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       The application of special gears, the development of planetary gears, the development of low vibration and low noise gears are some characteristics of gear design. In order to achieve the goal of miniaturization of gears, the bearing thrust of existing involute gears can be increased. . Hard tooth surface technology is widely used in many countries to improve the hardness to reduce the size of the device; special tooth profile represented by circular arc gear can also be applied. The main transmission system of Shipborne helicopter developed by Britain and France adopts circular arc gear, which greatly reduces the height of the reducer. With the trend that marine power is replaced by medium-speed diesel engines, the use of high-power planetary gears on large ships is indeed effective; now in large transmission devices such as metallurgy, mining, cement mill, planetary gears with its advantages of small size, good coaxiality and high efficiency are used more and more.

       Gear manufacturing process 

       Since the 1970s, the manufacturing accuracy of all kinds of gears has been improved by about one grade, some even by 2-3 grades. The precision of low speed gears has been improved from 8-9 grades to 7-8 grades. The gear of the machine tool is increased from 6~8 to 4~6. The gear of the mill is raised from 7~8 to 5~6. For small and medium-sized gears with small modulus, the hobbing efficiency has been greatly improved due to the development of high-performance gear hobbing machine and the improvement of tool materials. Using multi head hob, the cutting speed can reach 90m/s under large feed and condition. For example, if the quenched and tempered steel gears with modulus about 3 are machined with superhard hobs, the excessive cutting can reach 200m/s. To improve the gear shaping efficiency, it is restricted by the cutter reciprocating motion mechanism of the gear shaping machine. Recently, new structures such as tool unloading, hydrostatic bearing, and stiffness enhancement of tool holder and column have been developed, and the efficiency has been greatly improved. The stroke number of the new gear shaper can reach 2000 times / minute. 　  

       Development trend of gear steels

       1. Low alloy steel containing Cr, Ni and Mo.

       2, boron steel.

       3. Carbonitriding steels.

       4. Free cutting steel.

       Due to the lack of Ni and Cr in China, 20CrMnTi carburizing steels are often used or boron containing rare earth steels are used. Heavy duty machines often use 18CrMnNiMo carburizing steel or medium carbon alloy steel. The machine tool industry is nitrided with edible 40Cr, 38CrMoAl steel and 25Cr2MoV steel for high speed gear.