Gear is an indispensable part in mechanical transmission. It needs special equipment and cutting tools for processing. The processing technology is complex, the production efficiency is low, and the raw materials are wasted. Moreover, after cutting the tooth shape, it cuts off the metal streamline and reduces the comprehensive mechanical properties of the parts. These have always been the main disadvantages of cutting gears. In recent decades, people have paid more and more attention to forming gears by forging in order to reduce costs and enhance market competitiveness.
As early as the early 20th century, German scientists proposed the precision forging technology of gears. Due to the advantages of this technology, it reflects extremely significant economic benefits in practice. Industrial developed countries continue to conduct in-depth research on this technology in theory and practicability, so that the precision and shape complexity of precision forgings are continuously improved and more widely used in the forming of various materials. In the 1960s, precision forming technology was applied to the production of tooth blank. In the 1970s, affected by the low cost of automobiles and motorcycles, gear forging technology had a great development. By the 1980s, forging technology had been quite mature and suitable for mass production. China began to study the forging process in the 1960s and achieved great development in the 1970s and 1980s. Compared with some developed countries, there are still some deficiencies in China’s precision forging technology. In terms of metal deformation law: n. A Abdul of the University of Birmingham, UK, studied the influence of friction coefficient, number of teeth Z and root circle diameter on metal deformation law and forming force in gear forming process. Then, Op Grover also used the upper bound element method to study the influence of parameters such as modulus and number of teeth on forming force. N.R.Chitkara and M.A.Bhutta of the University of Manchester in the United Kingdom established the mathematical model of the dynamic and admissible velocity field by combining the energy method and the upper limit method.
Chen Fuxiao of Henan University of science and technology and others used the same method to study the law of metal plastic deformation by changing the die structure and selecting appropriate process parameters, and proposed a new method (superplastic extrusion method), which can reduce the forming force and improve the forming quality. In 2010, Tan Xianfeng and Lin Zhiping of Nanchang University carried out a detailed theoretical analysis on the plastic forming process of cylindrical gear according to the fullness of tooth profile forming obtained from the experiment and the theory of upper bound method, and analyzed the stress state in the plastic deformation and the stress of the die. Wang Huajun and others studied the metal flow law in the process of tooth shape forming, designed and manufactured the preform and rolling experimental die of driven spiral bevel gear, and carried out lead test.
In die design and forming process: tuncer, Birmingham University, UK In the 1980s, C and T.A. Dean developed a new die structure combined with the characteristics of closed forging. The tooth profile is formed by internal shunting method. This method can not only reduce the forming force, but also form the tooth profile with good quality, but the tooth surface still needs cutting and processing. At present, Japan’s plastic forming technology is very superior. Since the 1980s, K. Kondo of Nagoya University and K. ohga of Shizuoka National Institute of technology have put forward the idea of shunting to further reduce the forming force and applied it to the gear precision forging process. The two shunting methods of decompression hole and decompression shaft proposed by them can effectively reduce the forming load and improve the service life of the die, Avoid folding defects in the middle of the blank.
In early China, due to the constraints of technology and equipment, the forged products had low precision and short die life, which could not be applied to industrial production. With the rapid development of industry, the research on precision forged gears was carried out gradually. Kou Shuqing, Fu Peifu and Yang Shenhua of Jilin University put forward the method of pre forging (closed die forging) before final forging (diversion) to study the metal flow in the extrusion process of metal materials at room temperature. Through the finite element numerical simulation method and combined with the upper bound element method, the metal flow law in the forming process was observed and analyzed theoretically. Zhang Qingping and Zhao Guoqun of Shandong University put forward a two-step forming process scheme to form spur gear. On the premise of the process scheme of “closed die forging inward shunting”, they further studied the design method of die tooth profile, determined the die structure and the best process parameters, and studied the change law of forming force and the filling fullness of corners by changing different shunting schemes, so as to reduce the forming load, The purpose of improving forming quality. Feng Wenjie and others studied the influence of whether the die has boss on the forming force by using the finite element simulation software. Through the analysis, it can be found that adding boss can effectively reduce the forming force. Song Ying and Feng Wenjie have effectively improved the service life of the mold by changing the structure and manufacturing process of the mold. Using finite element simulation software, Fang Yuan analyzed the influence law of relevant parameters of tooth profile on its forming process, and proposed a new forging process, that is, pre forging (closed forging) first and then final forging (split forging). The numerical simulation of the scheme was carried out. The simulation results show that the process has the advantages of small stress value, small forming load and uniform temperature distribution. Huang Tao and others summarized the process flow and development status of gear, and studied and numerically simulated the precision forging process of driven spiral bevel gear. Tian Fuxiang and others proposed a new closed precision forging process, and designed the floating die according to this process, which solved the flash problem existing in the open die forging of spiral bevel gear. The proposed one fire two forging process (the blank only needs to be heated once to complete the rough forging and fine pressing in turn) fully improved the working efficiency.
In terms of forming quality and accuracy control: wanheim, Y. Qin and others consider the elastic expansion and change the die size in combination with the reverse repair method, so as to reduce the dimensional error of formed parts and improve the accuracy of workpieces; By changing the size parameters of the die and combined with the forming force, a feasible die structure is designed. O. Eyercioglu et al. Further modified the structural parameters of the die by changing the corresponding die parameter values and using the finite element numerical simulation software for simulation analysis, and verified the accuracy of the numerical simulation results through experiments, so as to improve the accuracy of the workpiece.
Abdel of the University of Birmingham, UK Rahman et al. Studied the relevant factors affecting the tooth profile accuracy (such as the number of gear teeth, modulus, forming temperature, post-treatment, etc.), and analyzed the influence of various parameters on the hardness of gear precision forgings. The research results show that the tooth profile accuracy is affected by the process flow and die structure. Liu Hua of Xi’an Jiaotong University and others combined with the finite element numerical simulation software to simulate the metal plastic forming process of spur gear. By changing the tooth shape size, it is concluded that the elastic deformation of die and the elastic recovery of tooth directly affect the accuracy of formed parts. They are combined with the experimental verification to verify the accuracy of their conclusions. According to the design method of closed and closed precision hot forging die for driven spiral bevel gear of automobile rear axle, Zheng Jianshe puts forward a set of new process procedures to form spiral bevel gear. Combined with the closed forging precision forming process scheme, Lin Xue and others simulated the cold closed forging, studied the change law of metal elastic deformation, drew the die elastic curve and tooth profile change curve according to the results, corrected the tooth profile die by using the inverse compensation method, and designed the tooth die electrode.
From the existing research results, the precision forging process of spur gear is close to maturity and can be well applied to industrial production. However, for spiral bevel gear, because its tooth surface is arc-shaped and the metal flow path is complex in the forming process, it is not easy to fill the die cavity, so the product quality is not easy to be guaranteed. Under the condition of ensuring the product forming quality, more blank volume will be required to fill the die tooth cavity, resulting in excessive forming force, die rupture and increasing the investment cost of the die. Therefore, the application of precision forging technology in spiral bevel gear machining is difficult.