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Chapter 7 Pneumatic and Hydraulic Systems

Pneumatic Systems7 Pneumatic systems are designed to move loads by controlling pressurized

air in distribution lines and pistons with mechanical or electronic valves.

7 Air under pressure possesses energy which can be released to do useful

work.

7 Examples of pneumatic systems: dentists drill, pneumatic road drill,

automated production systems.


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cylinders and valves

compressor

reservoir

distributionlines

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Components of a Pneumatic System

motor airtreatment

9 Compressoris the power source of a pneumatic system. It is usually driven by a

motoror an internal combustion engine. The compressed air is first stored in a

strong metal tank calledreservoir.

9 Before entering thecylindersandvalves,the compressed air has to pass through

theair treatment devices,includingair filterto remove dust and moisture,pressure regulatorto adjust pressure, andlubricatorto spray lubrication oil.


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Air Filter -- to remove dust and moisture

air

filter

condensedwater

water releasevalve


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4

Pressure Regulator -- to adjust pressureadjust screw

ventilation hole

spring

diaphragm

valve


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Lubricator -- to spray lubrication oil

siphontube

lubrication oil


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6

Pneumatic Actuator -- Cylinder

7 Cylinder is the actuator in the pneumatic system. When compressed air flows

into a cylinder, energy stored in the air will release, transferring into kineticenergy to do work.

compressed air exhaust


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Example 1. Calculating the force produced by a cylinder

7 The input air pressure is 0.5 MPa, which means the air would exert a force of

0.5N on each square millimeters. If the area of the piston is 3 00mm2

, then thetotal force produced by the cylinder will be:

force = pressure xpiston area

= 0.5 N/mm2x 3 00mm2

= 150 N300mm 2

compressed air 0.5MPa

0.5MP


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Pneumatic MotorPiston Type and Vane Type

output shaft

piston rod

piston

inlet

outlet


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Example 2. Pneumatic Drill

control lever

disk valveair inlet air duct

piston air flow

air outlet air flow

anvil

spring

blade

(a) (b)


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Pressure Control ValveRelieve Valve

adjust screw9 Relief valve, also known assafety valve,

is used to maintain the desired pressure.

spring

ball valve


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Pressure Control ValveReduce Valve

adjust screw

spring

diaphragm


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IN IN

OUT OUT

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Directional Control Valve Two Port Valve (2/2)

1' Directional control valves are

commonly described by anx/y

designation, wherex is the number of

ports andy is the number of positions.

1' 2/2 valve: 2 ports, 2 positions.

1' The two port valve is similar to the

single pole single throw switch inelectric circuits.


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Directional Control Valve Three Port Valve (3/2)

9 The three port valve is similar to the

single pole double throw switch in

electric circuits.

3exhaust 3 2

2 a i r s u p p l y s p r i n g 11


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Example 3. Pneumatic Punching Machine (I)

single acting cylinder

three port valve (3/2)

punchingmold


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Example 3. Pneumatic Punching Machine (II)

32

1

32

1

(a) (b)


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Directional Control ValveOne Way Valve

9 The one way valve allows air flow from only one direction. It is similar to

the diode in electric circuits.

ball valve


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2

1

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Directional Control ValveShuttle Valve

7 A shuttle valve has three ports and contains a small rubber piston which is

free to move between port 1A and 1B within the valve.7 If air enters the valve through port 1 A or 1B, the piston is pushed to the

other side and air can only escape through port 2.

2

1 1

1B1A1B

valve2

1A

2


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2

1

Example 4: Dual Control Pneumatic Punching Machine

a

3

1

B

3b

A

2

C

D


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2

1

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Flow Control Valvethe Flow Regulator

1' Air can pass through the

regulator in either direction.

1' If air enters from left, the ball

valve is pushed open and air

can flow through the valveunrestricted.

finger screw

needle valve

1' If air enters from right, theball valve is closed so that air

can only pass through the

regulator.

1' The flow of air can be

controlled by turning a finger

screw.

OUTIN

ball v a l v e

spring


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Control of Double Acting Cylinders (I)

five port

valve(5/2)

flow control valve


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Control of Double Acting Cylinders (II)

9 Unlike a single acting cylinder, a double acting cylinder does not contain a

return spring. Movements in both directions are powered by compressed air.

9 The flow control valve makes the downward movement of piston 2 slower

than that of piston 1. However, both pistons move upward at the same speed.

3

5

1

1 2


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Air Operated Valves

9 In the valves described so far, the spool which controls the flow of air is

moved mechanically, by a button or lever.

9 In order to be automated, direction control valves in the pneumatic systems

have to be controlled by air pressure or electrical signals.

9 In air operated valves, the spool is moved by air pressure.

4 2 2

513 3 1


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Example 5: Application of Air Operated Valves

144 2

12

5 1

21

safe region

3 3

12


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Pneumatic Solenoid Valves

7 The spool position is moved by an electrical solenoid, and can controlled

electronically.

electrical solenoid

spool

(a) (b)

+V +V


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Hydraulics

1' The working fluid in a hydraulic system is incompressible. Thus a hydraulic

system can move large loads.

50N

400N

Pascals Law

100mm2

800mm2

A B


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Hydraulic Systems

9 Pneumatic systems are open systems, always processing new air, and air is

simply exhausted to the atmosphere. Hydraulic systems are closed systems,

always recirculating the same oil.

cylinder

hydraulicvalves

oil pumpmotor

oilreservoir


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7 Only a small force is required

by the operator to raise the

heavy load. The large piston

can be stopped at any point

because the oil cannot be

compressed.

smallpiston

handle

one-wayvalve

valverelease screw

Example 6. Hydraulic Jack

large

piston


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Hydraulic Actuators

9 In a hydraulic system, the actuators transferring hydraulic energy into

mechanical motion are hydraulic cylinders and hydraulic motors.9 There are 3 types of hydraulic motors : gear pump, vane pump and axial

piston pump.

low pressure oil

high pressure oil


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Example 7. Hydraulic Brakes

brake pedal

brakepads

caliper

piston

disc

master cylinder

brake fluid disc

wheelcylinder

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brake fluid


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Example 8. Hydraulic Control loop

filter

controlvalve cylinder

oilreservoir

relief valve

pump


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Pneumatic System

7 Air is easily available

7 Fast response

7 Air is non-flammable

7 Continuous variable transmission

Hydraulic System

7 High output force

7 Accurate hydraulic pressure

7No corrosion

7 Continuous variable

transmission

Comparison between Pneumatic and Hydraulic Systems

Advantages


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Pneumatic System

Output force is limited

Compressibility of air

Corrosion may occur

Pipe length is limited

Hydraulic System

Fluid might leak out

Fluid will degrade due to heat

Fluid flow speed is limited

Pipes are complicated

Working fluid is often flammable.

Comparison between Pneumatic and Hydraulic Systems

Disadvantages

Electrical Linear Actuator