Fluid power, Hydraulic & penumatic

Chapter 9

Actuators

Workhorses of the System

Objectives
 Describe the construction and operation of
basic 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
reducing hydraulic shock.
 Compare the design and operation of various
types of hydraulic motors.

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

Objectives
 Contrast the operation of fixed- and variable-
speed 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
hydraulic cylinders.


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


Hydraulic Cylinders
 A basic cylinder consists of:
– Piston
– Piston rod
– Barrel


Hydraulic Cylinders
 Parts of a typical cylinder


Hydraulic Cylinders
 The piston forms sealed, variable-volume
chambers in the cylinder
 System fluid forced into the chambers drives
the piston and rod assembly
 Linear movement is produced


Hydraulic Cylinders
 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


Hydraulic Cylinders
 Various seals are used in a cylinder


Hydraulic Cylinders
 Rod wipers prevent
contamination from
entering on rod
retraction

IMI Norgren, Inc.


Hydraulic Cylinders
 Cylinders are typically classified by operating
principle or by construction type
– Single-acting or double-acting
– Tie rod, mill, threaded end, or one piece


Hydraulic Cylinders
 Single- and double-acting cylinders

Single-acting Double-acting

Hydraulic Cylinders
 Single-acting cylinders exert force either on
extension or retraction
 They require an outside force to complete the
second motion


Hydraulic Cylinders
 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


Hydraulic Cylinders
 Effective piston area is reduced on retraction
due to the rod cross section


Hydraulic Cylinders
 Volume is reduced on retraction


Hydraulic Cylinders
 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


Hydraulic Cylinders
 Tie-rod cylinder


Hydraulic Cylinders
 Mill cylinders

Yates Industries, Inc.


Hydraulic Cylinders
 Threaded-end cylinder

Bailey International Corporation


Hydraulic Cylinders
 One-piece cylinder has the cylinder barrel
welded to the ends
 Produces a compact actuator
– Cost effective to manufacture
– Cannot be serviced (throwaway)


Hydraulic Cylinders
 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 rigid under load, but
cylinder can generate force in only one direction


Hydraulic Cylinders
 Typical hand-operated jack


Hydraulic Cylinders
 Telescoping cylinders are available for
applications 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


Hydraulic Cylinders
 Telescoping cylinders

Star Hydraulics, Inc.


Hydraulic Cylinders
 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


Hydraulic Cylinders
 Cylinder cushioning device


Hydraulic Cylinders
 A variety of mounting configurations are used
to attach the cylinder body and rod end to
machinery
– Fixed centerline
– Fixed noncenterline
– Pivoting centerline
– Expected cylinder loading is the major factor in the
selection of the mounting style


Hydraulic Cylinders
 Head-end flange mount


Hydraulic Cylinders
 Fixed-noncenterline mount


Hydraulic Cylinders
 Pivoting-centerline, clevis mount


Hydraulic Cylinders
 Pivoting-centerline, trunnion mount


Hydraulic Cylinders
 The force generated by a cylinder is calculated
by multiplying the effective area of the piston
by the system pressure


Hydraulic Cylinders
 Effective cylinder piston area


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

Ef = Sp × Pa
where:
Ef = extension force
Sp = system pressure
Pa = piston area

(Calculations require consistent units of measure in
these formulas)

Hydraulic Cylinders
 Force generated during the retraction of a
double-acting cylinder with a single-ended rod
is calculated as:
Rf = Sp × (Pa – Ra)
where:
Rf = retraction force
Sp = system pressure
Pa = piston area
Ra = rod area

Hydraulic Cylinders
 Speed at which the cylinder extends or retracts
is 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
travel


Hydraulic Cylinders
 Extension speed of a double-acting cylinder
with a single-ended rod is calculated as:
Es = Fr × (Cg ÷ Pa)
where:
Es = extension speed
Fr = flow delivery rate
Cg = cubic inches in one gallon
Pa = piston area


Hydraulic Cylinders
 Calculate retraction speed of a double-acting
cylinder with single-ended rod as:
Rs = Fr × [Cg ÷ (Pa – Ra)]
where:
Rs = retraction speed
Fr = flow delivery rate
Pa = piston area
Ra = rod area

Hydraulic Cylinders
 Flow rate to produce a desired extension or
retraction speed is calculated as:
Fr = (Ea × Cs) ÷ Cg
where:
Fr = system flow rate
Ea = effective piston area
Cs = cylinder speed


Hydraulic Cylinders
 Hydraulic cylinder manufacturers provide
detailed specifications concerning:
– Construction
– Physical size
– Load capacity


Hydraulic Cylinders
 This information includes basic factors such as:
– Bore
– Stroke
– Pressure rating
– Other details, such as service rating, rod end
configurations, and dimensions


Hydraulic Cylinders
 Typical manufacturer’s catalog page

Bailey International Corporation


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


Limited-Rotation
Hydraulic Actuators
 Most common designs of
limited-rotation actuators are:
– Rack-and-pinion
– Vane
– Helical piston and rod


Limited-Rotation
Hydraulic Actuators
 Rack-and-pinion limited rotation actuator

IMI Norgren, Inc.


Limited-Rotation
Hydraulic Actuators
 Vane limited-rotation actuator


Limited-Rotation
Hydraulic Actuators
 Helical piston and rod limited-rotation actuator


Limited-Rotation
Hydraulic Actuators
 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


Limited-Rotation
Hydraulic Actuators
 Limited-rotation actuators are used in this
robotic arm

IMI Norgren, Inc.


Hydraulic Motors
 Hydraulic motors are called rotary actuators
 They convert fluid pressure and flow into
torque and rotational movement


Hydraulic Motors
 Typical hydraulic motor application


Hydraulic Motors
 All basic hydraulic motors consist of three
component groups:
– Housing
– Rotating internal parts
– Power output shaft


Hydraulic Motors
 Parts of a typical
hydraulic motor


Hydraulic Motors
 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


Hydraulic Motors
 Primary parts that produce the rotating motion
in most hydraulic motors are either:
– Gears
– Vanes
– Pistons


Hydraulic Motors
 Four requirements of a motor


Hydraulic Motors
 Displacement of a hydraulic motor indicates
the volume of fluid needed to turn the output
shaft one revolution
– Fixed displacement
– Variable displacement


Hydraulic Motors
 In a fixed-displacement motor:
– Internal geometry cannot be changed
– Same volume needed per output shaft revolution


Hydraulic Motors
 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
 Hydraulic motors may be classified by the type
of 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


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


Hydraulic Motors
 The most common internal gear motor has a
gerotor design

Courtesy of Eaton Fluid Power Training


Hydraulic Motors
 The specially shaped gear teeth of the gerotor
form 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


Hydraulic Motors
 An orbiting gerotor motor is a variation of the
basic 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


Hydraulic Motors
 Orbiting gerotor motor



Hydraulic 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 the chambers to increase, turning
the motor shaft
– Basic vane motor is fixed displacement with an
unbalanced bearing load


Hydraulic Motors
 Basic vane motor


Hydraulic Motors
 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


Hydraulic Motors
 A basic, balanced vane motor


Hydraulic Motors
 Vane motors are available as either fixed or
variable displacement
 The variable-displacement feature allows an
operator to change the speed of a motor without
changing the system flow rate


Hydraulic Motors
 In variable-displacement designs, the chamber
in 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


Hydraulic 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


Hydraulic Motors
 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


Hydraulic Motors
 Axial piston motor

The Oilgear Company


Hydraulic Motors
 Axial piston motors are available in two
configurations:
– Inline
– Bent axis


Hydraulic Motors
 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


Hydraulic Motors
 Inline piston motor

The Oilgear Company

Hydraulic Motors
 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


Hydraulic Motors
 Bent-axis piston motor



Hydraulic Motors
 A number of alternate motor designs are used
in 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


Hydraulic Motors
 Hydraulic motors may be incorporated into
circuits 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


Hydraulic Motors
 Motors in series


Hydraulic Motors
 Motors in parallel


Hydraulic Motors
 Motors in parallel with flow control


Hydraulic Motors
 Braking circuits are used to slow hydraulic
motors 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


Hydraulic Motors
 Braking circuit


Hydraulic Motors
 An open-loop hydraulic motor system uses a
layout 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


Hydraulic Motors
 Closed-loop hydraulic motor systems
continuously 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


Hydraulic Motors
 Replenishment circuit


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

MDMA Equipment—Menomonie


Hydrostatic Drives
 Hydrostatic drives provide effective
transmission of power and allow easy
adjustment and control of:
– Output shaft speed
– Torque
– Horsepower
– Direction of rotation


Hydrostatic Drives
 When compared to conventional transmissions,
hydrostatic 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
conditions
– Provide an almost infinite number of input/output
speed ratios


Hydrostatic Drives
 Hydrostatic drives may be open or closed
circuits
– 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


Hydrostatic Drives
 Open circuit design


Hydrostatic Drives
 Closed circuit design

Sauer-Danfoss, Ames, IA


Hydrostatic Drives
 Four combinations of pump/motor
arrangements can be used
– Fixed-displacement pump and motor
– Fixed-displacement pump and variable-
displacement motor
– Variable-displacement pump and fixed-
displacement motor
– Variable-displacement pump and motor


Hydrostatic Drives
 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


Hydrostatic Drives
 Fixed-displacement pump and variable-
displacement 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
is reversible


Hydrostatic Drives
 Variable-displacement pump and fixed-
displacement motor:
– Torque output is fixed
– Horsepower and output shaft speed are variable
– Output shaft rotation may be reversed if pump is
reversible


Hydrostatic Drives
 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


Hydrostatic Drives
 Hydrostatic drives are typically considered
hydrostatic transmissions when both the pump
and motor have variable displacement
 This combination allows manual or automatic
control of torque, speed, and power output


Hydrostatic Drives

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


Hydrostatic Drives
 Integral construction combines all of the
transmission parts into a single housing
 Nonintegral construction involves separate
pump, motor, and accessories connected by
hoses or tube assemblies


Review Question
A(n) _____ cylinder can exert force during
both the extension and retraction strokes.

double-acting


Review Question
A(n) _____ is the system component that
converts fluid pressure and flow into linear
force and movement.

hydraulic cylinder


Review Question
List the three basic configurations used to
mount cylinders to equipment.

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


Review Question
The three conceptual component groups that make up
any hydraulic motor are:
A. 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.


Review Question
To vary the displacement of a vane motor, a
movable _____ is used to change the size of the
pumping chambers.

cam ring


Review Question
List the four possible pump/motor
arrangements that 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
displacement and the motor a fixed displacement, and
D. both pump and motor have variable displacement.


Review Question
During retraction, what is the effective area of
the piston of a double-acting cylinder?

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


Review Question
A cylinder that has externally mounted metal
rods holding the ends on the barrel is called
a(n) _____ cylinder.

tie-rod


Glossary
 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
that allows the cylinder to pivot during extension
and retraction.


Glossary
 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.


Glossary
 Cushioning
– 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
extension and retraction strokes.


Glossary
 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.


Glossary
 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.


Glossary
 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
to the input of a machine. Available in either open-
or closed-circuit designs.


Glossary
 Limited-rotation actuator
– 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
use in industries such as foundries and steel mills.


Glossary
 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.


Glossary
 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.


Glossary
 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.


Glossary
 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.


Glossary
 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
only one path for the current or fluid to follow as it
moves through the circuit.


Glossary
 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.


Glossary
 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
are attached to the barrel of the cylinder by threads.


Glossary
 Tie-rod cylinder
– 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.


Glossary
 Trunnion mount
– 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.


Fluid power, Hydraulic & penumatic

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