chapter 9 actuators - pbworks

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Chapter 9Chapter 9

ActuatorsActuators

Workhorses of the SystemWorkhorses of the Systemf yf y

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ObjectivesObjectives

Describe the construction and operation of basic hydraulic cylinders limited-rotationbasic hydraulic cylinders, limited rotation actuators, and motors. Compare the design and operation of various

types of hydraulic cylinders. Select appropriate cylinder design options

available for mounting hydraulic cylinders and

© Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only.3

reducing hydraulic shock. Compare the design and operation of various

types of hydraulic motors.

ObjectivesObjectives

Contrast the operation of fixed- and variable-speed hydraulic motorsspeed hydraulic motors. Describe the construction and operation of a

basic hydrostatic transmission. Size hydraulic cylinders and motors to correctly

meet system force and speed requirements. Interpret manufacturer specifications for


Interpret manufacturer specifications for hydraulic cylinders.

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Hydraulic CylindersHydraulic Cylinders

Actuators are the components used in a h d li t t id t i dhydraulic 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


mechanical force and movement


A basic cylinder consists of:– Piston

– Piston rod

– Barrel


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Parts of a typical cylinder



The piston forms sealed, variable-volume h b i th li dchambers in the cylinder

System fluid forced into the chambers drives the piston and rod assembly

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


from entering the cylinder during rod retraction


Various seals are used in a cylinder


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Rod wipers prevent t i ti fcontamination from

entering on rod retraction


IMI Norgren, Inc.


Cylinders are typically classified by operating i i l b t ti tprinciple or by construction type

– Single-acting or double-acting

– Tie rod, mill, threaded end, or one piece


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Single- and double-acting cylinders


Single-acting Double-acting


Single-acting cylinders exert force either on t i t tiextension or retraction

They require an outside force to complete the second motion


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Double-acting cylinders generate force during b th t i d t tiboth extension and retraction– Directional control valve alternately directs fluid to

opposite sides of the piston

– Force output varies between extension and retraction



Effective piston area is reduced on retraction due to the rod cross sectiondue to the rod cross section


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Volume is reduced on retraction



External tie rod bolts are used to secure the d th ti d li d d iends 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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Tie-rod cylinder



Mill cylinders


Yates Industries, Inc.

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Threaded-end cylinder


Bailey International Corporation


One-piece cylinder has the cylinder barrel ld d t th dwelded 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-t d j koperated jacks

– Rod is basically the same diameter as the inside of the cylinder barrel

– Large-diameter rod is more rigid under load, but cylinder can generate force in only one direction



Typical hand-operated jack


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Telescoping cylinders are available for li ti i i l t i di tapplications requiring long extension distances

– 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 the actuator that raises the box on a dump truck


Telescoping cylinders


Star Hydraulics, Inc.

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Cylinders often use hydraulic cushions– Provide a controlled approach to the end of the

stroke

– Reduces the shock of the impact as the piston contacts the cylinder head



Cylinder cushioning device


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A variety of mounting configurations are used t tt h th li d b d d d d tto attach the cylinder body and rod end to machinery– Fixed centerline

– Fixed noncenterline

– Pivoting centerline


g

– Expected cylinder loading is the major factor in the selection of the mounting style


Head-end flange mount


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Fixed-noncenterline mount



Pivoting-centerline, clevis mount


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Pivoting-centerline, trunnion mount



The force generated by a cylinder is calculated b lti l i th ff ti f th i tby multiplying the effective area of the piston by the system pressure


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Effective cylinder piston area



Force generated during the extension of a double-acting cylinder with a single-endeddouble acting cylinder with a single ended rod is calculated as:

Ef = Sp Pawhere:Ef = extension forceSp = system pressure


Sp system pressurePa = piston area

(Calculations require consistent units of measure in these formulas)

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Force generated during the retraction of a d bl ti li d ith i l d d ddouble-acting cylinder with a single-ended rod is calculated as:Rf = Sp (Pa – Ra)where:Rf = retraction force


Sp = system pressurePa = piston areaRa = rod area


Speed at which the cylinder extends or retracts i d t i d bis determined by:– Physical volume per inch of cylinder piston travel

– Amount of fluid entering the cylinder

Effective area of the piston is used to calculate the volume of the cylinder per inch of piston


the volume of the cylinder per inch of piston travel

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Extension speed of a double-acting cylinder ith i l d d d i l l t dwith a single-ended rod is calculated as:

Es = Fr (Cg Pa)where:Es = extension speedFr = flow delivery rate


Cg = cubic inches in one gallonPa = piston area


Calculate retraction speed of a double-acting li d ith i l d d dcylinder with single-ended rod as:

Rs = Fr [Cg (Pa – Ra)]where:Rs = retraction speedFr = flow delivery rate


Cg = cubic inches in one gallonPa = piston areaRa = rod area

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Flow rate to produce a desired extension or t ti d i l l t dretraction speed is calculated as:

Fr = (Ea Cs) Cgwhere:Fr = system flow rateEa = effective piston area


Cs = cylinder speedCg = cubic inches in one gallon


Hydraulic cylinder manufacturers provide d t il d ifi ti idetailed specifications concerning:– Construction

– Physical size

– Load capacity


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This information includes basic factors such as:– Bore

– Stroke

– Pressure rating

– Other details, such as service rating, rod end configurations, and dimensions


g ,


Typical manufacturer’s catalog page


Bailey International Corporation

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LimitedLimited--RotationRotationHydraulic ActuatorsHydraulic Actuators

Limited-rotation devices are actuators with an t t h ft th t t i ll li toutput 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


several revolutions


Most common designs of limited-rotation t tactuators are:

– Rack-and-pinion

– Vane

– Helical piston and rod


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Rack-and-pinion limited rotation actuator


IMI Norgren, Inc.


Vane limited-rotation actuator


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Helical piston and rod limited-rotation actuator



Limited-rotation actuators are used to perform b f f ti i i t f i d t i la number of functions in a variety of industrial

situations– Indexing devices on machine tools

– Clamping of workpieces

– Operation of large valves


p g

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Limited-rotation actuators are used in this b tirobotic arm


IMI Norgren, Inc.

Hydraulic MotorsHydraulic Motors

Hydraulic motors are called rotary actuators

They convert fluid pressure and flow into torque and rotational movement


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Typical hydraulic motor application



All basic hydraulic motors consist of three tcomponent groups:

– Housing

– Rotating internal parts

– Power output shaft


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Parts of a typical h d li thydraulic motor



System fluid enters the housing and applies t th t ti i t l tpressure 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 i t h d li t ithin most hydraulic motors are either:– Gears

– Vanes

– Pistons



Four requirements of a motor


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Displacement of a hydraulic motor indicates th l f fl id d d t t th t tthe 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


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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



Hydraulic motors may be classified by the type f l d li d t th b i f th t tof load applied to the bearings of the output

shaft– Unbalanced indicates the output shaft is loaded

from one side, side loading the shaft bearings

– Balanced indicates the bearing load is balanced by


use of two inlet ports arranged opposite of each other and two outlet ports similarly arranged

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The external gear hydraulic motor is the most d i l t f th b i t tcommon and simplest of the basic motor types

– Fixed displacement

– Unbalanced load on the bearings



The most common internal gear motor has a gerotor designgerotor design


Courtesy of Eaton Fluid Power Training

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The specially shaped gear teeth of the gerotor f i bl l h b th t llform variable-volume chambers that allow system fluid flow and pressure to turn the motor output shaft

Gerotor motors are fixed-displacement units operating with an unbalanced bearing load


operating with an unbalanced bearing load


An orbiting gerotor motor is a variation of the b i t d ibasic gerotor design– Uses a fixed outer gerotor gear with internal teeth

and an inner gear with external teeth

– Center point of the inner gear orbits around the center point of the fixed gear with internal teeth


– Motor operates at a slower speed, but has a higher torque output

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Orbiting gerotor motor




Basic vane motor has a slotted rotor located off center in a circular chamber and fitted withoff 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 the chambers to increase, turning


the volume of the chambers to increase, turning the motor shaft

– Basic vane motor is fixed displacement with an unbalanced bearing load

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Basic vane motor



Balanced vane motors evenly distribute the l d th b iload 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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A basic, balanced vane motor



Vane motors are available as either fixed or i bl di l tvariable displacement

The variable-displacement feature allows an operator to change the speed of a motor without changing the system flow rate


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In variable-displacement designs, the chamber i hi h th t d t iin which the rotor and vanes operate 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


Piston motors are available having either fixed i bl di l tor 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


revolution

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Two basic classifications of piston motors are i l i t d di l i taxial 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



Axial piston motor


The Oilgear Company

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Axial piston motors are available in two fi ticonfigurations:

– 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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Inline piston motor


The Oilgear Company


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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Bent-axis piston motor




A number of alternate motor designs are used i i li d h d li li tiin specialized hydraulic applications– Screw motor designs for quiet, continuous

operation

– Special piston-motor designs allowing the direct mounting and drive of wheels for off-road, heavy-


transport vehicles

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Hydraulic motors may be incorporated into i it i i ll l ticircuits using series or parallel connections– Series circuits: total system pressure is determined

by adding the loads placed on each unit

– Parallel circuits: each motor receives full system pressure; loads must be matched or equal flow


supplied to each motor if constant speed is desired from each unit


Motors in series


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Motors in parallel



Motors in parallel with flow control


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Braking circuits are used to slow hydraulic t t tmotors to a stop

– Inertia of a heavy rotating load can continue to turn the motor shaft

– Braking occurs when fluid discharged from the motor outlet port is forced to pass through an


adjustable pressure control valve before returning to the reservoir


Braking circuit


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An open-loop hydraulic motor system uses a l t t i l f b i h d li tlayout typical of a basic hydraulic system– Pump moves fluid from a reservoir, through a

directional control valve, to the motor

– Fluid is then returned from the motor to the reservoir through the same control valve



Closed-loop hydraulic motor systems ti l i l t fl id b t thcontinuously circulate fluid between the pump

and the motor without returning it to a system reservoir

These systems use a replenishment circuit to replace fluid lost through leakage


replace fluid lost through leakage

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Replenishment circuit


Hydrostatic DrivesHydrostatic Drives

Hydrostatic drive systems consist of the basic components typically found in other hydrauliccomponents typically found in other hydraulic motor circuits


MDMA Equipment—Menomonie

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Hydrostatic drives provide effective t i i f d lltransmission of power and allow easy adjustment and control of:– Output shaft speed

– Torque

– Horsepower


p

– Direction of rotation


When compared to conventional transmissions, h d t ti d ihydrostatic drives:– Have a high power output–to–size ratio

– May be stalled under full load with no internal damage

– Accurately maintain speed under varying load


y p y gconditions

– Provide an almost infinite number of input/output speed ratios

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Hydrostatic drives may be open or closed i itcircuits– Open circuit has the layout of a basic hydraulic

motor circuit

– Closed circuit has the outlet of the pump directly connected to the inlet of the motor and the outlet of


the motor directly connected to the inlet of the pump


Open circuit design


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Closed circuit design


Sauer-Danfoss, Ames, IA


Four combinations of pump/motor t b darrangements can be used

– Fixed-displacement pump and motor

– Fixed-displacement pump and variable-displacement motor

– Variable-displacement pump and fixed-


p p pdisplacement motor

– Variable-displacement pump and motor

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Fixed-displacement pump and motor:– Maximum horsepower, torque, and output shaft

speed are fixed

– Pump and motor have fixed displacement, so these characteristics cannot be changed



Fixed-displacement pump and variable-di l t tdisplacement motor:– Maximum horsepower is fixed

– Torque and speed are variable

– Due to use of a relief valve, efficiency is lowered

– Output shaft rotation may be reversed if the pump


Output shaft rotation may be reversed if the pump is reversible

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Variable-displacement pump and fixed-di l t tdisplacement motor:– Torque output is fixed

– Horsepower and output shaft speed are variable

– Output shaft rotation may be reversed if pump is reversible



Variable-displacement pump and motor:– Horsepower, torque, output shaft speed are variable

– Output shaft direction is reversible

– Most versatile of the four pump/motor combinations


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Hydrostatic drives are typically considered h d t ti t i i h b th thhydrostatic transmissions when both the pump and motor have variable displacement

This combination allows manual or automatic control of torque, speed, and power output



Two different general techniques are used Two different general techniques are used in the construction of hydrostatic transmissions– Integral

– Nonintegral


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Integral construction combines all of the t i i t i t i l h itransmission parts into a single housing

Nonintegral construction involves separate pump, motor, and accessories connected by hoses or tube assemblies


Review QuestionReview Question

A(n) _____ cylinder can exert force during b th th t i d t ti t kboth the extension and retraction strokes.

double-acting


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A(n) _____ is the system component that t fl id d fl i t liconverts fluid pressure and flow into linear

force and movement.

hydraulic cylinder



List the three basic configurations used to t li d t i tmount cylinders to equipment.

A. Fixed centerline, B. fixed non-centerline, and C. pivoting centerline.


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The three conceptual component groups that make up any hydraulic motor are:y yA. Rotor, vanes, and eccentric.B. Housing, rotating internal parts, and power output shaft.C. Housing, reciprocating internal parts, and power input shaft.D. Rotating internal parts, power input shaft, and power output

shaft.


B. Housing, rotating internal parts, and power output shaft.


To vary the displacement of a vane motor, a bl i d t h th i f thmovable _____ is used to change the size of the

pumping chambers.

cam ring


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List the four possible pump/motor arrangements th t b d ith h d t ti tthat may be used with a hydrostatic system.

A. Both pump and motor have fixed displacements, B. pump has a fixed displacement and the motor a variable displacement C pump has a variable


variable displacement, C. pump has a variable displacement and the motor a fixed displacement, and D. both pump and motor have variable displacement.


During retraction, what is the effective area of th i t f d bl ti li d ?the piston of a double-acting cylinder?

The cross-sectional area of the piston minus the cross-sectional area of the rod.


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A cylinder that has externally mounted metal d h ldi th d th b l i ll drods holding the ends on the barrel is called

a(n) _____ cylinder.

tie-rod


GlossaryGlossary

Barrel– The component containing the cylinders of an

axial piston hydraulic pump.

Clevis mount– A cylinder rod and cap mounting configuration

involving a C-shaped casting and a mounting pin


g p g g pthat allows the cylinder to pivot during extension and retraction.

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GlossaryGlossary

Closed circuit– A hydraulic circuit design in which pump output is

returned directly to the pump inlet after passing through a hydraulic motor. The design is commonly used with hydrostatic drive systems.


GlossaryGlossary

CushioningA d i f t i fl id li d th t– A design feature in fluid power cylinders that reduces fluid flow near the end of the extension or retraction stroke to decelerate piston movement, which avoids both noise and component damage.

Double-acting cylinder– Cylinders that may be powered both on the


Cylinders that may be powered both on the extension and retraction strokes.

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GlossaryGlossary

Effective piston area– The area of a piston that contributes to the force

generated by system pressure. For example, the effective area of a cylinder piston during retraction is the area of the piston minus the cross-sectional area of the piston rod.


GlossaryGlossary

Fixed-centerline mount– A cylinder-mounting design in which the load

carried by the cylinder rod and piston is supported at the centerline of the cylinder barrel, which is fixed to a machine member.


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GlossaryGlossary

Head– The height of a column of water or other liquid

necessary to develop a stated pressure.

Hydrostatic drive– A fluid power drive system using a hydraulic pump

and motor to transmit the power of a prime mover


p pto the input of a machine. Available in either open-or closed-circuit designs.

GlossaryGlossary

Limited-rotation actuatorA t t d i th t i il d– An actuator design that primarily produces rotational movement of one revolution or less. Various designs are available using a rack and pinion, vane, or helical shaft.

Mill cylinder– A hydraulic cylinder constructed of heavy steel for


A hydraulic cylinder constructed of heavy steel for use in industries such as foundries and steel mills.

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GlossaryGlossary

Open circuit– A hydraulic circuit that uses the layout of a basic

hydraulic motor circuit with a directional control valve to control motor direction and a reservoir to hold surplus fluid.


GlossaryGlossary

Orbiting gerotor motor– A variation of the gerotor motor that uses the

internal-toothed gear of the gerotor set as a fixed gear. The external-toothed gear orbits following the internal-toothed gear. This produces higher torque/lower speed output.


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GlossaryGlossary

Parallel circuit– An electrical or fluid power circuit that

simultaneously provides multiple paths for the current or fluid to follow as it moves through a circuit.


GlossaryGlossary

Pivoting-centerline mount– A clevis or trunnion mounting that allows the

cylinder to follow an arc as it powers a machine member. The load remains concentrated on the centerline of the cylinder.


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GlossaryGlossary

Replenishment circuit– A circuit used with closed-loop hydraulic systems

that provides makeup fluid to replace any fluid lost from leakage during system operation.

Series circuit– An electrical or fluid power circuit that provides


p ponly one path for the current or fluid to follow as it moves through the circuit.

GlossaryGlossary

Single-acting cylinder– A cylinder design that exerts force only on

extension or retraction and depends on some outside force to complete the second movement.


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GlossaryGlossary

Telescoping cylinder– A linear actuator constructed of several nested tubes

that can extend a distance equal to several times the actuator’s retracted length.

Threaded-end cylinder– A linear actuator design in which the cap and head


g pare attached to the barrel of the cylinder by threads.

GlossaryGlossary

Tie-rod cylinderA linear actuator design in which the cap and head– A linear actuator design in which the cap and head components are secured to the barrel of the cylinder by external tie rods that run between those components.


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GlossaryGlossary

Trunnion mountA cylinder mounting method that places fittings on– A cylinder mounting method that places fittings on the sides of cylinders, allowing the cylinder to pivot as it extends and retracts to move a machine member.


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