Shock absorber working principle 

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... This section provides an introduction to the theory and triggering factors of aeration and cavitation in shock absorbers based on Dixon [2] and the analytical work conducted by the authors [18] and supported with experimental studies [19]. The aeration phenomenon in a shock absorber is defined as a process by which gas, typically nitrogen, is circu- lated through, mixed with, or dissolved in oil being used as a working fluid in shock absorbers. Gas is included in shock absorbers under certain pressure, separately from the oil, to provide compressibility to allow for the rod displacement volume compensation. Theory states [2,21] that a liquid exposed to a soluble gas (i.e. the liquid comes into contact with the atmosphere of a gas that can dissolve in it) is in one of three forms: liquid-gas solution, liquid-gas bubble emulsion or foam. The liquid-gas solution is prone to bubble formation when the pressure of the liquid-gas solution falls below the so- called saturation pressure. In this state, the liquid is no longer capable of retaining all the gas in its dissolved form and therefore bubbles occur. The solubility of gas in a liquid is directly proportional to the absolute pressure above the liquid surface (Henry’s law), and normally decreases with rising temperature [2]. All of the mentioned liquid-gas mixtures can be considered as liquid with pockets of gas or vapor. The dissolved gas has a significant influence on the oil mixture and thus on the shock absorber's behavior. The presence of gas bubbles is the cause of the damping force loss in the shock absorber. It is an undesirable and negative effect visible as asymmetry of the force displacement characteristic and should be minimized. Fig. 1 shows the influence of aeration on the damper performance based on the force-displacement characteristic obtained for a sequence of 1500 cycles. The energy of hydraulic friction absorbed by a shock absorber caused an increase in its temperature. The damper was cycled with high velocity of 1.5 m/s, three sequences (the first, the middle, and the last) of 500 cycles each were plotted to show deterioration of the force-displacement characteristic. Modeling the dynamics of gas bubble formation and transport is a task very difficult for several reasons. Most important ones are difference between time scales in which aeration processes occur (order of minutes) and the time scales of oil flow through a damper (order of seconds), existence of uncontrollable parameters on which bubble size depends and the bubble size itself (e.g. oil impurities and sharp edges), re-absorption of gas from bubbles surface, etc. The cavitation phenomenon occurs, when oil does rupture under the influence of tensile stress, the rupture of the fluid manifests itself as a number of very small cavities in the oil [1]. The process of cavitation depends among other considerations, on the purity of the liquid and the rate at which the liquid is stressed. Cavitation is the formation of pockets of vapor in a liquid. When the local ambient pressure at a point in the liquid falls below the liquid's vapor pressure, the liquid undergoes a phase change to a gas, creating "bubbles," or, more accurately, cavities, in the liquid. Changing temperatures alter the vapor pressure of a liquid dramatically, making it easier or harder for the local ambient pressure to dip below the vapor pressure to cause cavitation. The violent collapse of cavitation or aeration bubbles results in the production of noise as well as the possibility of material damage to nearby solid surfaces [1]. Noise is a consequence of the momentary large pressures that are generated when the contents of the bubble are highly compressed. This also results in a micro flow in the liquid caused by the volume displacement of a growing or collapsing cavity. A larger number of col- lapsing gas bubbles decreases the bulk modulus of the gas-oil mixture, and produces. Cavitation and aeration occur at restrictions where potential pressure energy is converted into kinetic energy increasing flow velocity and dramatically decreasing the pressure in the oil locally. Valve systems used in hydraulic dampers should be designed to minimize the possibility of occurrence of local low-pressure regions which contribute to the formation of gas or cavity bubbles. The remaining content of the paper is divided into five sections. Section 2 presents the working principles of shock absorbers and valve systems, while Section 3 provides an introduction to the methodology used to simulate structure-fluid interactions in a valve system and illustrates the process of fluid-structure model development. Section 4 discusses the proposed valve design optimization method, while Section 5 reports experimental results of this optimization process. Lastly, Section 6 presents the summary of the paper. This section presents the fundamental working principles of a hydraulic shock absorber. The hydraulic double-tube damper presented in Fig. 2 consists of a piston moving within a liquid-filled cylinder. As the piston is forced to move within the cylinder (pressure tube), a pressure differential is built across the piston and the liquid is forced to flow through valves located in the piston and the base-valve assembly. The presence of the piston divides the cylinder space into two chambers: (i) the rebound chamber, that portion of the cylinder above the piston and (ii) the compression chamber, that portion below the piston. The action of the piston transfers liquid to and from the reserve chamber, which surrounds the cylinder, through the base-valve assembly located at the bottom of the compression chamber. Two types of valves are used in the shock absorber: (1) intake valves and (2) control valves. The intake valves are basically check valves which provide only slight resistance to flow in one direction and prevent flow in the opposite direction when the pressure differential is reversed. Control valves are preloaded through a valve spring to prevent opening until a specified pressure differential has built up across the valve. The two working phases of a hydraulic shock absorber are distinguished as the compression phase and rebound phase. During the compression phase the rod is tucked into the damper, compression chamber volume decreases and oil flows through the piston compression intake valve (piston intake) and the base compression control valve (base valve) accordingly, to the rebound and reserve chambers. During the rebound phase the rod is rejected from the damper, the compression chamber volume increases and oil flows through the piston rebound control valve (piston intake) and base rebound intake valve (base intake) accordingly, to the rebound and reserve chamber. The piston and base valves has to be balanced during operation which requires to maintain the differential pressure over the piston has to be greater than the sum of differential pressure over the base valve and the gas pressure in the reserve chamber. This requires to adjust the pressure-flow characteristics of piston and base valves to meet valve balance conditions during a compression stroke. Valve unbalance results in an effect that the pressure in the rebound chamber becomes lower than the atmospheric pressure during a compression stroke. This low pressure causes cavitation or gas release from oil-gas mixture in whole rebound chamber volume. The paper considers a specific type of shock absorber valve, i.e. the clamped piston compression valve presented in Fig. 3. Such a valve system consists of a combination of disc springs, referred to further in the paper as a stack of discs or a disc stack. The number of discs, their diameters and thickness, directly affects the operational pressure-flow characteristics of the valve system. A valve system operation can be split into three regimes. In the first regime, there is only a small flow through bleeds of a very small area below 1mm2 in the so-called orifice disc while the stack of discs is complete- ly closed (Fig. 3a). The damping forces produced by the valve are therefore very small, similar to a drive along a smooth road such as a highway. The stack of discs starts opening in the second regime providing a typical range of damping forces (Fig. 3b). The last regime corresponds to the case when the stack is fully opened and the restriction is provided by the profiled channels in the piston component (Fig. 3c). This regime covers off-road conditions or violent ma- neuvers on the road. This work focuses on the second and third regime, corresponding to the minimum (initial) opening and the maximum opening of the valve system. Valve systems are characterized by the pressure-flow characteristics which are obtained during component- level tests with the use of a hydraulic test-rig equipped with a pump, a flow tool where the valves are assembled and measured, hydraulic lines, and a data acquisition system. The controller regulates the flow through the valve allowing a pressure-flow characteristic of a valve to be captured. The pressure is a differential pressure measured before and after the flow tool, evaluating the pressure drop across the valve assembly for a given flow rate. A hydro-mechanical valve system model usable from an engineering application perspective should be able to reproduce essential properties of a valve system during operation in a shock absorber. This requires a combination of two sub-models: (i) a finite element mechanical (stress/strain) model, and (ii) a flow model. The mechanical model obtains (i) stress in discs, (ii) displacement between the orifice and a valve seat; both as a function of the pressure load. The opening of a disc stack can also be expressed as a function of an outflow area vs. pressure load. If the shock absorber geometry is known, then the flow model allows the outlet flow rate through the valve system as a function of the pressure load to be obtained. The input of the ...