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1 Chapter 7 Pneumatic and Hydraulic Systems Pneumatic Systems 7 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: dentist’s drill, pneumatic road drill,  automated production systems.

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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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    2

    cylinders and valves

    compressor

    reservoir

    distributionlines

    2

    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

    4

    Pressure Regulator -- to adjust pressureadjust screw

    ventilation hole

    spring

    diaphragm

    valve

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    5

    Lubricator -- to spray lubrication oil

    siphontube

    lubrication oil

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    6

    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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    7

    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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    9

    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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    1010

    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

    12

    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

    17

    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

    19

    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

    29

    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