates; the centrifugal force Fc and the gyro moment Mg occur, followed by the addition of the spindle speed; the centrifugal force Fc and the gyro moment Mg also increase sharply; the bearing has a large touch stress. Then incurs increased bearing conflicts, increased temperature rise, reduced accuracy, and reduced lifespan. Thus; to improve the high-speed function of such bearings; should try to find the addition of Fc and Mg. From the angular touch ball bearings Fc and Mg calculation formula It is known that reducing the density of the ball data, the diameter of the ball and the touch angle of the ball are all beneficial to reduce Fc and Mg; therefore, today's high-speed spindles use small ball bearings with a touch angle of 15° or 20°. However, the ball diameter cannot To reduce too much; basically only 70% of the standard series of ball diameter; to avoid weakening the rigidity of the bearing; the more important thing is to seek improvement on the ball data.
Compared with GCr15 bearing steel; the density of tantalum nitride (Si3N4) ceramics is only 41%; the ball made of tantalum nitride is much lighter; the centrifugal force and gyro moment that occur naturally during high-speed reversal are also small. More. Together with this; the elastic modulus and hardness of tantalum nitride ceramics are 1.5 times and 2.3 times that of bearing steel; and the coefficient of thermal expansion is only 25% of bearing steel; this can improve the rigidity of bearings. And the number of life; also make the cooperation gap of the bearing under different temperature rise conditions. The problem is also swayed and swayed (5) 骋 幌 幌 幌 挥 玫 玫 èŠ èŠ èŠ èŠ é«‘è›± é«‘è›± é«‘è›± é«‘è›± é«‘è›± é«‘è›± é«‘è›±Qian Wei. The theory indicates that the ceramic ball-angle touch ball bearing can improve the speed of the ball bearing by 25%~35%; however, the price is higher.
Foreign bearings with inner and outer rings of steel and rolling bodies are collectively referred to as hybrid bearings. The current hybrid bearings have new developments: First, ceramic materials have been used to manufacture cylindrical roller bearings; ceramic cylindrical hybrid bearings have been presented on the market. Second, the inner and outer rings of the bearing are made of stainless steel (for example, FAD's nitrided stainless steel Crodinur30), especially the inner ring; because the coefficient of thermal expansion of stainless steel is 20% smaller than that of bearing steel; natural at high speed reversal; The increase in the contact stress caused by the thermal expansion of the inner ring is subject to pressing.
It is well known that the dmh value is the speed factor that expresses the high-speed function of the rolling bearing (dm is the uniform diameter of the inner and outer rings of the rolling bearing; the unit mm, h is the rotational speed of the bearing; unit r/min). The high-speed function of the angular touch ball bearing is not only The touch angle, diameter and data of the ball are related; and it is closely related to the smooth method of the bearing. The current rolling bearing has three methods: smooth grease, smooth oil mist and smooth oil and gas; among them, the oil mist is smooth, although it works well; but it pollutes the environment and damages workers. Ankang; has been rarely used abroad.
Grease smoothness is the most simple and environmentally friendly method of smoothing. Because the fat is simply transformed under ultra-high speed operation, its dmh value is low; the bearing is only 80×104 when it is a steel ball; 110×104 (the dmh value of the new generation of cryogenic bearings developed by FAG can also be added by about 10%). Today, the most used high-speed spindle bearings are oil and gas smoothing methods; it is a punctual and quantitative supply of bearings. Oil and gas mixture; make the best trace of all parts of the bearing smooth and reduce the pollution to a small. Select oil-stained steel ball or ceramic ball-angle touch ball bearing; its dmh value can usually reach 140×104 and 210×104 If a special oil and gas smoothing method is selected, the dmh value of the ceramic ball-angle touch ball bearing can reach 250×104 or even higher, which is a kind of special oil and gas smoothing method; its dmh value has reached 280×104 in the laboratory) .
High-speed electric spindle rolling bearings are available in a variety of ways; however, typical two-diagonal contact ball bearings with “O†plan for the front and rear bearings are preferred. Because the rear bearings are also angular contact ball bearings; ball bearings are usually provided. The rear bearing can be moved along the axial direction of the housing; so that the main shaft can be self-expanded to the rear after being heated. Generally speaking; the angular touch ball bearing needs to be preloaded in the axial direction to work normally; the preloading is larger; the bearing is The higher the stiffness, the greater the temperature rise. The simpler method is: according to the speed of the electric spindle and the load to be accepted; choose an optimal fixed preload value, the better method is the preloading energy Adjusted with the spindle speed change; reduce the preload at high speed; add preload at low speed.
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