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Fluid Power Systems (ME353)
Fall 2012
Lecture 6
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Actuators
Workhorses of the System
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Hydraulic Cylinders
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Hydraulic Cylinders
Actuators are the components used in a
hydraulic system to provide power to a required
work location
Cylinders are the hydraulic system components
that convert fluid pressure and flow into linear
mechanical force and movement
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A basic cylinder consists of:
– Piston
– Piston rod
– Barrel
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The piston forms sealed, variable-volume chambers in the
cylinder
System fluid forced into the chambers drives the piston and rodassembly
Linear movement is produced
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Seals prevent leakage between:
– Piston and cylinder barrel
– Piston rod and head
– Barrel and its endpieces
Wiper seal, or scraper, prevents dirt and water from entering the cylinder
during rod retraction
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Single- and double-acting cylinders
Single-acting Double-acting
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Single-acting cylinders
– The circuit generates force in the extension only
– They require an outside force to complete the second motion
Double-acting cylinders generate force during both extension
and retraction
– Directional control valve alternately directs fluid to opposite
sides of the piston – Force output varies between extension and retraction
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Effective piston area is reduced on retraction
due to the rod cross section
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Volume is reduced on retraction
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Classification according to construction Type: External tie rod bolts are used to secure the ends on the Tie-Rod cylinder
design
– Commonly found on heavy industrial machines
– External tie rods increase chance of damage and promote accumulation
of dirt
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Mill cylinders
Yates Industries, Inc.
Threaded-end cylinder
One-piece cylinder has the cylinder barrel welded to the ends Produces a compact actuator
– Cost effective to manufacture
– Cannot be serviced (throwaway)
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Hydraulic Ram is commonly
used in hand-operated jacks – Rod is basically the same
diameter as the inside of the
cylinder barrel
– Large-diameter rod is more rigidunder load, but cylinder can
generate force in only one
direction (no volume for the
hydraulic fluid in the rod side)
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Telescoping cylinders are available for applications
requiring long extensiondistances
– Rod is made up of several
tubes of varying size nested
inside of the barrel – Each tube extends,
producing a rod longer than
the cylinder barrel
– Typical example is theactuator that raises the box
on a dump truck
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Cylinders often use Hydraulic Cushions Devices
– Provide a controlled approach to the end of the stroke
– Reduces the shock of the impact as the piston contacts thecylinder head
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A variety of mounting configurations are used to attach the cylinder body
and rod end to machinery
– Fixed centerline
– Fixed-noncenterline
– Pivoting centerline (clevis mount or trunnion mount)
– Expected cylinder loading is the major factor in the selection of the mounting style
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The force generated by a cylinder is calculated by multiplying
the effective area of the piston by the system pressure
Aext > Aret
Fext > Fret (Constant pressure P)
Vext < Vret (Constant Flow Rate Q)
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Hydraulic cylinder manufacturers provide detailed
specifications concerning:
– Construction
– Physical size
– Load capacity
This information includes basic factors such as:
– Bore
– Stroke
– Pressure rating – Other details, such as service rating, rod end configurations,
and dimensions
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Typical manufacturer’s catalog page
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Limited-Rotation Hydraulic Actuators
Limited-rotation devices are actuators with an output shaft that typically
applies torque through approximately 360° of rotation Models are available that are limited to less than one revolution, while others
may produce several revolutions
Most common designs of limited-rotation actuators are:
– Rack-and-pinion
– Vane
– Helical piston and rod
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Rack-and-pinion
Vane limited-rotation actuator
Helical piston and rod
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Limited-rotation actuators are used to perform a number of
functions in a variety of industrial situations
– Indexing devices on machine tools
– Clamping of workpieces
– Operation of large valves
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Hydraulic Motors
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Hydraulic motors are called rotary actuators
They convert fluid pressure and flow into
torque and rotational movement
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All basic hydraulic motors consist of three
component groups:
– Housing
– Rotating internal parts
– Power output shaft
System fluid enters the housing and applies pressure to the rotating internal
parts
This, in turn, moves the power output shaft and applies torque to rotate a
load
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Primary parts that produce the rotating motion in most hydraulic
motors are either:
– Gears
– Vanes
– Pistons
Four requirements of a motor
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Displacement of a hydraulic motor indicates the volume of
fluid needed to turn the output shaft one revolution
– Fixed displacement – Variable displacement
In a fixed-displacement motor:
– Internal geometry cannot be changed
– Same volume needed per output shaft revolution
In a variable-displacement motor:
– Internal geometry can be changed
– Displacement per shaft revolution can be adjusted
– Motor can operate at variable speeds with a constant input
flow
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1- Gear Hydraulic Motors
The external gear hydraulic motor is the most
common and simplest of the basic motor types
– Fixed displacement
– Unbalanced load on the bearings
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The most common internal gear motor has a
gerotor design
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2- Hydraulic Vane Motors Basic vane motor has a slotted rotor located off center in a circular
chamber and fitted with movable vanes – Space between the vanes creates a number of variable-sized chambers
– Forcing fluid into the small-size chambers causes the volume of thechambers to increase, turning the motor shaft
– Basic vane motor is fixed displacement with an unbalanced bearing
load
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Balanced vane motors evenly distribute the load on the bearings
– Achieved by operating the rotor and vanes in a slightly oblong chamber
– Allows two inlet ports and two outlets ports to be used in the motor
– Placing ports opposite each other balances bearing loading
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Vane motors are available as either fixed or variable displacement
The variable-displacement feature allows an operator to change the speed ofa motor without changing the system flow rate
In variable-displacement designs, the chamber in which the rotor and vanesoperate is contained in a moveable ring
– When the center point of the rotor and ring are concentric, the
displacement is zero
– Moving the ring so the center points are not concentric increases the
motor displacement and changes motor speed
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3- Hydraulic Piston Motors
Piston motors are available having either fixed or variable displacements
In variable-displacement designs, the length of the piston stroke is changed
to vary the volume of fluid needed to rotate the motor one revolution
Two basic classifications of piston motors are axial piston and radial piston
– An axial piston motor has pistons with centerlines parallel to the axis of
the output shaft
– A radial piston motor has pistons with centerlines perpendicular to the
axis of the output shaft
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Axial piston motors are available in two configurations:
– Inline
– Bent axis
In an inline piston motor:
– Centerline of the barrel is concentric with the centerline of the power
output shaft
– A swash plate transmits force from the pistons to the shaft
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In a bent-axis piston motor:
– Centerline of the barrel is at an angle to the centerline of the output shaft
– A universal joint and other fittings are used to transmit force between the
barrel and the output shaft
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