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ACADEMIC TEXTBOOK
FLUID POWER CONTROL SYSTEMS
The lecture: 15 hours
Kielce University of Technology
Faculty of Mechatronics and Machine Design
Author: Ryszard Dindorf
Kielce, 2011/2012
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CONTENTS
1. Fluid power basic
2. Hydraulic control system
2.1. Introduction
2.2. Hydraulic components
2.2.1. Hydraulic pumps
2.2.2. Hydraulic motors
2.2.3. Hydraulic accumulators
2.2.4. Hydraulic cylinders
2.2.5. Hydraulic valves
2.2.5.1. Direction control valves
2.2.5.3. Flow control valves
2.2.5.3. Pressures control valves
2.2.5.4. Check valves
2.3. Hydraulic fluids
2.4. Hydraulic circuits
3. Pneumatic control system
3.1. Introduction
3.2. Air compressors
3.3. Pneumatic actuators
3.3.1. Single acting cylinders
3.3.2. Double acting cylinders
3.3.3. Magnetic cylinders
3.3.4. Rodless cylinders
3.3.5. Semi-rotary actuators
3.3.6. Bellows
3.3.7. Pneumatic muscles
3.4. Pneumatic valves
3.4.1. Directional control valve
3.4.2. Shut-off valve
3.4.3. Flow control valve
3.4.4. Pressure valve
3.5. Pneumatic circuits
References
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1. FLUID POWER BASIC
The term fluid power refers to energy that is transmitted via a fluid under pressure [13]. With
hydraulics, that fluid is a liquid such as oil or water. With pneumatics, the fluid is typically
compressed air or inert gas. The basis of fluid power systems is pressurized fluids. The fluid
power systems are generally grouped under the two broad classifications of hydraulic power
systems and pneumatic power systems (Fig.1.1).
Figure 1.1. Classifications of fluid power systems
Hydraulic systems use liquids (oil), while pneumatic systems use gas (air). In the hydrostatic power
systems the power is transmitted by increasing mainly the pressure energy of a liquid. Pneumatic systems
are power systems using compressed air as a working medium for the power transmission. Other fluids
(water, water-based fluids, synthetic fluids) are often used in special applications. Fluid power is
one of the three types of power transfer systems, namely fluid power systems, mechanical
systems and electrical systems, commonly used today. Each of the systems transfer power from a
prime mover (source energy) to an actuators that complete the task (work) required of the system.
Figure 1.2 shows the power transmission in a fluid power system.
Figure 1.2. Power transmission in a fluid power system
Although hydraulic and pneumatic systems share the characteristics of energy transfer by means
of fluid pressure and flow, differences affect how and where they are applied [1]. These
differences include: accuracy of actuator movement, operating pressure, actuator speed,
component weight and cost. Fluid compressibility is the inherent characteristic that produces the
difference between hydraulic and pneumatic systems. A gas (air) is compressible, while a liquid
can be compressed only slightly. Compressibility produces a more “spongy” operation in
pneumatic systems, which is not suitable where highly accurate movement is required. Hydraulic
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systems can operate at much higher pressures than pneumatic systems. There are five functions
that are basic to system operation of any fluid power system, Table 1.1: energy conversion, fluid
distribution, fluid control, work performance and fluid conditioning.
Table 1.1. The basic functions of a fluid power system [1]
Energy conversion
Fluid power systems do not generate energy, but transform it into a form that can be
used to complete a task. The process begins with a prime mover pressurizing a fluid.
It ends with an actuator using the energy stored in the pressurized fluid to perform
work.
Fluid distribution
The operation of fluid power systems requires the distribution of fluid to the
components in the system. Various types of lines are involved in this function.
Valves and other components also serve to assist in fluid distribution.
Fluid control
Fluid power systems require the control and regulation of the fluid in the system to
perform the tasks desired by the system designer. A number of different
components are used to control fluid flow rate, direction, and pressure in a system.
Control of these three elements allows the system to provide the desired operating
characteristics.
Work performance Using the energy stored in the pressurized fluid of the system is the primary
function of a fluid power system. This process involves actuators that convert the
energy stored in the fluid to linear or rotary motion to perform the desired work.
Fluid conditioning
Fluid power system performance and service life require a fluid that is clean and
provides lubrication to system components. This function involves storing fluid,
removing dirt and other contaminants, and maintaining proper system operating
temperature.
Hydraulic power systems use the prime mover (electric motor, diesel engine) to drive a pump
that pressurizes a fluid, which is then transferred through valves, pipes and hoses to an actuators
(motors, cylinders). Hydraulic power systems have gained wide scale use and applicability in
technologically driven industrial manufacturing process; manipulators, robots; transport, road
vehicles, rail, shipping, aircraft; public services, road cleaning, health, maintenance, elevators;
military vehicles, aerospace etc. Hydraulic system operating pressure ranges from a few 10 MPa
to several 50 MPa. Pressures of more than 60 MPa are used in special situations. Industrial
systems commonly operate in the low (below 7 MPa) to moderate (below 21 MPa) pressure
range. In hydraulic systems in low-pressure region (p<10 MPa) the influence of entrained gas on
the bulk modulus is substantial. Hydraulic systems operate at higher pressures, requiring the use
of stronger materials and more-massive designs to withstand the pressure. Hydraulic systems
applications tend to involve equipment that handles heavier weights, requiring both higher system
operating pressure and physical strength of machine parts.
Pneumatic power systems normally operate between 0.5 MPa to 0.85 MPa (5.0-8.5 bar). The
air compressor converts the mechanical energy of the prime mover (electric motor, diesel engine) into
pressure energy of the compressed air. This transformation facilitates the transmission and the control of
power. An air preparation process is needed to prepare the compressed air for use. The air preparation includes
filtration, drying, and the adding of lubricating oil mist. The compressed air is stored in the compressed air
reservoirs and transmitted through rigid and/or flexible lines. The pneumatic power is controlled by means
of a set of pressure, flow, and directional control valves. Pneumatic systems are commonly used when
high-speed movement is required in an application. A speed for a pneumatic rotation motor of
over 20,000 revolutions per minute (rpm) is possible. Rapid-response pneumatic cylinder
operation is also possible with pneumatic systems. These designs are generally found in situations
involving lighter loads and lower accuracy requirements. System operating pressure affects the
structure of components. Pneumatic systems operate at much lower pressures and, therefore, can
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be manufactured using lightweight materials and designs that minimize the amount of material.
Pneumatic systems tend to involve applications where the ease of handling and lightweight is
critical for effective operation of the tool or system.
Fluid power systems are made up of component groups containing parts designed to perform
specific tasks [1]. These component groups are in operation together to perform the work desired
by the system designer. The work may involve simple or complex tasks, but the component
groups perform specific system functions that are basic to all fluid power systems. The structure
of typical hydraulic power systems involve five component groups, Table 1.2: power unit (pump,
accumulator), actuators (motors, cylinders), conductors (pipe line, hoses), control valves, fluid
conditioning (filter, heat exchanger).
Table 1.2. The structure of hydraulic power system [1]
Power unit
The power unit group of components deals primarily with the energy-conversion
function of the system. The unit consists of a prime mover, pump, and reservoir.
The prime mover is the source of energy for the system. The energy produced by
the prime mover turns the pump, which produces fluid flow that transmits energy
through the system. The reservoir serves as a storage unit for system fluid. It also
performs fluid maintenance functions.
Actuators
The actuators group of components performs the work done by the system. These
components convert the energy in the system fluid to linear or rotary motion. The
basic actuators are cylinders for linear motion and motors for rotary motion. A
variety of cylinder and motor designs are used to produce the specific motion
needed to complete the work required of the system.
Conductors
Fluid distribution is the primary function of the conductors group of components.
Pipes, hoses, and tubes serve as the conductors that confine and carry system fluid
between the pump and other system components. Conductors perform such tasks as
the intake of fluid for the pump, distribution of fluid to and from control valves and
actuators, transmission of sensing and control pressures, and the draining of liquids
that have accumulated in components.
Control valves
Three different types of valves are required to perform the fluid control function in a
fluid power system. The valves in the control valves group are: directional control
valves, pressure control valves and flow control valves. Directional control valves
provide control over fluid flow direction in sections of a system to start, stop, and
change the direction of actuator movement. Pressure control valves are used to limit
the maximum pressure of the system or in a section of the system. Flow control
valves provide control over fluid flow rate in a section of a system to control the
rate of movement of an actuator.
Fluid conditioning
The fluid conditioning group involves maintaining and conditioning system fluid.
This requires removal of dirt and moisture from the fluid and assuring proper
operating temperature. Specially designed components are available to perform
these tasks. However, basic systems often maintain the fluid using other system
components that perform the task as a secondary function. Filters are used to
remove dirt and moisture from systems, although a properly designed reservoir can
perform this task under certain conditions. Maintaining the proper system operating
temperature can require the use of a heat exchanger. However, this task is usually
performed by dispensing heat through the reservoir, system lines, and other
components.
Summing up, the pneumatic system is: clean, fast, intrinsically safe, overload safe, inexpensive
for individual components; hydraulic system is: easy controllable, produces extremely large
forces, requires high pressures, requires heavy duty components. The dangers of the use of
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compressed air include: air embolism, hose/pipe whipping, noise, crushing/cutting. The dangers
of working with high pressure oil can be infinitely more drastic: high pressure oil injection, oil
burns, crushing/cutting, carcinogens.
All main hydraulic and pneumatic components can be represented by simple symbols. Each
symbol shows only the function of the component it represents, but not its structure. Symbols can
be combined to form hydraulic or pneumatic diagrams. A diagram describes the relations between
each component, that is, the design of the system. Notice the use of standard, internationally
recognized symbols, see ISO 1219-1, ISO 1210-2. Just a few of the hydraulic and pneumatic
symbols are compared in Table 1.3.
Table 1.3. Comparison of hydraulic and pneumatic symbols
Differences
Pumps and compressor
differ only by filling in
the direction arrow.
Cylinders and other
linear actuators also
differ with respect to
supply and direction
arrows.
Motors and other rotary
actuators also differ
with respect to supply
and direction arrows.
Supply and pilot arrows
are filled right black or
left white.
Hydraulic valves have
a crossover to tank.
Pneumatic valves tend
to have two exhaust
outlets to atmosphere.
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Hydraulic symbol
Pneumatic symbol
Hydraulic pump
Compressor
Hydraulic double acting cylinder
Hydraulic motor
Hydraulically actuated and supplied
3/2 pilot spring
Hydraulic circuit
Pneumatic double acting cylinder
Pneumatic motor
Pneumatically actuated and supplied
3/2 pilot spring
Pneumatic circuit
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2. HYDRAULIC CONTROL SYSTEM
2.1. INTRODUCTION
The hydraulic system transmits the hydraulic power by the controlled circulation of
pressurized fluid, usually oil, water or water-basis fluids, to actuators (cylinders, motors) that
convert it into a mechanical output capable of doing work on a load [14]. Hydraulic power
systems have greater flexibility than mechanical and electrical and can produce more power than
such systems of equal size. They also provide rapid and accurate responses to controls. As a
result, hydraulic power systems are extensively used in modern aircraft, automobiles, heavy
industrial machinery, manipulators and robots, and many kinds of machine tools.
Any device operated by a hydraulic system may be called a hydraulic device, but a distinction
has to be made between the devices which utilize the impact or momentum of a moving fluid and
those operated by a thrust on a confined fluid i.e. by pressure [2]. This leads us to the subsequent
categorization of the field of hydraulic systems into: hydrodynamic system and hydrostatics
system. Hydrodynamic system deals with the characteristics of a liquid in motion, especially
when the liquid impacts on an object and releases a part of its energy to do some useful work.
Hydrostatic system deals with the potential energy available when a liquid is confined and
pressurized. This potential energy, also known as hydrostatic energy, is applied in most of the
hydraulic systems. This field of hydraulics is governed by Pascal's law. It can thus be concluded
that pressure energy is converted into mechanical motion in a hydrostatic device whereas kinetic
energy is converted into mechanical energy in a hydrodynamic device. Hydrostatic drives,
typically rated from 7.5 to 220 kW, incorporate positive displacement pumps and positive
displacement motors. Mechanical energy is transmitted from the prime mover to the pump. The
pump imparts energy through a fluid to a hydraulic motor. The prime mover can be an electric
motor, a gasoline or diesel engine, or a take-off from the main machine drive. Hydrostatic drives
offer several features that make them particularly adaptable to many adjustable speed
applications: infinitely adjustable stepless change of speed, torque, and power; no damage even if
stalled at full load; operation in both directions of rotation at controlled speeds; set speeds held
accurately against driving or braking loads; small size and low weight per power output.
The hydraulic power can be divided up into the power supply section, the power control
section and the drive section (working section). The power supply section is made up of the
energy conversion part and the pressure medium conditioning part. In this part of the hydraulic
system, the hydraulic power is generated and the pressure medium conditioned. The following
components are used for energy conversion – converting electrical energy into mechanical and
then into hydraulic energy: electric motor, internal combustion engine, coupling, pump, pressure
indicator (manometer), protective circuitry (relief valve). The power supply unit provides the
necessary hydraulic power – by converting the mechanical power from the drive motor. The most
important component in the power supply unit is the hydraulic pump. This draws in the hydraulic
fluid from a reservoir (tank) and delivers it via a system of lines in the hydraulic installation
against the opposing resistances. Pressure does not build up until the flowing liquids encounter a
resistance. The power is supplied to the drive section by the power control section in accordance
with the control problem. The following components perform this task: directional control valves,
flow control valves, pressure valves, check valves (non-return valves). The drive section of a
hydraulic system is the part of the system which executes the various working movements of a
machine or manufacturing system.
Practically, every hydraulic system of any arbitrary hydrostatic drive system consists of
pressure energy generators (pumps, accumulators), pressure energy actuators (motors, cylinders),
resistance elements (valves, conduits), auxiliary elements (filters, heat exchangers, tanks), and
control elements (control valves).
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2.2. HYDRAULIC COMPONENTS
2.2.1. Hydraulic pumps
Hydraulic pumps are used to convert mechanical energy (torque, speed) into hydraulic energy
(flow, pressure). The basic hydraulic pumps are shown in Table 2.1.
Table 2.1. Hydraulic pumps
External gear pump
Internal gear pump
Gerotor pump
Screw pump
Unbalanced vane pump
Balanced vane pump
Swashplate axial piston pump
Bent axial piston pump
Inside impinged radial piston pump
Outside impinged radial piston pump
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The hydrostatic pumps work on the positive displacement principle [3]. Positive displacement
pumps have an expanding cavity on the suction side and a decreasing cavity on the discharge
side. Liquid flows into the pumps as the cavity on the suction side expands and the liquid flows
out of the discharge as the cavity collapses. The volume is constant given each cycle of operation.
The positive displacement pumps can be divided in two main classes: reciprocating, rotary.
Displacement is by means of pistons, vanes and gears. In addition, a hydraulic pump will either
have a constant or variable displacement. The four most common types of hydraulic pump are the
gear, screw, vane and reciprocating:
 Gear pumps are displacement machines. Depending on their construction they are sub-divided
into external gear pumps, internal gear pumps and gerotor gear pump. External gear pumps are
widely used, especially in the mobile hydraulic sector. As the gears rotate, the hydraulic fluid
is carried via the external spaces between the teeth from the suction side to the pressure side.
These pumps are of simple design, cost-effective and robust and operate within a wide speed
range (500 to 6000 rpm) at relatively high pressures (up to 30 MPa). Internal gear pump is a
pump generally with two gears, an external internally-toothed hollow wheel with an internally
rotating pinion. By rotating both wheels the oil is carried in the spaces between the gear teeth.
Due to the long arc over which gear meshing takes place, the pump works quietly up to
pressures of 30 MPa. The gerotor pump operates in accordance with the planetary principle.
The rotor has one gear less than the internally geared stator. As a result of the way the internal
and external gears mesh, a good seal is obtained without an additional sickle seal. The pump
operates quietly up to pressures of 16 MPa.
 Screw pumps are similar to internal gear pumps in their main characteristic of possessing an
extremely low operating noise level. Screw pumps contain 2 or 3 worm gears within a
housing. The worm gear connected to the drive has a clockwise thread and transmits the rotary
movement to further worm gears, each of which has anti-clockwise threads. An enclosed
chamber is formed between the threads of the worm gears. This chamber moves from the
suction port to the pressure port of the pump without a change in volume. This produces a
constant, uniform and smooth flow and hence operation tends to be very quiet.
 There are two types of vane pump in a common use: single stroke (unbalance) and double
stroke (balance). With both designs the displacement chamber is formed between the circularshaped stator, the rotor and the vanes. The vanes may be moved radially inside the rotor. What
differs them is the form of the ring which limits the stroke movement of the vanes. In these
pumps, a number of vanes slide in slots in a rotor which rotates in a housing or a ring. The
housing may be eccentric with the center of the rotor, or its shape may be oval. In some
designs, centrifugal force holds the vanes in contact with the housing, while the vanes are
forced in and out of the slots by the eccentricity of the housing. In one vane pump, light
springs hold the vanes against the housing; in another pump design, pressurized pins urge the
vanes outward. During rotation, as the space or chamber enclosed by vanes, rotor, and housing
increases, a vacuum is created, and atmospheric pressure forces oil into this space, which is
the inlet side of the pump. As the space or volume enclosed reduces, the liquid is forced out
through the discharge ports.
 In the reciprocating pump it is this back-and-forth motion of pistons inside of cylinders that
provides the flow of fluid. Reciprocating pumps, like rotary pumps, operate on the positive
principle - that is, each stroke delivers a definite volume of a liquid to the system. Axial piston
pumps convert rotary motion of an input shaft to an axial reciprocating motion, occurring at
the pistons. This is accomplished by a swashplate that is either fixed or variable in its degree
of angle. As the piston barrel assembly rotates, the pistons rotate around the shaft with the
piston slippers in contact with and sliding along the swashplate surface. With the swashplate
vertical, no displacement occurs because there is no reciprocating motion. As the swashplate
increases in angle, the piston moves in and out of the barrel as it follows the angle of the
swashplate surface. With the swashplate vertical, no displacement occurs because there is no
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reciprocating motion. As the swashplate increases in angle, the pistons move in and out of the
barrel as it follows the angle of the swashplate. During one half of the circle of rotation, the
piston moves out of the cylinder barrel and generates an increasing volume. In the other half of
the rotation the piston moves into the cylinder barrel and generates a decreasing volume. This
reciprocating motion draws fluid in and pumps it out. Bent axis piston pump consists of a
drive shaft which rotates the pistons, a cylinder block, and a stationary valving surface facing
the cylinder block bores which ports the inlet and outlet flow. The drive shaft axis is angular in
relation to the cylinder block axis. Rotation of the drive shaft causes rotation of the pistons and
the cylinder block. In radial piston pumps, the pistons are arranged radially in a cylinder block,
they move perpendicularly to the shaft centerline. Two basic types are available: one uses
cylindrically shaped pistons, the other ball pistons. They may also be classified according to
the porting arrangement: check valve or pintle valve. They are available in fixed and variable
displacement, and variable reversible (over-center) displacement. When filling the workspace
of the pumping pistons from "inside" (e. g. over a hollow shaft) it is called an inside impinged
(but outside braced) radial piston pump. If the workspace is filled from "outside" it is called an
outside impinged radial piston pump (but inside braced). The pumping volume may be fixed
or variable.
2.2.2. Hydraulic motors
Hydraulic motors convert hydraulic energy into mechanical energy. In order to fulfil exactly
the wide range of requirements (speed, torque, power) different design principles need to be
applied. The basic hydraulic motors are shown in Table 2.2.
Table 2.2. Hydraulic motors
Gear motor
Gerotor motor
Vane motor
Axial piston motors
Bent axis piston motor
Radial piston motor
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Hydraulic motors are classified as rotary actuators [3]. But strictly speaking, the term rotary
actuator is reserved for a particular type of the unit whose rotation is limited to less than 360°.
Hydraulic motors are used to transmit fluid power through linear or rotary motion. They resemble
pumps very closely in construction. However, as already understood, pumps perform the function
of adding energy to a hydraulic system for transmission to some remote point, while motors do
precisely the opposite. They extract energy from a fluid and convert it to a mechanical output to
perform useful work. To put it more simply, instead of pushing on the fluid as the pump does, the
fluid pushes on the internal surface area of the motor, developing torque. Since both the inlet and
outlet ports in a motor may be pressurized, most hydraulic motors are externally drained. The
three most common types of hydraulic motors are the gear, vane and piston:
 Gear motors are very similar in design to gear pumps. They are differ in the axial pressure
field is different and gear motors have a drain case port, as they are designed for changing
directions of rotation. The fluid flowing to the hydraulic motor acts on the gears. A torque is
produced, which is output via the motor shaft. Gear motors are often used in mobile
hydraulics. Annular gear motor, e.g. planetary motor, which works on the planetary principle.
Gear motors are normally limited to operating pressures of around 15 MPa and operating
speeds of 2,400 rpm. They are available with a maximum flow capacity of 550 dcm3/min.
Hydraulic motors can also be of the internal gear type. The internal gear type motors can
operate at higher speeds and pressures. They also have greater displacements than the external
motors. Screw-type motors also form part of gear motors. As in the case of pumps, screw-type
hydraulic motors use three meshing screws. The rolling screw set results in an extremely quiet
operation. Screw type motors can operate at pressures up to 21 MPa and can have
displacement volumes up to 0.227 dcm3.
 The internal construction of the vane motors is similar to that of a vane pump, however the
principle of operation differs. Vane motors develop torque by virtue of the hydraulic pressure
acting on the exposed surfaces of the vanes, which slide in and out of the rotor connected to
the drive shaft. As the rotor revolves, the vanes follow the surface of the cam ring because
springs are used to force the vanes radially outward. Vane motors are universally of the
balanced design type. Since vane motors are hydraulically balanced, they are fixed
displacement units. These motors can operate at pressures of up to 17 MPa and speeds up to
4,000 rpm. The maximum flow usually delivered by these motors is in the range of
950 dcm3/min. The vane-type motors have more internal leakage as compared with the piston
type and are therefore not recommended for the use in servo control systems.
 Piston motors are also similar in construction to that of piston pumps. Piston motors can be
either fixed or variable displacement units. They generate torque through pressure acting at the
ends of pistons, reciprocating inside a cylinder block. To put it rather simply, piston-type
hydraulic motors use single-acting pistons that extend by virtue of fluid pressure acting on
them and discharge the fluid as they retract. The piston motion is translated into circular shaft
motion by different means such as an eccentric ring, bent axis or with the help of a swashplate.
The piston motor design usually involves incorporation of an odd number of pistons. This
arrangement results in the same number of pistons receiving the fluid as the ones discharging
the fluid, although one cylinder may get blocked by the valve crossover. On the contrary, with
the number of pistons being even and one getting blocked, there would be one more piston
either receiving or discharging the fluid leading to speed and torque pulsations. Piston motors
are the most efficient of all motors. They are capable of operating at very high speeds of
12,000 rpm and also pressures up to 35 MPa. Large piston motors are capable of delivering
flows up to 1,500 dcm3/min. Piston motors are further categorized as radial and axial piston
motors. In an axial piston motor, the rotor rotates on the same axis as the pistons. There are
basically two types of axial piston motor design. They are in-line piston motor (swashplate
type) and bent-axis type.
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2.2.3. Hydraulic accumulators
Hydraulic accumulators have various tasks to fulfil in a hydraulic system: energy storage, fluid
reserve, emergency operation, equalising of forces, damping of mechanical and pressure shocks,
leakage oil compensation, oscillation damping, pulsation damping, vehicle suspension,
recuperation of deceleration energy, maintaining pressure constant and flow compensation
(expansion tanks) [15]. There is always an equilibrium between pressure of the hydraulic fluid
and the counter pressure generated by the weight (weight loaded accumulator), the spring (spring
accumulator) or the gas (gas pressure accumulator). The basic hydraulic accumulators are shown
in Figure 2.1.
Figure 2.1. Hydraulic accumulators: a) weight loaded, b) spring, c) piston, d) bladder, e) diaphragm
Gas pressurized accumulators are catagorized according to their separating element (between
the gas and fluid) into bladder accumulators, piston accumulators and diaphragm (membrane)
accumulators [11]. With this hydraulic accumulators gas is used as loading medium for the
hydraulic fluid. Bladder accumulators contain a fluid and a gas chamber separated by a gastight
bladder. The fluid chamber around the bladder is connected to the hydraulic circuit, so that when
pressure is increased this chamber fills, squeezing the bladder and compressing the gas. When the
pressure drops the compressed gas expands, forcing the accumulated fluid into the circuit. Piston
accumulators comprise a fluid and a gas chamber with a piston as a gastight separating element.
The gas side is pre-filled with nitrogen. The fluid chamber is connected to the hydraulic circuit,
so that when the pressure rises the piston accumulator absorbs fluid and the gas is compressed.
When the pressure drops the compressed gas expands, displacing the accumulated pressure fluid
in the circuit. Diaphragm accumulators comprise a steel container which is resistant to
compression and is usually either spherical or cylindrical in shape. The separating element inside
the accumulator is a diaphragm made of an elastic material (elastomer). There are two types of
diaphragm accumulator available:
 In the screwed model the diaphragm is held in position by screwing the top and bottom part to
clamping nuts. It is possible to exchange the diaphragm in this model. At the bottom of the
diaphragm in the centre there is a valve plate, which prevents the diaphragm being from pulled
out when fluid is connected.
 In welded accumulators the diaphragm is pressed into the lower part before circular seam
welding is carried out. By using a suitable welding process and by situating the diaphragm
correctly, this ensures that the elastomer material is not damaged when the welding is carried
out. It is not possible to exchange the diaphragm.
2.2.4. Hydraulic cylinders
The hydraulic cylinder is the connecting element between the hydraulic circuit and the
operating machine. It carries out linear (translatory) movements in order to transmit forces.
Cylinders are linear actuators whose output force or motion is in a straight line. Their function is
to convert hydraulic power into linear mechanical power. Hydraulic cylinders extend and retract
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to perform a complete cycle of operation. Their work applications, as earlier discussed, may
include pulling, pushing, tilting and pressing. The type of cylinder to be used along with its
design is based on a specific application. Hydraulic cylinders are suitable for lifting, lowering and
locking operations and for load shifting. Cylinders with a piston are the most common form of
hydraulic cylinder. The mechanical force is produced by the action of the hydraulic fluid on the
piston. The basic hydraulic cylinders are shown in Table 2.3.
Table 2.3. Hydraulic cylinders
Single acting cylinder for pushes
Single acting cylinder for pulling
Single acting plunger cylinder
Double acting cylinder, single rod
Double acting cylinder, double rod
Double acting tandem cylinder
Single acting telescope cylinder
Double acting telescope cylinder
The simple basic hydraulic cylinders [2]:
 Cylinders with spring return are single acting cylinders. They are used in applications where
an external, restoring force does not exist. Return springs may be situated either within the
cylinder or mounted onto the cylinder as a separate component. As these springs can only
carry out limited strokes and exert limited forces, they are mainly to be found in "small
cylinders".
 The plunger is a single acting piston which is used in plunger cylinder. This is a cylinder
model with only one piston area and hence it can only transfer forces due to pressure. Plunger
cylinders are used wherever a definite direction of force will ensure a return of the piston to its
starting position. The examples of this are upstroke presses, lifting devices, etc. Retraction of
the piston can only occur through the weight of the piston or due to an external force being
applied.
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 Double acting cylinders have two opposing effective areas which are of the same or different
sizes. They are fitted with two pipe ports which are isolated from each other. By feeding fluid
via ports "A" or "B" the piston may transfer pulling or pushing forces in both stroke directions.
Two types of double acting cylinder exist: single rod cylinder and double rod cylinder. Single
rod cylinders have a differential piston which is fixed to a piston rod smaller in diameter than
the cylinder diameter. The maximum power that can be transmitted depends on the piston area
for outward travel and on the annulus area for return travel. The respective areas to be filled
are equal in length because of the stroke, but vary in volume owing to the difference between
piston and annulus size. The stroke speed is thus inversely proportional to the area. The
differential piston is a double acting piston in a single rod cylinder. As it is fixed on one side
to a piston rod with a smaller diameter, it has two effective working areas of different sizes.
 Double rod cylinders have a piston firmly fixed to two piston rods of a smaller diameter. The
maximum force that can be transmitted depends, in both directions, on the same annulus area
and also on the maximum permissible operating pressure. This means that at the same
operating pressure equal forces act in both directions. As the areas, stroke lengths and
therefore also the space to be filled are the same on both sides, it follows that the speeds will
also be the same. For special applications there are versions of double rod cylinders with
different piston rod diameters. In double acting cylinders operating in tandem, there are two
cylinders which are connected together in such a way that the piston rod of one cylinder
pushes through the bottom of the other cylinder to its piston area. By using this arrangement
the areas are added together and large forces may be transferred for relatively small external
diameters without increasing the operating pressure.
 Telescopic cylinders are a special design of hydraulic cylinder that provide an exceptionally
long output travel from a very compact retracted length. If the pistons are placed under
pressure via port (A), the sections extend one after another. The pressure is dependent on the
size of the load and the effective area. Hence the piston with the largest effective area extends
first. At constant pressure and flow the extension begins with the largest force and the lowest
speed and finishes with the smallest force and the highest speed. In double acting cylinders the
pistons are extended in the same way as in single acting cylinders. The order in which the
individual stages are retracted depends on the size of the annulus area and on the external load.
The piston with the largest annulus area returns first to its starting position. In double acting
cylinders the pistons are extended in the same way as in single acting cylinders. The order in
which the individual stages are retracted depends on the size of the annulus area and on the
external load. The piston with the largest annulus area returns first to its starting position via
pipe port (B) when it is placed under pressure.
2.2.5. Hydraulic valves
A valve is a control device used for adjusting or manipulating the flow rate of a liquid
(hydraulic fluid) in a pipeline. The valve essentially consists of a flow passage whose flow area
can be varied. The external motion can originate either manually or from an actuator positioned
pneumatically, electrically or hydraulically, in response to some external positioning signal. This
combination of the valve and actuator is known as a control valve or an automatic control valve.
Control is achieved by influencing the start, stop, direction, pressure and flow of the hydraulic
fluid operating in the system.
Valves may be sub-divided [3]:
 according to their function into directional valves, pressure control valves, flow control valves
and non-return valves,
 according to their construction into poppet valves, spool valves, or rotary spool valves,
 according to their spool positions: 4/3-way (3 positions) directional valves, 4/2-way (2
positions) directional valves and other,
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 according to the ports controlled: 3/2-way (3 ports) directional valves, 4/3-way (4 ports)
directional valves and other,
 according to their open and closed loop characteristics: sequence valves, proportional/servo
valves and other,
 according to their type of control into pilot valves and main valves,
 according to their type of actuation into muscle power (manually or foot actuation),
mechanically (roller shaft or roller), electrically (solenoids) or pressure (pneumatic or
hydraulic actuated) or a combination of these.
Basically there are three types of control valves [3]:
1. Direction control valves: Direction control valves determine the path through which a fluid
traverses within a given circuit. In other words, these valves are used to control the direction
of flow in a hydraulic circuit. It is that the component of a hydraulic system that starts, stops
and changes the direction of the fluid flow. Additionally the direction control valve actually
designates the type of hydraulic system design, either open or closed. An example of their
application in a hydraulic system is the actuator circuit, where they establish the direction of
motion of a hydraulic cylinder or a motor.
3. Flow control valves: The fluid flow rate in a hydraulic system is controlled by flow control
valves. Flow control valves regulate the volume of oil supplied to different parts of a hydraulic
system. Non-compensated flow control valves are used where precise speed control is not
required, since the flow rate varies with the pressure drop across a flow control valve.
Pressure-compensated flow control valves are used in order to produce a constant flow rate.
These valves have the tendency to automatically adjust to changes in pressure.
2. Pressure control valves: Pressure control valves protect the system against overpressure
conditions that may occur either on account of a gradual build up due to decrease in fluid
demand or a sudden surge due to opening or closing of the valves. Pressure relief, pressure
reducing, sequencing, unloading, brake and counterbalance valves control the gradual buildup
of pressure in a hydraulic system. Pressure surges can produce instantaneous increases in
pressure as much as four times the normal system pressure and that is the reason why pressure
control devices are a must in any hydraulic circuit. Hydraulic devices such as shock absorbers
are designed to smoothen out pressure surges and also to dampen hydraulic shock.
2.2.5.1. Direction control valves
Direction control valves are used to control the direction of flow in a hydraulic circuit. They
are primarily designated by their number of possible positions, port connections or ways and the
manner in which they are actuated or energized [4]. For example, the number of porting
connections is designated as ways or possible flow paths. For example, the number of porting
connections are designated as ways or possible flow paths. A four-way valve would have four
ports: P (Pump), T (Tank), A (A chamber of actuator) and B (B chamber of actuator). A threeposition valve is indicated by three connected boxes. There are many ways of actuating or
shifting the valve. They include push button, hand lever, foot pedal, mechanical, hydraulic pilot,
air pilot, solenoid and spring (see Fig.2.2).
Figure 2.2. Simple actuating the 4/3-way directional control valve
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Although they may be designed as rotary or poppet style, the spool type directional control is the
most common. This design consists of a body with internal passages that are connected or sealed
by a sliding spool along the lands of the valve. Direction control valves may also be categorized
as normally open and normally closed valves. This terminology would normally accompany the
direction control valves, as reflected in the examples of two-position valves given below.
Normally open and normally closed valves are compared in Table 2.4. The spool type directional
control valves in industrial applications are sub-plate or manifold mounted. The porting pattern is
industry standard and designed by valve size. Directional control valve sizing is according to flow
capacity which is critical to the proper function of the valve. Flow capacity of a valve is
determined by the port sizes and the pressure drop across the valve. A direct acting direction
control valve can be actuated either manually or with the help of a solenoid. Direct acting
indicates that some method of force is applied directly to the spool, causing the spool to shift.
Single and double solenoid control valves are available with DC 12 or 24 volts solenoids or AC
50/60 Hz 120 or 230 volt solenoids. For the control of systems requiring high flows pilot operated
directional control valves must be used due to the higher force required to shift the spool. The top
valve, called the pilot valve, is used to hydraulically shift the bottom valve, or the main valve.
Table 2.4. Normally open and normally closed valves
Solenoid operated 2/2-way valve, normally closed
Solenoid operated 2/2-way valve, normally open
Solenoid operated 3/2-way valve, normally closed
Solenoid operated 3/2-way valve, normally open
The directional control valve actually designates the type of circuit. One can categorize most
hydraulic circuits into two basic types: closed center and open center, as shown in Figure 2.3.
Figure 2.3. Basic hydraulic circuits: a) closed center, b) open center
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Open center circuits are defined as circuits whose route pump flow back to the reservoir
through the directional control valve during neutral or dwell time [4]. This type of a circuit
typically uses a fixed volume pump, such as a gear pump. If flow were to be blocked in neutral or
when the directional control valve is centered, it would force flow over the relief valve. This
could possibly create an excessive amount of heat and would be an incorrect design. A closed
center circuit blocks pump flow at the directional control valve, in neutral or when centered. We
must utilize a pressure compensated pump, such as a piston pump, which will de-stroke; or an
unloading circuit used with a fixed volume pump. Directional control valve (4/3-way - four ports,
three-position, solenoid operated) incorporates a neutral or center position which designates the
circuit as open or closed, depending on the interconnection of the P and T ports, and designates
the type of work application depending on the configuration of the A and B ports. The four most
common types of three-position valves are: closed type, open type, closed type, flow type, and
tandem type, shown in Figure 2.4.
Figure 2.4. Directional control valves: a) closed type, b) open type, c) float type, d) tandem type
2.2.5.3. Flow control valves
Flow control valves are used to regulate the flow rate of oil supplied to different areas of
hydraulic systems. One of the most important applications of flow control valves in hydraulic
systems is in controlling the flow rate to actuators (cylinders and motors) to regulate their speeds.
Any reduction in flow will in turn, result in a speed reduction at the actuator. Flow control valves
are used to influence the speed of movement of actuators. By changing the opening to flow, the
fluid is either closed loop or open loop controlled at the throttling position. Dependent on their
behaviour the flow control valves may be divided into throttling valves and pressure compensated
valves. In addition, both types may operate dependent upon pressure difference or independently
of pressure difference across them. Some factors, which should be considered during the design
stage of a flow control valve are [2]:
• The maximum and minimum flow rates and the fluid density, which affect the size of the valve.
• The corrosive property of the fluid, which determines the material of construction of the valve.
• The pressure drop required across the valve.
• The allowable leakage limit across the valve in its closed position.
• The maximum amount of noise from the valve that can be tolerated.
• The means of connecting the valve to the process i.e. screwed, flanged or butt welded.
There are many different designs of valves used for controlling flow. Many of these designs
have been developed to meet specific needs. Flow control valves are classified as: fixed or nonadjustable, adjustable or throttling and pressure compensated, they are shown in Figure 2.5. The
flow through throttle valves is dependent on the pressure difference at the throttling point i.e. the
greater the pressure difference the larger the flow. Throttle valves are used where a constant
operating resistance is given or where speed variation with variable loads has no effect or is
actually desirable. The amount of flow through an orifice will remain constant as long as the
pressure differential across the orifice does not change. Changing load or upstream pressure will
change the pressure drop across the valve.
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Figure 2.5. Flow control valves: a) fixed (non-adjustable), b) adjustable (throttling), c) pressure
compensated
2.2.5.3. Pressure control valves
The term pressure valve includes all valves which change the operating pressure of a system or
part of a system to a pre-determined value [9]. Depending on the seal of the throttle area we
differentiate between two types of pressure valve: spool valve and poppet valve. According to the
function the pressures valves are sub-divided into relief valves, sequence valves, pressure
unloading valves and pressure reducing valves. The two basic pressure control valve design types
are: direct-acting pressure control valves and pilot-operated pressure control valves. For accurate
control of pressure and force in a hydraulic circuit, five different types of pressure control valves
have been developed. These are given below along with their graphical representation, as shown
in Figure 2.6.
Figure 2.6. Pressure control valves: a) relief valve, b) reducing valve, c) unloading valve,
d) counterbalance or brake valve, e) sequence valve
 The most widely used type of pressure control valve is the pressure relief valve since it is
found in practically every hydraulic system (Fig. 2.6a). It is a normally closed valve whose
function is to limit the pressure to a specified maximum value by diverting the pump flow
back to the tank. The primary port of a relief valve is connected to system pressure and the
secondary port connected to the tank. When the poppet in the relief valve is actuated at a
predetermined pressure, a connection is established between the primary and secondary ports
resulting in the flow getting diverted to the tank.
 Pressure-reducing valves are normally open pressure control valves that are used to limit
pressure in one or two legs of a hydraulic circuit (Fig. 2.6b). Reduced pressure results in a
reduced force being generated.
 Unloading valves are remotely piloted, normally closed pressure control valves, used to direct
flow to the tank when pressure at a particular location in a hydraulic circuit reaches a
predetermined value (Fig. 2.6c). A counterbalance valve again is a normally closed pressure
control valve and is particularly used in cylinder applications for countering a weight or
overrunning load.
 Brake valves are normally closed pressure control valves that are frequently used with
hydraulic motors for dynamic braking (Fig. 2.6d). The operation of these valves involves both
direct and remote pilots connected simultaneously. During running, the valve is kept open
through remote piloting, using system pressure. This results in eliminating any back pressure
on the motor that might arise on account of downstream resistance and subsequent load on the
motor or cylinder.
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 A sequencing valve again is a normally closed pressure control valve used for ensuring a
sequential operation in a hydraulic circuit, based on pressure (Fig. 2.6e). In other words,
sequencing valves ensure the occurrence of one operation before the other.
2.2.5.4. Check valves
A check valve is installed in a hydraulic system to control the direction flow of hydraulic fluid.
The check valve allows free flow of fluid in one direction, but no flow or a restricted one - in the
other direction. There are two general designs in check valves [3]. One has its own housing and is
connected to other components with tubing or hose. Check valves of this design are called in-line
check valves. In the other design, the check valve is a part of another component and is called an
integral check valve. It will not be covered because its operation is identical to the in-line check
valve. The symbol for check valves (non-return valves) is a ball which is pressed against a sealing
seat (Fig.2.7). This seat is drawn as an open triangle in which the ball rests. The point of the
triangle indicates the blocked direction and not the flow direction. Pilot controlled check valves
are shown as a square into which the symbol for the check valve is drawn. The pilot control for
the valve is indicated by a control connection shown in the form of a broken line. The pilot port is
labelled with the letter X.
Figure 2.7. Symbols of check valves: a) unloaded, b) spring loaded, c) pilot-controlled
Check valves are a simple but important part of a hydraulic system [9]. Simply stated, these
valves are used to maintain the direction that fluid flows through a system. And since check
valves are zero leakage devices we can use them to lock hydraulic fluid from the cylinders. This
section has been designed to help you understand how the different valves function and the
strategy of where they are used in the system. In-line check valves are classified as directional
control valves because they dictate the direction the flow can travel in a portion of the circuit.
Because of their sealing capability many designs are considered to have zero leakage. The
simplest check valve allows free flow in one direction and blocks flow from the opposite
direction. This style of check valve is used when flow needs to bypass a pressure valve during
return flow, as a bypass around a filter when a filter becomes clogged, or to keep flow from
entering a portion of a circuit at an undesirable time.
2.3. HYDRAULIC FLUIDS
The hydraulic working fluid is the single most important component of any hydraulic system.
It serves as a lubricant, heat transfer medium, sealant and most important of all, a means of
energy transfer. Hydraulic fluids are basically non-compressible in nature and can therefore take
the shape of any container [4]. This tendency of the fluid makes it exhibit a certain advantage in
the transmission of force across a hydraulic system. The use of a clean, high-quality fluid, is an
essential prerequisite for achieving efficient operation of the hydraulic system. This has
necessitated the development of modern fluids designed specifically for application in hydraulic
systems. Although hydraulic fluid types vary according to application, the four common types
are:
1. Petroleum-based fluids which are the most common of all fluid types and widely used in
applications where fire resistance is not required.
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2. Water glycol fluids used in applications which require fire resistance fluids.
3. Synthetic fluids used in applications where fire resistance and nonconductivity is required.
4. Environment-friendly fluids that end up causing minimal effect on the environment in the event
of a spill.
The first major category of hydraulic fluids is the petroleum-based fluid, which is the most
widely used type. The crude oil that is quality refined can be used for light services. Additives
should be added to these fluids in order to maintain the following characteristics: good lubricity,
high viscosity index, oxidation resistance. The primary disadvantage of a petroleum-based fluid is
that it is flammable.
The hydraulic fluids have the four essential primary functions of power transmission, heat
dissipation, lubrication and sealing to accomplish all of which should possess the following
properties: ideal viscosity, good lubricity, low volatility, non-toxicity, low density, environmental
and chemical stability, high degree of incompressibility, fire resistance, good heat-transfer
capability, foam resistance and most importantly, easy availabihty and cost-effectiveness. It is
quite obvious that no single fluid can meet all the above requirements and it is therefore essential
that only the fluid that comes closest to satisfying most of these requirements be selected for a
particular application. The following components are used to condition the hydraulic fluid: filter,
cooler, heater, thermometer, pressure gauge, reservoir (tank), filling level indicator. The tank in a
hydraulic system fulfils several tasks: acts as an intake and storage reservoir for the hydraulic
fluid required for operation of the system; dissipates heat; separates air, water and solid materials;
supports a built-in or built-on pump and drive motor and other hydraulic components, such as
valves, accumulators, etc. Hydraulic filters can be arranged in various different positions within a
system (pressure line filter, pump inlet filter, return flow filter, by-pass flow filtering).
2.4. HYDRAULIC CIRCUITS
A hydraulic diagram is a compilation of hydraulic graphic symbols, interconnected, showing a
sequence of operational flow. In short, they explain how a circuit functions. Correct diagram
reading is the most important element of hydraulic troubleshooting. Although initially most
circuits may appear complicated, recognizing standard symbols and systematic flow tracings
simplifies the process. A hydraulic system can be divided into the following sections: the signal
control section and the power section. The signal control section is divided into signal input
(sensing) and signal processing (processing). Signal input may be carried out: manually,
mechanically, contactlessly (electrically). Signals can be processed by the following means:
operator, electricity, electronics, pneumatics, mechanics, hydraulics.
The basic functional requirements common to all hydraulic systems are as follows: hydraulic
source (pumps), the means of distributing the hydraulic power (pipes and flexible hose), the
means of controlling the hydraulic power (pressure and flow valves), the means to provide load
actuation (cylinders and motors). A hydraulic drive system is a transmission system that uses
pressurized hydraulic fluid to drive hydraulic machinery, equipment and devices. A simple
hydraulic circuits, for example Figure 2.8, which shows a cylinder and a motor actuated circuits
illustrating basic system components [6]. The hydraulic circuit requires a tank with its fluid, a
pump, a pressure-relief valve, a directional control valve, and a cylinder (motor) to provide the
force (torque) to move the load. Pump (1) draws oil from tank (2), and the pump output line will
contain high-pressure filter (3) to prevent dangerous particles from passing into the system and
causing damage. Pressure-relief valve (4) is required to the working pressure and also to protect
the system from catastrophic failure should the pump output flow not be required by the load.
Operation of directional valve (5), usually by means of an electrical signal, allows fluid to flow in
either of two directions, as indicated on the valve symbol. Actuators – cylinder (6) and motor (7)
will move in the appropriate direction, depending on the input signal selected.
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Figure 2.8. Two simple basic hydraulic circuits: a) cylinder drive, b) motor drive, 1 – pump, 2 – tank,
3 – high pressure filter, 4 – pressure relief-valve, 5 – directional valve, 6 –cylinder, 7 – motor
The speed of motors (Fig.2.9) can be controlled by using a number of different methods [12]:
 meter in circuits – a flow control configuration in which the valve is located downstream of
the pump and upstream of the actuator,
 meter out circuits – a flow control configuration in which the valve is located downstream of
the actuator,
 bleed off – a flow control configuration in which a valve directs flow back to the tank when
actuated.
Meter-in control refers to the use of a flow control at the inlet to an actuator (motors, cylinders)
for use with actuators against which the loads is in opposition to the direction of movement. For
overrunning load forces and/or those with a large mass, meter-out is used where the actuator
outlet flow passes through the restrictor (throttle valve).
Figure 2.9. Basic speed control of motors: a) meter-in, b) meter-out, c) bleed off,
1 – pump, 2 – relief valve, 3 – throttle, 4 – motor
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3. PNEUMATIC CONTROL SYSTEM
3.1. INTRODUCTION
The pneumatic system is a system that uses compressed air to transmit and control energy [5].
A compressor is powered by an electrical drive or diesel engine. Atmospheric air is drawn
through a dust filter and compressed by a factor of up to ten times. Freshly compressed air is hot
and contains moisture concentrated from the natural humidity of the atmosphere. Processing takes
place through an after-cooler and water-separator before the air passes to an air receiver for
storage. A typical industrial pneumatic system (compressed air installation) consists of three main
parts: compressed air production plant (compressor), distribution system (installation) and
application equipment (cylinders, motors, air devices, air tools) (Fig.3.1).
Figure 3.1. Pneumatic system: 1 – compressor, 2 – air receiver, 4 – air treatment unit, 4 – control calve,
5 – actuator (cylinder)
The compressed air in a pneumatic system must be conditioned before if can be used by
components. There are three types of air preparation process, Figure 3.2: filtering contaminants
out of the air, regulating the air pressure level, lubricating the air.
Figure 3.2. Air treatment unit (FRL unit): 1 – air filter with water separator, 2 – pressure regulator, 3 –
gauge, 4 – lubricator
Pneumatic control systems are widely used in our society, especially in the industrial sectors
for the driving of automatic machines, automatic production lines, mechanical clamps, etc. [16].
Many factories have equipped their production lines with compressed air supplies and movable
compressors. There is an unlimited supply of air in our atmosphere to produce compressed air.
Moreover, the use of compressed air is not restricted by distance, as it can easily be transported
through pipes. After use, compressed air can be released directly into the atmosphere without the
need of processing. Pneumatic components are extremely durable and cannot be damaged easily.
The designs of pneumatic components are relatively simple. They are thus more suitable for use
in simple automatic control systems. Compared to electric and hydraulic components, pneumatic
components are more durable and reliable, compressed air is less affected by high temperature,
dust, corrosion, etc. Pneumatic systems are safer than electromotive systems because they can
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work in inflammable environment without causing fire or explosion. Apart from that, overloading
in pneumatic system will only lead to sliding or cessation of operation. Unlike electromotive
components, pneumatic components do not burn or get overheated when overloaded. The
operation of pneumatic systems do not produce pollutants. The air released is also processed in
special ways. Therefore, pneumatic systems can work in environments that demand high level of
cleanliness. One example is the production lines of integrated circuits. As pneumatic components
are not expensive, the costs of pneumatic systems are quite low. Moreover, as pneumatic systems
are very durable, the cost of repair is significantly lower than that of other systems. Although
pneumatic systems possess a lot of advantages, they are also subject to many limitations. As
pneumatic systems are powered by the force provided by compressed air, their operation is
subject to the volume of the compressed air. The speeds of rectilinear and oscillating movement
of pneumatic systems are easy to adjust and subject to few limitations. The pressure and the
volume of air can easily be adjusted by a pressure regulator. As the volume of air may change
when compressed or heated, the supply of air to the system may not be accurate, causing a
decrease in the overall accuracy of the system. As the cylinders of pneumatic components are not
very large, a pneumatic system cannot drive loads that are too heavy. Compressed air must be
processed before use to ensure the absence of water vapour or dust. Otherwise, the moving parts
of the pneumatic components may wear out quickly due to friction. As air can easily be
compressed, the moving speeds of the pistons are relatively uneven. Noise will be produced when
compressed air is released from the pneumatic components. Pneumatic components can be
divided into two categories: components that produce and transport compressed air, components
that consume compressed air. The usefulness of using compresses air as a power source is as:
cleanliness, pressure is transmitted undiminished in all direction throughout the system, low cost,
the best solution for the jig and fixture systems, automation lines, pick and place in electronics
industry. This indicates that the pneumatic systems are not suitable for heavy duty in terms of
load.
3.2. AIR COMPRESSORS
A compressor is a machine that compresses air or another type of gas from a low inlet pressure
(usually atmospheric) to a higher desired pressure level [10]. This is accomplished by reducing
the volume of the gas. As shown in Figure 3.3, there are two basic compressor types: positivedisplacement and dynamic.
Figure 3.3. Air compressor types
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Air compressors are generally positive displacement units and are either of the reciprocating
piston type or the rotary screw or rotary vane types [5]. Positive displacement air compressors
work by filling an air chamber with air and then reducing the chamber’s volume. Dynamic
compressors impart velocity energy to continuously flowing air or gas by means of impellers
rotating at very high speeds. The velocity energy is changed into pressure energy both by the
impellers and the discharge volutes or diffusers. In the centrifugal-type dynamic compressors, the
shape of the impeller blades determines the relationship between air flow and the pressure (or
head) generated. Reciprocating compressors work like bicycle pumps. A piston, driven through a
crankshaft and connecting rod by an electric motor, reduces the volume in the cylinder occupied
by the air or gas, compressing it to a higher pressure. Single-acting compressors have a
compression stroke in only one direction, while double-acting units provide a compression stroke
as the piston moves in each direction. Large, industrial reciprocating air compressors are doubleacting and water-cooled. Multi-stage, double-acting compressors are the most efficient
compressors available, and are typically larger, noisier, and more costly than comparable rotary
units. Reciprocating compressors are available in sizes from less than 0.75 kW to more than 440
kW. The most common type of rotary compressor is the helical-twin, screw-type (also known as
rotary screw or helical-lobe). Male and female screw-rotors mesh, trapping air, and reducing the
volume of the air along the rotors to the air discharge point. Rotary screw compressors have low
initial cost, compact size, low weight, and are easy to maintain. Rotary screw compressors may be
air- or water-cooled. Less common rotary compressors include sliding-vane, liquid-ring, and
scroll-type.
3.3. PNEUMATIC ACTUATORS
A pneumatic actuator converts compressed air energy into mechanical motion. The motion can
be rotary or linear, depending on the type of actuator. Some types of pneumatic actuators include:
rod cylinders, rodless cylinders, rotary vane, rotary rack and pinion, clamping cylinders,
pneumatic muscles, bellows, speciality actuators that combine rotary and linear motion, vacuum
generators. Pneumatic actuators, of which cylinders are the most common, are the devices
providing power and movement to automated systems, machines and processes. A pneumatic
cylinder is a simple, low cost, easy to install device that is ideal for producing powerful linear
movement over a wide range of velocities, and can be stalled without causing internal damage.
Adverse conditions such as high humidity, dry and dusty environments can be easily tolerated.
The diameter or bore of a cylinder determines the maximum force that it can exert and the stroke
determines the maximum linear movement that it can produce. Cylinders are designed to work at
different maximum pressures up to 1.6 MPa. The pneumatic cylinder has the following general
characteristics: diameters 2.5 to 320 mm, stroke lengths 1 to 2000 mm, available forces 2 to
45000 N at 6 bar, piston speed 0.1 to 1.5 m/s [6]
3.3.1. Single acting cylinders
A single acting cylinder has only one entrance that allows compressed air to flow through [6].
With single acting cylinders, compressed air is applied on only one side of the piston face. The
other side is open to atmosphere. Single acting cylinders use compressed air for a power stroke in
one direction only. The return movement of the piston is effected by a built-in spring or by the
application of an external force. The spring force returns the piston to its start position with a
reasonably high speed under no load conditions. For single acting cylinders with no spring, some
external force acting on the piston rod causes its return. Most applications require a single acting
cylinder with the spring pushing the piston and rod to the instroked position. For other
applications sprung outstroked versions can be selected. Single acting cylinders are therefore only
available in stroke lengths of up to 100 mm. Single acting cylinders are used in stamping,
printing, moving materials, etc. Figures 3.4 and 3.5 shows both types of single acting cylinder.
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The cylinder can be arranged to have a powered instroke or a powered outstroke. The spring in a
single acting cylinder is designed to provide sufficient force to return the piston and rod only.
Figure 3.4. Single acting cylinder with spring return: a) – sprung instroked, b) – sprung outstroked,
Figure 3.5. Single acting cylinder no with spring: a) - normally instroked external force returns,
b) - normally outstroked external force returns
3.3.2. Double acting cylinders
Double acting cylinders use compressed air to power both the outstroke and instroke [6]. This
makes them ideal for pushing and pulling within the same application. Superior speed control is
possible with a double acting cylinder, achieved by controlling the exhausting back pressure.
Double acting cylinders are used particularly when the piston is required to perform a work
function in both directions of motion. As the effective area of the piston is small, the thrust
produced during retraction is relatively weak. The impeccable tubes of double acting cylinders are
usually made of steel. Non cushioned cylinders will make metal to metal contact between the
piston and end covers at the extreme ends of stroke. They are suitable for full stroke working only
at slow speeds which results in gentle contact at the ends of stroke. Double acting non cushioned
cylinders are shown in Figure 3.6.
Figure 3.6. Double acting non cushioned cylinder: a) – single rod, b) – through double rod
For faster speed, external stops with shock absorption are required. These should be positioned to
prevent internal contact between the piston and end covers. If large masses are moved by a
cylinder, cushioning is used in the end positions. Before reaching the end position, a cushioning
piston interrupts the direct flow of air to the outside. For the last part of the stroke the speed is
slowed to reduce impact on the cylinder. Cushioned cylinders have a built in method of shock
absorption. Small bore light duty cylinders have fixed cushions which are simply shock absorbing
discs fixed to the piston or end cover (Fig.3.7a). Other cylinders have adjustable cushioning. This
progressively slows the piston rod down over the last part of the stroke by controlling the escape
of a trapped cushion of air (Fig.3.7b).
Figure 3.7. Double acting cushioned cylinder: a) – fixed cushioned, b) – adjustable cushions
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3.3.3. Magnetic cylinders
Magnetic cylinders have a band of magnetic material around the circumference of the piston
and are fitted with a nonmagnetic cylinder barrel [8]. The magnetic field can be imagined as the
shape of a donut around the barrel. This will travel with the piston as the piston rod moves in and
out. By placing magnetically operated switches on the outside of the barrel, one at each end for
example, signals will be received each time the piston rod completes a stroke (Fig.3.8).
Figure 3.8. Magnetic cylinder: a) – single acting normally instroked, external force returns, fixed
cushioned, b) – double acting, adjustable cushions
3.3.4. Rodless cylinders
For some applications it is desirable to contain the movement produced by a cylinder within
the same overall length taken up by the cylinder body. For example, action across a conveyor
belt, or for vertical lifting in spaces with confined headroom [8]. The object to be moved is
attached to a carriage running on the side of the cylinder barrel. A slot, the full length of the
barrel, allows the carriage to be connected to the piston. Long sealing strips on the inside and
outside of the cylinder tube prevent loss of air and ingress of dust. The slot is unsealed only
between the lip seals on the piston as it moves backwards and forwards. Direction and speed
control is by the same techniques as applied to conventional cylinders. Rodless cylinders are
shown in Figure 3.9.
Figure 3.9. Rodless cylinder: a) – double acting, cushioned, b) – double acting, magnetic cushioned
3.3.5. Semi-rotary actuators
There are many applications that require a turning or twisting movement such as turning
components over in a drilling jig or providing a wrist action on a pick and place device. For these
applications semi-rotary actuators (rack-and-pinion, skotch-yoke, vane) can be used (Fig.3.10).
Figure 3.10. Pneumatic symbol of semi-rotary actuator
There are different designs available [8]. Many are based on cylinders that use a rack and a pinion
or a skotch yoke to convert the linear movement to rotation. Cylinder based semi-rotary actuators
typically use two single-acting pistons to generate a linear movement and a gear to transform the
piston displacement into a rotation. The gear can be designed as rack and pinion. The advantage
of the design is the uniform torque for a constant pressure. For a constant pressure their output
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torque depends on the angle of rotation. A single-vane actuator has a cylindrical chamber in
which a vane connected to a drive shaft rotates through an arc to 270°. Two ports are separated by
a stationary barrier. Differential pressure applied across the vane rotates the drive shaft until the
vane meets the barrier. Rotation is reversed by reversing pressure fluid at the inlet and outlet
ports. A double-vane actuator has two diametrically opposed vanes and barriers. This
construction provides twice the torque in the same space as a single-vane actuator, however
rotation is generally limited to 100°.
3.3.6. Bellows
Bellows are durable single acting concertina like actuators which extend when inflated and are
similar to the air suspension units seen on large trucks [8]. They provide powerful short strokes
and have all round compliance allowing them to bend in any direction. When air is introduced,
they cylinder extends and the bellows diameter decreases. The longer the stroke, the more the
force decreases. There are three basic types: single convolution, double convolution and triple
convolution (Fig.3.11). Single, double and triple convolution types provide a range of strokes
with power developed from nominal diameters in the range 70 mm to 546 mm. These actuators
can be used as air springs and are ideal for isolating the vibration of supported loads from the
actuators base mounting.
Figure 3.11. Pneumatic symbol of bellows: a) – single convolution, b) – double convolution,
c) – triple convolution
3.3.7. Pneumatic muscles
Pneumatic muscle is a relatively new actuator (Fig.3.12). The pneumatic muscle as a
pneumatic actuator may be described as: light weight, lower cost, smooth, flexible, powerful,
damped, compliant. It may be characterized as an elastic, single-acting, cylinder pulling the load
in the axial direction. The pulling force generated by pneumatic muscles is big in relation to their
mass and cross-section. A pneumatic muscle is constructed of a radially deformable pipe made of
rubber, latex or silicone, braided with elastic radially tensile net. The net fixed to the muscle ends
functions as a kind of artificial tendons. The muscle filled with compressed air deforms radially
(expands) and gets shorter. The strains occurring in the muscle depend upon outer axial load. The
force generated by pneumatic muscle is a function of pressure, initial length, contraction degree
and its material properties. Initially big force decreases to zero after the critical contraction degree
has been reached. Pressure control enables to change the degree of muscle’s contraction and the
value of pulling force. The pulling force of the pneumatic muscle in reference to its cross-section
amounts to about 300 N/cm2.
Figure 3.12. Pneumatic muscle actuator: 1 – pneumatic symbol, 2 – overall symbol
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3.4. PNEUMATIC VALVES
The function of valves is to control the pressure or flow rate of pressure media. Depending on
design, these can be divided into the following categories: directional control valves, shut-off
valves, flow control valves, pressure control valves [6].
3.4.1. Directional control valve
Directional control valves are primarily used to control the direction of flow between the
components of a pneumatic circuit [11]. Due to their internal resistance they also throttle the air
flow, an effect that is usually not welcome. There are several ways to distinguish between
directional control valves: number of ports or openings (ways), number of switching positions or
internal stable states, internal design, e.g. spool, poppet or diaphragm, type of operation, e.g.
electrical, pneumatical or manual. The working ports are usually labelled 2 and 4, the exhaust
ports 3 and 5 and the supply 1; the control ports of pneumatically operated valves are not counted.
Some typical valves and their symbols are shown in Table 3.1.
Table 3.1. Typical configurations of directional control valves
Name valve
Symbol valve
2/2-way valve NC (Normally Closed), direct electrically
operated
3/2-way valve NC (Normally Closed), electrically
operated, with pilot valve
3/2-way valve NO (Normally Opened), electrically
operated
4/2-way valve, mechanically operated
5/2-way valve, electrically operated, monostable
5/2-way valve, electrically operated, bistable
5/3-way valve, pneumatically operated
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The switching positions are represented by squares. Internal connections between ports for a
given position are indicated by an arrow or arrows within the square [7]. In circuit diagrams
valves are always shown in the neutral position, i.e. valve not actuated. For 2/2-way and 3/2-way
valves it is important to distinguish between those that are open from the supply to the working
port when no operating signal is applied and those that are closed. This information is often
appended to the valve description as 2/2-way valve NC for normally closed or 3/2-way valve NO
for normally opened. There are several configurations for the middle position of 5/3-way valves.
All ports may be closed, as in Table 8, or the working ports may be connected to each other and
the pressure supply or to the exhausts. For double acting cylinder, 5/2-way valves are often used.
The type of actuation, monostable or bistable, depends on the application and the required
behaviour of the drive during and after an emergency shut-down. If the drive is to remain in the
state before the emergency, bistable valves are used. They require an electrical signal to change
the position of the spool which remains otherwise unchanged due to friction forces. This is a
typical situation for horizontal drives. In assembly applications vertical drives are required to
move upwards to clear the working space when an emergency occurs to avoid collisions with
parts that are out of control. In this case, monostable valves that require an electrical signal to
move the drive downwards are used. The operating signal can be a force applied through a lever,
a pressure from another valve or an electric voltage. This signal acts either directly on the main
stage or with the help of an additional valve, called pilot valve. This valve is a power amplifier
and reduces the required energy of the command signal, e.g. the force of a manually operated
valve or the electric current. In the main stage the flow control is often done by a sliding spool
that opens or blocks passages between the ports. Other designs use poppets or diaphragms,
especially for valves with two positions and two or three ports.
3.4.2. Shut-off valve
A shut-off valve is defined in ISO 5598 as a “valve whose main function is to prevent flow”
[7]. Examples are non-return valves, non-return valves with override, shuttle valves or twin
pressure valves, quick exhaust valve, as shown in Table 3.2.
Table 3.2. Simplified symbols of shut-off valves
Name valve
Symbol valve
Name valve
Non-return valve
Non-return valves
with override
Shuttle valve
“OR”
Twin pressure valve
“AND”
Quick exhaust valve
Quick exhaust valve
with silencer
Symbol valve
Non-return valves or check valves allow almost unrestricted flow of air in one direction while
blocking it in the opposite direction. As one manufacturer states, “free flow in one direction; fully
leak-proof shut off in the other”. Non-return valves with override combine in parallel a non-return
valve and a 2/2-way directional control valve that can bypass the non-return function. The bypass
is usually operated pneumatically at port 12. The required pressures to “switch on” or to “reset”
the bypass depend on the operating pressure. Non-return valves with override can be used as a
safety device to hold a cylinder in position even if there is a leakage in the supply line.
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The shuttle valve “OR” selects the higher of two inlet pressures. It allows air to flow from one
of the supply ports, port 1 or port 12, to port 2 depending on the pressure levels at port 1 and port
12; the air flow from the other supply port is blocked. Assuming that p 12 < p1, air can flow in both
directions, e.g. from 1 to 2 or from 2 to 1, depending on the pressures at the ports 1 and 2. If the
pressures at the ports 1 and 12 are equal, the shuttle valve will remain actuated in the original
position.
The twin pressure valve (Two-pressure) “AND” selects the lower of two inlet pressures. It
allows air to flow from one of the supply ports, port 1 or port 12, to port 2 depending on the
pressure levels at port 1 and port 12; the air flow to the other supply port is blocked. Assuming
that p12 > p1, the air can flow in both directions, e.g. from 1 to 2 or 2 to 1, depending on the
pressures at the ports 1 and 2.
Some cylinder applications require a very high piston speed which can be achieved by using a
quick exhaust valve. This valve allows exhausting the air from the cylinder through a large
exhaust port and a silencer thus bypassing the tubing and main control valve. As a result, the back
pressure is lower than in a circuit where the air has to pass through tubing and the directional
control valve to be exhausted. Increases of up to 50 % of the piston’s speed can be achieved
depending on the cylinder type and loading.
3.4.3. Flow control valve
A flow control valve is defined in ISO 5598 as a “valve whose main function is to control the
flow rate” [7]. The flow control action can be operative in one or both directions. The simplest
form of a flow control valve is bi-directional flow regulator (throttling valve) and uni-directional
flow regulator, as shown in Figure 3.13.
Figure 3.13. Pneumatic symbols of control valve: a) bi-directional flow regulator,
b) uni-directional flow regulator
A needle’s tapered nose is used to throttle the flow by reducing the cross-sectional area of the
flow path. Simple tapers tend to produce a parabolic flow curve, whereas complex tapers tend to
give 35–45 % increased flow per turn of the adjusting screw. Throttling valves can be used as
shut-off valves, but over-torquing may score the nose and seat. The uni-directional flow regulator
is a one-way flow control valve or throttle/non-return valve allows free flow in one direction and
the controlled flow in the other direction. These valves are typically used for the speed control of
cylinders. In many cases they are designed to be mounted directly at the cylinder ports. There are
two types: one to give free flow into the cylinder and restricted flow out and the other where the
flow into the cylinder is restricted and the flow out free. For meter-out control the non-return
valve allows almost unrestricted flow from the directional control valve to the cylinder while the
exhaust flow from the cylinder is throttled to build up a back pressure that slows down the piston
movement. For both flow directions there is an influence of the setting of the needle.
3.4.4. Pressure valve
A pressure control valve is defined in ISO 5598 as a “valve whose function is to control
pressure” [7]. In pneumatics, this usually means the reduction of the supply pressure produced by
the compressor by throttling and is carried out by pressure regulators. But for some applications
there are also pneumatic relief valves which open a passage to vent compressed air, for example
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to the atmosphere, if a critical pressure is reached. They are either used as safety devices to
prevent the bursting of vessels or as charging valves in lorries to allow safe operation of several
consumers with only one compressor.
Pressure regulators, also known as pressure reducing valves, are used to reduce the pressure
acting on downstream components of a pneumatic circuit (Fig.3.14a). A pressure reducing valve
is normally installed in a combined assembly of filter, regulator and sometimes lubricator to
prepare compressed air just before delivering it to pneumatic equipment or machinery. Other
typical applications are the control of pressure for air tools such as screwdrivers and impact
wrenches or controlling pressure to cylinders.
A pressure relief valve opens a passage from the high pressure side to the low pressure side
when the supply pressure exceeds a given value (Fig.3.14b). They are typically attached to the
pressure vessel of a compressor to act as a safety device before the vessel bursts due to an
excessive delivery of the compressor. Relief valves are typically not used for pressure regulation
which is done in pneumatic systems by pressure reducing valves to avoid wasteful discharge of
compressed air to the atmosphere.
Figure 3.14. Pneumatic symbols of pressure valve: a) pressure reducing valve (adjustable relieving),
b) pressure relief valve (adjustable, tapped exhaust port)
3.5. PNEUMATIC CIRCUITS
The basic method of controlling the speed is by controlling the flow in or out of the cylinder.
The simplest way is to place a restrictor (throttling valve) on the appropriate port but this reduces
the thrust and wastes energy through friction. Two simple basic pneumatic circuits diagram are
shown in Figure 3.15.
Figure 3.15. Basic pneumatic circuits: a) double acting cylinder drive, b) single acting cylinder drive,
1 – 5/2 way directional valve (manual operated), 2 – uni-directional flow regulator, 3 – double acting
cylinder (adjustable cushions), 4 – 3/2 way directional valve (electric operated), 5 – uni-directional flow
regulator, 6 – single acting cylinder (spring returns the piston)
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For the majority of applications, best controllability results from uni-directional flow regulators
(control valve) fitted to restrict the flow out of the cylinder and allow free flow in [8]. The
regulator fitted to the front port controls the outstroke speed and the one fitted to the rear port
controls the instroke speed. Speed is regulated by controlling the flow of air to exhaust which
maintains a higher back pressure. The higher the back pressure the more constant the velocity
against variations in load, friction and driving force. On the other side of the piston the full power
driving pressure is quickly reached.
The graph below (Fig.3.16), shows the behaviour of pressure and speed during the stroke of a
typical double acting cylinder fitted with flow regulators [8]. If speed is controlled by fitting unidirectional flow regulators the other way round, velocity will not be as constant, or, as
controllable. The back pressure will quickly exhaust and the restricted flow on the other side of
the piston will slowly build to just enough pressure differential to cause movement. Precise
speeds are difficult to adjust as the variables in load and friction represent a higher percentage of
the total load. Also, for fast speeds on adjustable cushion types the cushioning will be less
effective. For very slow speeds and light loads the movement can be jerky. It is caused by the
difference between static and dynamic friction. Pressure builds up to break the piston out of static
friction then the lower dynamic friction allows it to accelerate. The restricted flow cannot keep up
with it so the pressure drops and the piston stops. The sequence is then repeated.
Figure 3.16. Speed/pressure graph of pneumatic cylinder (source [8])
When analyzing or designing a pneumatic circuits, the following four important considerations
must be taken into account: safety of operation, performance of desired functions, efficiency of
operation, costs [10]. When designing a pneumatic drive, there can be many objectives which
usually lead to different choices of components: minimum stroke time, i.e. maximum number of
cycles per unit time; optimal usage of compressed air; minimum-size circuit members; minimum
component price; standardisation to reduce the number of spare parts [7]. Examples of simple
basic pneumatic circuits: meter-out control, meter-in control. However, there are several basics
circuits, which can be integrated into the final circuit. Generally, the actuation of a cylinder is
effected via a directional control valve. The choice of such a directional control valve (number of
connections, number of switching positions, type of actuation) is dependent on the respective
application [6]. A directional control valve can be used as an input, processing or control element.
The distinguishing feature for the allocation of the individual components to the respective groups
of elements is the configuration within a pneumatic system.
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REFERENCES
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2.
3.
4.
5.
Daines J.R:. Introduction to Fluid Power. The Goodheart-Willcox Publisher, Illinois 2009.
Chapple P.: Principles of hydraulic system design. Coxmoor Publishing Company, Oxford 2003.
Watton J.: Fundamentals of Fluid Power Control. Cambridge University Press, Now York 2009.
Doddannavar R., Barnard A.: Practical hydraulic systems. Elsevier, Newnes 2005.
Improving Compressed Air System Performance a sourcebook for industry. Lawrence Berkeley
National Laboratory Washington, DC, 2003.
6. Croser P., Ebel F.: Pneumatic. Basic level. Festo Didactic GmbH & Co., Denkendorf 2000.
7. Beater P.: Pneumatic Drives. System Design, Modelling and Control. Springer-Verlag, Berlin 2007.
8. The Norgren Guide to Specifying Pneumatic Actuators. Norgren, Inc, Lichfield UK, 2010.
9. Control valves. http://www.school-up.com.
10. Pneumatic Systems. Basics, Components, Circuits and Cascade Design.
http://foriums.hydraulicspneumatics.com.
11. System Design, Modelling and Control. Directional control valves. http://www.globalspec.com.
12. Basic Hydraulic Circuit Design. http://www.toolingu.com.
13. Fluid Power basic, by Peter Nachtwey, Delta Computer Systems, Inc.
14. Fluid power. Nonresident Training Course. Albert Beasley, Naval Education and Training
Professional Development and Technology Center.
15. Engineering for Hydraulic & Pneumatic (E.H.P.), http://www.ehp-eg.com/contact-us.
16. Pneumatic systems. Technological Studies. http://resources.hkedcity.net.
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