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Mechanical transmission of CNC machine tools

Oct 15, 2019


First, the ball screw nut pair


The static and dynamic conflict coefficients of the ball screw are actually nothing different. It eliminates backlash and preloads, helping to improve positioning accuracy and stiffness. The ball screw is made by a specialized factory.


In order to improve the sensitivity, positioning accuracy and avoidance of the feed system, it is necessary to reduce the conflict of the CNC punch feed system and reduce the difference between the static and dynamic conflict coefficients. Therefore, the ball screw pair is often used to form a linear motion tissue that is not too long.


The transmission power of the ball screw pair is as high as 85%-98%, which is 2-4 times that of the general sliding screw pair. The ball screw pair has a collision angle of less than 1° and is therefore not self-locking. If the ball screw drive drives the lifting movement (such as the headstock or the lifting table), it is necessary to have a brake device.


Second, the coupling


The bypass coupling uses a tapered clamping ring to transmit the load, so that the power transmission has no directional clearance.


The flanged coupling structure is simple, cost-effective, and can transmit large torque. It is often used in low speed, five kinds, and the rigidity of the shaft is large and the centering is good. His primary flaw is the high demand for the alignment of the two axes. If there is displacement and skew between the two shafts, the rescue will cause additional load in the machine, which will deteriorate the working condition.


A coupling is a device used to engage two shafts that are sent to the tissue for transport and torque transfer together. The types of couplings currently available are hydraulic, electromagnetic and mechanical. Mechanical couplings are the most widely used.


The sleeve coupling has a simple structure and a small radial dimension, but the loading and unloading is difficult (the shaft needs to be moved axially). Moreover, the two axes are strictly aligned, and radial or viewpoint errors are not allowed, so they are bound to be bound when used.


Third, the slowdown organization


1) Synchronous toothed belt


Synchronous toothed belt drive is a new type of belt drive. He uses the toothed shape of the toothed belt to sequentially mesh with the gear teeth of the pulley to transmit motion and power. Therefore, it has the advantages of belt transmission, gear transmission and chain transmission, and has no relative sliding, uniform transmission precision and high transmission precision. Moreover, the toothed belt has high strength, small thickness and light weight, so it can be used for high speed transmission. The toothed belt does not need to be specially tensioned, so the load acting on the shaft and the bearing is small, and the transmission power is also high, and it has been widely used in numerically controlled machine tools. The primary number and standard of the synchronous toothed belt are as follows:


· Modulus The modular modulus is m=p/π, which is a primary basis for the scale calculation of the toothed belt.


• Pitch The pitch p is the interval between two adjacent teeth on the pitch line. Since the strength layer does not change during the operation, the center line of the strength layer is defined as the pitch line (neutral layer) of the toothed belt, and the circumference L of the pitch line is used as the nominal length of the toothed belt.


Other parameters The other parameters and dimensions of the toothed belt are the same as those of the involute rack. The calculation formula of the toothed tooth profile is different from that of the involute rack because the pitch line of the toothed belt is on the strong layer, not in the middle of the tooth height.


The method of marking the toothed belt is: modulus × width × number of teeth, that is, m × b × z.


2) Gear transmission equipment


Gear transmission is a widely used mechanical transmission, and there are gear transmissions in the transmission equipment of various machine tools. There are two intentions for selecting a gear transmission device in a servo feed system of a numerically controlled machine tool. One is to change the output of the high-speed torque servo motor (such as stepper motor, DC and communication servo motor, etc.) to the input of the low-speed and high-torque performance component; the other is to make the ball screw and the workbench The moment of inertia is a proprietary smaller specific gravity in the system. In addition, the open loop system also ensures the required accuracy of motion.


In order to minimize the influence of the flank clearance on the machining accuracy of the CNC machine, it is often adopted in the structure to reduce or eliminate the lost motion of the gear pair. For example, the double-gear gear misalignment method is used, the offset center distance is adjusted by using the partial pain sleeve, or the axial gap adjustment method is adopted to eliminate the gear backlash.


Compared with the selection of the synchronous toothed belt, the gear reduction equipment is selected in the CNC machine feed chain, which is more prone to low frequency vibration. Therefore, the damper is often equipped with a damper to improve the dynamic function.


In general, a typical CNC machine tool closed-loop control feed system consists of azimuth comparison, expansion components, drive units, mechanical transmission equipment and detection reaction components. In the meantime, mechanical transmission equipment is an important part of azimuth control. The mechanical transmission device referred to herein refers to the entire mechanical transmission chain that changes the rotational motion of the drive source into a linear motion of the workbench, and includes a central transmission structure such as a gear device and a screw nut pair.