Study of vibrational behaviour of shock absorber in harshness frequency range 20-100 Hz

Abstract

The main task of the shock absorber is to provide good comfort to the vehicle by damping the relative movement between the wheel axle and the body. The damping force in the shock absorber is created by the viscous resistance of the working oil as it passes through the valve systems between the various chambers. At low frequencies (below 1 Hz) and large relative displacements of the shock absorber (over 5 mm), the hysteresis of the gas filled in the shock absorber has a slight damping effect also. The fluid and gas forces are the main components of the total force that acts in the damper during its compression and extension in the frequency range up to 20 Hz. These forces counteract the disturbing forces of road unevenness and reduce the level of vibrations that are transmitted to the body at the mounting points. Road disturbances with a frequency above 20 Hz are characterized by small displacements (amplitudes). Small displacements of the piston rod (less than 1 mm) do not provide the necessary pressure in the chambers to overcome the resistance of the valve springs and actuate the fluid damping. In this frequency range, the compressibility of the working fluid also affects the force in the shock absorber. The work examines the vibrational behaviour of a telescopic shock absorber in the frequency range 20-100 Hz – known as the harshness range in NVH (Noise, Vibration and Harshness) studies. For this purpose, a hydrodynamic test bench is used, which is equipped with a force sensor. The pressures in each chamber of the shock absorber are measured. The pressure in the extension chamber is the indicator of the damping force. The results show that the fluid damping of the shock absorber decreases with the increase of the disturbance frequency. Above 50 Hz, the vibrations passing through the shock absorber are damped only by its rubber mounts. The force transmitted through the shock absorber increases and it is linearly proportional to vibration accelerations.