Lightweight And Fatigue Research On Coach Body Frame

For bus body frame, lightweight design is a mainstream trend of passenger carmanufacturing. The lighter the coach quality, the less the fuel consumption and theless the emission. Besides, the safety of the passenger can be enhanced. It’s veryimportant to study lightweight design of a bus.In this paper, a bus body frame was studied, and the finite element model of itsbody frame was established in the form of beam element with the finite elementanalysis software ANSYS Workbench. The strength, stiffness, modal and fatigue lifeof the model were computed, and then the ways to reduce the weight of the bus framebased on the calculation results were studied, and then the strength, stiffness andmodal of the improved bus frame were validated. The main work and conclusions areas follows:(1) The strength and stiffness in four kinds of working conditions of the buswere calculated, including bending, torsion, emergency braking and sharp turns. Thecalculation results are: For bending and torsion condition, the safety factor of the busbody frame is beyond3, which meets the safety factor of experienced requirements,in details, for bending condition, the safety factor is beyond3and for torsioncondition, the safety factor is beyond1.7. For the bending condition, the maximumopening deformation is1.68mm, and for the torsional condition, the maximumopening deformation reaches5.42mm at the position of back door, which should beimproved, except this, all the other results are within the safety range, so the strengthand stiffness are in accordance with the safety index and can meet the requirements.For the emergency brake and the sharp turn condition, the maximum stress and thedisplacement of the bus body frame is30.318MPa,38.178MPa and5.5127mm,6.9573mm,respectively. For the two conditions, the strength and stiffness meet therequirements.(2) To calculate the mode of the bus body frame, the results are: the first-ordertorsional frequency of the bus body frame is9.297Hz, and the first-order vertical bending frequency is17.985Hz. These two frequencies are bigger than the resonantfrequency that is4Hz of the suspension and smaller than the idling frequency that is24.33Hz of the engine, which can prevent the resonance of the bus body frame; thedifference between the two frequencies is beyond3Hz, and this can prevent theoccurrence of coupling within themselves; the first eight natural frequency values ofthe bus body frame are within the range from7.866Hz to22.102Hz, which can avoidthe idling frequency of the engine, which is within the range from24.33Hz to25.67Hz, and avoid the excitation frequency of the road which is3Hz, this canmaintain the stability and reduce the noise of the bus during the period of driving.(3) Two lightweight improvement programs were proposed. The strength,stiffness and frequency of the two lightweight model were verified, the results showthat: For program one, for bending condition, compared with the original model, themaximum stress is31.696MPa, which has a slight increase, and the maximumopening deformation is0.60mm. For torsion condition, the maximum stress is56.889MPa, the opening deformation of the back door is reduced from5.42mm to4.68mm, and the values of other parts are within5mm, the strength and stiffness meetthe requirements, and the low-level frequency values are within the range from8.227Hz to21.286Hz, which meet the frequency requirements. For program two, forbending condition, the maximum stress is35.845MPa, the maximum deformation is5.8088mm, for torsion condition, the maximum stress is55.400MPa, the maximumdeformation is9.3448mm, the maximum relative displacement difference(the openingdeformation value)of the door pillars is around2mm, so the strength and stiffnessmeet the requirements, the low-level frequency is within the range from8.119Hz to21.170Hz, which also meet the frequency requirements.(4) Quasi-static fatigue calculation and analysis of the body frame were studiedin the fatigue module, the least cycle number among the element nodes is located atthe connections between the front door pillars and the chassis, the result is1.4675×105; so the damage degree is6.814×10-6, which is the maximum damage degree inthe bus body frame. And quasi-static fatigue calculation and analysis for the improvedbus body frame shows that the fatigue life of all the nodes still has a large marginwhen the s-n curve of Q235steel is taken as the standard, thus the fatigue performance of the bus body frame meets the requirements and it can improve thematerial utilization the body frame.(5) After the improvements, the weight of the bus body frame is reduced by175.1kg and167.2kg for the first program and the second program, respectively. Thetwo programs both meet the strength, stiffness and frequency requirementstheoretically, the weight of the bus frame is reduced by about6%at last.