Development of precision forging technology for ho

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Development of gear precision forging technology

three kinds of automotive gears such as cylindrical spur gear, cylindrical helical gear and synchronous gear have been studied and tested. The manufacturing process adopts warm forging and cooling treatment, and uses finite element simulation to analyze the forging process and design the die, so as to ensure the accuracy of the gear. After three years of research, we have mastered its basic technology, and the next step will be factory field test

key words: gear; Precision forging; Computer simulation; Die design and manufacturing

1 Introduction

gear precision forging has developed greatly in recent decades. More and more manufacturers and users attach importance to manufacturing gears by forging. It is generally believed that forging can improve the utilization rate of materials, improve productivity, improve the mechanical properties of gears, reduce costs and enhance market competitiveness. Especially for large-scale production in the automotive industry, gear precision forging has greater benefits and potential [1 ~ 3]

although gear precision forging has many advantages and has been used in the gauge of bevel gears, we will have a leading market position and good industrial dynamic die production, but there is still a long way to go before it is applied to the large-scale production of cylindrical spur gears and helical gears of a certain size. Especially for gears used in automobile power transmission, a set of practical and reliable production process needs to be established before they can be accepted by manufacturers

gear precision forging technology originated from Germany. As early as the 1950s, due to the lack of sufficient gear processing machine tools, the Germans began to trial produce bevel gears by closed hot die forging. The main feature is that a very new EDM process was used to manufacture the cavity of the forging die. In addition, the forging process is strictly controlled. On this basis, gear forging technology is further applied to the production of spiral bevel gears and cylindrical gears. However, in the forging of cylindrical gears, because the plastic flow direction of metal materials is perpendicular to its stress direction, its tooth shape is more difficult to form than that of bevel gears. The Forging Research of cylindrical gears began in the 1960s and developed greatly in the 1970s, mainly due to the pressure from the automotive industry to reduce costs. By the 1980s, the forging technology was more mature, which could achieve higher accuracy and consistency, so that the forged gears could be accurately positioned on the flow production line and suitable for mass production

The purpose of gear precision forging is to directly produce gears that do not need subsequent machining. If it can be forged at room temperature, the shape and size of the gear can be easily controlled, and the error caused by high temperature can be avoided. At present, many bevel gears and small cylindrical gears have been made by this method. When the overall size is suitable, the cold extrusion process can also be used to manufacture cylindrical straight and helical gears. However, most gears used in automobile transmission have large diameter and height, so it is not suitable to use extrusion technology. If closed die forging is used, it requires high pressure to make the metal material flow and fill the mold cavity, so this kind of gear needs to adopt hot forging or warm forging process. High temperature will bring about material oxidation, mold distortion, and affect the accuracy and surface quality of forgings. It is difficult to correct these errors with additional machining, and the cost should be increased. Especially when the subsequent grinding process is used to correct the error on the tooth profile, in addition to increasing the cost and prolonging the working hours, there is also the problem of gear positioning in the grinding process

at present, the generally agreed process is the combination of hot forging, warm forging and cold forging. Hot forging and warm forging can achieve high efficiency and high utilization of materials, while cold forging process can correct the errors of hot and warm forging process and improve surface quality. At the same time, the cold treatment process can also obtain residual compressive stress on the surface of gear teeth and improve the service life of gears

during my work at the University of Birmingham, the College of mechanical engineering where I worked has just completed a three-year research project funded by the British Engineering Scientific Research Association (EPSRC) and cooperated with seven British enterprises (gear manufacturing, mold manufacturing, gear users, forging mills and steel companies): precision forging of spur and helical gears. On the basis of years of research and practice, the project further discusses the mechanism of gear forging, and uses modern analysis means, such as computer simulation and design technology, in order to develop a productive and economically feasible forging processing technology to manufacture precision gears that no longer need subsequent processing on the tooth profile. The project studied and tested three kinds of gears: cylindrical spur gear, cylindrical helical gear and synchronous gear. Considering the economy of the whole process, precision forging is limited to the contour part, while the tooth end and inner hole are machined. The manufacturing process is warm forging and cooling. Gears meeting the shape requirements are obtained by warm forging, and an allowance of about 011mm is left in the contour. In the process of cold treatment, the warm forged gear is extruded through a precisely designed and manufactured die, so as to correct the error of the contour part and obtain a high-precision tooth surface. In the research process, the finite element method is used to analyze the forging process and design the die, so as to ensure the accuracy of the gear. After three years of research, we have mastered its basic technology, and the next step will be factory field test. At the same time, it is preparing to apply for the second phase of the project

2 warm forging process

as the project requires to find a practical production way suitable for the factory, a high-speed, single acting single crank mechanical press is selected for this study. Because the forgings are heated, the thermal expansion and cold shrinkage of the material and the deformation of the die must be considered. Therefore, the finite element method is used for accurate calculation. In addition, the forging process is simulated by finite element method to ensure the accuracy of forgings. The experiment shows that the error of forging steel gears between 850 ℃ and 950 ℃ can be controlled within the range of 0105mm

hollow cylindrical blanks are often used for forging hollow axisymmetric parts or gears. Figure 1 shows the design of a closed die forging for cylindrical spur gears. The right side of the figure shows the situation before forging, and the left side shows the situation after forging. The die is composed of an upper die (punch), a lower die (reverse punch), a mandrel and a contour cavity (as shown in the figure). The die cavity is supported by springs. During the forging process, the punch moves downward with the press slider and drives the die cavity downward. Because the punch only needs to seal the upper surface of the cavity without pressing into the cavity, the punch can be made into a simple shape. In this design, the punch is in the shape of a stepped cylinder. The reverse punch keeps stationary during the forging process and pushes the gear out of the cavity after forging. The mandrel is connected with the punch here to help the positioning of the blank. Since the cavity moves with the forging in the forging process, the friction between the cavity and the forging will help the metal flow, and the required load is also lower than that when the cavity is fixed

in the forging process, the basic function of the die is to make the parts form correctly. For this kind of gear forging die design, literature [4, 5] has carried out extensive and in-depth discussion. Due to the limitations of the types of forgings and equipment, there are many combinations of dies. Literature [4] has extensively studied the influence of die structure, die design and equipment on forging accuracy in general precision forging

the shape of forgings is not only affected by high temperature thermal expansion, but also related to the elastic deformation of the die, which is related to load and radial pressure. In gear forging, the corners of gear teeth are finally formed. It is in this final filling stage that the load rises sharply. The example in [6] shows that the last 013mm stroke of the punch (112% of the total deformation) will lead to an increase of 50% of the load. It can be considered from the mold design to reduce the load. For example, the introduction of chamfer can make the metal easy to flow into the upper and lower corners of the gear teeth; The resulting end allowance can be easily removed in the subsequent cutting process. In this way, the distortion of the die is reduced, the service life is prolonged, and the accuracy of the forging is improved. For the example of cylindrical spur gear involved in this paper, document [5] analyzes several possible die design schemes in detail: such as fixing the die cavity, chamfering the punch and punch, and using finite element analysis to analyze the flow of metal, the formation of gear teeth and the change of forging load under various circumstances. The influence of friction on deformation and load under various conditions is also discussed

warm forged cylindrical spur gears, helical gears and synchronous gears on this equipment. During warm forging, the blank is heated to 900 ℃ and the die is heated to 200 ℃, and water-based graphite is used as lubricant. Figure 4 shows the measurement results of the cylindrical spur gear after warm forging. It can be seen that the tooth shape of the gear is consistent, and the contour has an allowance of 0108mm ~ 011mm, so that it can be corrected in the subsequent cold treatment process. Since the temperature is controlled at about 900 ℃, the surface quality of gear teeth is also high, RA is about 3 m

3 cold treatment process

as the first stage of gear precision forging, hot and warm forging process is relatively easy to control, because the forged gear has a certain margin that can be adjusted. The cold forming process requires quite high accuracy. When the forged gear is extruded through the cold forming die, the shape should meet the final requirements without further processing. For gear teeth, the accuracy of the profile is required to be about 10 m, which puts forward very high requirements for the design of the die

in the gear cold forming (finishing) process, the gear is gradually squeezed through the cold forming die, 1 Add shims in the fixture; The internal stress and deformation of the die are related to the position of the gear. In particular, the change of radial pressure will determine the deformation of the die. If the appropriate die shape can be designed, the deformation of the die can be used. For example, when the gear is at the inlet and outlet of the mold, the force on the mold is relatively small, so the deformation is relatively small. When the gear is in the middle of the mold, the deformation of the mold is relatively large. This feature may be used to obtain gear teeth with a drum shape. Literature [6] has made theoretical analysis and Experimental Research on drum formation with angular hollow cylindrical parts

finite element method is used in gear cold forming (finishing) process to analyze die deformation, gear deformation and springback. The influence of the allowance of forged gear, die shape, size and structure on the final product is considered. Figure 5 is a finite model used to analyze the cold forming process of cylindrical spur gears. The deformation of the punch has little effect on the shape of the gear, so it can be treated as a rigid body in the finite element model. The deformation of the mold cavity directly affects the shape and size of the contour, so the mold is simulated as a deformed body. In the process of cold treatment, only the surface of the gear tooth is plastic deformed, and the interior of the gear tooth and most areas of the gear tooth are in the state of elastic variable cost, which is relatively lower than the holding mode to obtain mineral resources. The proportion of elastic recovery is very large, and the final shape of the contour can only be predicted by using the elastic-plastic material model

die for cold forming of cylindrical spur gears. The punch is integrated with the mandrel and installed on the upper slider of the press. There is a small gap between the mandrel and the forged gear hole, which is not used for positioning, but only for guiding. Because in the forging process, only the accuracy of gear teeth can be guaranteed, but the accuracy of inner holes cannot be guaranteed. The gear enters the mold cavity by the chamfer of the mold, mainly power lithium-ion batteries and supercapacitors

if it is used for the finishing of helical gears, the gears will rotate during the extrusion process. It is necessary to install a rolling bearing between the punch and the press, so that the punch can rotate with it. In addition, when extruding the helical gear, the forces on both sides of the gear teeth are asymmetric, and the deformation is also different. The gear can be extruded again in the opposite direction. In fact, spur gears can also be used in two ways

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