| A hydraulic Press is the general purpose forging machine. It is an important equipment in modern industry. Lower beam is the key component of a hydraulic press. Its deformation and other behavior affect the quality of products directly. Because of complex of the hydraulic press structure, it is can not get enough accuracy and reliability of the calculation to apply the traditional methods such as mechanics of materials. Although the finite element method applied in structural analysis provides powerful analytical means, the existing researches are confined to the structural analysis and size optimization only. Without structural topology optimization, it is difficult to achieve a through improvement to the original structure topology, so does the structure optimization.Evolutionary structural optimization (ESO), proposed by Xie Y.M and Steven G.P in 1993, applies the thought of biological evolution in structural topology optimization. In the optimization process the material units which are little contribution to the stiffness of the structure will be gradually removed, and the criteria of full stress is used to the design of structural topology optimization. The concept of ESO is simple and easy to carry out, specially, it is easy to fulfill that ESO integrates with the existing finite element software. The researchers at home and abroad interest ESO since it born. The subsequent development of BESO has solved the problem of the traditional ESO can not deal with. After more than 10 years of development, ESO has achieved many important results. A large number of published examples have proved the effectiveness of the algorithm.At present, the research on ESO methods are biased theory fields. The ESO theory and methods are still in the continuous development. But the engineering application of ESO is little. Many problems are still waiting to be solved in BESO, for example, too many parameters needed in evolution process. Because of those, the practical application of ESO has been restricted. Exploring new algorithm, new evolutionary rule and new application areas for ESO are the difficulties and focus of ESO research. In this study, the criteria in the process of optimization have been explored, and a new BESO method based on ratio of stress change is introduced. The ratio of stress change in the structures between the material removals is applied as the rule of element addition. Algorithm employs fewer parameters than other BESO methods and can easily evolve to the optimal topology of elastic structures. The example given shows that the method can obtain clear profile of topology and global optimal solution with higher reliability and efficiency. It is the further development and improved of traditional ESO method.As an example of the ESO methods in practical engineering application, the structure of a hydraulic press lower beam has been optimized applying ESO and the BESO which is given in this paper. Starting from the topology structure, according to the best stress transmitting course, the optimal structure of the lower beam is explored thoroughly. It is a useful exploration for the practical application of ESO. A more reasonable topology structure has been achieved, improving the performance of hydraulic press and raising China's hydraulic press design level. The results of the research are valuable for the practical application of engineering, and provide a new thought of design for the hydraulic press lower beam and other three-dimensional structures.
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