Manufacturing technology of bevel gear for the hot

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Abstract: the difficulty of bevel gear processing lies in the processing of teeth and the control of heat treatment deformation. For bevel gears with low accuracy requirements, the teeth can be processed on ordinary milling machines. Zheng Daqing, global senior vice president of BASF Greater China business and market development, added that through the analysis of the manufacturing process of a typical bevel gear on the concrete trailer pump, this paper introduces the tooth processing process and heat treatment process of the gear in detail

key words: bevel gear milling, normalizing, carburizing, quenching and annealing process

there is a paired bevel gear on the supporting wheel of concrete drag pump. The structure of one bevel gear is shown in Figure 1, and the material is 20CrMnTi. The main technical parameters of the gear are: the modulus of the big end face is 4, the number of teeth is 20, the tooth shape angle is 20 °, the tangential displacement coefficient is 0, the radial displacement coefficient is -0.27, the big end tooth height is 0.8mm, and the accuracy grade is 8CB gb11365, Root cone angle and other parameters are shown in Figure 1. Since the hardness of the tooth surface is required to be hrc56~64, the gear needs to be carburized and quenched, and the depth of the carburizing layer is required to be 0 2mm。 From the technical requirements and accuracy levels in the figure, the machining accuracy level of the gear is not high. To ensure the machining quality of the part, the key is to solve the process links of bevel gear machining and heat treatment. Because the part belongs to the trial production stage, in order to reduce the processing cost, we decided to use the formed gear milling cutter to process the bevel gear on the horizontal milling machine, And through the formulation of reasonable heat treatment process methods and parameters to ensure the processing quality of the gear

Figure 1 part structure diagram

1 bevel gear processing process and difficulty control

in order to ensure that the final heat treatment of the gear can reduce the deformation and obtain uniform structure and better mechanical properties, before machining, carry out high-temperature normalizing pretreatment on the gear blank, which can refine the structure, reduce the deformation of the parts during carburization and quenching, and improve the cutting performance at the same time. After normalizing, turn to machining. First, rough and finish the excircle and end face of the gear blank on the lathe, then mill the gear on the horizontal milling machine, and then insert the keyway on the slotting machine. After all machining is completed, carburize and quench and low-temperature tempering are carried out, and finally carry out post-processing. The process flow of the part is: normalizing → turning → milling → keyway → carburizing, quenching and annealing → sand blasting → rust prevention treatment. The process difficulties are gear milling and heat treatment

2 machining principle of gear milling method with formed gear milling cutter

machining straight bevel gears with formed gear milling cutter belongs to approximate forming method. Because the umbrella shape of spur bevel gear should be spherical involute in theory. Because the spherical surface cannot be expanded into a plane, the equivalent involute tooth profile on the back cone expansion surface is approximately used to design the tooth profile of the forming milling cutter. Because the tooth profile of spur bevel gear shrinks to the top of the cone, the equivalent involute tooth profile curvature on the developed surface of each back cone along the pitch cone bus is different. Therefore, the tooth profile of the milling cutter for spur bevel gears can only be designed according to the equivalent involute tooth profile on a back cone unfolding surface on the pitch cone bus, and the processed gear tooth profile can only be more accurate in a certain section, while there are certain errors in other sections. Therefore, the straight bevel gear forming milling cutter is suitable for rough machining and machining low precision straight bevel gears

3 gear milling method and process

Fig. 2 gear clamping

on the ordinary horizontal milling machine, the indexing head with rotary table is used for indexing. The specific method is to fix the gear blank on the dividing head, as shown in Figure 2. Adjust the inclination of the dividing head and pull the dividing head up to an angle, which is equal to the bevel angle of the bevel gear root δ f. Use the cutting mark method to align the symmetrical line of the milling cutter profile with the center of the gear blank, adjust the cutting depth, take the big end as the benchmark, raise the workbench by one big end full tooth height, and then mill the middle of the tooth slot first. Because the thickness of the straight bevel gear forming milling cutter used at this time is designed according to the tooth slot width slightly smaller than the small end of the bevel gear, so after the middle of the tooth slot is milled, there is a certain machining allowance at the big end and two sides of the gear, so, The above parts should also be processed. At this time, rotate the indexing head base in the horizontal plane for one time α Angle, so that the symmetrical line of the milling cutter profile is at an angle with the axis of the gear blank. At the same time, move the horizontal workbench appropriately (the movement can be determined by trial cutting). First mill the allowance on the side of the tooth groove (Fig. 3), and then rotate the base of the dividing head in reverse 2 α Angle, the horizontal workbench also moves in the opposite direction, milling the allowance on the other side of the tooth groove. Horizontal rotation angle of indexing head base α The calculation formula of is as follows:

where: m - module of the processed gear (mm)

R - pitch of the processed gear (mm)

substitute the relevant parameters of the gear into the above formula to calculate α After the angle, rotate the base of the dividing head according to the angle

Figure 3 Schematic diagram of tooth machining

4 tool selection

from the structure and technical parameters of the gear, the bevel gear belongs to the standard structure. Generally, the tooth shape of the standard straight bevel gear forming milling cutter is designed according to the large end back cone equivalent involute tooth shape, while the tooth thickness is designed according to the small end, and a certain fine cutting allowance is considered. Generally, there are 3, 8, 15 and 26 straight bevel gear forming milling cutters, and the tool number should be selected according to the equivalent number of teeth of the bevel gear. Equivalent number of teeth zv can be calculated according to the following formula:

where: Z - number of teeth of straight bevel gear

δ—— Pitch cone angle of straight bevel gear

according to the calculated equivalent number of teeth, the cutter can be selected reasonably through the selection table of standard straight bevel gear forming milling cutter number

5 heat treatment process method and process

as mentioned above, the gear blank is subject to overall high-temperature normalizing treatment before machining. The normalizing equipment is RT box furnace. The specific process is that when the furnace temperature rises to the charging temperature ℃, put the gear into the furnace. If the parts need to be stacked during loading, the stacking height should not exceed two layers, and then heat the parts to ℃, so that the structure changes completely. After holding for minutes, air cooling can be used. The normalizing process diagram is shown in figure 4:

Figure 4 normalizing process diagram

because the accuracy requirement of the gear is not very high, in order to reduce the cost of carburizing and quenching, the process method of direct quenching after carburizing can be adopted. The specific method is to put the parts into the carburizing basket without superposition. When the furnace temperature rises to 840 ℃, drop aviation kerosene. The dropping speed of kerosene is d/min. When the furnace temperature rises to 930 ℃, the parts begin to be loaded into the furnace. After the end of the exhaust period, put two test rods in. When the diffusion time of one test rod is 120 min, take out the quenching oil for cooling and inspect the depth of the hardening layer. Decide whether to cool down according to the inspection results. If the depth of the carburized layer is insufficient, the diffusion period will be extended to 150min. Carburization can be carried out according to the parameters specified in Table 1. In order to control the carbon potential concentration during carburization, the method of adding oxygen sensors to the "wind light" model (source: mobaiguan) made of Dow polyurethane in the carburizing furnace can be used for monitoring and control. The oxygen sensors convert the atmosphere state into potential difference through zirconia probe. The above difference is compared and calculated by the control system, Determine whether the concentration of carbon potential is appropriate, and automatically optimize and adjust it, so that the control accuracy of carbon potential can reach ≤± 0.05% C, which can effectively control the depth of carburized layer and improve the quality of carburized layer, so as to ensure that there is no massive and shaped cementite. RQ well type carburizing furnace is used for carburizing equipment. After the carburizing homogenization period, the parts are directly quenched after being discharged from the furnace. The quenching equipment adopts a special constant temperature oil bath, and the quenching medium is a special isothermal graded quenching oil. The medium temperature is constant between ℃. After cooling for 30min, it is discharged from the furnace for air cooling. Carburizing treatment parameter table

in order to eliminate the residual stress of quenched parts and prevent the generation of quenching cracks, quenched parts should be tempered at low temperature as soon as possible. The specific process method is that when the furnace temperature rises to about 160 ℃, the parts begin to be loaded into the furnace, and after holding for min, they are discharged from the furnace for air cooling. The tempering equipment is RJJ tempering furnace. Quenching and tempering process diagram is shown in figure 5:

Figure 5 quenching and tempering process diagram

6 post sequence processing

put the heat-treated parts on the rotary table of the sandblasting machine for sandblasting, select quartz sand computer to receive data, process it, and draw the particle size of the curve. During sandblasting, always turn the parts to make the sandblasting effect on all sides uniform. After the parts are sandblasted, soak them in antirust solution for min, and then dry them in the air. It should be noted that the antirust treatment must be completed within 24 hours after sand blasting

7 effect

the above method is simple and practical, and the processing of spur bevel gears can be realized without special equipment. It is economical and low cost. After testing the processed bevel gear, the size and shape and position accuracy meet the requirements, and the carburized layer depth is 1 3mm, the metallographic structure of the tooth is a single strip tempered martensite structure, and the hardness is HRC. The core hardness is HRC, which can fully meet the quality requirements of the part. (end)

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