The following points highlight the elements of a hydraulic system. The elements are: 1. Pumps 2. Hydraulic Valves 3. Hydraulic Circuits.
Element # 1. Pumps:
The pump develops the desired pressure. The pressure most commonly used in hydraulic machine tools varies from 5 to 80 kg/cm2.
The pumps used may be:
(i) Reciprocating pumps,
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(ii) Rotary pumps.
In reciprocating pump the liquid is displaced due to the reciprocating motion of the pistons or plungers. Fig. 11.62 shows the operation of vane pump in which vanes slide in radial slots of a rotor. The eccentricity between rotor and its housing can be varied. Vanes perform reciprocating motion relative to their slots due to eccentricity. Pressure developed is proportional to eccentricity.
Rotary type pumps may be screw type or gear pump (Fig. 11.63). A screw pump consists of two worms welded into a common shaft and having right hand and left hand threads. When the worm shaft rotates, the liquid supplied to the worms from the right and left flows towards the middle where the discharge pipe is arranged.
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A gear pump has two gears each with around 8 to 12 teeth to ensure a tighter mesh. As the gears rotate the liquid from the suction pipe is caught up by the teeth and is carried between their spaces to the discharge pipe.
Gear pumps are fixed delivery pumps, i.e., they deliver a constant volume at a constant pressure. For obtaining high pressure two pumps may be used in series. For greater volume, two pumps may be used in parallel.
Element # 2. Hydraulic Valves:
A valve is a hydraulic device mounted in the path of fluid flow. It can be used for changing the direction, pressure of volume of the fluid flowing in the hydraulic system.
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Relief valve is used to maintain a constant pressure at the valve intake by continuously discharging a certain amount of oil back to the tank. Safety valve is used to prevent the pressure from building up above a given maximum value.
Reducing valves are used to reduce the oil pressure and maintain the reduced pressure at the valve outlet. Flow control is achieved in a constant delivery circuit with the help of throttle valve. Direction control valves are used to change the direction of fluid flow.
Element # 3. Hydraulic Circuits:
Hydraulic circuits may be open circuit or closed circuit type. In open circuit the exhaust oil returns to the reservoir, keeping the oil cool due to the larger volume used. Fig. 11.64 shows an open circuit using fixed displacement pump. The piston speed is controlled by flow rate control valve (throttle valve) in the return line.
