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Chapter 7 Pneumatic Directional Control Valves KM

7.1 - Introduction

Pneumatic Directional control valves are used mainly to influence the path taken by air

stream. The main functions of these valves are to start stop and regulate the direction

of air flow and help distribution of air in the desired line. They are characterized by its

number of controlled connections or ways, the number of switching connections and

the method of actuation.

According to the method of construction D.C.V can be subdivided into following types:

1. Poppet valve - Here the connections are opened and closed by means of balls, discs

or cones.

2. Slide valve - Here the individual connections are linked together or closed by means

of spools.

7.2 Difference between poppet valve and slide valve

POPPET VALVE SLIDE VALVE

1

2

3

4

5

6

The valve element (ball, disc or

cone) sits on specially machined

and finished seat

Valve finishing is difficult and

costlier.

Construction is complicated.

Valve actuation possibility is limited.

Higher actuating force is required to

operate.

Wear and tear of the valve area is

not uniform.

The valve element is a sliding spool

sliding inside a bore in the valve

housing.

Valve bore and spool finishing is easier

and simple

Construction is simple.

All types of actuation are easily

adaptable.

Lesser actuating force is required to

operate.

Valve spool and bore generally get

uniform wear and tear.

7.3 Symbolic representation

Ports are designated using a number system in accordance with ISO 5599. Earlier

letter system was used to designate the ports. Pneumatic component symbols are as

per ISO 1219.


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Table 7.1 Port Markings

Port ISO 5599

numbering system

Lettering system remarks

Pressure port 1 P Supply port

Working ports 2

4,2

A

A,B

3/2 DCV

4/2 or 5/2 DCV

Exhaust ports 3

5,3

R

R,S

3/2 DCV

5/2 dcv

Pilot ports 12

14

Y,Z

Z

Connects port 1

to port 2

Connects port 1

to port 4

Table 7.2 ISO 1219 Graphic symbols

2/2 normally open DCV 2/2 normally closed DCV

3/2 normally open DCV 3/2 normally closed DCV

4/2 DCV 5/2 DCV

Table 7.3 Component designations

Pneumatic Actuators 1A, 2A

Processing elements and final control

elements

1V1, 1V2 etc.

2V1, 2V2 etc.

Input elements (or Signal elements) 1S1, 1S2 etc.

Power supply elements 0Z1, 0Z2 etc.


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7.4 Construction of Directional control valves.

A Directional control valve is generally specified as a port/position valve where ports

is the number of openings and position is the number of switching positions of the

valve.

7.4.1 3/2 poppet type DCV

The cross sectional view of the plunger operated 3/2 DCV in the normal and actuated

position is as shown in figure. Here a ball in conjunction with a simple valve seat is

used to control the flow. In the normal position, spring forces the ball against the valve

seat preventing the compressed air from flowing from the pressure port P to working

port A and working port A is open to exhaust port R. Actuation of the valve plunger

causes the ball to be forced away from the valve seat and working port A is open to

the pressure port P and exhaust port R gets closed.

7.4.2 - 3/2 Spool type D.C. Valve


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1. The cross sectional view of 3/2 spool type D.C.V in normal and actuated position

is shown in figure.

2. The valve element is a spool actuated by a push button and a spring .The spool

slides over the valve bore inside the valve housing.

3. In the normal position P is closed and A is open to R

4. When the push button is pressured due to movement of spool, the spring gets

compressed. Now connected to P and R close

5. When the actuating force on the push button is released the valve returns to

normal position connecting A to P closes.

7.4.3 - 5/2 Double pilot operated Suspended Disc type D.C. valve

Suspended seat valve is a 5/2 valve with sealing along different portions of the spool.

The disc seat seal connects port 1 to either port 2 or the port 4.As the valve is actuated

a disc seat seal connects P port to either a port or B port Secondary seals on the spool

pistons close which exhaust port is not required. The valve has a manual override

button at each end to manually operate the valve spool. A pneumatic signal at pilot

port Z causes the spool to switch as shown in figure b. Here paths from P to A and B

to R are open.A pneumatic signal applied to pilot part causes paths from P to B and

from A to S to open and port R is blocked.

Advantages

1. Insensitive to plugging by impurities

2. Requires low actuation force

7.4.4 - 5/3 Directional control valve.


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It has five working ports and three switching positions and with these valves double

acting cylinders can be stopped with the stroke range. This means a cylinder piston

under pressure in mid position is briefly clamped in the normally closed position and in

the normally open position piston can be moved unpressurised. If no signals are

applied at either of the two control ports the valve remains spring centered in mid

piston.

7.5 - Quick Exhaust Valve

1. Quick exhaust valve is used to increase the speed of a pneumatic cylinder by

avoiding lengthy return time through direction control valve from the cylinder so

that air energy can act quickly.

2. The principle of operation is to allow the cylinder to retract at its near maximum

speed by reducing the resistance to flow of exhausting air during motion of the

cylinder.

3. The valve consists of an inlet port P, outlet port A and an exhaust port R. A flexible

ring (poppet) moves inside the valve when pressure acts on it to block either the

inlet port or the exhaust port.

4. When there is an air flow from inlet port P, the exhaust port R is sealed by the

poppet due to compressed air acting on it, permitting the air flow to the cylinder.

5. When the compressed air is to be exhausted to atmosphere, poppet moves away

from the exhaust port and seals the inlet port due to air pressure acting on the

poppet.

6. Return air from the cylinder rapidly vents through the exhaust port R and silencer

and thus avoids the direction control valve as it normally happens.


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7.6 - Pneumatic Circuits

The structure of the circuit diagram should correspond to the control chain whereby

the signal flow is represented from the bottom to the top.

The control chain is as follows

Signal output ( ActuatorsCylinders and motors, Final control element)

Signal processing (DCV, PCV, and FCV)

Signal input (Push button, limit switch valves)

Power supply (air service unit)

Direct control of Cylinders

Cylinder is actuated directly through a manual or mechanically operated valve without

any intermediate switching of additional DCV. This type is limited to cylinder with piston

diameter smaller than 40mm.

Circuit 1: Direct Control of Single Acting Cylinder

Component

designation

Component name

0Z FRL unit1S 3way 2position DCV

1A Single acting cylinder


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Operation

1. In the initial position the air supply is shut off at port 1 and the cylinder rod is

retracted by spring return. Hence air from the piston chamber is exhausted from

port 2 to 3.

2. When the push button is pressed, 3/2 valve shifts the position. The air supply now

gets connected to single acting cylinder port from 2. The buildup of pressure causes

the piston rod of cylinder to extend against the force of the cylinder return spring.

3. As soon as the push button is released, valve shifts back to initial position and the

cylinder rod retracts.

Circuit 2: Direct Control of Double Acting Cylinder

Operation

1. A 5/2 Direction control valve controls the double acting cylinder. In the initial position

pressure is supplied to the cylinder rod side and the pressure on the piston side isexhausted from ports 4 to 5.

2. When the push button of the D.C.V is pressed supply air passes through the valve

from 1 to 4 and the piston rod advances. The displaced air flows to atmospheres

through ports 2 to 3.

3. Once the push button is releases the 5/2 valve shifts to initial position and the piston

rod retracts.

Component

designation

Component name

0Z FRL unit

1S 3way 2position DCV

1A Double acting cylinder


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Indirect control of cylinders

Cylinders with a large piston diameter requires high air and the control element

required to actuate it must have high nominal flow rate. Manually actuated valve will

not be suitable for such large valves as the force will be too less to move the spool.

Hence indirect control is preferred for such cylinders. In this method signal is generated

by another small valve, which will provide the force necessary to switch the main valve.

Circuit 3 Indirect Control of Single Acting Cylinder

Operation

1. In the initial position the single acting cylinder is retracted, port 1 of the valve 1V is

closed, port 2 is open to exhaust port 3.

2. When the push button is pressed valve 1S and pressure is applied to pilot port 12

of the valve 1V. Port 1 of the valve 1V now gets connected to port 2. Pressure

Component designation Component name

0Z FRL unit

1S 3/2 push button operated spring offset DCV

1V 3/2 pilot operated spring offset DCV



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applied to the cylinder piston causes the rod to extend. The signal at port 12

remains as long as the push button is operated.

3. When the push button is released, valve 1S returns to the initial position, signal air

in the pilot line connecting to 12 is exhausted to atmosphere through port 3.

4. Main valve 1V1 also returns to initial position and return spring causes the cylinder

to retract. Air from the cylinder is exhausted through main valve.

Circuit 4 Indirect control of double acting cylinder

Operation

1. In the initial position the double acting cylinder is retracted, port 1 of the valve 1V is

connected to port and rod end is pressurized, port 4 is open to exhaust port 5.

2. When the push button is pressed valve 1S and pressure is applied to pilot port 12

of the valve 1V. Port 1 of the valve 1V now gets connected to port 4. Pressure


0Z FRL unit

1S 3/2 push button operated spring offset DCV

1V 3/2 pilot operated spring offset DCV



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applied to the cylinder piston causes the rod to extend. The signal at port 12

remains as long as the push button is operated.

3. When the push button is released, valve 1S returns to the initial position, signal air

in the pilot line connecting to 12 is exhausted to atmosphere through port 3.

4. Main valve 1V1 also returns to initial position and air supply to port 2 causes the

cylinder to retract. Air from the cylinder is exhausted through port 5.

(Draw FRL unit at the bottom for the circuits from 5 to 9)

Circuit 5 Use of Quick Exhaust Valve in Single Acting Cylinder

1. The circuit consists of quick exhaust valve 1V installed in the path connecting to the

cylinder. The valve has a closable supply connection 1, a closable exhaust 3 and an

outlet 2.

2. When the push button is pressed, compressed air closes the exhaust port 3 of 1V and

is passed to the piston end from outlet port 2 and the piston rod advances.

3. When the push button is released, quick exhaust valve senses pressure at port 2 and

closes port 1 by pushing the poppet. This blocks the path to the DCV.

4. Exhaust port 3 of 1V opens and air rapidly exhausts through the large exhaust port 3,

bypassing the normal exhaust route through the direction control valve.

5. Rapid release of pressure in the piston end causes the cylinder to retract very fast. The

compressed air makes loud bang of noise when exhausted rapidly to atmosphere. To

bring the exhaust blasts within the allowed noise level efficient silencers are installed.

Component

designation

Component name

1S 3/2 push button operated

spring offset DCV

1V Quick exhaust valve



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Circuit 6 Use of Quick Exhaust Valve in Double Acting Cylinder

1. The circuit consists of quick exhaust valve installed near the cylinder rod end to

increase the extension speed of double acting cylinder.

2. When the push button of 1S is pressed, compressed air passes to the piston end

and the piston rod advances. Quick exhaust valve reads the pressure at port 2 and

closes port 1.

3. Air rapidly exhausts to atmosphere after passing through the silencer from port3.

Hence extension speed of the cylinder increases over the normal speed.

4. When the push button is released, air supply closes exhaust port 3 and the air

supply to the outlet port 2 causes the cylinder to retract. The retraction speed is

normal as the air from the piston will pass through the DCV.

Component

designation

Component name

0Z FRL unit

1S 5/2 push button operated

spring offset DCV

1V Quick exhaust valve.



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Circuit 7 Memory valve circuit

1. When the push button of the pilot valve 1S1 is pressed, signal is generated at pilot

port 14 of the main valve. Piston side of the cylinder is supplied with compressed

air and the piston rod advances. Push button is released as soon as the cylinder

starts extending.

2. Signal at port 14 is now lost but the valve 1V1 remains in the same position by

friction until an opposing signal is received. Signal air exhausts through port 3 of

the valve 1S1.

3. When the push button of the pilot valve 2 is pressed, signal is generated at port 12

of 1V1. The valve switches back to initial position. Compressed air is now supplied

to rod side of the cylinder and the position rod retracts. Push button is released

immediately and the cylinder retains the position due to memory function.


0Z FRL unit

1S1 and 1S2 3/2 push button operated DCV

1V1 5/2 double pilot operated DCV



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4. When pilot signals appear simultaneously at both pilot ports 14 and 12 of the main

valve, they produce equal and opposite forces on the valve spool and the latter

tends to remain stationary until one of the signals goes off this problem is called

signal conflict.

Flow control valves

They influence the volumetric flow of compressed air in both directions. One way flow

control valve throttles the air flow in one direction only. A check valve blocks the flow

of air in the bypass leg and the air can only flow through the regulated cross section.

In the opposite direction, the air can flow freely through the opened check valve. These

valves are used for the speed regulation of actuators. The two fundamental types of

throttling circuits for double acting cylinders are: supply air throttling and exhaust air

throttling.

Circuit 8 Supply air throttling Circuit

1. Here the flow control valve is installed in such a way that supply air entering the

cylinder at either strokes is throttled.

2. When the push button is operated compressed air passes through needle valve of

1V1 and the extension speed is controlled. Flow from the rod freely passes through

check valve of 1V2.

3. A load in the direction of movement of the cylinder accelerated beyond the set value

due to free escape on the outlet side.

4. When the push button is released, retraction speed of the cylinder is controlled.

Component

designation

Component name

0Z FRL unit


1V1 and 1V2 One way flow control

valve



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Circuit 9 Exhaust air throttling circuit

1. In this circuit one way flow control valve is installed on either ends in such a way

that air exhausting from the cylinder is throttled.

2. When the push button is pressed, air supply passes through one way flow control

valve, it bypasses needle valve and goes through check valve in to the piston end

and piston rod starts advancing.

3. Extension speed of the cylinder is controlled depending on the needle valve setting

of 1V2.

4. If the push button is released, air supply bypasses needle valve of 1V2 and goes

through check valve into the rod end. As the cylinder starts retracting speed is

controlled depending on the needle valve setting of valve 1V1.

5. This type is most commonly used for speed control of double acting cylinder

because smooth motion of cylinder is gained in this case. Here piston is loaded

between cushion of air the first cushion of air is due to supply air entering the

cylinder through check valve and the second cushion effect is due to exhaust air

leaving the cylinder at a slower rate. Not suitable for small volume cylinders as

effective pressure cannot build up sufficiently

Component

designation

Component name

0Z FRL unit


1V1 and 1V2 One way flow control

valve


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