fluid power - (me353)- lec6

8/17/2019 Fluid Power - (ME353)- Lec6

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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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