Present situation of precision forging technology of spur bevel gear

At present, the forging process of straight bevel gear mainly includes the following “two fire two forging” and “one fire two open die forging process of press, open cold forging process of rotary forging press, and re moving die forging process.

1) Open die forging process of friction press

When using friction press to forge straight bevel gear with open die, one forging can not ensure that every tooth cavity can be fully filled, that is to say, the yield is low, it is necessary to cut off the flash and forge again, so the open die forging of gear should be formed by two steps of pre forging and final forging. The open die forging of spur bevel gear with friction press includes warm forging, warm forging and cold forging.

For the warm forging finishing process, the temperature drop speed is fast because of the low heating temperature, large surface area and fast heat dissipation. When the workpiece is pre forged and the flash is cut, the temperature has dropped to a very low level, and the temperature of each pre forging can not be guaranteed to be the same, and the forging force fluctuates greatly. Therefore, it is not suitable to use residual temperature final forging after pre forging and trimming, but after cooling, cleaning and reheating. The forging process is as follows: blanking, heating, roughing, pre forging, air cooling, blast cleaning, heating, final forging, air cooling, inspection and warehousing.

The above process is also known as “two fire and two forging” process, which has been proved stable by long-term production and use. However, due to one more heating process and heating equipment, the production cost is higher. For this reason, the “one heat two forging” process of warm forging and finishing of hot forging has been developed. The process includes blanking, heating, thickening, pre forging, cleaning, final forging, air cooling, inspection and warehousing.

The workpiece temperature of the final forging step of the “one heat two forging” process depends on the residual heat after the workpiece pre forging and trimming, so the initial heating temperature of the workpiece is higher than the initial heating temperature of the “two heat two forging” process. There are two functions of the pre forging process: one is to remove the oxide skin formed in the heating process, the other is to obtain the shape and size needed for the centering during pre forging. The “one fire two forging” process is less than the “two fire two forging” process by one heating, which reduces the equipment investment and production cost, and has been adopted by many domestic manufacturers. However, because the process is warm forging finishing, the fluctuation of oxidation and cooling shrinkage will affect the further improvement of gear accuracy, and the tooth surface roughness is large. For this reason, a new process of warm forging forming and cold forging fine finishing is developed. The process is as follows: blanking, heating, roughing, pre forging, all flash edges, stacking and cooling in the bin to room temperature, sandblasting, cleaning, phosphating, saponification, final forging, all flash edges, inspection and warehousing.

It is necessary to increase the tonnage of final forging press and the steps of phosphating and saponification. However, it eliminates the influence of the final forging temperature fluctuation and high temperature oxidation on the products, further improves the precision of forging straight bevel gear and reduces the tooth surface roughness.

2) Open type cold forging process of rotary forging press

Swiss type rotary forging press is a cold forming press, which can only be used for cold forging. However, the relative deformation of the straight bevel gear teeth is large during forging, and the metal work hardening is serious during cold forging, and the total deformation can not be achieved once by rotary forging, Therefore, the two-step forming process of pre forging and final forging is: blanking, annealing and softening treatment, acid pickling, alkali washing and phosphating saponification, rotary forging, annealing and softening all the flying edges, phosphating saponification, rotary forging and final forging, normalizing all the flying edges, inspection and warehousing.

Before the pre forging and final forging of cold forging straight bevel gear with rotary forging press, it must be annealed and softened, and there are three times of heating together with heat treatment normalizing. Swiss rotary forging machine is expensive, the total equipment investment is high, so the production cost is high. However, the capacity of rotary forging press is equal to that of ordinary forging press of the same tonnage, which has no vibration and low noise, and can obtain higher gear accuracy and lower tooth surface roughness. Therefore, if the general forging press open warm forging is used as the pre forging, after trimming the flash, the material box is cooled instead of annealing and softening, and then the cold forging of the rotary forging press is used as the final forging, it should be a better production process.

3) Forging process of compound moving die

The basic actions of the compound die in the forging of the compound die include closing the die and locking the die cavity, single or multiple punches entering and leaving the die cavity, opening the die and discharging the ejector pin. In essence, the punch extrudes the workpiece in the closed cavity to deform it into a preset shape. It is characterized by forging the workpiece with high dimensional and shape accuracy at one time to achieve near net shape forging. It requires that the preform volume error is small, the mold cavity surface contacting the workpiece can bear high unit pressure, and the forging process force is large. Therefore, higher requirements are put forward for equipment capacity, rigidity of equipment and tooling and strength under working condition. At present, the technology of forging straight bevel gear with compound moving die has been used in production in Russia, Japan and so on. It is still in the stage of research and development in China and has not been widely used. The forging process of the compound die forging straight bevel gear only consists of two steps: blank making and forming. The process includes blank making, less non oxidation heating, forging, cooling, inspection and warehousing.

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