Research On Coordinated Matching Design Method Of Main Bearing Assembly Structure Based On Improved NSGA-?

Power density improvement is an inevitable trend in the development of a new generation of power.As one of the most important loaded assembly structures of diesel engine,the main bearing assembly structure is mainly composed of the engine block,the main bearing cap,the main bearing bush,the crankshaft and a number of connecting bolts.Its reliability problems such as the increase of crankshaft and main bearing coupling deformation and typical parts strength failure caused by the increase of power density play non-negligible impacts on the reliability of the whole engine.For the coordinated matching design of the main bearing assembly structure,"coordinated"is the design goal,which means the main bearing assembly structure presents comprehensive and optimal harmonious state under the action of external loads,that is,strength coordinated(increasing the strength safety factors of the engine block and the main bearing cap),deformation coordinated(reducing the main bearing bush deformation),contact strength coordinated(increasing the contact strength safety factor of surface between the engine block and the main bearing cap)and mass coordinated(mass controllable)."Matching"is the design method,which refers to the process of optimizing the performance of all aspects of the assembly structure by developing the coordination evaluation criteria of strength,deformation,contact strength and mass,and introducing multi-objective optimization theory to make the main bearing assembly structure reach the comprehensive optimum.Starting from the system level of the main bearing assembly structure,using the simulation analysis method,combined with the experimental verification,the research on the coordinated matching design method of the main bearing composite structure is carried out with the matching goals of strength,stiffness,contact strength and mass.The evaluation method of deformation coordination of main bearing was discussed.The influence laws of reliability of key components were revealed.And the traditional NSGA-II algorithm was improved.The main research contents are as follows:The establishment of the finite element model of the main bearing assembly structure is the basis of the research of this thesis.To take into account both the calculation cost and the calculation accuracy,two simplified models that considering the local stiffeners and ignoring the local stiffeners were proposed.Based on the simplified scheme,the local parameterized finite element model of the main bearing assembly structure is established.The verification experiments of strength and contact strength were designed and carried out to verify the calculation accuracy of the model.This work provided a model foundation for the research on the coordinated matching design method of the main bearing assembly structure.The research on the characterization method and calculation method of deformation coordination is the most complicated among the four coordinated factors of the main bearing assembly structure.Aiming at the problem that the traditional deformation evaluation indicators are difficult to judge whether there is movement interference between the main bearing bush and its mating pair,the maximum radial deformation of the main bearing bush was proposed as an evaluation index to characterize the deformation coordination of the main bearing.The definition of the maximum radial deformation of the main bearing bush was given.And the calculation method of the maximum radial deformation was discussed.The verification experiment of the maximum radial deformation is designed and carried out,which verifies the correctness of the evaluation method of the deformation coordination of the main bearing.The research on the influence laws of the reliability of the key components of the main bearing composite structure is the basis of the coordinated matching design.The sensitivity of the maximum radial deformation of the main bearing bush to the design parameters was analyzed based on the orthogonal experiment.The typical design parameters of the main bearing assembly structure in the influencing factors of bolt load,structural dimension and assembly tolerance were determined.The influence laws and mechanisms on typical design parameters on the strength,stiffness and contact strength coordination of the key components were explored.The NSGA-II multi-objective optimization genetic algorithm is the main solution method used in the coordinated matching design of the main bearing assembly structure.To address the limitations of low computational efficiency and the evolutionary process of the algorithm at the later stage affecting the distribution of Pareto optimal solutions in the original NSGA-II,an improved NSGA-II was proposed based on FSCS-ART algorithm and the hybrid adaptive strategy.It has been verified that the difference between the dispersion degreeM_Dispersionand the average distance of the nearest neighbor M_avg of the individuals generated by the improved method and traditional method are 0.1511 and 1.2916,respectively,indicating that the individuals generated by the improved method are more evenly distributed,which is conducive to enriching the diversity of the population.Besides,the average fitness of the improved algorithm is obviously closer to the maximum fitness curve than the average fitness of the traditional algorithm in late stage.It indicated that the calculation convergence of the improved algorithm is better,and it is more conducive to improve the calculation efficiency of coordinated matching design.Based on the above researches,aiming at the problem of unsatisfactory coordination among the inspection elements,the evaluation criteria for the coordination of the main bearing assembly structure were proposed based on AHP(Analytic Hierarchy Process).The mathematical model of multi-objective optimization of main bearing assembly structure was constructed by combining the research on the evaluation method of main bearing deformation coordination and the research on influence laws of reliability of key components.The improved NSGA-II was employed to solve the mathematical model,and the coordinated matching design scheme was determined.A coordinated matching design method of main bearing assembly structure was developed.Since then,a new way has been opened for the coordinated matching design of complex structures.And a series of conclusions and results with theoretical significance and practical engineering value were obtained.After optimization,the strength coordination of the assembly structure is increased by 13.91%,the deformation coordination is increased by 14.96%,the contact strength coordination is increased by 2.63%,the mass slightly increases but still meeting the lightweight design requirements,the overall coordination of the main bearing assembly structure is improved by 11.10%.The coordinated matching design goal of the main bearing assembly structure is achieved.Finally,the improved structures of the main bearing bush and main bearing cap were processed according to some optimized design parameters.The measurement experiment of the radial deformation of the main bearing,the first principal stress measurement experiment of the strength danger zone and the maximum contact pressure measurement experiment of the dangerous contact surface were designed and carried out.By comparing the experimental results of the original model and the improved model,the effectiveness of the coordinated matching design method of the main bearing assembly structure was verified.