Effect of Hydraulic Accumulator on the System Parameters of an Open Loop Transmission System

This paper denouements the study of operating parameters of a hydraulic transmission system with and without the application of hydraulic accumulator. In this respect, the hydraulic system, designed in the open circuit mode, consists of the fixed displacement hydraulic pump which gives pressured fluid to the hydro-motor and charges the accumulator as well. The load on the motor is controlled hydraulically by coupling the motor with the loading pump, which discharges the fluid through the Pressure Relief Valve (PRV). The pressure decay of the accumulator in the hydraulic system has been analysed and the parameters affecting the pressure decay has been sorted out. The effect of pressure damping in the hydraulic system has also been investigated for different sizes of accumulators under different loading conditions. The proposed hydraulic system is modelled using MATLAB-SimHydraulics software. The study concludes with the identification of some major operating parameters which affects the performance of the Bladder type Accumulator. EFFECT OF HYDRAULIC ACCUMULATOR ON THE SYSTEM PARAMETERS OF AN OPEN LOOP TRANSMISSION SYSTEM 304-2 Hydraulic accumulator stores the energy of the fluid by compressing the spring used in piston type accumulator or gas in bladder type accumulator. The gas accumulators are mostly used in mobile equipment where an elastic diaphragm separates gas and fluid working environment. The accumulators use nitrogen to keep the hydraulic fluid pressurized. When the fluid is flow into an accumulator, the nitrogen (N2) inside the accumulator is compressed by reducing its volume. The accumulators are based on the principle that gas is more compressible than mineral oil, used in the hydraulic system. Energy is stored by the volume of the hydraulic fluid that compresses the gas under pressure. If the fluid is released, it will flow out quickly, under the pressure of the expanding gas. The rate at which the compression and expansion of the gas takes place affects the gas state – which is defined by volume, pressure and temperature [5]. A hydraulic system is considered energy saving if it will have the following capabilities. First, it must improve the efficiency of the power source of the system or next, it must be a regenerative system to recover energy during degeneration or while the load is lowered. An energy saving HST system using accumulator was investigated through analysis and modelling [6]. 2. Physical model of hydraulic system The present study deals with the surge absorbing characteristics of a hydraulic accumulator. For this purpose, an open loop hydraulic system is considered which has some basic hydraulic components as shown in fig. 1. The hydrostatic system consists the variable speed electric motor (1) which gives variable mechanical power to the variable displacement pump (2). The pump gives pressured flow to the hydraulic motor (5) through the check valve (3). An accumulator (4) is used at the suitable location of the system, where availability of surge is maximum. To generate fluctuating load to the hydraulic system, a loading pump (6) is mechanically connected with the hydro-motor; the loading pump gives flow to the PRV (7). The load at the motor is adjusted by adjusting the set pressure of the PRV. The hydraulic accumulator is installed in between them to absorb pressure pulsation and shock. Also, it stores potential energy from the surge, which will be used during emergency condition. The surge in the system may come from the pulsating flow or the fluctuating load [7]. For instance, with a five cylinder piston pump there will be five pulsations per revolution which is unimportant. However, an external disturbance is introduced in the system, through the electric motor, to generate the surge in the present system. Accumulator positioned upstream of the pump acts as a pulsation damper. Fig. 1. Physical system of open-loop hydraulic accumulator The main theme of this paper is to find out the suitable size of accumulator which will give less pulsation as pulsation cannot be removed by 100% but can be filter to negligible level. The amount of pulsation absorb is also a function of pre-charge pressure. The specifications of the components used in the simulation of the Hydrostatic System (HST) system are tabulated in the table shown below: Table 1.Specification of hydraulic system Component parameter value Unit Displacement of hydraulic pump 19.8 Cc/rev Displacement of hydraulic motor 12.0 Cc/rev Displacement of loading pump 11.0 Cc/rev Valve pressure setting 50.0 Bar 3. Principle of operation The bladder type accumulator consists of a synthetic polymer rubber bladder like chloroprene, nitrile, etc. inside a metal (steel) shell. The bladder is filled with compressed gas. A poppet valve located at the discharge port closes the port when accumulator is completely discharged. This keeps the bladder from getting out into the system. The main advantage of bladder type accumulator is that it responds quickly for receiving and expelling oil flow of oil [8]. It has lighter weight, lower cost, and compactness. The gas usually nitrogen is pre-charged during empty of hydraulic fluid and it should never be less than 1⁄4 th of the working pressure [9]. Nitrogen is used due to non-poisonous, not combustible and does not combine easily with other element i.e. inert. The initial pre-charge gas pressure p1 is normally 90% of the minimum working pressure P3 of the hydraulic system. This is to prevent the accumulator bladder constantly closing the anti-extrusion check valve. The maximum system pressure P2 is the fluid pressure when the accumulator is fully charged. P2 should not be greater than 3 times the minimum working pressure or the elastomeric material of the bladder may be damaged. 5 International & 26 All India Manufacturing Technology, Design and Research Conference (AIMTDR 2014) December 12–14, 2014, IIT Guwahati, Assam, India