Pneumatic Systems Chapter 7 Pneumatic and Hydraulic Systems

Chapter 7 Pneumatic and Hydraulic Systems
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Pneumatic Systems
Pneumatic systems are designed to move loads by controlling pressurized
air in distribution lines and pistons with mechanical or electronic valves.
Air under pressure possesses energy which can be released to do useful
work.
Examples of pneumatic systems: dentist’s drill, pneumatic road drill,
automated production systems.

Components of a Pneumatic System
2
compressor
motor
reservoir
distribution
lines
air
treatment
cylinders and valves
Compressor is the power source of a pneumatic system. It is usually driven by a
motor or an internal combustion engine. The compressed air is first stored in a
strong metal tank called reservoir.
Before entering the cylinders and valves, the compressed air has to pass through
the air treatment devices, including air filter to remove dust and moisture,
pressure regulator to adjust pressure, and lubricator to spray lubrication oil.

Air Filter -- to remove dust and moisture
3
air
filter
condensed
water
water release
valve

Pressure Regulator -- to adjust pressure
adjust screw
ventilation hole
spring
diaphragm
4
valve

Lubricator -- to spray lubrication oil
5
siphon
tube
lubrication oil

Pneumatic Actuator -- Cylinder
6
Cylinder is the actuator in the pneumatic system. When compressed air flows
into a cylinder, energy stored in the air will release, transferring into kinetic
energy to do work.
compressed air
exhaust

Example 1. Calculating the force produced by a cylinder
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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 300mm 2 , then the
total force produced by the cylinder will be:
force = pressure × piston area
= 0.5 N/mm 2 × 300mm 2
= 150 N
2
300 mm
compressed 壓 縮 氣 air 體 0.5MPa

Pneumatic Motor – Piston Type and Vane Type
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piston rod
output shaft
piston
inlet
outlet

Example 2. Pneumatic Drill
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control lever
air inlet
air duct
disk valve
piston
air flow
air outlet
anvil
air flow
spring
blade
(a)
(b)

Pressure Control Valve – Relieve Valve
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Relief valve, also known as safety valve,
is used to maintain the desired pressure.
adjust screw
spring
ball valve

Pressure Control Valve—Reduce Valve
adjust screw
11
spring
diaphragm
減 壓 閥 表 示 符 號

Directional Control Valve – Two Port Valve (2/2)
12
IN
IN
OUT
OUT
Directional control valves are
commonly described by an x/y
designation, where x is the number of
ports and y is the number of positions.
2/2 valve: 2 ports, 2 positions.
The two port valve is similar to the
single pole single throw switch in
electric circuits.

Directional Control Valve – Three Port Valve (3/2)
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The three port valve is similar to the
single pole double throw switch in
electric circuits.
3
出 氣 口
exhaust
3
2
2
氣 壓 源
1
air supply
spring
彈 簧
1

Example 3. Pneumatic Punching Machine (I)
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single 氣 壓 缸 acting cylinder
方 three 向 控 port 制 閥 valve (3/2)
沖 punching 壓 模 mold

Example 3. Pneumatic Punching Machine (II)
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3
1
2
3
1
2
(a)
(b)

Directional Control Valve – One Way Valve
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The one way valve allows air flow from only one direction. It is similar to
the diode in electric circuits.
ball 球 valve 閥
止 回 閥 表 示 符 號

Directional Control Valve – Shuttle Valve
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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.
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
閥 valve
2
1A
1B
1A
1B
2
1
1
梭 動 閥 氣 壓 表 示 符 號

Example 4: Dual Control Pneumatic Punching Machine
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A
D
a
3
1
B
2
C
b
3
1
2

Flow Control Valve – the Flow Regulator
Air can pass through the
regulator in either direction.
If air enters from left, the ball
valve is pushed open and air
can flow through the valve
unrestricted.
If air enters from right, the
ball valve is closed so that air
can only pass through the
regulator.
The flow of air can be
controlled by turning a finger
screw.
IN
ball 球 valve 閥
spring 彈 簧
finger 調 整 screw 螺 栓
needle 針 形 閥 valve
OUT
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Control of Double Acting Cylinders (I)
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five 五 口 二 port 位 置 valve 方 向 閥 (5/2)
流 量 控 制 閥
flow control valve

Control of Double Acting Cylinders (II)
Unlike a single acting cylinder, a double acting cylinder does not contain a
return spring. Movements in both directions are powered by compressed air.
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.
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1 2
3
1
5

Air Operated Valves
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In the valves described so far, the spool which controls the flow of air is
moved mechanically, by a button or lever.
In order to be automated, direction control valves in the pneumatic systems
have to be controlled by air pressure or electrical signals.
In air operated valves, the spool is moved by air pressure.
4
2
2
5 1
3
3 1

Example 5: Application of Air Operated Valves
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4 2
14 12
5
1
3
安 safe 全 區 region
域
2
1
3
3
1 2

Pneumatic Solenoid Valves
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The spool position is moved by an electrical solenoid, and can controlled
electronically.
+V +V
electrical solenoid
電 磁 線 圈
spool 閥
(a)
(b)

Hydraulics
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The working fluid in a hydraulic system is incompressible. Thus a hydraulic
system can move large loads.
50N
400N
Pascal’s Law
100mm 2
800mm 2
A
B

Hydraulic Systems
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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
oil pump
motor
hydraulic
valves
oil
reservoir

Example 6. Hydraulic Jack
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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.
handle
small
piston
large
piston
one-way
valve
valve
release screw

Hydraulic Actuators
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In a hydraulic system, the actuators transferring hydraulic energy into
mechanical motion are hydraulic cylinders and hydraulic motors.
There are 3 types of hydraulic motors : gear pump, vane pump and axial
piston pump.
low pressure oil
high pressure oil

Example 7. Hydraulic Brakes
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brake fluid
disc
master cylinder
brake pedal
brake
pads
brake fluid
piston
disc
caliper
wheel
cylinder

Example 8. Hydraulic Control loop
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filter
control
valve
cylinder
oil
reservoir
relief valve
pump

Comparison between Pneumatic and Hydraulic Systems
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Advantages
Pneumatic System
Air is easily available
Fast response
Air is non-flammable
Continuous variable transmission
Hydraulic System
High output force
Accurate hydraulic pressure
No corrosion
Continuous variable
transmission

Comparison between Pneumatic and Hydraulic Systems
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Disadvantages
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.
Electrical Linear Actuator