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2. Components of Pneumatic System

Jun 04, 2018

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    Components of Pneumatic System

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    Pneumatic System

    Use compressed atmospheric air as the operating medium

    A pneumatic system is open

    Pressure rise in a gas can be distinctly leisurely

    The slow response of an air compressor necessitates storage

    of compressed air at the required pressure in a receiver vesselThe volume of this vessel is chosen so there are minimal

    deviations in pressure arising from flow changes in loads

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    Deviations in air pressure are smaller, and compressor control is

    easier if a large receiver feeds many loads.

    A large number of loads statistically results in a more even flow ofair from the receiver, also helping to maintain a steady pressure

    Therefore, compressed air is produced as a central service which

    is distributed around the site in a similar manner to electricity, gas

    and water.

    Pneumatic System

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    Behavior of a gas subjected to changes in pressure, volumeand temperature is governed by the general gas equation.

    PV = nRT

    A compressor increases air pressure by reducing its

    volume, resultant rise in temperature.

    A pneumatic system must therefore incorporate somemethod of removing this excess heat.

    Pneumatic System

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    Atmospheric air contains water vapour

    As compressed air is cooled, and if left the resultant water

    droplets would cause valves to jam and corrosion to formin pipes.

    An after-cooler must therefore be followed by a water

    separator.

    Often after coolers and separators are collectively

    called, primary air treatment units.

    Pneumatic System

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    Dry cool air is stored in the receiver, with apressure

    switch used to start and stop the compressor motor,maintaining the required pressure.

    Pneumatic System

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    Ideally, air in a system has a light oil mist to reduce chances of

    corrosion and to lubricate moving parts in valves, cylinders and so

    on.

    The exit air from the receiver passes through a unit which

    provides the lubricating mist along with further filtration and

    water removal.

    This process is commonly called secondary air

    treatment.

    Pneumatic System

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    Air in the receiver is held at a slightly higher pressure than

    needed to allow for pressure drops in the pipe lines.

    A local pressure regulation unit is then employed with thesecondary air treatment close to the device using air.

    Composite devices called service units comprising water

    separation, lubricator and pressure regulation are available

    for direct line monitoring close to the valves and actuators

    of a pneumatic system.

    Pneumatic System

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    Component parts of a pneumatic system

    Fig. Represents the components used in theproduction of a reliable source of compressed air

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    Compressor

    Positive displacement devices(where a fixed volume of air is

    delivered on each rotation of the compressor shaft) and

    Dynamic devicessuch as centrifugal or axial blowers.

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    A compressor is selected by the pressure it is required to

    work at and the volume of gas it is required to deliver.

    Pressure at the compressor outlet is called the workingpressure and is used to specify the compressor.

    Pressure at the operating point is called, the operating

    pressure and is used to specify valves, actuators and other

    operating devices.

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    Piston compressors

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    Piston compressors Most common type of compressor

    Piston descends during the inlet stroke the inlet valve opens and air is

    drawn into the cylinder.

    As the piston passes the bottom of the stroke, the inlet valve closes and

    the exhaust valve opens allowing air to be expelled as the piston rises

    Spring-loaded valves are used, which open and close under the action of

    air pressure across themGives significant pressure pulses at the outlet port

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    A compressor which produces one pulse of air per piston

    stoke is called a single acting compressor.

    A more even air supply can be obtained by the double

    acting action of the compressor

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    single stage compressor - go direct from atmospheric to

    required pressure in a single operation.

    If the exit pressure is above about 5 bar in a single-acting

    compressor, the compressed air temperature can rise to

    over 200oC and the motor power needed to drive the

    compressor rises accordingly.

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    pressures over a few bar it is far more economical to use amultistage compressor with cooling between stages

    Normally two stagesare used for pneumatic pressures of

    10 to 15 bar, but multistage compressors are available for

    pressures up to around 50 bar

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    Multistage compressors can be manufactured with multi-

    cylinders

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    Multi stage compressor - with a single cylinder and adouble diameter piston

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    Flexible diaphragm compressor

    There is contact between pistons and air which may introducesmall amounts of lubrication oil from the piston walls into the air.This very smallcontamination may be undesirable in food andchemical industries.common way of giving a totally clean supply by incorporating aflexible diaphragm between piston and air.

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    Piston compressors are used where high pressures (> 20 bar)and relatively low volumes (< 10,000 m3 hr-1) are needed, but

    are mechanically relatively complex with many moving parts.

    Many applications require only medium pressure (< 10 bar) and

    medium flows (around 10,000 m3 hr

    -1).

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    For these applications, rotary compressors have the advantage of

    simplicity, with fewer moving parts rotating at a constant speed,

    and a steady delivery of air without pressure pulses.

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    Rotary screw compressorConsists of two intermeshing rotating screws with minimal(around 0.05 mm) clearance.As the screws rotate, air is drawn into the housing, trapped between the screws and carried along to the discharge port,where it is delivered in a constant pulse-free stream

    Dry n Wet type ..

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    Vane compressor

    Rotary vane compressors consist of a rotor

    with multiple sliding vanes that are mounted

    eccentrically in a casing. As the rotor rotates,gas is drawn into areas of increasing volume

    and discharged as compressed gas from areas

    of small volume.

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    The lobe compressor (Roots blower)

    As the lobe impellers rotate, gas

    is trapped between the lobe

    impellers and the compressorcase where the gas is pressurized

    through the rotation of lobes and

    then discharged.

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    The lobe compressor (Roots blower)

    Positive displacement compressorHigh delivery volume but lowpressure (1-2 bar).

    Operating pressure is mainlylimited by leakage between rotorsand housing.To operate efficiently, clearances

    must be very small, and wearleads to a rapid fall in efficiency

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    Air receivers and compressor control

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    Its volume - reduces pressure fluctuationsarising from

    changes in load and from compressor switching.

    Air coming from the compressor will be warm and the

    large surface areaof the receiverdissipates this heatto the

    surrounding atmosphere.

    Any moisture left in the air from the compressor will

    condense out in the receiver, so outgoing air should be

    taken from the receiver top.

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    Air treatment

    Atmospheric air contains many harmful impurities (smoke, dust, water

    vapour) and needs treatment before it can be used.

    In general, this treatment falls into three distinct stages,

    First, inlet filtering removes particles which can damage the air

    compressor.

    Next, there is the need todry the air to reduce humidity and lower the

    dew point.

    This is normally performed between the compressor and the receiver

    and is termedprimary air treatment.

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    The final treatment is to remove moisture and dirt and the

    introduction of a fine oil mist to aid lubrication - is generallytermed secondary air treatment.

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    Filters

    Inlet filters are used to remove dirt and smoke particles beforethey can cause damage to the air compressor.

    Classified as dry filters with replaceable cartridges or wet filters

    where the incoming air is bubbled through an oil bath then

    passed through a wire mesh filter.

    Dirt particles get attached to oil droplets during the bubbling

    process and are consequently removed by the wire mesh.

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    Filters are classified according to size of particles they will

    stop. [Dust particles are generally larger than 10 m, whereas smoke andoil particles are around 1 m.]

    A filter can have a nominal rating or an absolute rating

    Micro-filters with removable cartridges passing air fromthe centre to the outside of the cartridge case will remove

    99.9% of particles

    Coarse filters, constructed out of wire mesh and called

    strainers, are often used as inlet filters

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    Air dryers

    When pneumatic components wear or become corroded as a

    result of moisture, they consume more compressed air - and

    lose energy efficiency.

    In bulk air systems a simple after cooler followed by a

    separator unit where the condensed water collects and can be

    drained off.

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    Figure shows a typical water trap and separator.

    Air flow through the unit undergoes a sudden reversal of

    direction and a deflector cone swirls the air

    Both of these cause heavier water particles to be flung outto the walls of the separator and to collect in the trap

    bottom from where they can be drained.

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    Lubricators

    Pneumatic system components and almost all pneumatic

    tools perform better when lubricated with oil.

    Lubricated valves, cylinders and air motors have properoperation and long service life.

    Too little oil can allow excessive wear and cause premature

    failure.

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    Lubricators

    Excessive oil in the pipeline is wasteful and can become a

    contaminantin the ambient area as it is carried out of tools

    and valves by the air exhaust.

    Intermittent lubrication may be the worst condition of all

    because the oil film can dry out and form sludge or varnish

    on the internal surfaces of the equipment.

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    As air enters the lubricator its velocity is increased by aventuri

    ring causing a local reduction in pressure in the upper chamber.

    The pressure differential between lower and upper chambers causes

    oil to be drawn up a riser tube, emerging as a spray to mix with the

    air.

    The needle valve adjusts the pressure differential across the oil jet

    and hence the oil flow rate.

    The air-oil mixture is forced to swirl as it leaves the central cylinder

    causing excessively large oil particles to be flung out of the air

    stream.

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    Pressure regulation Supplies air at constant pressureregardless of flow variation or upstream

    pressure.

    Helps operate the system more economically by minimizing the amount

    of pressurized air that is wasted.Helpspromote safety by operating the actuator at reduced pressure

    Extends component life because operating at higher-than-recommended

    pressures increases wear rate and reduces equipment life

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    Outlet pressure is sensed by a diaphragm preloaded with an adjustable

    pressure setting spring.

    The diaphragm falls if the outlet pressure is too high, and rises if thepressure is too low.

    If outlet pressure falls, the inlet poppet valve is pushed open admitting

    more air to raise pressure.If the outlet pressure rises, the diaphragm moves down closing the inlet

    valve and opening the central vent valve to allow excess air to escape from

    the load thereby reducing pressure.

    In a steady state the valve will balance;dithering between admitting and

    venting small amounts of air to keep load pressure at the set value.

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    Actuators:

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    Cylinders

    Pneumatic cylinders offer a straight rectilinear motion

    classified as light. Medium and heavy duty

    Functionally, cylinders may be single acting and double acting.

    They may be further classified as diaphragm cylinder, duplexcylinder, through rod cylinder etc.

    End position cushioning of cylinders - if the cylinders do not travel

    up to the end of stroke, designers need not go for cushioned

    cylinders.

    The piston rods of cylinders are given special treatment as it is the

    highly stressed part.

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    Single Acting Cylinder

    The compressed air is fed only in one side.

    Produce work only in one direction.

    The return movement of the piston is effected by a built-in

    spring or by application of an external force.

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    Construction of Single Acting Cylinder

    Generally, a single acting cylinder is made of the following elements:

    (i) The cylinder body (tube)

    (ii) Two end covers (one may be an integral part of the cylinder tube)

    (iii) A piston(iv) Piston rod

    (v) V-cup seal

    (vi) O-ring(vii) Bush at bearing to guide the piston rod

    (viii) Built-in spring.

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    The stroke islimited by the compressed length of the spring.

    There is possibility of thespring to bulge outand scratch the

    finely finished piston-rod surface, thereby damaging it.

    The air has to firstovercome the pressure of the springand

    hence somepower is lost before actual stroke of the piston

    starts.

    Spring Size effect ..

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    Using a bigger single acting (SA) cylinder is quite

    uneconomical and, therefore, such a cylinder should not

    be used for a stroke length of more than 100 mm in

    general.

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    Double Acting Cylinder

    The force exerted by the compressed air moves the

    piston in two directions

    Used particularly when the piston is required to

    perform work not only on the advance movement but

    also on the return.

    Buckling and bending must be considered

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    A DA cylinder consists of:

    (i) Cylinder tube

    (ii) Piston unit

    (iii) Piston rod

    (iv) Double cup packing on piston, rod packing of '0' rings(v) End covers

    (vi) Bronze rod guide

    (vii) Port connection

    (viii) Cushion-assembly (in the case of cushion cylinder)

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    Types of Double acting cylinder

    Through rod cylinder:Piston rod extended on both ends of

    the piston. Ensures equal force and speed on both sides of the

    cylinder

    Cushion end cylinder:Air at the end of its exhaust from the

    cylinder is regulated out, so that impact of the piston to the

    end can be avoided

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    Tandem Cylinder: two cylinders are arranged in series so

    that the force obtained from the cylinder is almost double

    Impact cylinder: piston rod is specially designed to

    withstand high force or impact. Cylinder can work at high

    velocity

    Cable cylinder: cable is attached to each side of the piston

    eliminating piston rod (Rod less cylinder), Gives longerstroke length on a smaller installation space

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    Rotary cylinder(Turn cylinder):Piston rod is having a gear

    profile meshing against a worm wheel in such a manner that

    with linear movement of the piston rod, the worm wheel

    rotates at specific angle

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    Cushion Assembly

    Gradual deceleration of the piston near the end of its stroke.

    It is especiallyhelpfulwhen the piston rod is connected to a

    heavy loadand the piston is at ahigh speed.

    It reduces the shockor the impact load on theend coversof

    the cylinder.

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    A cushion is a chamber of

    relatively small diameter into

    which a cushion nose or collar

    enters as the piston nears the end

    of its stroke so that air is trapped in

    the cylinder tube between the

    piston and cylinder cover and is

    bled-off slowly, reducing the rate

    of piston travel

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    The cover contains a cushion needle

    Passage to needle from the inside face of the cover

    Contains a ball check valve which has a passage between

    the inside face of the cover and the air inlet port.

    When the cushion nose or collar approaches the cover andenters the cushion bore, the air is trapped between the

    piston and the cover

    The cushion needle is set so that the air is bled-off to theport connection at the proper rate to slow down the piston

    and reduce the shock.

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    For the return stroke, if the air is fed via the cushion

    chamber itself, initial motion would be extremely slowsince the air acting only on the cushion nose section, the

    force available would be low, and possibly not enough to

    move the piston.

    The use of a ball-check valve will overcome this difficulty

    by allowing air to flow freely to the piston face for moving

    the piston away from the cushion bore on the return

    stroke.

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    Piston speed: Average speed 0.1 to 1.5 m/s

    Impact cylinder

    10m/sPiston force: Dependent on air pressure, the cylinder

    diameter and frictional resistance of the seating

    components

    Fth = AP

    A = Useful piston area

    P = Pressure of air

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    D = Diameter of the Pistond = Diameter of the Piston rodA1 = Area of the cross section of the pistonA2 = Effective area of the cylinder piston rod endP = Air pressureFf = Cylinder forward forceFr = Return force of cylinderFR = Friction forceFf = A1P FR Fr = A2P - FR

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    Air consumption = Piston stroke X Piston Area X Compression Ratio

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    Pneumatic Valves

    Valves are fluid power elements used for controlling and

    regulating the working medium,

    Start and stop pneumatic energy

    Control the direction of flow of compressed air

    Control the flow rate of the compressed air

    Control the pressure rating of the compressed air

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    There are various types of valves available in the family ofcompressed air system but according to their main

    function, they may be divided into four broad groups:

    (i) Direction control valves

    (ii) Direction control check valves or non-return valves

    (iii) Flow control valves

    (iv) Pressure control valves

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    Directional Control Valves D.C. Valves)

    Used mainly to direct the flow of the pressure fluid

    in the desired directions.

    The main functions of these valves are to start,

    stop and regulate the direction of air flow and help

    distribution of air in the desired line.

    They can be actuated to assume different positionsby various actuating mediums, viz. electrical,

    mechanical, pneumatic, or other modes of control.

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    Valve osition

    A direction control valve has two or three

    working positions generally. They are:

    (i) Normal or zero position (natural) orneutral position

    (ii) Working position (the working position

    can be two or more)

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    Direction control valve has generally two, three, four or five ports

    or openings - termed ways or ports.

    The ways are designated by letters or alphabets such as:P =compressor line port (pump in the case of hydraulics)

    R =exhaust port (T in the case of hydraulics)

    {A, B} =working ports to cylinders or motors

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    In direction control valves with spring return, theneutral

    position is defined as the position to which thevalve

    returns after the actuating force has been withdrawn.

    Neutral position is indicated as "0".

    The starting position or initial position is defined as the

    position taken up by the valve (due to spring in-case of

    spring actuated D.C. valve) after installation.

    The valve attains the working positions when actuated.

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    Valve symbol A valve position is represented by a square The number of squares is equal to the number of distinct positions that the valve can take upInside a square, the lines indicate the flow and the arrows,the directions of flow

    Cut-offs of air flow is shown by short traverse lines insidethe squareConnections to inlet and outlet ports are drawn only to aconnecting position, i.e. initial position or neutral.

    The other positions are obtained by shifting the set ofsquares until the connections match the correspondinglines in the square.

    The connections are denoted by English Capital Letters(alphabets)

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    The other positions are obtained by shifting the set ofsquares until the connections match the correspondinglines in the square.

    The connections are denoted by English Capital Letters(alphabets)

    The working line connections are indicated by A, B, C. The compressor line connection (the pressure source) isindicated by P, and return lines by R, S and T.

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    Types of control valve

    There are essentially three types of control valve: poppetvalves, spool valves and rotary valves

    Poppet valves

    In a poppet valve, simple discs, cones or balls are used inconjunction with simple valve seats to control flow.

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    Poppet valves are simple, cheap and robust

    A major disadvantage of poppet valves is the force needed

    to operate them.

    Large capacity valves need large valve areas, leading to

    large operating force.

    They are mainly found in low pressure pneumatic systems.

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    Spool valves

    Spool (or slide) valves are constructed with a spool moving

    horizontally within the valve body

    Raised areas called 'lands' block or open ports to give the

    required operation.

    The operation of a spool valve is generally balanced.

    In the valve construction pressure is applied to oppos

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    In the valve construction, pressure is applied to opposfaces D and E and low tank pressure to faces F and G.

    There is no net force on the spool from system pressuallowing the spool to be easily moved.

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    Advantage of spool valves; different operations can be achievedwith a common body and different spools.

    This obviously reduces manufacturing costs.

    S l l d b hif i h l

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    Spool valves are operated by shifting the spool.

    This can be achieved by button, lever or striker, or

    remotely with a solenoid.

    Solenoid-operated valves commonly work at 24 V DC or

    110 V AC.A solenoid can exert a pull or push of about 5 to 10 kg,

    adequate for most pneumatic spool valves, but is too low

    for direct operation of large capacity hydraulic valves.

    Here pilot operation must be used,

    Rotary valves

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    Rotary valves consist of a rotating spool which aligns withholes in the valve casing to give the required operation.Rotary valves are compact, simple and have low operatingforces.

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    Check valve

    Check valves only allow flow in one directionand, as such,are similar in operation to electronic diodes.

    Free flow direction is normally marked with an arrow onthe valve casing.

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    Shuttle valves

    A shuttle valve, also known as a double check valve, allowspressure in a line to be obtained from alternative sources.

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    Fast exhaust valve

    Used to vent cylinders quickly.

    Used with spring return (single-acting) pneumatic cylinders.

    Consists of a movable disc which allows port A to be

    connected to pressure port P or large exhaust port R.

    Fast exhaust valves areusually mounted local to, or directly

    onto, cylinders andspeed up response by avoiding any delay

    from return pipes and control valves

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    twin pressure valve - AND GATE.

    Here an output is produced if both the input signals are fed

    Sequence valves

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    The sequence valve is used where a set of operations are to becontrolled in a pressure related sequence.Typical example where a workpiece is pushed into position bycylinder 1 and clamped by cylinder 2

    S l V 2 i d h d li f li d

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    Sequence valve V 2 is connected to the extend line of cylinder

    1.

    When this cylinder is moving the workpiece, the line pressure is

    low, but rises once the workpiece hits the end stop.

    The sequence valve opens once its inlet pressure rises above apre-set level

    Cylinder 2 then operates to clamp the workpiece.

    A check valve across V 2 allows both cylinders to retract

    together.

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    Flow control valves

    Flow control valves influence the volumetric flow of thecompressed air in both directions.Ex: Throttle valve - used for speed control of cylinders. [Care -throttle valve does not close fully]

    One-way flow control valve

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    One way flow control valve

    The air flow is throttled in one direction only.

    In the opposite direction, the air can flow freely throughthe opened check valve

    Fundamentally, there are two types of throttling circuits for

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    y yp gdouble-acting cylinders:

    Supply air throttling Exhaust air throttling

    Supply air throttling: One-way flow control valves are installed sothat the air entering the cylinder is throttled.

    The exhaust air can escape freelythrough the check valve of thethrottle valve on the outlet side of the cylinder.

    The slightest fluctuations in the load on the piston rod, lead tovery large irregularities in the feed speed.

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    A load in the direction of movement of the cylinder accelerates the

    cylinder beyond the set value.

    Therefore supply air throttling can be used for single-acting and small

    volume cylinders.

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    Exhaust air throttling

    The supply air flows freely to the cylinder and the exhaust air

    is throttled.

    In this case, the piston is loaded between two cushions of air.

    Arranging throttle relief valves in this way contributes

    substantially to the improvement of feed behaviour.

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    Time delay valve

    The time delay valve is a combined 3/2-way valve, one way flow

    control valve and air reservoir.

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    The compressed air is suppliedto the valve at connection1.

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    p pp

    The control air flows into the valve at 12through a one-way flow

    control valve and depending on the setting of the throttling screw, agreater or lesser amount of air flows per unit of time into the air

    reservoir.

    When the necessary control pressure has built up in the air reservoir,the pilot spool of the 3/2-way valve is moved downwards.

    This blocks the passage from2 to 3.

    The valve disc is liftedfrom its seat andthus air can flow from 1 to 2.

    The time required for pressure to build up in the air reservoir is equal

    to the control time delay of the valve.

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    By using additional reservoirs, the time can be extended.An accurate switch-over time is assured, if the air is clean

    and the pressure relatively constant.

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    Time response with delay on

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    Time response with delay off

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    Signal shortening / signal switch off