# Optimization design of cold extrusion combined die for spur gear used in truck

Improving the service life of cold extrusion die is the key point of precision forming of large modulus spur gear. The traditional design method of cold extrusion combined die is to determine the size parameters by combining Latin American formula and empirical formula, but it is not suitable for the die with tooth shape in the inner ring. According to the analysis in the third chapter, the unit pressure acting on the inner wall of the cold extrusion die is very large. If the single-layer integral tooth die is used, it is likely to crack during operation, and simply increasing the wall thickness of the die can not improve the strength of the tooth die to avoid cracking. A hard die core with interference is pressed into a ductile ring to produce prestress. This combined die structure is a common and effective solution to solve die cracking.

In order to reduce the maximum equivalent stress at the inner wall of the die core, the structural parameters of the cold extrusion combined die for spur gear were optimized. The Kriging model of the structural parameters of the combined die and the maximum equivalent stress of the die core is constructed by the combination of experimental design and numerical simulation. The particle swarm optimization algorithm is used to globally optimize it, and the optimal diameter ratio and interference coefficient of the cold extrusion combined die are obtained and applied to the production practice.

The traditional design of combined die for cold extrusion of spur gear mostly adopts the formula derived from thick wall cylinder theory for conservative estimation and correction according to practical experience, but it is not applicable to the design of combined die for cold extrusion of large modulus spur gear with complex cavity shape. Therefore, aiming at the problem that the overall structure of the cold extrusion combined die for spur gear is too large when using the traditional empirical method, a new optimization design method of combined die is proposed in this chapter. The details can be summarized as follows:

① The finite element model for stress analysis of combined die in the stable forming stage of spur gear is established. The numerical simulation results show that the combined die designed by empirical method can meet the requirements of process design under preload and working state. However, due to the conservative selection of structural size and interference of combined die, the die core is in a state of great compressive stress, Although it can ensure normal operation, the die structure is too large, resulting in the waste of expensive die materials.

② In order to reduce the equivalent stress on the inner wall of the die core, the structural parameters of the cold extrusion combined die for spur gear were optimized. The Kriging model is used to establish the approximate model between the structural parameters of the combined die and the maximum equivalent stress of the die core, and the particle swarm optimization algorithm is used to find the optimal solution in the feasible solution space to obtain the optimal diameter ratio and interference coefficient of the combined die.

③ By comparing the structural parameters and numerical results of the combined die designed by the empirical method and the optimal design, it is found that the size of the combined die designed by the optimal design is less than the design size of the empirical method, and the overall size after optimization is only 68.75% of the design size of the empirical method. In the pre tightening and working states, the maximum equivalent stress of the inner wall of the optimized mold core is less than the empirical method, and there is no circumferential tensile stress in both States, which can avoid the failure of cemented carbide mold core due to tensile stress.

④ The optimized combined die is used for actual production, and the service life of the die is about 150000 pieces. Compared with the combined die designed by the original empirical method, the service life remains unchanged, and the die keeps teeth and does not crack during use. The optimized combined die can effectively reduce the overall size of the combined die and save die materials. It provides a quantitative method and basis for the design of cold extrusion combined die for spur gear parts.

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