rotary actuators ppt1

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    SUDHIR

    100926005

    M.Tech Manufacturing Technology

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    Pneumatic rotary actuators convert energy of compressed air into

    rotary mechanical energy.

    A Rotary actuator can be defined in terms of the torque it produces

    and its running speed.

    The torque of a rotary actuator can be specified in three ways:

    Starting torque is the torque available to move a load from rest.

    Stall torque must be applied by the load to bring a running actuator

    to rest.

    Running torque is the torque available at any given speed.

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    DUTY CYCLEPneumatic actuators have a 100 percent duty cycle. In fact, the harder

    they work, the better they work. Electric actuators are mostcommonly available with 25 percent duty cycle motors.

    STALLINGPneumatic actuators may be stalled indefinitely without overheating. In

    some instances, if determined to be safe, the supply air pressuremay be increased within the limits set by the manufacturer toovercome temporary high torque requirements.

    Electric actuators should not be stalled. Stalling an electric motor

    draws excessive current, which generates heat in the motor and cancause damage.

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    SPEED CONTROLThe ability to control the speed of a pneumatic actuator is one of the

    most underutilized advantages of the design. Speeds with a verywide range are easily achieved with many pneumatic rotaryactuators. The benefits of this are prevention of water hammer,controlled steam injection speeds and sequencing, to name a few.

    TORQUE WEIGHT RATIO

    Pneumatic actuators have an excellent power-to-weight ratio, up toabout 4000 lbf. in. (450 N-m). Electric actuators have a high torque-to-weight ratio above 4000 lbf. in.(450 N-m). It averages around

    123:1 for pneumatic actuators & 44:1 for electric actuators at 1500lbf. in. (170 N-m).

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    EFFECT OF THROTTLING ON TORQUEEFFECT OF THROTTLING ON TORQUE

    Every size reduction of the opening crosssection, whether of the supplyhose itself or connection pieces, affects the amount of

    supplied air.

    By throttling you reduce the

    speed of your motor and

    simultaneously, the required

    torque: that means that you

    reduce the motor performance.

    By exhaust throttling, on the

    other hand, you can set the

    speed of the motor withoutgreat loss of the torque.

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    PNEUMATIC ELECTRIC

    Simple, accurate and inexpensive

    speed control

    A pulsing circuit may be added to slow

    the operating speed

    Not subject to overheating; not

    sensitive to wet environment

    Motor designed to prevent current or

    temperature damage. Must be sealed

    from moisture; heater and thermostat

    required

    100 percent duty cycle 25 percent standard duty cycle. May

    be Upgraded

    May be stalled indefinitely Should not be stalled

    Torque-to-weight ratio averages 123:1at 1500 lbf. in (170 N-m) Torque-to-weight ratio averages 44:1at 1500 lbf. in (170 N-m)

    Inherently explosion-proof, spark-proof May be hazardous

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    Medical technologypharmaceutical industry

    Food processing industryShip building,underwater usage

    Paper industry

    Foundries, iron works and

    power plants

    Machine construction

    Automobile industry

    Chemical industry

    Air tools

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    safe

    for use in potentially

    explosive environments

    sterilisable

    for repeated use inclean-rooms

    light and compact

    only 1/5 of the weight and 1/3

    of the size of an electric

    motor of equivalent power

    overload safe

    can be loaded to standstill

    with no damages

    reversible

    can be set in both

    rotational directions

    easy to control

    smoothly controlled by

    altering the pressure or

    air quantity (throttling

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    sealed

    even for underwater usage

    insensitive to acids resistant to cleaning agents

    and suitable for the high

    standards of the food

    processing industry

    insensitive to vibrations

    insensitive to heatinsensitive to dust

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    Rotary actuators are designed based on their extent of

    operation:

    ` For continuous rotation(>360)

    Air motorsPiston motor

    Radial piston motor

    Axial piston motor

    Gear motor

    Turbine motor

    Vane motor

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    ` For reciprocating rotary motion(up to 360)

    Semi rotary actuators

    Vane type

    Rack and pinion typeRotation by linkage

    Combined linear/rotary devices

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    GEAR MOTORSGEAR MOTORS` Rotation is caused due to imbalance of forces on gears.

    ` Gear motors are used in applications with a very high power

    rating ( up to 44 kW/60 hp).

    ` Speeds up to 35,000 rpm can be achieved.

    ` The direction of rotation is also reversible for these motors.

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    Tooth-gear motors consist of two tooth wheels which turn with little

    play within a housing. One wheel is connected to rotate with thedrive shaft, the other generates torque. Force is exerted on two

    flanks in the turn direction and one flank against the turn direction.

    Exhaust air builds up in chambers between the tooth flanks and

    housing wall, then is guided to the exhaust side and rotational

    movement is generated.

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    TURBINE MOTORSTURBINE MOTORS` Used where very high speeds (up to 120,000 rpm) and

    low torques are required.

    ` Lubricated air is not required, as there is lesser wear

    due to reduced or no rubbing or sliding contact between

    rotating parts & body cavity.

    `

    Common application is, high speed dentists drill.

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    PISTON MOTORSPISTON MOTORS

    ` Piston motors operate at lower speeds than vane motors, thelimiting factor being the inertia of the reciprocating parts. Freerunning speed is usually 5000 rpm or less, with the maximum powerbeing developed at 2000 to 4000 rpm.

    ` The power range at normal pressure being 1.5 - 19 kW (2 - 25 hp).

    ` Piston motors develop high starting torque, which falls withincreasing speed. Both the torque and power are directly dependenton supply air pressure.

    ` They are positive displacement machines, so the air consumption isdirectly related to speed.

    ` Motors may have output gears to increase the torque and lower the

    speed.

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    RADIAL PISTON MOTORSRADIAL PISTON MOTORS

    The crank shaft of the motor is driven by the compressed air

    via reciprocating pistons and connecting rods. To ensuresmooth running several pistons are required. The power of

    the motor depends on input pressure, number of pistons,

    piston area, stroke and piston speed.

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    AXIALAXIAL PISTON MOTORSPISTON MOTORS

    The working principle of the axial piston motor is similar to that

    of the radial piston motor. The force from 5 axially arrangedcylinders is converted into a rotary motion via a swash plate.

    Compressed air is applied to two pistons simultaneously, the

    balanced torque providing smooth

    running of the motor.

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    VANEVANE MOTORSMOTORS

    Torque is developed by pressure difference on the vanes.

    The more usual vane motor is a single chamber, but it is also

    possible to make a dual chamber motor.

    An eccentric rotor is contained in bearings in a cylindrical chamber.

    Slots are arranged in the rotor. The vanes are guided in the slots of

    the rotor and forced outwards against the inner wall of the cylinder by

    centrifugal force or via springs. This ensures that the individual

    chambers are sealed.

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    ` For motors with one turn direction, an increased turn angle for the

    expansion of the air is available. Therefore, these motors reach a

    somewhat higher efficiency.

    ` Power range 0.1 - 17 kW (0.14 - 24 hp).

    ` Speed can be achieved up to 60,000 rpm.

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    During the operation of an air motor, the vanes are pressed

    against the wall of the rotor cylinder by the centrifugal force,which seals the working chambers against each other. If there

    are special requests in regard

    to the start-torque, suitable measures must be met to guarantee

    an automatic start.

    For example: spring-loaded vanes

    pin-guided vanes

    thrust-rings

    vacuum induction, etc.

    With the above options, it is possible to achieve a start-torque,

    which is approximately 1.5fold of the nominal torque.

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    VANE TYPEVANE TYPE` The possible total angular movement for a single vane is about 270

    . The total angular movement is adjusted by means of the stops.

    ` A double vane type has two vanes, two shoes and two barrels. It is

    capable of generating approximately twice the torque of the single

    vane, but the angle of rotation is limited to about 90 .

    ` Can produce a torque of up to 200 Nm for a single vane and 400

    Nm for a double vane.

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    RACK & PINION TYPERACK & PINION TYPE There are several designs available, operated by a single acting

    cylinder with spring return, a double acting cylinder or twin cylinders.The basic principal is the same in each: the piston rod of a pneumatic

    cylinder becomes a rack which rotates the pinion shaft.

    Up to 400 Nm is possible with this type of design

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    ROTATIONROTATION BY LINKAGEBY LINKAGE

    This is a Scotch yoke actuator, consists of piston, connecting rod &rotary pin fixed to the shaft.

    The extent of rotation is normally limited to around 90

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    COMBINED LINEAR & ROTARY DEVICESCOMBINED LINEAR & ROTARY DEVICES The rotation and linear motion are geometrically related through a

    cam mechanism. The shape of the cam track has to be chosen to

    produce exactly the right kind of motion.

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