Structural optimization for rack of boat-type tractor based on sensitivity analysis

In order to promote the mechanization of cultivation in paddy field, Chinese scholars have been devoted to the study of the boat-type tractor. The hull was the main load-carry part of the Chinese traditional boat-type tractor, and modifying the shape and size over and over again based on the experience was the conventional design method of the hull. However, it was hard to ensure the parameters of the static and dynamic property selected from a series of variable ranges were the optimal ones. In order to achieve superior static and dynamic performance of the boat-type tractor, the multi-objective optimization design was conducted on the established separable boat-type tractor rack. The supporting device of the established boat-type tractor was the rack, and the transmission was mounted on it. The transmission was powered by the engine, and then power was output to the gearbox and transfer case through the propeller shaft and finally to the driven wheels. Therefore the rack played an important role in guaranteeing the reliability and stability of the whole machine. First, pre-processing was operated; we extracted the middle surface from the three-dimensional (3D) model and assigned shell units. Full constraint was applied on the rack where the wheels were connected, and after this the finite element analysis of the constraint model was conducted on the rack. Through the modal analysis we found that the first-order modal frequency of the frame was close to the engine working frequency, and therefore we determined the first-order modal frequency as the optimized response. Due to the complexity of the rack structure, we analyzed the sensibility of thickness to the first-order modal frequency, and tank bracket, Beam 1, Beam 2, gearbox bracket, Beam 3, Beam 5 and main beam were defined as the design variables. Besides, we took the minimum volume as the optimization goal. Then, the multi-objective optimization design model was established and 6 groups of non-inferior solutions were obtained. In order to select the optimal design scheme from 6 groups of non-inferior solutions, we composed complex fuzzy matter-element matrix of characteristics and their corresponding values in the optimized system. After normalizing the matrix, the membership matrix was acquired. This matrix could help us determine the entropy weight of the evaluation indices, and thus we could get the advantages and disadvantages of each non-inferior solution. After obtaining the optimal solution, we redesigned the rack and carried out the modal analysis, and the statics check was also conducted on it. Comparing the results before and after optimization, we found that the first-order modal frequency increased from 25.78 to 34.84 Hz whose growth rate was more than 35%, the maximum static stress decreased from 51.18 to 37.75 MPa, and moreover the working frequency of the engine was away from the natural frequency of the boat-type tractor rack. The most important thing was that at the time we obtained the excellent performance of the rack the volume of the rack only increased by 2.4%. The result showed that the rack of the boat-type tractor optimized based on the fuzzy matter-element model and coefficient of entropy had good dynamic performance. The research provides theoretical and technical references for the design of the new boat-type tractor. © 2016, Editorial Department of the Transactions of the Chinese Society of Agricultural Engineering. All right reserved.