11

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.

Chapter 7 Pneumatic and Hydraulic Systems

22

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.

motor

compressor

reservoir

distribution lines

airtreatment

cylinders and valves

Components of a Pneumatic System

33

air

filter

condensedwater

water releasevalve

Air Filter -- to remove dust and moisture

44

Pressure Regulator -- to adjust pressureadjust screw

spring

valve

diaphragm

ventilation hole

55

Lubricator -- to spray lubrication oil

lubrication oil

siphontube

66

Pneumatic Actuator -- Cylinder

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

77

Example 1. Calculating the force produced by a cylinder

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 300mm2, then the total force produced by the cylinder will be:

force = pressure × piston area

= 0.5 N/mm2 × 300mm2

= 150 N

壓縮氣體 0.5MPa

2mm300

compressed air 0.5MPa

88

Pneumatic Motor – Piston Type and Vane Type

inlet

outlet

piston

piston rodoutput shaft

99

blade

spring

anvil

air outlet

piston

air inlet

control lever

air flow

disk valve

air flow

air duct

(a) (b)

Example 2. Pneumatic Drill

1010

Pressure Control Valve – Relieve Valve

adjust screw

spring

ball valve

Relief valve, also known as safety valve, is used to maintain the desired pressure.

1111

Pressure Control Valve—Reduce Valveadjust screw

spring

diaphragm

減壓閥表示符號

1212

Directional Control Valve – Two Port Valve (2/2)

OUT

IN

OUT

IN

Directional control valves are commonly described by an x/ydesignation, 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.

1313

2

3

1

Directional Control Valve – Three Port Valve (3/2)

The three port valve is similar to the single pole double throw switch in electric circuits.

1

23

彈簧

出氣口

氣壓源 1

23

彈簧

出氣口

氣壓源

exhaust

air supply spring

1414

Example 3. Pneumatic Punching Machine (I)

氣壓缸

方向控制閥

沖壓模

single acting cylinder

three port valve (3/2)

punching mold

1515

Example 3. Pneumatic Punching Machine (II)

13 2

13 22

(a) (b)

13 2

13 22

(a) (b)

1616

The one way valve allows air flow from only one direction. It is similar to the diode in electric circuits.

球閥

止回閥表示符號

ball valve

Directional Control Valve – One Way Valve

1717

1B1A

2

2

11

1B1A

2閥

梭動閥氣壓表示符號

valve

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.

Directional Control Valve – Shuttle Valve

1818

Example 4: Dual Control Pneumatic Punching Machine

123

123

A

Ba

b

C

D

1919

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.

球閥彈簧

調整螺栓

IN OUT

針形閥

finger screw

needle valve

springball valve

2020

Control of Double Acting Cylinders (I)

流量控制閥

五口二位置方向閥 five port valve (5/2)

flow control valve

2121

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.

513

513

1 2

2222

Air Operated Valves

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.

35 1

24

3 1

2

2323

Example 5: Application of Air Operated Valves

1 23

35

2 31

4 2

1

14 12

安全區域

1 23

35

2 31

4 2

1

14 12

安全區域safe region

2424

Pneumatic Solenoid Valves

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

+V +V

電磁線圈

閥

(b)(a)

electrical solenoid

spool

2525

Hydraulics

400N

50N

B A

800mm2

100mm2

Pascal’s Law

The working fluid in a hydraulic system is incompressible. Thus a hydraulic system can move large loads.

2626

Hydraulic Systems

oilreservoir

motoroil pump

hydraulicvalves

cylinder

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.

2727

Example 6. Hydraulic Jack

handle

smallpiston

one-wayvalvevalve

release screw

largepiston

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.

2828

Hydraulic Actuators

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.

high pressure oil

low pressure oil

2929

Example 7. Hydraulic Brakes

brake pedal

brake fluid

master cylinder

disc

caliperpiston

brakepads

disc

wheelcylinder

brake fluid

3030

Example 8. Hydraulic Control loop

oilreservoir

filter

cylinder

relief valve

pump

controlvalve

3131

Comparison between Pneumatic and Hydraulic Systems

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

3232

Comparison between Pneumatic and Hydraulic Systems

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