The Key Technology On Precision Forming Process By Hot Forging And Cold Comparison Finishing Of Gear Shafts

The gear shaft, as one of the power transmission parts, is widely used in automobile, electric power, shipbuilding, construction machinery and other fields. The traditional gear shaft processing method of teeth of gear shaft is cut on the preformed blank with low production efficiency and waste of material. The gear shaft is formed by the precision plastic forming method, which has the advantages of high materials utilizing, production efficiency, product quality and so on. Furthermore, the metal streamlines on the gear tooth are not cut off and distribute along the tooth profile, which results that the gear shaft near net forming has a good application prospect.The plastic forming method has been widely used in the forming of cylindrical gears and bevel gears with small modulus and small diameter. For large diameter and large modulus gear shaft, especially spiral tooth gear shaft, it is difficult to control the precision of the teeth during hot precision forging, and the surface quality of products is poor. Moreover, the load and die stress of the cold forming are severe. In this paper, the gear shaft with helical teeth is deformed and finished by hot forging and cold shrinkage finishing technology. The objective is the near-net shaping of the gear shaft.Since the long shaft of the gear shaft is unstable in the die with the traditional floating die method, this paper forward floating die method is presented in this paper. In order to solve the problem of the folding of lower endplate of the teeth, the dimension of the stepped shaft blank is studied. In order to the characteristic of the gear shaft with a blind hole, the size of blind hole is studied. That shows a small taper of the blind hole can promote the flow of metal in the teeth, which reduces the folding of upper endplate of the teeth caused by the metal flow direction. A larger blind hole depth can lead to a less stress of die, which prevents metal from flowing in the gap between the metal flow and the upper die.The shaft end pressure assisted forming process was designed in the gear shaft hot forging die structure to solve the problem of forming defects of the gear shaft. The mathematical model of shaft end pressure assisted forming was established to predict the auxiliary pressure and was verified by crrosponding experiment. The relationship between the auxiliary pressure and the shaft gears with different parameters was analysised. The results showed that the more teeth number was, the smaller auxiliary pressure was needed; and the more module was, the more auxiliary pressure was needed; and the larger diameter ratio was, the smaller auxiliary pressure was required.To avoid the defects of the traditional ironing finishing technology, compressing finishing was proposed to improve the precision and surface quality of helical gear. The die of compressing finishing was designed, and the key parameters of the die were optimized. Through the research on the stress distribution of the die and the blank, it can be found that the stress imbalance between the two ends of the die results in the difference of the end face. The influence of shrinkage on the helical line is studied, and it is found that the difference in the end face reduces with the decrease of the shrinkage obviously.The theoretical model of the involute profile of gear shaft compressing finishing die was established with relationship between die and workpiece. Based on the tooth profile model and helix angle correction principle, the formula for calculating the involute and helix of the tooth die was established. The cause of the inhomogeneity of helical direction of helical tooth was analyzed by the finite element simulation. The experimental results showed that the accuracy of the gear profile reached class 8, and the accuracy of the helical line reached class 10. After finishing, the surface quality was improved obviously, and there was no difference between the two tooth surfaces, and the surface roughness reached class 8.The microstructure distribution and change of different parts of gear shaft were observed and analyzed during the combined forming process. Moreover, the relationship between the microstructure and the hardness of the tooth shape is studied. The ferrite grain height width ratio of tooth surface and center position on reference circle increased by 18.9% and 12.7% respectively. The change of hardness was consistent with the change of microstructure. and the maximum position was near the reference circle, and the hardness increases by approximately 20%. and the tooth surface hardness was significantly higher than that at the center position.The achivements of the study were applied to forming the teeth of the power-input shaft with fine surface finish, high accuracy and fine microstructure, with material utilization increased. The paper provides theoretical and experimental foundation and has a great reference value for the research of the precision forming of gear shafts. And it also provides an important guidance for the industrial application of gear shafts manufacturing by precision forming process.