Development trend of gear manufacturing

Internationally, power transmission gear devices are developing in the direction of miniaturization, high speed and standardization. The application of special gears, the development of planetary gear devices and the development of low vibration and low noise gear devices are some characteristics of gear design. In order to achieve the purpose of miniaturization of gear devices, the bearing thrust of existing involute gears can be improved. Hard tooth surface technology is widely used in various countries to improve the hardness to reduce the size of the device; The special tooth profile represented by circular arc gear can also be applied. After adopting circular arc gear in the main transmission system of Shipborne helicopter developed by Britain and France, the height of reducer is greatly reduced. With the trend that ship power is replaced by medium speed diesel engine, it is effective to adopt high-power planetary gear device on large ships; Nowadays, in large transmission devices such as metallurgy, mining and cement rolling mill, planetary gears are more and more used because of their small volume, good coaxiality and high efficiency. Due to the large-scale development of mechanical equipment, the working parameters of gears have been improved. For example, the transmission power of high-speed gear is 1000-30000kw. The circumferential speed of the gear is 20 ~ 200m / S (1200-12000r / min), and the design working life is 5×104-10×104 hours; The circumferential speed of rolling mill gear has increased from a few meters per second to 20m / s, even 30 ~ 50M / s. The transmission torque torch reaches l00 ~ 200t m. The service life is required to be more than 20 ~ 30 years. The accuracy grade of these gears is generally 3 ~ 8. It also has high requirements for stability and noise. For high speed gears (including turbomachinery gears). When the circumferential speed exceeds 100M / s, due to the thermal effect in operation, it is required to correct the thermal deformation at the beginning of design to make the gear reach a normal meshing state during operation. Especially for high-speed and heavy-duty gears. More consideration should be given. Secondly, for low-speed and heavy-duty gears such as rolling mill gears, the elastic deformation of the whole gear device system caused by the increase of tooth surface load coefficient after using hard tooth surface gears becomes prominent, so sometimes the elastic deformation reflected on the tooth surface should be corrected. This technology of gear tooth modification is an important trend in the manufacturing of high-power, high-speed and heavy-duty gears. In terms of gear manufacturing technology, the focus is on the machining of hard tooth surface, especially the development of cutting and heat treatment process of large hard tooth surface gear, such as superhard cutting, internal gear hobbing, forming gear grinding, gear honing of large module gear, elastic grinding wheel polishing, gear tooth modification, deep sand carbon and other new processes are constantly tested and applied in production.

The development of gear manufacturing technology is largely reflected in the improvement of accuracy level and production efficiency. Since the 1970s, the manufacturing accuracy of various gears has generally increased by about one level. Some even level 2 ~ 3. The accuracy of general low-speed gears has increased from level 8 ~ 9 to level 7 ~ 8. The gear of machine tool is increased from 6 ~ 8 to 4 ~ 6, and the gear of rolling mill is increased from 7 ~ 8 to 5 ~ 6. For small and medium-sized gears with small modulus, due to the development of high-performance hobbing machine and the improvement of tool materials, the hobbing efficiency has been significantly improved. Using multi head hob, the cutting speed can be 90m / s under the condition of large feed and. If the hardened and tempered steel gear with modulus of about 3 is processed with superhard hob, the over cutting can reach 200m / s. improving the gear shaping efficiency is limited by the reciprocating motion mechanism of the tool of the gear shaping machine. Recently, after developing new structures such as tool unloading, hydrostatic bearing and enhancing the rigidity of tool holder and column, the efficiency has been significantly improved. The stroke number of the new gear shaper can reach 2000 times / min.

Due to the wide application of hardened gears and the increasing number of gears with high speed and high performance requirements, gear grinding is required to improve efficiency and quality. Generally speaking. Generating method is widely used in gear grinding, while forming method is less. Maag gear grinding method has high precision but low efficiency. Not suitable for regrinding. The single grinding wheel gear grinding machine produced by Niles and hofler has good rigidity, reliable precision, suitable for large feed rate processing and high efficiency. In recent years, grinding methods are also being improved to improve efficiency, such as reducing grinding times, compressing the developed length and shortening the tail cutting stroke; Therefore, the “K” grinding method proposed by Maag company and the “double-sided grinding method” proposed by Niles company have improved the actual grinding efficiency. At present, for batch grinding of medium and small use gears, worm wheel gear grinding machine is preferred, with high grinding efficiency. For grinding of large module gears, in addition to the grinding of teeth with a single grinding wheel that can be reground, the method of forming drawing is adopted. It is also an effective way of efficient grinding.

In addition, there are some new process methods, such as g-trac no765 track gear cutter developed by Gleason company of the United States. It can process 88 gears per hour, which is 3 ~ 4 times higher than that of ordinary gear hobbing machine. High efficiency gear cutting process with double cutter head. The cutting speed can reach 137m / s. It takes only 6 seconds to roughen and finish a helical gear with M = 1.5mm, outer diameter of 24.43mm and tooth width of 19mm, and its efficiency is 5 ~ 10 times that of this gear. The efficiency of multi cutter head gear shaper of American Michigan tool company is 5 ~ 10 times higher than that of ordinary gear shaper. The gear cold forming process, cold extrusion, hot rolling and other less and no cutting processes in the automotive industry also continue to obtain new development. As for gear materials and heat treatment, with the development of self facing gears, people pay more and more attention to them.

Development trend of gear steel; First, low alloy steel containing Cr, Ni and Mo; Second, boron steel; Third, carbonitriding steel; Fourth, free cutting steel. Due to the lack of Ni and Cr in China, 20CrMnTi carburized steel or boron containing rare earth steel are commonly used. 18CrMnNiMo carburized steel or medium carbon alloy steel is commonly used in heavy machinery. The machine tool industry uses 40Cr, 38CrMoAl and other steels and 25Cr2MoV steel for nitriding.

Gear heat treatment processes generally include carburizing (or carbonitriding), nitriding, induction quenching and quenching and tempering. At present, the general trend is to improve the tooth surface hardness. The bearing capacity of carburized and quenched gears can be 2 ~ 3 times higher than that of quenched and tempered gears. Table 111# shows the comparison of gears manufactured by different processing methods, including center distance, weight and safety factor.

Carburizing and quenching gears can obtain high surface hardness, wear resistance, toughness and impact resistance, and can provide high pitting corrosion and fatigue resistance. The performance of the core and carburized layer mainly depends on the heat treatment process selected. For example, when the gear is quenched and tempered to 360hb, the contact fatigue limit stress of the gear surface is pH. – 750N / mm2, for example, when the surface is quenched to hrc56-60 -, pj1500n / mm2, for example, when the surface is carburized to the same hardness Yi- 1200n / mm2, for hardened and tempered gears. Due to the improvement of gear tool materials, the tooth surface hardness of pinion has been increased to 360hb and that of large gear has been increased to more than 280hb.

Gas carburizing is mostly used for gear carburizing. Propane gas is usually generated from the propane gas generator and sent to the carburizing furnace for carburizing. Liquid injection carburizing furnace is also used to gasify the organic liquid in the furnace for carburizing. This method occupies less land, has low raw material and treatment costs, unstable furnace and short working time, which is conducive to saving energy and cost. The recently developed vacuum ionization carburizing method, especially for gears with deep carburizing requirements, can further shorten the time and reduce deformation.

Application of electronic computer in various industrial fields; It has promoted the development of various technologies. Similarly, the design and calculation of gears have made rapid progress. People can calculate, analyze and compare various possible design methods by using computers, and obtain ideal results through optimization. For example, in analyzing the contact area of tooth surface, calculating the angle between meshing line and relative speed, compiling programs for elastohydrodynamic lubrication calculation and geometric parameter calculation. In addition, there are programs for the calculation of gear modification and gear bearing capacity. China has compiled the program software of gb3480-83 standard for the calculation of bearing capacity of involute cylindrical gears for production application. In terms of gear processing, the whole gear cutting process can be controlled by computer to make the manufacturing quality stable and reliable. At present, China is studying the application of microcomputer to calculate the gear cutting adjustment card of spiral bevel gears, The machining deviation can be adjusted in time, so that the tooth surface contact can reach an ideal position, and the work efficiency is greatly improved. In addition, according to the principle of numerical control, the parabola modification of the helical tooth surface of hourglass worm by microcomputer has been applied to production. Although this work is still in its infancy in China, it is of great significance to improve the quality and technical level of gear manufacturing.

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