CHAPTER 4

ACTUATORS
(Hydraulic Cylinder and
Motor)
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4.1 INTRODUCTION
Hydraulic cylinders and motors form the hydraulic
circuit component called the actuators.
They extract energy from the pressurized fluid and
convert it to mechanical energy to perform linear or
rotary motions.
Hydraulic cylinders (linear actuators) extend and
retract a piston rod to exert a force on an external
load along a straight line path.
Hydraulic motors (rotary actuators) rotate a shaft to
provide a torque to drive the load along a rotary
path. The rotation could be limited or continuous.
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4.2 Hydraulic Cylinders
Can be classified in two:
1. Single-acting
2. Double-acting
Single acting design is the simplest type of hydraulic
cylinder.
It can exert a force only in the extending direction.
Retraction is accomplished by gravity or spring.
The double acting type delivers force in both
directions.
Extension and retraction are by hydraulic means.
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Single Acting cylinder

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Double-Acting Cylinder

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4.3 Hydraulic Cylinder Cushions
Cushioning devices are provided in the ends of the
hydraulic cylinders or as separate units when loads
must be decelerated to prevent the excessive impact
that can occur.
Deceleration starts when the tapered plunger enters the
opening in the cap and restricts the exhaust flow.
During the last small portion of the stroke, the oil
must pass through an adjustable opening. For
charging the piston a check valve that allows free
flow is incorporated in the system.

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Cushioning cylinder

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4.4 Hydraulic Motors
4.4.1 Introduction
These are other classes of actuators that extract energy
from a fluid and convert it to mechanical energy.
Two types of motors
• Rotary actuators – limited rotation
• Hydraulic motor – continuous rotation
(gear, vane, and piston configuration)

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4.4.2 Limited Rotation Hydraulic Motors
A limited rotation hydraulic motor (also called
oscillation motor or rotary actuator) provides a
specified degree of rotation.
They are used extensively in industry for actuating
clamping devices, material handling, rotating cams
for braking mechanisms, dumping, positioning and
turning and many others.

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4.4.3 Continuous Rotation Actuators
These provide sustained rotation in either direction
due to application of torque by the pressurized fluid.
Hydraulic motors can rotate continuously and as such
have the same basic configuration as pumps.
However, instead of pushing on the fluid as pumps do,
motors are pushed on by the fluid.
There are three basic types of hydraulic motors:
gear, vane, and piston motors

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Gear Motors
A gear motor develops torque due to hydraulic
pressure acting on the surface of the gear teeth.
The direction of rotation can be reversed by reversing
the direction of flow.
Volumetric displacement is fixed.
The motor is not balanced with respect to pressure
loads, thus producing a large side load.

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Torque development by a gear motor

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Vane Motors
These develop torque by the hydraulic pressure acting
on the exposed surfaces of the vanes.

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 Analysis of Torque capacity
For a single rotating vane
RR = outer radius of rotor shaft (m)
RV = outer radius of vane (m)
L = width of vane (m)
p = hydraulic pressure (Pa)
F = hydraulic force acting on vane (N)
A = surface area of vane in contact with oil (m2)
T = torque capacity (N.m)

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Force on vane:
F = pA = p(RV - RR)L
Torque acting on the mean radius of the vane:
 RV + RR 
T = p( R V − R R ) L  
 2 
pL 2
= ( R V − R 2R )
2

The volumetric displacement can be approximated by

VD =  ( R − R ) L
2
V
2
L

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• Substituting in the torque equation will give

pVD
T=
2

The above equation shows that the torque can be

increased by increasing the pressure or the
volumetric displacement or both.

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Piston motors
Here we have, as in pumps, the in-line type and the
bent axis type.
The swash plate type can be of the variable
displacement type by varying the inclination angle
of the swash plate.

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18
Motor displacement varies with swash plate
angle

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Theoretical torque, power, and flow rate
This is for a frictionless operation. The expression is
given by

Theoretical power (W) = TT(N.m) x ω(rad/s)

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VD ( m / rev ) x p( Pa )
TT ( N.m) =
2

Theoretical power (W) VD x p x 

=
2
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Considering no leakage the theoretical volume flow
rate will be
QT(m3/s) = VD (m3/rev) x N(rev/s)
Hydraulic Motor Performance
This topic will deal with volumetric efficiency,
considering leakages, and mechanical efficiency
which takes into account the frictional losses.
Finally the overall efficiency which will involve
both efficiencies will be discussed.

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Hydraulic Motor Performance
Range of overall efficiencies:
Gear motors – 70 to 75 %
Vane motors – 75 to 85 %
Piston motors – 85 to 95 %

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Motor Efficiencies
Volumetric efficiency (ηv)- Inverse of that of the pump
since it uses more flow than it should theoretically
due to leakage.
theoretical flow − rate Q T
V = = 
actual flow − rate Qa
Mechanical Efficiency (ηm) – Since additional torque
is delivered to overcome friction, the actual torque
delivered is lower.
actual torque delivered by motor Ta
m = =
theoretical torque to be delivered by motor TT
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Actual power delivered by motor = Ta ω
Overall efficiency
This is given by the product of the two efficiencies.
actual power delivered by motor
ηo = ηv ηm =
actual power delivered to motor
Ta 
o = 
p Qa
Usual terms used are
Denominator-hydraulic power
Numerator-brake power

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4.4.4 Hydrostatic Transmission
A system consisting of a hydraulic pump, a hydraulic
motor, and appropriate valves and pipes can be used
to provide adjustable speed drives for many practical
applications.
Such a system is called a hydrostatic transmission. A
prime mover, electric motor or gasoline engine will
be required.
Applications in tractors, rollers, loaders, and lift
trucks.

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Some of the advantages include:
• Infinitely variable speed and torque in either
direction and over the full speed and torque ranges
• Extremely high power-to-weight ratio
• Flexibility and simplicity of design
A power package unit with the circuit diagram is
shown below. This unit includes a bidirectional
variable flow pump driven by a motor and also a
bidirectional fixed displacement motor.

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Hydrostatic transmission system

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