Design Improvement of a Truck Chassis based on Thickness Goolla Murali Trainee Engineer Sigma Consultancy Service H.No.7-1-282/C/A/7 Balkampet, Hyderabad 500018, India Subramanyam.B (M.Tech) SriKalahasteeswara Institute of Technology.Sri kalahasti, Chittoor 517640 A.P. India. Dulam Naveen Trainee Engineer Sigma Consultancy Service H.No.7-1-282/C/A/7, Balkampet. Hyderabad 500018, India. Keywords: Truck chassis, Bending stiffness, Torsional stiffness, finite element analysis, HyperMesh. Abstract Automobile chassis usually refers to the lower body of the vehicle including the tires, engine, frame, driveline and suspension. Out of these, the frame provides necessary support to the vehicle components placed on it. Also the frame should be strong enough to withstand shock, twist, vibrations and other stresses. The chassis frame consists of side members attached with a series of cross members. Stress analysis using Finite Element Method (FEM) can be used to locate the critical regions which have the highest stresses. These critical regions are the ones that may cause the failures in the frame. Total capacity of the frame includes tonnage and engine weight, which is applied as a distributed load on the entire frame. Design improvements are made to improve the load carrying capacity of the structure. Thickness has been modified to study as a part of the improvement. The details will be discussed in the later part of the paper. HyperMesh and OptiStruct solver has been used for the FE calculations. Introduction Chassis is a French term which is now denotes the whole vehicle except body in case of heavy vehicles. In case of light vehicles of mono construction, it denotes the whole vehicle except additional fittings in the body. Chassis consists of engine, power train, brakes, steering system and wheels mounted on a frame. Automobile chassis usually refers to the lower body of the vehicle including the tires, engine, frame, driveline and suspension. Out of these, the frame provides necessary support to the vehicle components placed on it. Also the frame should be strong enough to withstand shock, twist, vibrations and other stresses. The chassis frame consists of side members attached with a series of cross members Stress analysis using Finite Element Method (FEM) can be used to locate the critical point which has the highest stress. This critical point is one of the factors that may cause the fatigue failure. The magnitude of the stress can be used to predict the life span of the truck chassis. The accuracy of prediction life of truck chassis is depending on the result of its stress analysis. Objective The objectives of this study are: i) To determine the torsion stiffness and bending stiffness of the truck chassis by using torsion analysis and bending analysis using finite element method. ii) To improve the stiffness of the truck chassis by changing the geometrical dimension and structural properties. Simulate to Innovate 1
Process Methodology The theoretical analysis has been carried out using the finite element analysis to obtain the torsion value of the truck chassis. The objectives of this analysis are to evaluate and perform the comparison analysis between theoretical and experimental results. The specification of the truck chassis as shown in figures below Figure1 : Dimension of the truck chassis (All dimensions in mm) Figure2: Dimension of the truck chassis (All dimensions in mm) i. Figure1 and figure2 shows the geometrical dimensions of the truck chassis ii. The model was imported into the finite element software, which the HyperMesh software simulation system. iii. The material and element properties of the chassis structure were then defined. The chassis properties were listed as below: a. Modulus of Elasticity, E = 200Gpa b. Coefficient of Poisson = 0.30 c. Mass density, ρ = 7850 kg/m 3 iv. 2D shell elements were used in the meshing procedure. v. The free-free boundary condition was adopted in order to obtain the natural frequencies and mode shapes vi. The normal mode analysis was executed in order to find the natural frequencies and mode shapes of the chassis. The frequency range of interest was set greater than 30Hz. Figure3: Truck chassis meshed model Simulate to Innovate 2
The truck chassis was constrained at rear-end of the structure for torsion analysis. The load was applied at the front-end structure and the applied load was 4300 N on both directions. Calculation of the Global Torsion Stiffness Figure 4: Torsion Test using HyperMesh software system The global twist angle of the chassis is a function of the vertical displacement between the two involved coil-over mounting points and their distance. θ f = 2δ/L f (radians), where; L f : traverse distance to the chassis between the front dial indicators δ: total deflection of truck chassis (δ d + δ e ) θ f = 2δ/L f (radians), = 2(15.13-14.65) / 761 = 1.3e-3 rad where, Torque can be derived as: T= [(F d )/2 + (F e )/2] x L s F d : force in the right front end of the structure F e : force in the left front end of the structure L s : traverse distance between the force application points T = [(F d )/2 + (F e )/2] x L s = [(4300/2) + (4300/2)] * 697 = 2997.1 N-mm = 2.997 kn-mm The torsion angle in the rear part of the structure, θt, is determined from the measured displacements in the extremities of the longitudinal rails, θ t =[(δ d ) + (δ e )] / L t = (15.13-14.65) / 881 = 5.4e-4 rad where; δ d and δ e are the measured vertical displacements in the dial indicators, separated laterally by a distance Lt. The global torsion stiffness was measured between the extremities of the chassis. For this load case torsion stiffness could be calculated by: K=T/θ (Nm/rad) and, θ = θ f - θ t Where, K: global torsion stiffness T: Torque θ f : front torsion angle of the structure Simulate to Innovate 3
θ t : rear torsion angle of the structure. θ = θ f - θ t = (1.3e-3) - (5.4e-4) = 7.6e-4 rad K=T/θ = (2.997) / 7.6e-4 = 3.9 e3 kn-mm/deg = 3.9 kn-m/deg For the bending stiffness analysis the truck chassis was constrained at the rear and front wheel locations. The load of 1.5 kn was applied at the centre of the wheel base (i.e., 0.75 kn is to be applied at the center of the wheelbase on respective side members as shown in the figure below). Figure5: Bending Test using HyperMesh software system. Calculation of the bending Stiffnesss Bending stiffness = (Total Applied load) /(Deflection at point of application of load) = 1500 / 0.69 = 2.2 kn/ /mm Results & Discussions A finite element analysis was performed on the model using OptiStruct. Maximum normal stresses from the finite element analysis were compared with the acceptance criteria to verify the model. The results of Finite element analysis were observed to be lesser than that of the acceptance criteria. 1st flexible mode natural frequency Bending Stiffness Torsion Stiffness Acceptance Criteria FEA Results > 30 Hz 30..08 > 3 kn/mm 2.2 kn/mm > 4 kn-m/deg 3.9 kn-m/deg Table 1 : Comparison of results with acceptance criteria before modification Figure6: Deflection on chassis in bending before modification Simulate to Innovate 4
Figure7: Deflection on chassis in torsion before modification In order to meet the acceptance criteria few modifications were done to the actual model. The modification on the chassis from FE model was analyzed in order to improve strength as well as reducing in vibration. For the modifications and analysis, the existing truck chassis were added with stiffness. Initially the thickness of the model, where the maximum deflection occurs in bending analysis was increased to certain value with acceptable limit. And one more cross beam was added at the centre of the wheel base to add stiffness to the model as shown in the figure below. Figure8: Modified Chassis Figure9: Deflection on chassis in bending after modification Figure10: Deflection on chassis in torsion after modification Simulate to Innovate 5
The following table shows the comparison of results before and after modification Before Modification After Modification Weight 138 kg 163 kg 1st flexible mode natural frequency 30.08 Hz 30.16 Hz Bending Stiffness 2.2 kn/mm 3.26 kn/mm Torsion Stiffness 3.9 kn-m/deg 5.9 kn-m/deg Conclusion: Table 2: Comparison of weight before and after modification The objective of the analysis had successfully achieved in the area of improvement of torsion stiffness based on the result gained from the finite element analysis, further enhancement of the current chassis had been done through the chassis FE model in order to improve its torsional stiffness as well as reduce the vibration level. Series of modifications and tests were conducted by adding the stiffener in order to strengthen and improved the chassis stiffness as well as the overall chassis performances. ACKNOWLEDGEMENTS The authors would like to thank Altair HyperWorks, INDIA and DesignTech, hyderabad for their support throughout the execution of our paper and for providing significant information on chassis frames and making themselves available throughout our work. REFERENCES [1] Vishal Choudhari, Deepak Nidgalkar and Dinesh Chennappa "Case study of process Automation: Chassis Stiffness", Altair CAE Users Conference 2005. [2] Wesley Linton, Analysis of Torsional Stiffness and Design Improvement Study of a Kit Car Prototype, Cranfield University, Setember 2002. [3] Murali M.R. Krisna, Chassis Cross-Member Design Using Shape Optimization, Internasional Congress and Exposition Detroit, Michigan. February 23-26, 1998. [4] I.M. Ibrahim, D.A.Crolla and D.C. Barton, Effect of Frame Flexibility on the Ride Vibration of Trucks, Department of Mechanical Engineering, University of Leeds LS2 9JT, U.K. August 1994. Simulate to Innovate 6