Anna GE 6253 - Structure of a Hydrostatic Drive

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Structure of a hydrostatic drive Fluid power systems five basic parts Pumps Reservoirs Actuators Valves and Lines Aggregate Pump motor Fluid reservoir Pressure relief valve Filter Piping Control elements Actuator Valves determining the path pressure flow working fluid rate of the Elements doing work Linear Rotational Pumps In the area of fluid power two categories of pump symbols are used i e hydraulic or pneumatic The basic symbol for the pump is a circle containing one or more arrow heads indicating the direction s of flow with the points of the arrows in contact with the circle Hydraulic pumps are shown by solid arrow heads Pneumatic compressors are represented by hollow arrow heads A variable displacement pump is a device that converts mechanical energy to hydraulic fluid energy The displacement or amount of fluid pumped per revolution of the pump s input shaft can be varied while the pump is running Many variable displacement pumps are reversible meaning that they can act as a hydraulic motor and convert fluid energy into mechanical energy Reservoirs Reservoirs provide a location for storage of the hydraulic fluid or compressed gas Pneumatic reservoirs are usually simple tanks and their symbology is usually some variation of the cylinder shown in Figure Hydraulic reservoirs can be much more complex in terms of how the fluid is admitted to and removed from the tank An actuator in a fluid power system is any device that converts the hydraulic or pneumatic pressure into mechanical work Actuators are classified as linear actuators and rotary actuators Linear actuators have some form of piston device Rotary actuators are generally called motors and may be fixed or variable The difference between them is that the point of the arrow touches the circle in a pump and the tail of the arrow touches the circle in a motor Figure illustrates a four way three position valve and how it operates to vary the flow of the fluid Valves are the most complicated symbols in fluid power systems a valve opens closes or throttles the process fluid In fluid power systems it is common for a valve to have three to eight pipes attached to the valve body with the valve being capable of routing the fluid in any number of combinations of input and output flow paths Simple Hydraulic Power System Line Diagram of Simple Hydraulic Power System A spool valve is a special type of flow directing valve used in pneumatic and hydraulic systems to direct the pressurized fluid to different locations Components of a hydraulic system Oil reservoir it is a storage tank for oil It consist of tank filter strainer baffle plates drain plug Oil pump the purpose of pump is to supply a flow of fluid to hydraulic system Pressure relief valve it is used to control pressure If pressure in a system increases above a pre set level this valve opens to release the fluid out of the system Direction control valve it is used to control the direction of flow of working fluid so as to control the direction of motion of the actuator Flow control valve it is used to control flow rate of flow of working fluid so as to control the speed of actuator Pressure control valve it is used to maintain the desired pressure level in the various parts of circuit Actuators it is used to convert air pressure delivered by air circuit into applied force and motion Components of Pneumatic system Air compressor it is a machine which increases the pressure of air by reducing its volume Heat exchanger it is a device that transfer heat through a conducting wall from one fluid to another Air dryer it is a moisture separator which removes moisture from the compressed air before it is fed to air receiver tank F R L unit Filter Regulator Lubricator Air filter it separates solid contaminants from compressed air and it supplies clean air to pneumatic system Air regulator it maintains constant pressure in the system Air lubricator it reduces friction between internal moving parts of the system A typical hydraulic system 1 pump 2 oil tank 3 flow control valve 4 pressure relief valve 5 hydraulic cylinder 6 directional control valve 7 throttle valve Functions of the components shown in Fig 1 1 are as follows 1 The hydraulic actuator is a device used to convert the fluid power into mechanical power to do useful work The actuator may be of the linear type e g hydraulic cylinder or rotary type e g hydraulic motor to provide linear or rotary motion respectively 2 The hydraulic pump is used to force the fluid from the reservoir to rest of the hydraulic circuit by converting mechanical energy into hydraulic energy 3 Valves are used to control the direction pressure and flow rate of a fluid flowing through the circuit 1 Off 2 Forward 3 Return Motor The piping shown in Fig 1 1 is of closed loop type with 4 External power supply motor is required to drive the pump 5 Reservoir is used to hold the hydraulic liquid usually hydraulic fluid transferred from the storage tank to one side of the piston and returned back from the other side of the oil piston to the tank Fluid is drawn from the tank by a pump that produces fluid flow at the required level of pressure If the fluid pressure exceeds the required level then the actuator and valves 6 Piping system carries the hydraulic oil from one place to another 7 Filters are used to remove any foreign particles so as keep the fluid system clean and efficient as well as avoid damage to the 8 Pressure regulator regulates i e maintains the required level excess fluid returns back to the reservoir and remains of pressure in the hydraulic fluid there until the pressure acquires the required level Figure 1 2 shows the components of the hydraulic system using The working fluid which is the hydraulic oil is stored in a reservoir When the electric motor is switched ON it runs a positive displacement pump that draws hydraulic oil through a filter and delivers at high pressure The pressurized oil passes through the regulating valve and does work on Oil from the other end of the actuator goes back to the tank via return line To and fro motion of the cylinder is controlled using directional control symbols actuator valve Cylinder movement is controlled by a three position change over a control valve 1 When the piston of the valve is changed to upper position the pipe pressure line is connected to port A and thus the load is raised 2 When the position of the valve is changed to lower position the pipe pressure line is connected to port B and thus the load


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Anna GE 6253 - Structure of a Hydrostatic Drive

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