Analysis And Improvement About Cracked Mainbeam Of JS6105H Frame

In the design process of new bus models, it is a very important job to judge the reasonableness and static, dynamic performance of the chassis frame structure, then to optimization it.Because of the complexity of the chassis frame structure, it is difficult to get optimal solution by the way of classical mechanics. In the forepart of frame design, the introduction of finite element analysis techniques can not only reduce the intensity cushion design of the component, but also greatly increase the reliability of the chassis frame.Based on market demand, we developed the JS6105H bus, it adopt engine postposition, back suspend is longer, and the force of the back of the chassis frame is larger. Unreasonable for the local structure of the original chassis frame structure, some of the vehicle appeared cracks on the back of the beam. So, the design to improve the structure has been done on the cracking part of the crossbeam and carling, and finite element analysis methods was used to improve the results evaluated.This paper describes a complete modeling, material parameter setting, mesh, load imposed, the process of restraint and coupling setting:Frame modeling: we will finish the frame 3D modeling with UG software developed by USA, then output iges format file, then input it to ANSYS software and mesh the model used shell element SHELL63, through connections to the various components of the overall model assembled into finite element model.Body modeling: for the body frame mainly formed of rectangular steel tube, we select beam element BEAM4. simplify and extract nodes coordinates based on 2D drawings, complete modeling directly in the pre-processing module of ANSYS software, input the coordinates directly to create the nodes, then connect nodes to generate unit. Suspension simplified: adopt vertical-type leaf spring independent suspension, simplify steel bracket to vertical beam, simplify leaf spring to the level flexible beam, rigid unit and BEAM4 elements will be used to simulate rigid beam and flexible Beam.Restriction setting: four bearing points of Finite Element Analysis model was taken in the point correspond to the four actual support points of the leaf spring, in the overall coordinates, the X, Y, Z freedom of the supporting points on right anterior leaf spring, in the same way, bound the right rear leaf spring supports Y,Z freedom, the left front point leaf spring supports X, Z freedom, and freedom of Z supports on left rear leaf spring.In order to get a accurate contrast of the change of the stress of the structure before and after frame crack, the body finite element modeling is made the same to the suspension finite element modeling, as well as the stress and restrictions. Only the frame structure is slightly changed. The results are as follows.Under the analysis of vehicle loaded with vertical deflection ,There is only a slightly change of the finite element model of leaf spring and it' s fully consistent with the actual situation, which demonstrates that finite element modeling, loading, the constraint can get a accurate simulation of the actual status.The frame is under low-stress state under the circumstances of full loading and static state before the improving design. The maximum Von-Mises equal stress is 162MPa in the right frame of the upper and lower beam bending outside corner, which is fully consistent with the beam crack. However, the corresponding part - the left beam only got a stress of 78MPa. The conjunction of inside beam and carling got a stress of 122MPa.After improving design, the original outside beam bending stress downgraded to 33-50MPa and the conjunction of inside beam and carling downgraded to 62-64MPa. The stress of the left and right carling is basically symmetrical. According to the calculations, vibration frequency declined slightly after improving design. The reason for the decline of the vibration frequency is the decrease of torsional stress as a result of increasing flexibility within the lower beam joints beams, and meanwhile reduces the torsional stiffness of the frame here.By the improving design of the cracking part of the beam, stress effects decreased significantly under the circumstances of slight increase material and working hours, particularly in the high-stress regions of the original structure. The overall frame uniform stress distribution becomes even, and further enhances the strength of the material utilization. It has been proved that the finite element analysis is quite exact and the change of the structure is effective on frame crack problem.