Design, Analysis and Optimization of a Shock Absorber

Transcription

1 Tarım Makinaları Bilimi Dergisi (Journal of Agricultural Machinery Science) 2014, 10 (4), Design, Analysis and Optimization of a Shock Absorber Durmuş Ali BİRCAN 1, Abdulkadir YAŞAR 2 1 Dept. of Mechanical Engineering, Çukurova University PO Box 01460, Sarıcam, Adana, Turkey abircan@cu.edu.tr 2 Dept. of Mechanical Engineering, Ceyhan Eng. Fac., Çukurova University PO Box 01950, Ceyhan, Adana, Turkey yasar@cu.edu.tr Received (Geliş Tarihi): Accepted (Kabul Tarihi): Abstract: A suspension system or shock absorber is a mechanical device designed to smooth out or damp shock impulse, and dissipate kinetic energy. In a vehicle, it reduces the effect of traveling over rough ground, leading to improved ride quality, and increase in comfort due to substantially reduced amplitude of disturbances. The design of spring in suspension system is very important. In this study, a shock absorber is designed and a 3D model is created using CATIA. The model is also modified by changing the wire diameter (thickness) of the spring. Structural and modal analyses are conducted on the shock absorber by varying material as stainless steel and aluminum alloy using ANSYS. The analysis is executed by considering hydraulic oil density and loads, vehicle weight and 4 persons. Also, comparison models were created for two materials and hydraulic fluid to verify optimum material for spring and shock absorber. And, results were analyzed and optimized in the Minitab program by using Taguchi method. Key words: Shock absorber, ANSYS, CAD, FEA, Taguchi method INTRODUCTION A shock absorber is a suspension component that controls the up-and-down motion of the vehicle s wheels. Pneumatic and hydraulic shock absorbers commonly take in the form of a cylinder with a sliding piston inside. The cylinder is filled with a fluid hydraulic fluid or air. This fluid-filled piston/cylinder combination is a dashpot. The shock absorbers duty is to absorb or dissipate energy. One design consideration, when designing or choosing a shock absorber, is where that energy will go. In most dashpots, energy is converted to heat inside the viscous fluid. In hydraulic cylinders, the hydraulic fluid will heat up, while in air cylinders, the hot air is usually exhausted to the atmosphere. In other types of dashpots, such as electromagnetic ones, the dissipated energy can be stored and used later. In general terms, shock absorbers help cushion cars on uneven roads. Shock absorbers are an important part of automobile and motorcycle suspensions, aircraft landing gear, and the supports for many industrial machines. Large shock absorbers have also been used in structural engineering to reduce the susceptibility of structures to earthquake damage and resonance. In a vehicle, it reduces the effect of traveling over rough ground, leading to improved ride quality, and increase in comfort due to substantially reduced amplitude of disturbances. Without shock absorbers, the vehicle would have a bouncing ride, as energy is stored in the spring and then released to the vehicle, possibly exceeding the allowed range of suspension movement. Control of excessive suspension movement without shock absorption requires stiffer springs, which would in turn give a harsh ride. Shock absorbers allow the use of soft springs while controlling the rate of suspension movement in response to bumps. The need for dampers arises because of the roll and pitches associated with vehicle maneuvering and from the roughness of roads. In Turkey, road quality is generally below average and poor. The design and construction of shock absorbers are demanding tasks that require advanced calculations and theoretical knowledge (Dixon, 2007). Main components of shock absorber, as shown in Figure 1, consist of piston rod and bearings are used for the main function is lubrication of total shock absorber. Other parts are; spring, piston ring, pressure chamber and outer body. 293

2 Design, Analysis and Optimization of a Shock Absorber In this study, a shock absorber is designed and a 3D model is created using CATIA. The model is also modified by changing the wire diameter of the spring. Structural modal analyses are conducted by varying material for spring for stainless steel and aluminum alloy using ANSYS. The analysis is executed by considering hydraulic oil density and loads, vehicle weight and 4 persons. Structural analysis is done to validate the strength of the spring. Also, comparison models were created for two materials and hydraulic fluid to verify optimum material for spring and shock absorber. And, results were analyzed and optimized in the Minitab program by using Taguchi method. Figure 1. Section view of shock absorber LITERATURE REVIEW This literature review is carried out to understand the present practices and theories in shock absorber design and analysis. Kim (1993) performed an analysis of a twin tube damper with focus on implementation into a vehicle suspension system. Kim's model included chamber compliance and fluid compressibility which yielded a differential equation for the chamber pressures that was solved using the Runga Kutta Method. Incorporating damping data into a quarter car model, the frequency response of the sprung mass and tire deflection were calculated numerically. Ferdek et al. (2012) designed a physical and mathematical model for a twin-tube hydraulic shock absorber, using oil as the working medium. To analyze the model, methods of numerical integration were incorporated. The effect of the amplitude and frequency of the excitation, as well as the parameters describing the flow rate of oil through the valves, were examined. The basic characteristics of the damping force were also obtained. Titurus et al. (2010) investigated the possibility of precise experimental identification of steady damper characteristics. The velocity sensitive and nominally symmetric hydraulic dampers were considered. Czop et al. (2010) presented the model of a complete system, consisting of a variable damping shock absorber and a servo-hydraulic tester, used to evaluate the vibration levels produced by a shock absorber. Also, same group (2011) gives the configurations of a typical valve system including three basic regimes of operation, which correspond to the amount of oil flowing through a valve cavity. The aim of this work was to propose a finite element fluid flow model, which can be used in order to reduce the velocity of fluid flow through a cavity of a shock absorber valve. Lee (2005) suggested a new mathematical model of displacement sensitive shock absorber to predict the dynamic characteristics of automotive shock absorber. The performance of shock absorber is directly related to the vehicle behaviors and performance, both for handling and ride comfort. Poornamohan (2012) designed shock absorber for 150 cc bike using PROE. The model was also changed by changing the thickness of the spring. They have performed the structural analysis and modal analysis on the shock absorber by varying material for spring, to steel and Beryllium Copper. Martande (2013), focused on to develop new correlated methodologies that will allow engineers to design components of shock absorbers by using FEM based tools. The different stress and deflection values in shock absorber components have been obtained using FEA tools and compared with analytical solutions. Percentage error is calculated and it is found that percentage error is less than 15%. MATERIALS and METHOD Design Calculations for Helical springs for Shock absorber The analytical calculations for the considered shock absorber assembly were done using the basic design calculations. The pre-required data considered are as follows: 294

3 Durmuş Ali BİRCAN, Abdulkadir YAŞAR Materials: AISI 304 Structural Steel and AISI 6061 Aluminum Alloy (Material properties can be seen in Table 1) Bauccio (1993) Diameter of wire: d=8mm and 10mm Hydraulic oil: TMS Oil 500 and TMS Oil 514 (Oil properties can be seen in Table 2) POAŞ (2014) Mean diameter of a coil D=58 mm Total number of coils = 11 Height of spring = 220mm Weight of vehicle = 1200kg Weight of 1 person=80kg, for 4 person= 80 4=320Kg Distribution of suspension = 25%,25%,25%,25% of 1200/4 = 300 kg for each wheel Gravity force=10 2 /s Considering loads as: W 1 =300 Kg = 3000 N and W 2 =300+{(4*80)/4}=3800 N Shock absorber weight is neglected Assumption about k values of spring is change with material. In our analysis hydraulic cylinder is used. Suspension system; there is no toe, caster and camber angle in suspension system. Oil pressure and density are uniformly distributed in the particular chambers Pressure-flow characteristics of all restrictions are given as monotonic functions. Design of Shock Absorber The shock absorber was designed in CATIA software. CATIA, feature based, parametric solid modeling program, is the standard in 3D product design, featuring industry-leading productivity tools that promote best practices in design. The design procedure is to create a model, view it, assemble parts as required, then generate any drawings which are required. See Figures 2 to 4. Figure 2. (a)upper and (b) Lower part of the shock absorber Table 1. Material properties of AISI 304 and AISI 6061 Aluminum Alloy Properties AISI 304 AISI 6061 Density 8 g/cc 2.7 g/cc Hardness, Brinell Hardness, Vickers Tensile Strength, Ultimate 505 MPa 310 MPa Tensile Strength, Yield 215 MPa 276 MPa Modulus of Elasticity 139 GPa 71,9 GPa Poisson's Ratio 0,29 0,33 Shear Modulus 86 GPa 26 GPa Figure 3. Spring model Table 2. Properties of TMS Oil 500 and TMS Oil 514 Properties Standards TMS Oil TMS Oil C ASTM ,720 0,860 g/ml Flash point COC, C ASTM D Kinematic Viscosity 40 C'de mm2/s 100 C'de mm2/s ASTM D ,3 10,4 75,1 15,9 Viscosity Index ASTMD Figure 4. Shock absorber assembly model 295

4 Design, Analysis and Optimization of a Shock Absorber Finite Element Analysis (FEA) The finite element method is a powerful tool for the numerical procedure to obtain solutions to many problems encountered in engineering analysis. Structural, thermal and heat transfer, fluid dynamics, fatigue related problems, electric and magnetic fields, the concepts of finite element methods can be utilized to solve these engineering problems. In this method of analysis, a complex region is discretized into simple geometric shapes called finite elements the domain over which the analysis is studied is divided into a number of finite elements as shown in Figure 5. The software implements equations that govern the behavior of these elements and solves them all; creating a comprehensive explanation of how the system acts as a whole. These results then can be presented in tabulated or graphical forms. This type of analysis is typically used for the design and optimization of a system far too complex to analyze by hand. Created geometric models are transferred to the ANSYS program to be done FEA. Selection of element type on the mathematical model, creating the mesh form, determining the contact areas, boundary conditions, environment and material properties and the type of analysis have been made in the program interface. Figure 5. Shock absorber meshed model Taguchi Method The Taguchi method involves reducing the variation in a process through robust design of experiments. Taguchi developed a method for designing experiments to investigate how different parameters affect the mean and variance of a process performance characteristic that defines how well the process is functioning (Roy,2010). The experimental design proposed by Taguchi involves using orthogonal arrays to organize the parameters affecting the process and the levels at which they should be varies. Instead of having to test all possible combinations like the factorial design, the Taguchi method tests pairs of combinations. This allows for the collection of the necessary data to determine which factors most affect product quality with a minimum amount of experimentation, thus saving time and resources (Montgomery, 2005). The general steps involved in the Taguchi Method are as follows: 1. Define the process objective, or more specifically, a target value for a performance measure of the process. 2. Determine the design parameters affecting the process. Parameters are variables within the process that affect the performance measure such as. 3. Create orthogonal arrays for the parameter design indicating the number and conditions for each experiment. The selection of orthogonal arrays is based on the number of parameters and the levels of variation for each parameter. 4. Conduct the experiments indicated in the completed array to collect data on the effect fn the performance measure. 5. Complete data analysis to determine the effect of the different parameters on the performance measure. While there are many standard orthogonal arrays available, each of the arrays is meant for a specific number of independent design variables and levels. For example, if one wants to conduct an experiment to understand the influence of 3 different independent variables with each variable having 2 set values (level values), then an L8 orthogonal array might be the right choice. The L8 orthogonal array is meant for understanding the effect of 3 independent factors each having 2 factor level values. This array assumes that there is no interaction between any two factors. In this study, the taguchi parameter design phase is served the objective of determining the optimal shock-absorber parameters to achieve the lowest stress value. The following are the questions considered in this study; The relationship between the control factors (dimensions of the spring, forces, materials) and output response factors(stresses) 296

5 Durmuş Ali BİRCAN, Abdulkadir YAŞAR The optimal conditions of the parameters of shock absorber. To accommodate two control factors into the experimental study, a standardized Taguchi-based experiment design L8 (2 3 ) was chosen to be used in this study and is shown in Table 3. There are 8 experimental runs that need to be conducted with the combination of levels each control factor (A-C). The selected parameters are displayed in Table 4 with their codes and values. Table 3. The basic Taguchi L8 (2 3 ) orthogonal array Test A B C Figure 7. Von misses stress for Test 3 and 3000N Table 4. Parameters, codes and levels used for orthogonal array Control Factors Code Level 1 Level 2 Wire Diameter A 8 mm 10 mm Hydraulic oil Density B 0,720 g/ml 0,860 g/ml Materials C AISI 304 AISI 6061 RESULTS of FINITE ELEMENT ANALYSIS For finite element analysis, CAD model of shock absorber is created and imported in to FEA software ANSYS. For stress analysis, constraints are applied at the one side of coil and the force is applied on the centre of other side of coil. By giving these conditions, Von misses stresses in shock absorber components have been obtained as shown in Figure 6 to Figure 9. Figure 8. Von misses stress for Test 3 and 3800N Figure 6. Von misses stress for Test 1 and 3000N load Figure 9. Von misses stress for Test 8 and 3800N 297

6 Design, Analysis and Optimization of a Shock Absorber Transferring the results in to Minitab ANSYS Von Misses stress results were transferred in to Minitab that created design in program. In Taguchi design of experiment is used. After unfixed properties of design program serves a basic Taguchi L8 (2 3 ) orthogonal array and ANSYS results were transferred as an input in the C4 and C5 array. Figure 10 shows the process. Table 5. Optimum design results Figure 10. Updated Taguchi L8 (2 3 ) orthogonal array The collected experimental data were analyzed by using Minitab15.0 software for the determination of the effects of each parameter on shock absorber. The optimum combination is obtained by choosing the level with the lowest S/N ratio for each control factor. Based on performance statistics analysis, the level 1 of parameter B, the level 2 of parameter A and B are the best levels, respectively. Therefore, the optimal combination is A2B1C2 and named as Test 6. The larger wire diameter of the spring was selected to optimum design due to better resistance of the stress and deformation. Meanwhile, the larger thickness would be projected as the best response given in the ideal situation. The figures 10 ad 11 show the results for the control factor of thickness (A) at Level 2(10mm) For the oil density, lower force values illustrate the optimum selection by considering existing stress at the absorber. When we analyzed the resultant stress existed due to forces, lower stresses are observed. So from the figures, the control factor of force (B) at Level 1(3000N) showed the best results. A comparison of prediction results with experimental materials results for spring. Both training data testing data were compared in Table 5. The Aluminum alloy shows the better characteristic and smallest resultant force value would be the ideal situation. Figure 10. S/N ratio of optimum design Figure 11. The optimum design by considering all parameter and effects. CONCLUSION In this study, the taguchi method was used to determine optimal design parameters for the absorber using steel and aluminum materials with different wire diameters under varying applied load conditions. The experimental results were evaluated using FEM analysis with ANSYS. The following conclusions may be drawn: In this study, shock absorber was designed a by using 3D parametric software CATIA, To validate the strength of design, structural and modal analysis was executed by varying spring material as: AISI 304 and AISI The optimum levels of the control factors for minimizing the Von Misses stresses using S/N rates were determined. The optimal conditions 298

7 Durmuş Ali BİRCAN, Abdulkadir YAŞAR for Von Misses stresses at A2B1C2 (Wire diameter=10mm, oil density=0,720 and Material=AISI 6061 respectively). According to the results of statistical analyses, the most significant parameter is the material properties Another significant point is that thickness of spring; while rising at spring diameter meanwhile stresses decrease smoothly. So, this case shows the importance of thickness and resistance of spring under changing load conditions. Forces which are build up on absorber and spring, causing serious stress differences. Maximum values are changing proportionally according to the size of the force. REFERENCES Dixon, J. C., The Shock Absorber Handbook. 2nd Ed., John Wiley & Sons Ltd, England Kim D., Analysis of Hydraulic Shock-Absorber and Implementation on the Vehicle Suspension Systems. M.S. Thesis, Seoul National University, S. Korea. Ferdek, U., Łuczko, J., Modeling and Analysis of a Twin-Tube Hydraulic Shock Absorber. Journal of Theoretical and Applied Mechanics 50, 2: Titurus, B, A Method for the Identification of Hydraulic Damper Characteristics from Steady Velocity Inputs. Mechanical Systems and Signal Processing 24: Czop P., Pawe R., A Computational Fluid Flow Analysis of A Disc Valve System Journal of KONES Powertrain and Transport, Vol. 18, No. 1. Czop, P., A high-frequency first-principle model of a shock absorber and servo-hydraulic tester. A Journal of Mechanical Systems and Signal Processing. 25(6): Lee, C., Study on the Damping Performance Characteristics Analysis of Shock Absorber of Vehicle by Considering Fluid Force. Journal of Mechanical Science and Technology. 19(2): Poornamohan, P., Design and Analysis of a Shock Absorber. IJRET DEC 1(4): Martande, S., Design and Analysis of Shock Absorber. IJAIEM 2(3): Bauccio, M ASM Metals Reference Book. Third edition, Materials Park, OH. Montgomery, D. C., Design and Analysis of Experiments. New York, John Wiley and Sons. Roy, R A Primer on the Taguchi Method. 2nd Edition, Society of Manufacturing Engineers, Michigan. 299