Development of gear precision forging technology

Gear precision forging has developed greatly in recent decades. More and more manufacturers and users pay attention to manufacturing gears by forging. It is generally believed that the method of forging gears can improve the utilization of materials, improve productivity, improve the mechanical properties of gears, reduce costs and enhance market competitiveness. Especially for large-scale production in automobile industry, gear precision forging has greater benefits and potential.

Although gear precision forging has many advantages and has been used in the large-scale production of bevel gears, it is still a long way from being applied to the large-scale production of cylindrical spur gears and helical gears of certain sizes. Especially for gears used in automobile power transmission, a set of practical and reliable production process needs to be established in order to be accepted by manufacturers.

Gear precision forging technology comes from Germany. As early as the 1950s, due to the lack of sufficient gear processing machine tools, the Germans began to trial produce bevel gears by closed hot die forging. The main feature is that a very new EDM process was used to manufacture the cavity of forging die. In addition, the forging gear process is strictly controlled. On this basis, gear forging technology is further applied to the production of spiral bevel gear and cylindrical gear. However, in the forging of cylindrical gear, because the plastic flow direction of metal material is perpendicular to its stress direction, its tooth profile is more difficult to form than bevel gear. The Forging Research of cylindrical gears began in the 1960s and developed greatly in the 1970s, which is mainly due to the pressure from the automobile industry to reduce costs. By the 1980s, the gear forging technology was more mature and could achieve higher accuracy and consistency, so that the forged gears could be accurately positioned on the flow production line and suitable for mass production.

The purpose of gear precision forging is to directly produce gears without subsequent cutting. If the gear can be forged at room temperature, the shape and size of the gear can be easily controlled and the error caused by high temperature can be avoided. At present, many bevel gears and small cylindrical gears have been made by this method. When the overall size is suitable ‚ Cold extrusion process can also be used to manufacture cylindrical spur and helical gears. However, most of the gears used in automobile transmission have large diameter and height, so they are not suitable for extrusion process. If closed die forging is used, high pressure is required to make the metal material flow and fill the die cavity, so this kind of gear needs hot forging or warm forging process. High temperature will lead to material oxidation, die distortion, and affect the accuracy and surface quality of forgings. It is difficult to correct these errors with additional machining, and the cost should be increased. Especially when the subsequent grinding process is used to correct the error on the tooth profile, in addition to increasing the cost and prolonging the working hours, there is also the problem of gear positioning in the grinding process.

At present, the generally agreed process is the combination of hot forging, warm forging and cold forging. Hot forging and warm forging can achieve high efficiency and high utilization of materials. Cold forging process can correct the errors of hot and warm forging process and improve surface quality. At the same time, the cold treatment process can also obtain residual compressive stress on the gear surface and improve the service life of the gear.

While working at Birmingham University, ZHY gear senior consultant engineer has just completed a 3-year research project funded by the British Engineering Science Research Association (EPSRC) and cooperated with 7 enterprises in the UK (gear manufacturing, mold manufacturing, gear users, forging mills and steel companies): precision forging of cylindrical spur gear and helical gear.

Based on years of research and practice, the project further discusses the mechanism of gear forging, and uses modern analysis means, such as computer simulation and design technology, in order to develop a manufacturing and economically feasible forging gear processing technology to manufacture precision gears that do not need subsequent processing on the tooth profile.. The project studied and tested three kinds of gears: cylindrical spur gear, cylindrical helical gear and synchronous gear. Considering the economy of the whole process, gear precision forging is limited to the contour part, while the tooth end and inner hole are machined. The manufacturing process is warm forging and cold treatment. The gear meeting the shape requirements is obtained by warm forging, and an allowance of about 0 ∙ 1mm is reserved in the contour part. In the process of cold treatment, the warm forged gear is extruded through a precision designed and manufactured die, so as to correct the error of the contour part and obtain a high-precision tooth surface. In the research process, the finite element method is used to analyze the forging gear process and design the die, so as to ensure the accuracy of the gear. After three years of research, we have mastered its basic technology, and the next step will be factory field test. At the same time, it is preparing to apply for the second phase of the project.

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