Research On The Key Technology Of Near-net Shaped Forming For Gears Used In Car

Gears are widely used as an important part in power transmission device in car. In recent years, with the rapid development of automobile, the demand on gear's manufacturing becomes much strict. For the cold precision gear forging has a lot of advantages when compared to conventional gear shaping, such as material saving, better mechanical properties and higher productivity etc, cold precision forging has become an alternative method to conventional method in gears manufacturing. This paper studied the closed precision die forging process systematically.To avoid edge contact, reduce vibration and noise, increase the stability during power transmission, the gear modification theory and method were studied. The method of calculating modification quantity in teeth length direction and the method to establish the center of crown teeth face were given. Based on the character of base pitch variation during the gears working, the calculation method of modification quantity in teeth tips was obtained.Closed die forging is a main near/net shaped forming method for gear parts; it is the best process for producing modified bevel gears until now. Rigid-plastic Finite element method (FEA) was employed to analyze the bevel gear forging process, damage distribution was obtained and the forming load was predicted, that laid the foundation of optimizing process for bevel gear forging. FEA results show that the material flows to the bottom of the teeth cavity die first, and then it flows into the teeth cavity in radial direction until the die cavity is full filled, meanwhile, it flows backward. The damage became higher from the small end to the big end of the bevel gear. Because of the confluence of material, a small peak appears in the middle of the damage distribution curve along the teeth tip. The load keeps rising during forging, when the cavity is full filled, the load rise sharply, since the material can flows into the process compensation space, the load increase can be effectively inhibited.A die set for bevel gear forging was invented; its structure and principle of work were described. A curved parting-face die and a plane parting face die were designed. Based on the divided flow forging theory, principle and method for designing process compensation space was given. The fracture of the teeth tip in a curved parting.face die and the crack at the bottom corner in a plane parting face die were analyzed. According to the analysis results, a non-uniform assembled die structure was proposed. A three-layer assembled die was optimized by a combined method of classic thick-wall cylinder theory, Matlab programming and FEA, and its life was obviously increased.Digitized design and manufacturing technique was proposed. A matrix steel was used as die material instead of high-speed steel and the heat treatment hardness was decreased to fit the milling requirement. It is successful to use milling as an alternative method to electro-spark machining to manufacture bevel gear cavity die, which greatly increased the accuracy of the die and the manufacture efficiency.Based on the characters of helical gear, a number of near/net shaped forging processes were analyzed by classic plastic theory and FEA method. According to the analyse results, the single punch forging with a process compensation space at the big radii-side of the die cavity bottom were decided as the best forging case. Based on the forging characters and selected forging case a die set with a rotational reject device was designed, that laid a foundation for a further study.Based on the results above (except for helical gear), experiments for near/net forging of bevel gears were carried on Y28-400/400 double action press. It verified the forging process, digitized die modeling and manufacturing technique, non-uniform assembled structure die. A product line has been built in Dongfeng finished gear plant in Shiyan, the products has a better accuracy than the seventh grade of Chinese standard GB11365-89 and has a teethsurface roughness of Ra=0.4～0.8μm, the teeth can be used in car without any furthermachining, that means the cold forging technology of bevel gears is producing remarkable technical and economic benefit.