Large module gear in power transmission of heavy industry (m ≥ 3) is essential. With the rapid development of heavy truck, agricultural motor vehicle and other industries, the demand for large module gears is increasing day by day. The manufacturing of spur gears is a continuous production process. Precision forging spur gears should not only ensure continuous and efficient production, but also ensure the reliability of molds. With the development of cold extrusion technology, the production of spur gears This net near forming manufacturing method is more and more used in production. After cold extrusion, the tooth surface of spur cylindrical gear is smooth, the metal streamline is continuous, and the tooth bearing capacity is high. Therefore, cold extrusion precision plastic forming process is gradually becoming the main method of spur cylindrical gear processing. Firstly, this chapter analyzes the research object and the original forming process, establishes the finite element model of the original forming process straight tooth cylindrical gear through cold extrusion, and carries out numerical simulation. By comparing with the production results, the accuracy of the numerical simulation results is verified. Finally, aiming at the shortcomings of the original forming process scheme, a new forming process scheme is proposed.
The research object is the large module spur gear in a truck wheel reducer. Figure 1 is the part drawing of the spur gear, and Figure 2 is the physical drawing of the part. Modulus M = 4, number of teeth z = 16, pressure angle α= 20 °, displacement coefficient X = 0.45. The tooth surface accuracy is required to be high, and the tooth profile and tooth direction accuracy are required to reach grade 8 accuracy specified in GB / t10095-2001.
Further design the cold extrusion drawing of the research object according to part figure 1, as shown in Figure 3. The diameter of tooth tip circle of cold extrusion part is Φ 74.2mm, the design machining allowance is 0.3mm on one side to ensure that the tooth top can be seen after passing through the outer circle; The machining allowance of the upper and lower end faces is 2.8mm in total, the machining allowance of the inner hole is 0.375mm on one side, and the two end faces and inner holes are the key and important dimensions of the spur gear, so a certain machining allowance is reserved; The tooth surface is directly formed by cold extrusion without machining, so there is no machining allowance. Figure 4 shows the annular extrusion blank. The blank is made by hot forging. After forging, spheroidization annealing treatment is carried out. The spheroidization rate is greater than 90% and the hardness is less than 150hbs. After heat treatment, the blank is made by turning. The outer surface of the ring blank needs to be turned to see light, and a small amount of invisible light is allowed for the inner hole and end face.
The original forming process scheme of the spur gear is: hot forging blank spheroidizing annealing shot blasting Turning Blank phosphating saponification cold extrusion shaving, and the material is 20CrMnTi. Cold extrusion adopts the continuous through extrusion forming process of “part pressing part”, as shown in Figure 5. The design of extrusion die is shown in Figure 6. The main process parameters of extrusion die include die tooth top fillet R and die entry angle α、 Splitting angle thickness T and sizing band length L. The cold extrusion forming of spur gear is a complex three-dimensional large deformation process. The design of die has a great impact on the forming quality and die service life of spur gear, and the process parameters restrict each other, which brings great difficulties to the selection of process parameters. According to experience, the fillet of the tooth top of the female die is 1.2mm, the die entry angle is 45 °, the thickness of the splitting angle is 2.5mm, and the length of the sizing belt is 5mm.
The cold extrusion formed spur gear is shown in Figure 7. The tooth tops of the upper and lower ends of the spur gear are not fully filled. Measure the formed spur gear with vernier caliper, the upper end is not full, the length is 1.5-2mm, the upper end face is concave about 2mm, and there are longitudinal burrs; The lower end is not full, the length is 5-7mm, the lower end face is convex about 2.5-3.5mm, the tooth top collapse angle of the lower end is more serious than that of the upper end, and the machining allowance of the upper and lower end faces is large. Although the cold extrusion has carried out appropriate machining allowance compensation for the tooth top in the design, there is still no light at the tooth top at the lower end of the spur cylindrical gear after the extrusion passes through the outer circle, which can not meet the design size requirements.
Randomly select a cold extruded spur gear and test it on the 3906 spur gear measuring center. The two important indicators of spur gear accuracy test are the total deviation of tooth profile and the total deviation of helix. Test the total deviation of tooth profile and the total deviation of helix on the left and right tooth surfaces of the four teeth selected at the approximate quartile position along the circumference of spur gear, The measurement results are shown in Figure 8. The test results of the total deviation of the tooth profile of the cold extruded spur gear are shown in Figure 2.8 (a). The minimum total deviation of the tooth profile is the left tooth surface of the 13th tooth, and the tooth profile accuracy grade is grade 6; the maximum total deviation of the tooth profile is the right tooth surface of the 13th tooth, and the tooth profile accuracy grade is grade 9. The test results of the total deviation of the helix of the cold extruded spur gear are shown in Figure 8 (b) It is shown that the helix of the four test teeth is disordered and irregular. The minimum total deviation of the helix is the left tooth surface of the 9th tooth, and the accuracy grade of the tooth direction is 9; the maximum total deviation of the helix is the left tooth surface of the 13th tooth, and the accuracy grade of the tooth direction is 11. According to the test results, the accuracy of the cold extruded spur gear is 11 ~ 12 or worse, and the precision forming is not achieved The accuracy requirements of spur gear design.