UNIT-II

HYDRAULIC CIRCUITS

Introduction• A hydraulic circuit consists of various components like pumps, actuators and

control valves and they are arranged in a systematic manner to perform usefultask.

The following are the important factors to be considered while designing a hydrauliccircuits.1. Safety of operation2. Performance of desired function3. Efficiency of operation.Single acting hydraulic cylinder circuit (Reciprocation circuit)

• The following circuit shows a two position, two way, manually operated, springoffset. Direction Control Valve (DCV) which is used to control the operation of asingle acting cylinder.

• In the right envelope flow path configuration (ie. in the spring offset mode) thepump flow goes to the tank via the .pressure relief valve. The spring in thecylinder retracts the piston to send the oil back to the tank.

• When the valve is shifted to the left envelope flow path configuration, the pumpflow extends the piston. Once complete extension is over, the pump flow goes throughthe pressure relief valve.

• If we deactivate the DCV, then it allows the piston to retract as the DCV shifts toright envelope flow path configuration mode. (ie spring offset mode).

• During this spring offset mode as well as at the end of full extension mode, thefull pump flow goes to the tank through the pressure relief valve.

Double acting hydraulic cylinder circuit(Reciprocation circuit)The following circuit shows the control of a double acting hydraulic cylinder.

The circuit consists of three position, four way direction control valve.• Then the four way valve is in centre position (spring centred position i.e., tandew

design), the full pump flow goes to tank and the cylinder is hydraulically locked.• When the four way valve is shifted to the left envelop flow path configuration, 6

flows from port P through port A and hence the piston is extended against theforward.

• Now, the oil in the rod end of the cylinder flows back freely to the tank througithe four way valve from port B through port T. If this oil is not allowed to flowback t tank from the rod end of the cylinder, then the piston cannot extend.

• When the four way valve is deactivated, then it occupies the spring centeredenvelope position and the cylinder is once again hydraulically locked.

• When the four way valve is shifted to the right envelope flow path configuration,the piston retracts as oil flows from port P through port B. Now oil in the left sideof the piston is returned to the tank via the four way valve from port A to port T.

Continuous reciprocation circuit• The following circuit shows the continuous reciprocation of a hydraulic cylinder.

This continuous reciprocation is achieved by two sequence valves Each valvesenses a stroke completion by the corresponding pressure buildup.

• Until the particular stroke of the cylinder is completed, each check valve andcorresponding pilot line prevents shifting of the four way valve.

• The check valves are used to allow pilot oil to leave either end of the DirectionControl Valve while pilot pressure is applied to the opposite end.

• This permits the Direction Control Valve to shift as required.

• The fixed displacement pump delivers the oil at a constant rate to the DirectionControl Valve (DCV) which is a three position, four way manually operated andspring centered.

• When the right envelope flow path configuration is activated, the port B isconnected to pressure port P Hence the oil is forced into the piston rod end (1)through port B.

• This causes the piston to move to the left and hence the tool head moves in theforward direction to accomplish the forward stroke.

• This is known as working stroke. At this time, the oil in the left side end of thepiston (2) is returned to the reservoir (or tank) through port A.

QUICK RETURN MOTION CIRCUITS

Hydraulic Circuit for a shaping machine (also for planing machine)• The figure shows the hydraulic circuit for a shaping machine. This is a continuous

reciprocating circuit.• It uses a double acting cylinder. This circuit consists of the following

components.1. Fixed Displacement pump2. Pressure relief valve

• When the left envelope flow path configuration is activated, the port A isconnected to the pressure port P. So the oil is forced into the cylinder in the pistonend (2) through port A.

• This causes the piston to move to the right. So the tool head moves in the reversedirection. This is known as return stroke. At the same time, the oil in the cylinderat the piston rod end (1) is drained through port B to the tank.

• The speed of the tool head of shaper machine is directly proportional to the speedof the piston. Also, the speed of the piston is directly proportional to the area onwhich the fluid pressure acts. During forward stroke of the piston, the fluidpressure acts over the area A which is smaller than A (Because A contains pistonrod).

• Therefore the speed of piston to the right side (return stroke) is faster than thespeed of piston to the left side (forward stroke) i.e., The return stroke is faster thanthe working stroke. Hence, this quick return motion is achieved.

• The same circuit can be used for planing machine with quick return motion. Inthis case, the cylinder and piston will be fitted to the planing machine tableinstead of tool head.

Sequencing circuit• Sequencing circuit is shown in figure. The sequence valve is closely related to the

pressure relief valve.• It is used when a set of operations are to be controlled in a pressure related

sequence.

• In this circuit shown, the work piece is pushed into the proper position by cylinder1 and it is clamped by cylinder 2.Here, the sequence valve is connected to theextension line of cylinder 1.

• When the line pressure is more, cylinder 1 moves the work piece and hence theline pressure becomes low. But, once the work piece hits the end stop, the linepressure again rises.

• The sequence valve opens once the inlet pressure rises above a preset level. Nowthe cylinder 2 starts operating and clamps the work piece.

• The check valve across the sequence valve V allows both cylinders to retracttogether. Therefore, the sequence valve causes operations in a hydraulic circuit ina sequential manner, Another example is shown here.

• The sequence of operation of two double acting cylinders are controlled by twosequence valves.

• When the DCV is shifted to left envelope configuration, the left cylinder 1extends fully a then the right cylinder 2 extends. When the DCV is shifted to rightenvelope configuration, the right cylinder 2 retracts fully and then the left cylinderretracts.

• This sequence of cylinder operation is controlled by the two sequence valves.When the DCV is in center position, both cylinders are hydraulically locked.

• This type of circuits are used in production operation. For example, one cylinder(1) will extend and clamp the work piece and then another cylinder (2) will extendto drive spindle to drill a hole in the work piece.

• The cylinder (2) then retracts and the drill spindle is retracted. And then, cylinder(1) retracts the remove the work piece.

SYNCHRONISING CIRCUITS

The following circuit shows the synchronizing circuits.• When the DCV is in the left envelope mode, the fluid from the pump is forced to

the blank end of the cylinder 1, and fluid from the rod end of cylinder 1 is forcedto the blank end of the cylinder 2. From the rod end of the cylinder 2, the fluidreturns to the tank.

Hydraulic cylinder Synchronizing circuit

Thus, the cylinders are connected in series to operate in synchronization.For the two cylinders to be synchronized, the piston area of cylinder 2 must be equal tothe difference between the areas of the piston and rod for cylinder 1.whereA = Area of piston in cylinder 2A = Area of piston in cylinder 1A = Area of piston rod in cylinder 1.Also, the pump should be capable of delivering a pressure force P A in cylinder 1 toovercome the loads F and F acting on both cylinders.

ACCUMULATOR CIRCUITS

Accumulator• Accumulator is used as an auxiliary power source. It is a device which stores the

potential energy of the fluid held under pressure by an external source against some dforce come from gravity, mechanical springs and compressed gases.

• The stored potential energy in the accumulator acts as a quick secondary source ofpower and does useful work as required by the system.

The following are the three basic types of accumulators used in hydraulic systems.1. Gravity type2. Spring loaded type3. Gas loaded type

Weight loaded (or) Gravity type accumulator• This type of accumulator consists of vertical steel cylinder containing piston with

packing to prevent leakage. A dead weight is added on the top of the piston.

• This dead weight due to gravitation force provides the potential energy in theaccumulator.

• This type of accumulator provides a constant fluid pressure through out the fullvolume output of the unit regardless of rate and quantity of output.

• But this type of accumulator is extremely heavy weight and large size and hence itis not suitable for mobile equipment.

Spring type accumulator

Gas loaded accumulator• Gas loaded accumulators are also called hydro pneumatic accumulators. They

work with Boyle’s law of gases, which states that for a constant temperatureprocess, the pressure of a gas varies inversely with its volume.

• The gas volume of the accumulator will become half if the pressure is doubled.The compressibility of gases is the reason for the storage of potential energy.

• This potential energy forces the oil out of the accumulator when the gas expandsdue to the decrease

There are two types of gas loaded accumulator namely.1. Non-separator type2. Separator type.

Non-separator type Gas loaded accumulator• This type consists of a fully enclosed shell containing a gas charging valve on the

top and an oil port at the bottom.• In this type, the gas is filled up at the top and the heavy oil at the bottom of the

shell. There is no separator between the gas and oil and thus the pressurized gaspushes the oil directly.

• This type has ability to handle large volumes of oil. But the disadvantage of thistype is that the absorption of gas in the oil due to absence of a separator.

• The entrapped gas in the oil will cause cavitations and damage to the pump.Absorption of gas in the oil also makes oil compressible and causes the spongyoperation of the hydraulic actuators.

Separator type gas loaded accumulator• In this type, there is a separator between the gas and oil. This separator effectively

utilizes the compressibility of the gas.

There are three major types of separator type gas loaded accumulator which are givenbelow.1. Piston type2. Diaphragm type3. Bladder type

Accumulator circuitsThe following are the most common applications of accumulators.1. Accumulator as an auxiliary power source2. Accumulator as a compensator for an internal (or) external leakage3. Accumulator as an emergency power source4. Accumulator as a hydraulic shock absorber.

Accumulator as an auxiliary power source• In this circuit, the accumulator is used to store oil delivered by the pump during a

part of the working cycle.• The accumulator then releases this stored oil upon demand to complete the

remaining cycle. Therefore, it is used as a secondary (auxiliary) power source toassist the pump.

• When the four way DCV is at the left envelope mode, oil flows from theaccumulator to the blank end of the cylinder.

• This makes the piston extend until it reaches the end of its stroke. When thecylinder is in the fully extended position, the accumulator is charged by the pump.

• When the four way DCV is at the right envelope mode, oil flows from the pumpand accumulator to retract the piston rapidly.

• To supply adequate oil during retraction stroke, the accumulator size can beselected accordingly.

Accumulator as a compensator for internal (or) external leakage• A leakage may occur during an extended period of time during which the system

is pressurized but not in operation.• To prevent this leakage, accumulator can be used.

• The pump charges the accumulator and system until the maximum pressuresetting on the pressure switch is obtained.

• Once the maximum pressure is obtained, the contacts on the pressure switch openautomatically to stop the electric motor which drives the pump.

• Then the accumulator supplies leakage oil (stored already) to the system during along period of time until the system pressure drops to the minimum pressuresetting of the pressure switch.

• Once the system pressure drops to the minimum setting of the pressure switch, itcloses the electrical circuit of the pump motor and pump recharges the system.

• Thus the accumulator is used as a leakage compensator and saves electrical powerand reduces heat in the hydraulic system.

Accumulator as an emergency power source: (Safety circuits)• Sometimes, the pump (or) electrical power failure may occur and the normal

supply of oil pressure will be lost. Even at this condition, the piston should beretracted for safety purpose.

• This retraction may be accomplished by an accumulator as an emergency source.

• In this circuit, a three way, solenoid actuated DCV is used along withaccumulator. When the DCV is at the left envelope mode, oil flows to the blankend of the cylinder and also through check valve into the accumulator and rod endof the cylinder.

• During the extension of the cylinder, the accumulator is charged. If the pump (or)electrical power fails, then DCV shifts to right envelope mode.

• Then, the pressurised oil stored in the accumulator, is forced to the rod end of thecylinder to retract the piston to its starting position.

Accumulator as a hydraulic shock absorber (Industrial circuit)• In most of the industrial circuits, the accumulators are used to eliminate (or)

reduce the high-pressure pulsations (or) hydraulic shock.• Hydraulic shock is also known as water ‘hammer is cause by the sudden stoppage

(or) deceleration of a hydraulic fluid flowing at relatively high velocity in a pipeline.

• This water hammer develops a compression wave near rapidly closing valve. Thiscompression wave travels at the speed of sound, back and forth along the entirelength of the pipe until its energy is finally dissipated by friction.

• It creates rapid pressure pulsations (or) high pressure surges which will causegreat damage to the hydraulic system components.

• To suppress the pressure pulsations (or) high pressure surges, an accumulator canbe installed near the rapidly closing valve. Thus, the accumulator is used as ahydraulic shock absorber.

Safety circuits (Fail-safe circuits)• Fail-safe circuits are designed to prevent accident to the operator (or) damage to

the equipment.• These circuits prevent the system from accidentally falling on the operator and

they also prevent overloading of the system. The fail safe circuit is shown infigure.

• This circuit prevents the cylinder from accidentally falling in the event a hydraulicline ruptures (or) a person ignorantly, carelessly, operates the DCV when thepump is not operating.

• In order to lower the cylinder, the pilot pressure from the blank end must push thepiston to pilot open the check valve at the rod end side to allow the oil to returnthrough the DCV to the tank.

• This happens when the push button is pressed to admit pilot pressure actuation ofthe DC’G (or) when the DCV is directly manually actuated while the pump isoperating.

• The pilot operated DCV allows free flow in the opposite direction to retract thepiston when this DCV returns to bottom envelope mode (ie spring offset mode).

• Another safety circuit shown here is designed to protect an operator fromaccident. In order to extend or retract the cylinder, the operator must actuate bothmanually operated valves with the help of push buttons. Also, the operator can notviolate this safety rules by tying down one of the buttons, because it is necessaryto release both buttons to retract the cylinder.

• For safety purpose, if the both buttons are pressed, then the main three positiondirectional control valve is pilot - actuated to extend the cylinder. And similarly,if the both buttons are released, then only the cylinder retracts.

Pressure Intensifier circuit (or) Punching press circuit• Very high pressures can be developed by a pressure intensifier using a low-

pressure pump.• Thus we can eliminate the high pressure pump in. the punching press with the use

of pressure intensifier.• The press circuit is shown here which consists of a pilot check valve, a sequence

valve and a pressure intensifier.

• When the system pressure attains the sequence valve pressure setting, theintensifier starts to operate.

• Then the pressure intensifier increases the system and this high pressure output ofthe intensifier closes the pilot check valve and the blank end of the piston toperform the punching operation.

• In this circuit, check valve is used instead of a regular check valve to permit theretraction of the The pressure intensifier should be installed near the cylinder tokeep the high lines as short as possible.

•

• The pump delivers the Oil at a constant rate to the three position, 4 way DCV.This DCV is manually operated and spring centered.

• When the DCV is at the left envelope mode, the oil flows from the pump tocylinder end 1 through port A and forces the piston to move to the right. Now themachine table moves to right.

• At the same time, oil from the cylinder end 2 is forced to the tank through port B.• When the DCV is at the right envelope mode, the oil flows from the pump to

Cylinder end 2 through port B and forces the piston to the left.• Thus, the machine table is moved to the left At the same time, oil from cylinder

end 1 is forced to the tank through port A.• The piston is provided with piston rods at both ends. The pressure of the fluid in

both ends is uniform sinëe there is no change in cross sectional area. Hence, thespeed of the piston and thereby, the speed of the machine ‘table is uniform in bothdirections.

Hydraulic fork lift circuits (Meter-in circuit)• The following hydraulic circuit shows the lift circuit used to lift and lower the

heavy load and red hot load with safety.• The fluid is metered into the cylinder when the DCV is shifted to direct the fluid

to the cylinder rod end. (ie. DCV is in left envelope mode).• That is why, it is known as meter-in circuit. The feed direction is shown by the

arrow during lifting.

• The fluid is passing through a compensated flow control valve before entering therod end of the cylinder. Now the piston is moving up.

• During lifting, the speed should be controlled. For speed control, the fluidentering the rod end should be measured and metered.

• For example the lift cylinder on a hoist is used to lift the basket of red hot partsgently out of the furnace without damaging the furnace walls.

• The basket is then swing over the quench tank and dropped quickly to the quenchoil.

• When the DCV is the right envelope mode i.e., when it is shifted to let thecylinder drop the load, the fluid can enter the blank end of the cylinder withoutrestriction. Now, the fluid on the rod end will flow through the integral checkvalve.

Hydraulic fork lift circuits (Meter-in circuit)• The following hydraulic circuit shows the lift circuit used to lift and lower the

heavy load and red hot load with safety.• The fluid is metered into the cylinder when the DCV is shifted to direct the fluid

to the cylinder rod end. (ie. DCV is in left envelope mode).• That is why, it is known as meter-in circuit. The feed direction is shown by the

arrow during lifting.