The basic function of shock absorbers used in vehicle suspension systems is to prevent vehicles from prolonged oscillation resulted by the inhomogeneity on the surface of roads.
There are several types of dampers used in suspension systems. The most common one is a hydraulic or telescopic shock absorber. The telescopic twin-tube shock absorber is widely used for vehicles, due its simple structure and reliable security. The hydraulic shock absorber is mounted concentrically with a coil spring between the lower suspension arm and the vehicle body. In order to provide damping to the suspension system, the shock absorber dissipates energy in the compression stroke (jounce) and extension stroke (bounce) whenever a vehicle moves in shock on the road. The shock absorber is the key element of suspension systems. It is used to dissipate energy and therefore, it reduces vertical oscillation of vehicles caused by the unevenness of road surfaces.
The hydraulic shock absorber is made up of a piston rod, three chambers (compression, rebound and reserve chambers) and four valves (rebound, rebound intake, compression and compression intake-valves). The rebound and compression chambers are completely filled with viscous fluid, whereas the reservoir has only a partial filling. The rest space of the reservoir is filled with high-pressure atmospheric air or nitrogen gas.
Friction exists in all mechanical systems that have parts with relative movement. In the hydraulic shock absorber, the frictional force is caused by the contacted parts.
Previous studies have explained that the property of friction force between piston and cylinder, excessively high viscous oil property inside the shock absorber and the stiffness of the piston valve have a negative effect on the behavior of hydraulic shock absorbers.
Kwak Yong Song, a researcher at the Robotics Institute, has considered the damping characteristics of shock absorbers by the effect of friction in the guide bearing, and then investigated the damping characteristics of suspension systems.
He used the simulation program MATLAB Simulink for the dynamic response (time-displacement characteristic) to the impulse loading of an automobile suspension system.
The result has shown that the friction at the shock absorber guide bearing has a practically negligible effect on the suspension system input amplitudes of more than 40mm, but has a significant effect on the input amplitudes of less than 30mm.
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