Experimental Investigation of Effects of Shock Absorber Mounting Angle on Damping Characterstics Tanmay P. Dobhada Tushar S. Dhaspatil Prof. S S Hirmukhe Mauli P. Khapale Abstract: A 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 bumps while traveling over rough ground, leading to improved ride quality, and increase in comfort. This paper focuses on studying the effects of shock absorber mounting angle on its damping characteristics when provided with different size of bumps. A shock absorber test rig is developed which provides the damping characteristics at different load-speed-bump-angle combination. The model is shown to give an excellent representation of the damper under investigation. The performance of the damper model is examined as the parameters are varied, and the results are discussed. Keywords automotive damper, testing, angle, parameter estimation I. INTRODUCTION A conventional suspension system for a wheeled vehicle consists of mechanical springs and dampers and has several functions to perform. Two of its main functions are to isolate the body from the terrain over which it is travelling and to keep the tyre in contact with the surface. The characteristics of these components are fundamental to the performance of the suspension system in achieving these objectives. Due to high-speed road transportation, people put forward higher requirements on the handling and ride comfort performances of a vehicle. Thus, in this paper the damping characteristics of pneumatic damper are examined experimentally. The objective of this research is to find an optimum angle at which the shock absorber can be mounted in two-wheeler such that it exhibits better damping characteristics and provide better comforthandling package. 19 D J Purdy from the Royal Military College of Science in the United Kingdom investigated adjustable automotive dampers in 1999, resulting in theoretical and experimental results. In this paper the dynamics of an adjustable gas-pressurized damper are examined both experimentally and theoretically. Using the test rig Purdy adjusted the valves on the provided shock absorber and was able to produce force-velocity curves for various adjustments. Purdy discusses the work of Surace, who found that under continuous operation the effects of temperature can cause the characteristics of the damper to vary. Therefore Purdy used a sinusoidal input rather than random testing to enable the results to be closely analyzed so that the influence of temperature could be isolated. Purdy s approach is similar to the approach adopted for this research, only difference is instead of sinusoidal input bumps of varying intensity are given by means of cam and follower system A common approach to shock absorber and vehicle suspension testing is to use a quarter car model. These are explained thoroughly in Weispfenning s journal of 1997 which discusses fault detection and diagnosis of vehicle vertical dynamics. Weispfenning highlights that worn dampers cause longer braking distances, lead to faster wear of the tires and deteriorate the handling of the car, especially during cornering. A quarter car model is a simplified model useful for investigation into shock absorber, which allows modeling and development of concept such as semi-active suspension with varying parameter. This allows researchers to obtain a force velocity curve for the car s suspension. For this study similar basic approach is being used to design the test rig.
Shaohua Li in 2012 published paper on dynamical test and modeling for hydraulic shock absorber on heavy vehicle under harmonic and random loadings. The aim of paper was to find the suitable loading condition during damper dynamic test and develop a testing and analysis methodology for obtaining the dynamic properties of shock absorbers for use in vehicle dynamic simulation. For this study similar methodology is used but test rig designed is different and the dynamic testing will be done for the damper of light weight capacity. From the overall literature review it is found that very limited work has been done on the testing of damper at different angles and determining optimum mounting angle of damper. This experimental research work presents a model to calculate the damping characteristic which includes amplitude of vibration transmitted (Force Transmissibility), damping coefficient and natural frequency of the system. This paper starts by describing the construction and working of test rig. The response of the damper is examined at different angle-load-speed combination. Finally, the results are discussed and conclusions are drawn. II. DESIGN OF TEST RIG A. DC Motor: Torque Calculations: T= F x R where, T = Torque (N m) F= Force (N) R = Distance of application of force from center of motor shaft(m) Total Force = Maximum Sprung weight + Weight of brackets = 58.86 + 4.905 = 63.765 N T= 63.765 x 0.02 T= 1.273 N-m = 13 Kg cm Motor Selected: Specifications: 12V DC Motor, Torque: 15Kg.cm, Maximum Speed: 500rpm B. Damper: Type: Pneumatic Damper Stroke Length: Maximum: 9.5 cm Minimum: 1 cm C. Design of cam and follower system: 1. Selection of Cam and follower: a. Selection of Cam: i. According to Shape: Radial or Disc Cam ii. According to Follower Movement: iii. Rise-Dwell-Return Cam According to Constraint of follower: Preload Spring Cam b. Selection of follower: Roller Follower Reasons: i. In roller followers the sliding motion of the knife edge follower is replaced by rolling motion. ii. This rolling motion reduces the wear due to friction. 2. Selection of follower motion: Cycloidal motion Reasons: 1) As shown in fig1 the jerk is maximum at beginning, mid and end of each stroke and zero at one fourth and three fourth of each stroke. 2) The jerk is having finite value throughout the cycle. Therefore cycloidal curve is best suited for high speed cams. Fig. 1 Displacement, Velocity, Acceleration and Jerk Diagrams 20
Displacement Diagram and Cam Profile for Cycloidal motion of follower: Specifications of Test Rig: TABLE I. TEST RIG COMPONENTS AND THERE FUNCTIONS Sr. No Component Name Materia l Capacity/ Rating Function in Test Rig 1. Pneumatic Damper - 5Kg Test Component Fig. 2 Displacement Diagram 2. Electric motor - 12V D.C Motor Max.Torqu e-15 Kgcm Speed- 500rpm To operate cam and give bumps 3. Upper mounting bracket Wood - Upper mounting of damper at different angles and mounting weights 4. Lower mounting bracket Wood - Lower mounting of damper and mounting of follower Fig.3 Cam Profile 5. Base Wood - To support all the components in test rig acting as a foundation 6 Cam Wood - To provide artificial bumps 7. Follower Wood To transmit the bumps to the damper Fig.4 Manufactured Cam and Follower 21
thus we can find optimized mounting angle of shock absorber. IV. RESULTS A.The graphs obtained by testing damper at different angle when load was 5Kg and speed 60rpm are as follow: 1.Graph of Displacement Vs Time when damper mounted at 80 0 with horizontal plane: Fig.5 Shock Absorber Test Rig III. EXPERIMENTAL PROCEDURE The working of the test rig can be explained in following steps with the help of fig 5. 1. The electric motor is connected to the dimmerstat whose output is 12V DC. 2. When speed is given, the output shaft starts rotating which rotates cam mounted over it and thus providing artificial bumps 3. The motion of cam is transmitted to damper through the follower. 4. The weights which act as sprung mass of vehicle are mounted on the upper bracket for initial deflection of the damper. 5. When bumps are provided some vibrations are absorbed by the damper providing damping effect. 6. Remaining vibrations are transmitted to the mass which are recorded by means of strip chart recorder. 7. These recorded waveforms are used to determine damping characteristics at different angles and Fig.5 Results when damper mounted at 80 0 2. Graph of Displacement Vs Time when damper mounted at 90 0 with horizontal plane: Fig.6 Results when damper mounted at 90 0 22
B.The graphs obtained by testing damper at different angle when load was 5Kg and speed 80rpm are as follow: 1.Graph of Displacement Vs Time when damper mounted at 80 0 with horizontal plane: 2. Damping ratio: ξ = ξ = 0.09/6.29 ξ=0.0143 Hence, the value obtained at different angles is as follows: Sr.No. TABLE II. Mounting Angle ( 0 ) LOGARITHMIC DECREMENT AND DAMPING RATIO Speed (Rpm) Logarithmic Decrement(δ) Damping Ratio (ξ) 1. 80 60 0.09 0.014 2. 90 60 0.028 0.0045 3. 80 70 0.040 0.00101 4. 90 70 0.0510 0.00129 Fig.7 Results when damper mounted at 80 0 3. Graph of Displacement Vs Time when damper mounted at 90 0 with horizontal plane: Conclusion 1. The rate of decay of amplitude is measured by parameter known as logarithmic decrement. 2. The rate of decay of amplitude is proportional to the amount of damping present in the system. 3. Thus larger the damping greater is the rate of decay of amplitude and thus better comfort. 4. Thus, from Table II we can conclude that better damping is obtained when the damper is mounted at an inclined angle of 80 0 at any speed. Fig.8 Results when damper mounted at 90 0 Figures 5, 6, 7 and 8 indicate the force transmitted to the mass above upper bracket. Hence from the readings obtained we can calculate the damping ratio to compare damping characteristics: Sample Calculations for Speed= 60rpm, Angle=80 0 : 1. Logarithmic Decrement: Logarithmic Decrement is defined as the natural logarithm of, the ratio of any two successive amplitudes on the same side of mean position. δ= (1/n) log e [X o /X n ] δ=(1/10) log e [0.5/0.2] δ=0.09 References [1]. Purdy, D. 2000, "Theoretical and experimental investigation into an adjustable automotive damper", Proceedings of the Institution of Mechanical Engineers, Part D: Journal of Automobile Engineering, vol.214, no. 3, pp. 265-283. [2]. Weispfenning, T. 1997, "Fault Detection and Diagnosis of Components of the Vehicle Vertical Dynamics", Meccanica, vol. 32, no. 5, pp. 459-472. [3]. Robertas Pečeliūnas Influence of Shock-Absorber Parameters on Vehicle Vibrations During Braking Solid State Phenomena Vol. 113 (2006) pp 235 240 [4]. Weihua Li, MR damper and its application for semi-active control of vehicle suspension system Mechatronics, 12 (7), 963-973. 23