| T-section rails are produced by cutting machining in which bending straightening is the last process. In order to improve rail straightness and accuracy, it is also an essential part in the production route of a rail. The development of precision automatic straightening equipment is to stimulate the study on press straightening theory inevitably, and the theoretical research on straightening technology must be on the basis of elastic-plastic bending and springback mechanism. Therefore, this paper studies the bending springback characteristics of T-section rails based on elastic-plastic theory, experimental research and finite element simulation method respectively, then has a systematic research on the anti-bending rail straightening process.The main contents of this thesis are as follows:Chapter1introduces the scientific background, purpose of this subject and the state-of-art concerning the press bending and straightening process. Besides, the structure and contents of this paper are elaborated in this chapter.For the material hardening directly affects the accuracy of bending deformation calculation, chapter2studies the bending deformation of T-section rails, considering the work-hardening materials. The analytical formulas for the springback and residual curvatures are given based on elastic-plastic theory. The three-point bending experiments are also carried out on the universal testing machine. By comparing the results of theoretical analysis and experimental data, the influence of the changes in material models and parameters on rail bending deformation calculation and precision straightening is explored.The model of load-deflection relationship can not only reflect the whole elastic-plastic deformation of the rail bending process(plastic deformation, elastic-plastic deformation and elastic rebound after unloading), but also provide a new method for straightening calculation, which can be applied directly in the control system for press bending straightening. So chapter3studies the load-deflection relations of T-section rails based on elastic-plastic theory. The analytical expressions for the load-deflection relations in the loading process are given. To verify the model, experiments are carried out respectively on a universal testing machine and a self-made hydraulic straightening machine.Chapter4focuses on the experiment study and the finite element analysis of T-section rail bending process and combines both together. First, the prediction model of springback is developed using artificial neural network approach. The relationships between the loading stroke and springback stroke under certain initial deflection of a rail are analyzed. Then numerical simulations using finite element method are also performed to investigate the effect of material properties on springback. A neural network for identification of material parameters is developed by experiment and simulation data. At last, the finite element model is validated with experimental results of springback. For the T-section elevator guide raid is a long rail with high precision, which bending shape is often complex with two more curvatures that need to be straightened for two more steps, in chapter5, the study of rail bending process is further expanded to multi-step bending and straightening process. A model of bending deformation in the section-by-section bending process of a long guide rail is proposed. To achieve the purpose of section-by-section straightening, a calculation method for the loading stroke is introduced.Chapter6studies the residual stresses that occur in a T-section rail bending process, considering the special cross section of the rail that is symmetrical in horizontal direction but nonsymmetrical in vertical direction. The elastic-plastic bending theory is used to analyze the residual stress distributions of the rail and the analytical expressions are obtained. The possibilities of reverse yielding during the step of unloading are also analyzed.Chapter7developed an automatic control system for press bending straightening. Many kinds of technologies such as mechanical, measurement, automatic control, computer software and so on are integrated in this system. Then based on the elastic-plastic bending theory, the load-deflection model of rail bending and the experimental data respectively, the chapter summarizes three methods to calculate the loading stroke for straightening, for integration into the developed control system. |
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