Fluid Power Diagrams Explained

MODULE TITLE : APPLICATIONS OF PNEUMATICS AND
HYDRAULICS
TOPIC TITLE : FLUID POWER DIAGRAMS
LESSON 1 : GRAPHICAL SYMBOLS AND STANDARDS
APH - 1 - 1
© Teesside University 2011

Published by Teesside University Open Learning (Engineering)
School of Science & Engineering
Teesside University
Tees Valley, UK
TS1 3BA
+44 (0)1642 342740
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________________________________________________________________________________________
INTRODUCTION
________________________________________________________________________________________
There are several ways to transmit power or control movement:
• mechanical – clutches, gears, levers
• electrical – motors, switchgear, generators
• hydraulic – valves, pumps, motors, cylinders
• pneumatic – valves, compressors, cylinders.
In industry, automatic movement can normally be achieved in three ways:
• electrical device
• hydraulic device (fluid-based systems)
• pneumatic device (gas-based systems).
The study of hydraulics and pneumatics deals with the use and characteristics
of fluids. Hydraulics or pneumatics is the controlled transmission of energy by
pressurised liquids (oil) or compressed air, also called fluid power. Almost
anything that requires movement involves fluid power. Hydraulic power is
usually used for precise control of large forces, e.g. the rudder control system
in aircraft, while pneumatic power is for rapid and light forces, such as the
door control system on a bus.
Before the 1950s, pneumatics was most commonly used as a working medium in
the form of stored energy. During the 1950s, the sensing and processing roles
developed in parallel with working requirements. This development enabled
working operations to be controlled using sensors for the measurement of
machine states and conditions. The development of sensors, processors and
actuators has led to the introduction of pneumatic systems. In more recent times,
pneumatics has played a more and more important role in the development of
technology for actuation. The individual elements have further developed with
changes in materials, manufacturing and design processes.
1
Teesside University Open Learning
(Engineering)

The use of hydraulic equipment is increasing steadily throughout the
industrialised world and also in those underdeveloped countries where
agricultural and earth-moving equipment is imported.
In 1988, the hydraulics industry was worth £200 million in the UK and $6000
million in the USA. The market for fluid power equipment in 1999 was in
excess of €21 billion and in 2000 was €25 billion.
Britain is the 5th largest market after the United States, Japan, Germany and
Italy, with around 4% of the total at an estimated £590 million (in 2000). Of
this, hydraulics accounts for two thirds and pneumatics for one third. The
industry employs around 10,000 people in the UK. The industry of fluid
power has a significant manufacturing base in Britain, where there are
approximately 120 companies selling fluid power equipment.
Fluid power systems are used in almost every branch of engineering to control
and transmit power. It is essential that any engineer involved in the design,
maintenance or application of these systems, has an understanding of the
principles involved.
The systems used to transmit power using fluids are complex, and therefore a
set of standard symbols has been developed to represent fluid power systems.
This lesson will introduce the more commonly used fluid power symbols.
2
(Engineering)

________________________________________________________________________________________
YOUR AIMS
________________________________________________________________________________________
On completion of this lesson you should be able to:
• recognise the common fluid power symbols
• sketch the basic types
• understand how functional information is used
• identify valve port connections using the standard numbering system
• understand the difference between finite and infinite position valves.
________________________________________________________________________________________
STUDY ADVICE
________________________________________________________________________________________
Frequent sketching from memory of the common symbols is one of the best
methods of learning them.
3
(Engineering)

________________________________________________________________________________________
GRAPHICAL SYMBOLS AND STANDARDS
________________________________________________________________________________________
GRAPHIC SYMBOLS AND DESCRIPTIONS OF COMPONENTS
A typical fluid power system may include the following components:
• a pump, or air compressor, to convert mechanical power to fluid power
• a cylinder, or motor, to convert fluid power to linear or rotary mechanical
power
• a valve to control the direction and amount of flow
• filters and regulators to condition the fluid
• hose, tube and couplings to conduct the fluid
• seals to contain the fluid
• accumulators and reservoirs to store the fluid
• instruments such as pressure switches, flow meters, transducers to
monitor the performance of the fluid power system.
FIGURE 1 shows the construction drawings of a check valve or non-return
valve, from which we can see that the components used in fluid power circuits
are often complex and difficult to draw. Therefore, a method has been adopted
of representing their function using graphical symbols. FIGURE 2 illustrates
the functional and the conventional graphic symbol for the check valve shown
in FIGURE 1. Comparing them you can see how useful and important the
graphic symbols are in engineering.
4
(Engineering)

FIG. 1 The construction and operation of a check valve
FIG. 2
AP
(a) Functional symbol
AP
Freeflow
(b) Conventional symbol
Freeflow
Return
flow
blocked
Free
flow
P
AA
A
P
P
5
(Engineering)

The development of fluid power systems is assisted by a uniform approach to
the representation of the elements and the circuits. The symbols used for the
individual elements must display the following characteristics:
• function
• actuation and return actuation methods
• number of connections (all labelled for identification)
• number of switching positions
• general operation principles
• simplified representation of the flow path.
A symbol does not represent the following characteristics:
• size of dimensions of the component
• particular manufacturer and methods of construction or costs
• orientation of the ports
• any physical details of the element
• any unions or connections other than junctions.
Therefore, we can say that graphic symbols identify the components and their
function, but do not provide any information about their designs.
STANDARDS
Symbols are described in various national documents, such as DIN 24300,
BS 2917, ISO 1219 and the new ISO 5599, CETOP RP3 plus the original
American JIC and ANSI symbols. There are variations of these standards
throughout the world; however, the differences are normally so small that an
understanding of the symbols used here will allow most circuit diagrams to be
interpreted. Most of the symbols used in this text comply with BS 2917/ISO
1219.
6
(Engineering)

It is not possible within this lesson to cover every piece of equipment and its
relevant symbol: only the underlying principles of symbolic representation and
the common symbols used will be presented.
Once you have gained an understanding of the basic symbols it is possible to
interpret circuit diagrams which are used to represent system operations.
A symbol consists of lines, shapes and numbers which identify the purpose and
method of operation of the component being represented.
BASIC SYMBOLS
Lines
Lines are most commonly used in circuit diagrams to represent fluid
conductors (pipes). There are several different kinds of line in use, dependent
upon the function. For instance, the fluid may be “power fluid” (being used to
power an actuator) or it may be “control fluid” (being used to control the
operation of a valve).
Some of the more common lines are shown in FIGURE 3.
7
(Engineering)

FIG. 3
Working line, return line, or feed line
Pilot control line
Exhaust or drain line
Mechanical connection
Used to show several components
assembled in one unit
Flexible pipe connecting moving parts
An electric cable
Interconnected pipes and fluid
flows between them
Continuous
Long dashes
Short dashes
Double line
Chain dot
Curved
Electric line
Pipe junction
The pipes cross but are not connected
Air is allowed to bleed off to atmosphere
Crossed pipes
Air bleed
or
8
(Engineering)

The Circle and Semicircle
The circle and semicircle are used to represent several components. The size
of the circle (shown in FIGURE 4) and additional information within or across
the circle dictate its specific function.
FIG. 4
The circle is commonly used to represent rotating components, such as pumps,
motors and compressors. Similar circles are used to represent gauges, non-
return valves, rotary connections, and mechanical links or rollers. FIGURE 5
shows some graphical symbols of fluid power components associated with
circles.
FIG. 5
M
Pressure
gauge
Electric
motor
Non-return
valve
Mechanical
roller control
As a rule, energy conversion units
(pump, compressor, motor)
Measuring instrument
Non-return link roller, etc
Mechanical link, roller, etc
Semi-rotary actuator
9
(Engineering)

I should also point out that we use the symbols shown in FIGURE 6 to express
the simplified pressure resource, which also relate to the circle.
FIG. 6
The Square and Rectangle
Squares and rectangles are used to symbolise valves. Valves with an infinite
number of positions are represented within a single square (pressure and flow
control valves). These valves can assume any position between fully open and
fully closed.
Valves which can only take up specific positions have each specific position
represented by a square, as shown in FIGURE 7.
FIG. 7
Single square – infinte positions
Two-squares – two positions
Three squares – three positions
1
1
2
2 3
(a) Pneumatic (b) Hydraulic
10
(Engineering)

The Diamond
The diamond represents conditioning equipment being used to control the
quality of the fluid and includes filters, lubricators, water separators, coolers,
and so on. Some examples associated with the diamond symbol are shown in
FIGURE 8. From this diagram we can see that the specific function is again
dictated by additional symbols within the diamond.
FIG. 8
Now, the symbols so far are incomplete: for them to have meaning they require
additional information with respect to the function that each of them performs.
This additional information is given in the form of a functional symbol.
Arrows
Arrows are used to indicate flow path and the direction a fluid takes within a
component; the arrows may be used with or without a tail and are often drawn
both ways. Some examples are shown in FIGURE 9.
Filter Cooler Heater Lubricator
11
(Engineering)

FIG. 9
Miscellaneous
Other symbols associated with flow and frequently used in fluid power
systems are shown in FIGURE 10.
FIG. 10
A solid triangle is used to indicate
hydraulic flow through a line
A hollow triangle is used to indicate
pneumatic flow through a line or
exhaust
to atmosphere
Two curved lines as shown
indicate reduction in area
Arrows used to indicate flow
path through a component
A sloping arrow indicates
adjustable setting
12
(Engineering)

PUMPS, MOTORS AND COMPRESSORS
Pumps, motors and compressors all deal with the conversion of energy and are
represented by circles. Pumps and compressors convert mechanical energy
from the input shaft into fluid energy and create a flow of fluid in the system;
this is illustrated using a direction triangle pointing outward as shown in
FIGURE 11 below.
FIG. 11
From FIGURE 11, it can be seen that the pump using oil as the fluid uses a
blocked-in triangle while the compressor or pneumatic actuator uses a triangle
in outline only indicating pneumatic operation.
If the devices are motors, which take in fluid energy and convert it to
mechanical energy, the triangles are reversed and point into the centre of the
circle as shown in FIGURE 12.
FIG. 12
Fixed displacement pneumatic
motor
Fixed displacement hydraulic
motor
Fixed displacement pump
(flow-rate not variable)
Compressor
Fixed displacement pneumatic
semi-rotary actuator
13
(Engineering)

If the device can have its capacity varied (the speed of the motor or the output
volume of the pump) it is indicated with a sloping arrow as shown in
FIGURE 13.
FIG. 13
The symbols for pumps and motors up to now have only included one triangle.
If two triangles are present it indicates that there are two directions of flow, i.e.
a motor may have its direction of rotation reversed or a pump may have its
inlet and outlet reversed.
Using the principles developed for symbols identify the following in FIGURE 14.
FIG. 14
(a) ............................................................................................................................................
(b) ............................................................................................................................................
(c) ............................................................................................................................................
________________________________________________________________________________________
(a) (b) (c)
Variable capacity pneumatic
motor
Variable capacity hydraulic
pump
14
(Engineering)

(a) Variable capacity bi-directional pneumatic motor.
(b) Variable capacity bi-directional hydraulic pump.
(c) Variable capacity uni-directional hydraulic pump.
CYLINDERS
Cylinders are linear actuators that are described by their type of construction
and method of operation. They are classified as either single-acting or double-
acting.
Single acting cylinders just have one port, i.e. only one piston surface can be
pressurised with working fluid. These cylinders are returned either by the
effect of external forces, indicated by the symbol with the open bearing cap, or
by a spring. FIGURE 15 shows different types of single acting cylinders.
FIG. 15
Double acting cylinders have two ports for supplying either side of the pistons
with working fluid. From FIGURE 16, it can be seen for a double acting
cylinder with single piston rod that the piston area is greater than the annular
piston surface. Conversely, the symbol for the cylinder with two-sided piston
rod shows that these areas are of the same size.
Single acting cylinder
return by external force
Single acting cylinder
with spring return
Single acting telescopic
cylinder
15
(Engineering)

FIG. 16
VALVE SYMBOLS
Valves are used in fluid power systems to control pressure, flowrate and
direction of the fluid. The function of valves is to direct and regulate the flow
of fluid from compressor (if the working fluid is gas) or pump (if the working
fluid is liquid) to the various load devices.
Directional Control Valves (DCVs)
Directional control valves are identified by two numbers and additional
functional information; a typical example would be:
3/2 manually operated DCV
Double acting cylinder
with single piston rod
Double acting cylinder
with double ended piston rod
Double acting telescopic cylinder
Double acting cylinder with
single ended piston cushioning
Double acting cylinder with end
piston cushioning at both ends
Double acting cylinder with adjustable
end piston cushioning at both ends
16
(Engineering)

The first number (3) refers to the number of ports (holes where pipe
connections are made); the second (2) to the number of control positions. The
additional information, in this case ‘manually operated’, tells us how the valve
is operated. Other information sometimes given is the type of valve internal
mechanism (spool or poppet type) and the port size and type of thread.
FIGURE 17 illustrates two 3/2 manually operated spring return DCVs for a
pneumatic system and hydraulic system respectively, without any additional
functional information (apart from the actuating mechanism on the left).
FIG. 17
The ports are shown as lines protruding from the valve square; these would
normally be connected to the pipework system or to exhaust. There are
standard number and letter systems used for valve port identification as listed
in TABLE 1.
1
2
3
(a) Pneumatic DCV (b) Hydraulic DCV
P
A
T
17
(Engineering)

TABLE 1
It is rare to come across valves with more than 5 ports, although they are
available for certain applications.
The valve port connections are always shown to the ‘normal’ or ‘at rest’
position of the valve. In the case of FIGURE 17 this is the box next to the
spring.
Let’s consider the simplest of directional control valves – the 2/2 DCV
manually operated spring return which is normally closed and add the
functional information (arrows).
In the un-operated condition, as shown in FIGURE 18, no flow is possible
between ports 1 and 2 or P to A. When the valve is operated (imagine the box
on the left taking the right position) the inlet port and outlet port become
connected allowing flow to take place. The actuating mechanism is given a
number or letter which relates to the ports that become connected when
operated – in this case 12 (ports 1 and 2 or P and A connected).
Port
Working lines or service ports
Pressure (power) supply
Tank/exhaust
Designation
Hydraulic system Pneumatic system
A, B, C and so on
P
R, S, T, and so on
2, 4 and so on
(even number)
1
3, 5 and so on
(odd number)
18
(Engineering)

FIG. 18 A 2/2 DCV N/C (normally closed)
FIGURE 19 shows a similar valve which is normally open (N/O) and closed
when activated.
FIG. 19 A 2/2 DCV N/O (normally open)
There are several different flow path configurations for directional control
valves; we will illustrate some of the more common configurations.
FIGURE 20 shows a 3/2 DCV with its functional information. Is the valve shown in
this diagram normally open or normally closed? Is it applied to a pneumatic or
hydraulic piston?
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...................................................................................................................................................
________________________________________________________________________________________
(a) Pneumatic system (b) Hydraulic system
1
2
P
A
(a) Pneumatic system (b) Hydraulic system
12
1
2 12
P
A
19
(Engineering)

FIG. 20
The valve is ‘normally closed’. Ports 1 and 2 will become connected when the
actuating mechanism is operated. This valve is used in a pneumatic system.
The easiest way to understand the operation of the valve is to show how it is
used to control a simple actuator. A cylinder is one type of actuator.
FIGURE 21 shows a 3/2 DCV being used to control the operation of such a
cylinder. In the normal position (N/C) the main air supply is blocked (port 1)
and the cylinder connection (port 2) is connected to atmosphere via port 3; the
cylinder is held in the retract position by spring force. When the valve is
activated, air is allowed to enter the cylinder via the new connection between 1
and 2 causing the cylinder to out-stroke. When the valve is de-activated
connections 2 and 3 are re-made and 1 blocked, allowing the spring to return
the cylinder and exhaust the trapped air to atmosphere through port 3.
FIG. 21
12
1
2
3
1
2
3
12
20
(Engineering)

Produce a sketch of the above cylinder being operated by a 2/2 DCV and explain any
problems that may occur and how they may be overcome.
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21
(Engineering)

If your answer looks like that in FIGURE 22, there will be a problem with the
cylinder operation. We would find that when the DCV was operated the
cylinder would extend, but when the DCV was released the cylinder would
remain extended because air trapped between the piston and port 2 would have
no means of exhausting.
FIG. 22
There are two possible solutions:
(1) Use a 3/2 DCV as shown in FIGURE 21.
(2) Use two 2/2 DCVs as shown in FIGURE 23.
12
1
2
22
(Engineering)

FIG. 23
Valve 1 would be operated to extend the cylinder (valve 2 remaining closed)
and valve 2 operated to allow the trapped air to exhaust.
What would happen if both valves were operated simultaneously?
...................................................................................................................................................
...................................................................................................................................................
________________________________________________________________________________________
The mains air supply would be connected to atmosphere via valve 2 and
therefore the cylinder would fail to move.
12
1
2
12
2
1
Valve 1 Valve 2
23
(Engineering)

The 5/2 DCV
This valve is commonly used to control the operation of a cylinder which is
powered in both directions (double acting). In this case, as shown in
FIGURE 24, the valve is operated by the application of a pneumatic control
signal at either end. The valve is drawn in the normal condition (where the
cylinder is ‘at rest’) and the valve ends are numbered 12 and 14 respectively.
FIG. 24
The operation of the valve and cylinder is as follows. In the ‘at rest’ condition
mains air is connected to the cylinder rod end via ports 1 and 2, a signal being
present at valve actuator 12. When the signal is removed from 12 and applied
to 14 the valve changes position and the mains air is connected to port 4; this
allows the cylinder to be out-stroked exhausting the air from the rod end via
port connection 2 to 3.
4 2
12
35
1
14
24
(Engineering)

Methods of operation
Methods of actuation of directional control valves are dependent on the
requirements of the task. The operation types could be mechanical, pneumatic
or hydraulic, electrical and combined actuation. When applied to DCV,
consideration must be given to the method of initial actuation of the valve and
also the method of return actuation. Normally these are two separate methods,
which are both shown on the symbol either side of the position boxes.
FIGURE 25 shows common examples widely used in DCVs.
FIG. 25
FIGURE 26 shows a 4/3 solenoid operated, spring return hydraulic DCV, in
the centre position, pressure line unloaded to tank and load locked.
FIG. 26
BA
TP
General manual
Hand level
Foot pedal
Indirect pneumatic pilot
Direct liquid pilot
Spring return
Push button
Roller stem
Solenoid
Detent
25
(Engineering)

Exercise
Describe the operation method used in the valve shown in
FIGURE 27.
FIG. 27
Solution
This is a 5/3 push button operated spring return pneumatic DCV, the middle
position closed.
More detailed descriptions on DCVs will be given later in our study.
PRESSURE CONTROL VALVES
Pressure control valves are represented by using squares. The flow direction is
indicated by an arrow. The valve ports can be labelled as:
• P (pressure port) and T (tank) or A and B in hydraulic systems
• 1 (pressure port) and 3 (exhaust) or 2 and 4 in pneumatic systems.
The position of the valve within the square indicates whether the valve is
normally open or normally closed.
24
35 1
26
(Engineering)

The Pressure Regulator
Pressure regulating valves are generally adjustable against spring compression.
The symbols are distinguished according to the following types:
• pressure sensing: downstream, upstream or external
• relieving or non-relieving and fluctuating pressure
• adjustable or fixed settings.
The pneumatic pressure regulating valve shown in FIGURE 28 is held
normally open by the variable control spring allowing a flow of air between
ports 1 and 2 until such time as downstream pressure acting via the pilot line
produces sufficient force to overcome the spring and cause the valve to close.
FIG. 28
Pressure Relief Valve
The function of this valve is to provide protection against over-pressurisation
of the system. This is done by opening at some pre-determined limit and
allowing excess fluid to escape, either to atmosphere or to a holding tank, as
shown in FIGURE 29.
1 2
27
(Engineering)

FIG. 29
The valve is held normally closed by the variable spring until pressure in the pilot
line (upstream) produces sufficient force to overcome it; the valve then opens
allowing excess fluid to escape, thus limiting the pressure within the system.
FLOW CONTROL VALVES
The function of these valves is to control the rate of fluid flow and hence
control actuator speed. This is normally achieved using a variable orifice.
Fixed Restriction
FIGURE 30 shows a fixed restriction in a pneumatic system. This is the
simplest type of flow control: a fixed size of orifice is inserted into the system.
The rate of flow is dependent upon orifice size and the up and downstream
pressure values.
FIG. 30
1 2
1 2
28
(Engineering)

Variable Restriction
If the size of the orifice is adjustable, the rate of flow is easily increased or
decreased. The symbol for a variable restriction used in pneumatic system is
shown in FIGURE 31.
FIG. 31
Variable Restriction with Integral Check Valve
This valve gives a variable flowrate in one direction (from P to A in hydraulic
systems) but allows free unrestricted flow of the fluid in the reverse direction
(A to P) as shown in FIGURE 32.
FIG. 32
The pneumatic circuit diagram shown in FIGURE 33 illustrates the use of
some of the more common components.
AP
1 2
29
(Engineering)

FIG. 33
Identify each of the numbered components and state their purpose within the circuit.
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________________________________________________________________________________________
6
5
4
3
2
1
4 2
12
35
1
14
3
2
4
6
5
1
30
(Engineering)

Pressure source: used to supply the power to the system.
Pressure regulating valve: used to control the downstream pressure.
Pressure gauge: used to monitor the downstream pressure.
5/2 double pilot operated DCV: used to control the direction of cylinder
motion.
Double acting cylinder: used to convert fluid power into mechanical
power via straight line motion.
Variable restrictor with integral check: used to control the speed of the
actuator during out-stroke but to allow unrestricted flow and hence
uncontrolled speed during retract.
THE SHUTTLE VALVE
This valve, normally applied in pneumatic systems, is used when a signal is to
be sent from either one of two positions; it is sometimes known as an “OR”
valve, as shown in FIGURE 34.
FIG. 34
3
21
6
5
4
3
2
1
31
(Engineering)

A signal at either 1 or 2 will result in an output at 3, at the same time blocking
the opposite input, e.g. if an input signal is present at 2 the ball immediately
moves to the left blocking port 1, allowing a flow to take place between 2 and 3.
If signals are present at both 1 and 2 the stronger of the two will pass through.
THE QUICK EXHAUST VALVE
This valve, again applied in pneumatic systems, is used when air must be
rapidly exhausted from a cylinder, normally to allow rapid retraction of a
cylinder. FIGURE 35 shows the valve symbol and a typical application.
FIG. 35
Port 1 is connected to the controlling valve, port 2 to the cylinder and port 3 to
atmosphere. When the cylinder is out-stroking the valve acts like a shuttle
valve blocking off port 3 and allowing a flow from 1 to 2. When the cylinder
is retracting rapidly, instead of the exhausting air taking a restricted path via
the DCV, the valve shuttles back, connecting port 2 to 3 which allows the air to
escape rapidly to atmosphere.
This concludes the lesson on graphical symbols. Only the common
components and their symbols have been discussed. Explanations of the less
common symbols will be given as and when they are used.
3
2
1
To DCV
Air escapes freely
to atmosphere
Rapid retract
32
(Engineering)

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NOTES
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33
(Engineering)

________________________________________________________________________________________
SELF-ASSESSMENT QUESTIONS
________________________________________________________________________________________
1. What are the following symbols generally used to represent?
(i) continuous lines
(ii) circles
(iii) squares
(iv) diamonds.
2. What do the symbols shown in FIGURE 36 represent?
FIG. 36
3. Explain the standard number and letter systems used for valve port
identification.
4. Explain the operation of the valve and cylinder shown in FIGURE 37.
(a) (b)
34
(Engineering)

FIG. 37
12
1
2
3
35
(Engineering)

________________________________________________________________________________________
ANSWERS TO SELF-ASSESSMENT QUESTIONS
________________________________________________________________________________________
1. Check your answers with the descriptions on pages 8, 9, 10 and 11.
2. (a) Represents a pump using oil as the fluid.
(b) Represents a compressor.
3.
4. The diagram shows a 3/2 manually operated spring return DCV being
used to control the operation of a single-acting spring return cylinder. In
the normal position (N/C) the main air supply is blocked (port 1) and the
cylinder connection (port 2) is connected to atmosphere via port 3; the
cylinder is held in the retract position by spring force. When the valve is
activated, air enters the cylinder via ports 1 and 2, causing the cylinder to
outstroke. When the valve is de-activated connections 2 and 3 are re-
made and port 1 blocked, allowing the spring to return the cylinder and
exhaust the trapped air to atmosphere.
Port
Tank/exhaust
Designation
Hydraulic system
(letter system)
Pneumatic system
A, B, C and so on
P
R, S, T and so on
(number system)
2, 4 and so on
(even number)
1
3, 5 and so on
(odd number)
36
(Engineering)

________________________________________________________________________________________
SUMMARY
________________________________________________________________________________________
Graphical symbols are necessary to produce circuit diagrams that include
complex components, which would otherwise be impossible to draw.
Graphical symbols are made up of lines, circles, squares, and diamonds and
functional information is then added.
Valves may have either a finite or infinite number of positions; pressure and
flow controls are infinite while directional controls are normally finite. There
is a standard port identification code for DCVs:
TABLE 1 (reproduced)
An understanding of the graphical symbols used for fluid power is essential if
further work on circuitry is to be understood.
Port
Tank/exhaust
Designation
Hydraulic system
(letter system)
Pneumatic system
A, B, C and so on
P
R, S, T and so on
(number system)
2, 4 and so on
(even number)
1
3, 5 and so on
(odd number)
37
(Engineering)

Fluid Power Diagrams Explained

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