Fluid Power Diagrams and Questions (Mechanical engineering Pneumatic & hydraulic)

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MODULE TITLE : APPLICATIONS OF PNEUMATICS AND

HYDRAULICS

TOPIC TITLE : FLUID POWER DIAGRAMS

LESSON 1 : GRAPHICAL SYMBOLS AND STANDARDS

APH - 1 - 1

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.

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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.

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________________________________________________________________________________________

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.

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________________________________________________________________________________________

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.

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FIG. 1 The construction and operation of a check valve

FIG. 2

(a) Functional symbol

Freeflow

(b) Conventional symbol

Freeflow

Return flow

blocked

Free flow

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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.

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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.

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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

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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

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

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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

2 3

(a) Pneumatic (b) Hydraulic

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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

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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

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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

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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

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(a) Variable capacity bi-directional pneumatic motor.

(b) Variable capacity bi-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

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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

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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.

(a) Pneumatic DCV (b) Hydraulic DCV

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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

R, S, T, and so on

2, 4 and so on

(even number)

3, 5 and so on

(odd number)

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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?

...................................................................................................................................................

________________________________________________________________________________________

(a) Pneumatic system (b) Hydraulic system

2 12

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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

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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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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.

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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.

2 12

Valve 1 Valve 2

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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

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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

General manual

Hand level

Foot pedal

Indirect pneumatic pilot

Direct liquid pilot

Spring return

Push button

Roller stem

Solenoid

Detent

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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.

35 1

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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

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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

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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.

1 2

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FIG. 33

Identify each of the numbered components and state their purpose within the circuit.

........................................................................................................................................

________________________________________________________________________________________

4 2 12

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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

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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.

To DCV

Air escapes freely to atmosphere

Rapid retract

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________________________________________________________________________________________

NOTES ________________________________________________________________________________________

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________________________________________________________________________________________

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)

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FIG. 37

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________________________________________________________________________________________

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.

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

Working lines or service ports

Pressure (power) supply

Tank/exhaust

Designation

Hydraulic system

(letter system)

Pneumatic system

A, B, C and so on

R, S, T and so on

(number system)

2, 4 and so on

(even number)

3, 5 and so on

(odd number)

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________________________________________________________________________________________

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

Working lines or service ports

Pressure (power) supply

Tank/exhaust

Designation

Hydraulic system

(letter system)

Pneumatic system

A, B, C and so on

R, S, T and so on

(number system)

2, 4 and so on

(even number)

3, 5 and so on

(odd number)

Teesside University Open Learning (Engineering)

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setdistillerparams << /HWResolution [2400 2400] /PageSize [612.000 792.000] >> setpagedevice