Pneumatic System

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Chapter 7 Pneumatic and Hydraulic Systems Pneumatic Systems 9

Pneumatic systems are designed to move loads by controlling pressurized air in distribution lines and pistons with mechanical or electronic valves.

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

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Examples of pneumatic systems: dentist’s drill, pneumatic road drill, automated production systems.

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Components of a Pneumatic System reservoir compressor motor

distribution lines

cylinders and valves

air treatment

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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.

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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.

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Air Filter -- to remove dust and moisture

air

filter

condensed water water release valve

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

valve

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Lubricator -- to spray lubrication oil

siphon tube lubrication oil

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Pneumatic Actuator -- Cylinder 9 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

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Example 1. Calculating the force produced by a cylinder 9 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

300mm 2

= 150 N

0.5MPa 壓縮氣體 compressed air 0.5MPa

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

inlet

outlet

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Example 2. Pneumatic Drill control lever disk valve

air inlet

air duct air flow

piston air outlet

air flow

anvil spring blade

(a)

(b)

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Pressure Control Valve – Relieve Valve 9 Relief valve, also known as safety valve, is used to maintain the desired pressure.

adjust screw

spring

ball valve

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Pressure Control Valve—Reduce Valve adjust screw spring diaphragm

減壓閥表示符號

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

IN

OUT

OUT

9 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. 9 2/2 valve: 2 ports, 2 positions. 9 The two port valve is similar to the single pole single throw switch in electric 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. 出氣口 exhaust

3 2 1

3

air氣壓源 supply 1

2

spring 彈簧

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Example 3. Pneumatic Punching Machine (I)

single 氣壓缸acting cylinder

方向控制閥 three port valve (3/2)

沖壓模 punching mold

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Example 3. Pneumatic Punching Machine (II)

3 1 3

2

2

1

(a)

(b)

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Directional Control Valve – One Way Valve 9

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

ball球閥 valve

止回閥表示符號

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Directional Control Valve – Shuttle Valve 9

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.

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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

1A

閥 valve

1B

2

1A

1B

2 1

1

梭動閥氣壓表示符號

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Example 4: Dual Control Pneumatic Punching Machine

A D

3

2 1

a

C

B

3

b

2 1

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Flow Control Valve – the Flow Regulator 調整螺栓 finger screw

9 Air can pass through the regulator in either direction.

針形閥 valve needle

9 If air enters from left, the ball valve is pushed open and air can flow through the valve unrestricted. 9 If air enters from right, the ball valve is closed so that air can only pass through the regulator. 9 The flow of air can be controlled by turning a finger screw.

OUT

IN

ball 球閥 valve

彈簧 spring

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Control of Double Acting Cylinders (I) 五口二位置方向閥 five port valve (5/2)

流量控制閥 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.

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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. 1

3 1 5

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.

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In order to be automated, direction control valves in the pneumatic systems have to be controlled by air pressure or electrical signals.

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

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2

5 1 3

2

3

1

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Example 5: Application of Air Operated Valves

4

14

2

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51 3 安全區域 safe region

2

3 1

3 1

2

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Pneumatic Solenoid Valves 9

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

+V

electrical 電磁線圈 solenoid

閥 spool

(a)

(b)

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Hydraulics 9

The working fluid in a hydraulic system is incompressible. Thus a hydraulic system can move large loads. 50N 400N

Pascal’s 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 hydraulic valves oil pump motor oil reservoir

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Example 6. Hydraulic Jack 9 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 one-way valve valve release screw

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.

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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 fluid disc master cylinder brake pedal brake pads brake fluid wheel cylinder

caliper piston disc

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Example 8. Hydraulic Control loop filter

control valve oil reservoir

relief valve

pump

cylinder

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Comparison between Pneumatic and Hydraulic Systems Advantages Pneumatic System

Hydraulic System

9 Air is easily available

9 High output force

9 Fast response

9 Accurate hydraulic pressure

9 Air is non-flammable

9 No corrosion

9 Continuous variable transmission

9 Continuous variable transmission

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Comparison between Pneumatic and Hydraulic Systems Disadvantages Pneumatic System

Hydraulic System

ƒ Output force is limited

ƒ Fluid might leak out

ƒ Compressibility of air

ƒ Fluid will degrade due to heat

ƒ Corrosion may occur

ƒ Fluid flow speed is limited

ƒ Pipe length is limited

ƒ Pipes are complicated ƒ Working fluid is often flammable.

Electrical Linear Actuator