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8/8/2019 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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