CHAPTER 11 MEM 341

CHAPTER 11:Pneumatic Circuits and
Applications

11.0 Pneumatic System
11.1 Graphic symbol
11.2 Basic pneumatic circuits
11.3 Pneumatic circuit application

11.0 Pneumatic System

 A pneumatic circuit consists of a variety of components
arranged so that a useful task can be performed, such as:

 compressors
 receivers
 filters
 pressure regulators
 lubricators
 air dryers
 actuators
 control valves
 conductors

 In a pneumatic circuit, the force delivered by a cylinder and the
torque delivered by a motor are determined by the pressure levels
established by pressure regulators placed at the desired location in
the circuit.

 The linear speed of the pneumatic cylinder and the rotational
speed of an air motor is determined by flow control valves placed
at the desired locations in the circuit.

 The direction of flow in various flow paths is established by the
proper location of directional control valves. After the pressurized
air is spent driving the actuators, it is then exhausted back into the
atmosphere.

 When analyzing or designing a pneumatic circuit, the following four
important considerations must be taken into account:

 Safety of operation
 Performance of desired function
 Efficiency of operation
 Cost

11.1 Graphical symbols





Basic pneumatic circuits

 Pneumatic circuits are similar to hydraulic circuits
 One difference is that no return lines are used in pneumatic

circuits because the exhausted air is released directly into the
atmosphere.
 Also, no input device (i.e. pump) is shown, because pneumatic
circuits use a centralized compressor as their source of energy.

Circuit #1: Control of a Single Acting Air Cylinder

Figure above shows a basic pneumatic circuit consisting of a 3-way
DCV, single acting cylinder.

 The return stroke is accomplished by a compression spring located at the rod
end of the cylinder

 When the push button valve is actuated, the cylinder extends
 The cylinder retracts when the valve is deactivated
 Needle valves V1 and V2 permit speed control of the cylinder extension and

retraction strokes

Circuit #2: Control of a Double Acting Air Cylinder

Figure above shows a basic pneumatic circuit consisting of a 4-way
DCV, double acting cylinder.

 Double acting cylinder requires a 4-way DCV
 Actuation of the push-button valve extends the cylinder
 The spring offset mode causes the cylinder to retract under air power.

Circuit #3: Indirect Control of a Double Acting Air
Cylinder

Describe what happens when the push
button is pressed and released.

1. Pressing the push button and then
releasing it will allow the
compressed air to enter the system
via port 2. Air pressure will then
activate the 5/2 DCV moving it into
its left position.

2. Once the 5/2 DCV is in its left position, compressed air will enter the double-
acting cylinder through its blank end, hence extending the piston rod

3. Once the piston rod is fully extended, it will mechanically activate the
roller operated 3/2 DCV. Activation of this DCV will then allow compressed
air to enter it and in turn pushes back the 5/2 DCV into its right position.

4. Once the 5/2 DCV is in its right position, compressed air will enter the
double-acting cylinder through the rod end, hence retracting the piston
rod.

Cylinder cycle timing system
Figure shows a circuit that employs a limit valve to provide a timed cylinder extend and
retract cycle.When push-button valveV3 is momentarily actuated, valveV2shifts to
extend the cylinder.When the piston rod cam actuates limit valveV4, it shiftsV2 into its
opposite mode to retract the cylinder. Flow control valveV1 controls the flow rate and
thus the cylinder speed.

Two – step speed control
A two-step speed control system is shown.The operation is as follows, assuming that flow
control valve V3 is adjusted to allow a greater flow rate than valve V4. Initially the cylinder is
fully retracted.When pushbutton valve V1 is actuated, air flow goes through valves V2,V3 and
the shuttle valve V5 to extend the cylinder at high speed.When the piston rod cam actuates
valve V6, valve V2 shifts.The flow is therefore diverted to valve V4 abd through the shuttle
valve. However, due to the low flow setting of valve V4, the extension speed of the cylinder is
reduced.After the cylinder has fully extended, valve V1 is released by the operator to cause
retraction of the cylinder.

Two-handed safety control circuit



11.3 Application of Pneumatic Circuit

The application of pneumatic circuits is very extensive. The
following are some examples.
a. Transport system

I. Figure below shows a simplified industrial transport system.
II. When the button is pushed, the cylinder will push one of the goods

from the shelf onto the transport belt.
III. When the button switch is released, the cylinder will retract

automatically.

b. Vehicle door operation system

I. Pneumatic circuits can be used to operate the doors of public
vehicles.

II. Assuming the opening and closing of the doors are controlled by two
button switches ON and OFF.

III. When the button is switch ON is pressed, the doors will open. When
the button switch OFF is pushed, the doors will close.

IV. Figure below shows a pneumatic circuit that can be used to operate
the doors of vehicles.

Tutorial Questions

1. What does the circuit of Figure 1 accomplish when the manual shutoff valve V1 is
opened?

Figure 1

2. Consider the circuit of Figure 2
a.What happens to the cylinder when valve V4 is depressed?
b.What happens to the cylinder when valve V5 is depressed?

Figure 2

3. For the circuit of Figure 3, what happens to the cylinders in each case?
a.Valve V1 is actuated and held
b.Valve V1 is released and valveV2 is actuated and held.ValvesV3 andV4 are sequence valves.

Figure 3
4. For the circuit in Question 3, based on Figure 3, cylinder 1 will not hold against a load while

cylinder 2 is retracting. Modify this circuit by adding a pilot check valve and appropriate piping so
that cylinder 1 will hold while cylinder 2 is retracting.

5.The pneumatic system of Figure 4 contains a double-rod cylinder controlled by two
three-way, two-position directional control valves. Describe the four operating
conditions for this system.

Figure 4