Fluid power introduction

Definition
• It is the use of a confined fluid flowing under
pressure to transmit power from one location to
another.
• It is one of the three commonly used methods of
transmitting power in an industrial setting.
• Others are Electrical and Mechanical power
transmissions.
• Two of the most important advantages of fluid
power transmission are its ability to multiply
force and its flexibility to change direction quickly
without damage to the system.

Hydraulics versus Pneumatics
• Fluid power is divided into two areas:
Hydraulics and Pneumatics.
• Hydraulics is the transmission of power
through a liquid, most commonly petroleum
based oil.
• Pneumatics is the use of a gas, usually air.
• The fundamental difference between air and
oil is that air is compressible, while oil is
relatively incompressible.

Hydraulics versus Pneumatics
• In general, hydraulic systems are usually
preferred for applications that require:
1. High power/large load capacity
2. Precise positioning, and
3. Smooth movement.
• Pneumatics is well suited for applications that
require;
1. Low power/light to moderate load capacity
2. Low to medium precision, and
3. Quick response

Introduction to Pneumatic Systems
• A pneumatic system is a system that uses
compressed air to transmit and control
energy.

• Pneumatic systems are used in controlling
train doors, automatic production
lines, mechanical clamps, etc

Examples for Pneumatic Systems

Advantages of Pneumatic Systems
• High Effectiveness – use of compressed air in not restricted by
distance & can easily be transported.
• High durability and reliability – Extremely durable and not
damaged easily.
• Simple Design - more suitable for use in simple automatic control
systems.
• High adaptability to harsh environments - compressed air is
less affected by high temperature, dust, corrosion, etc.
• Safety - Because they can work in inflammable environment without
causing fire or explosion.
• Easy selection of speed and pressure
• Environment friendly - The operation of pneumatic systems do not
produce pollutants.
• Economical - The costs of pneumatic systems are quite low.

Disadvantages of Pneumatic Systems
• Relatively low accuracy - The volume of air may change
when compressed or heated, the supply of air to the system may
not be accurate.
• Low loading - As the cylinders of pneumatic components are
not very large, a pneumatic system cannot drive loads that are
too heavy.
• Processing required before use - Compressed air must be
processed before use to ensure the absence of water vapour or
dust.
• Uneven moving speed - As air can easily be
compressed, the moving speeds of the pistons are relatively
uneven
• Noise - Noise will be produced when compressed air is released
from the pneumatic components.

Main Pneumatic Components
• Pneumatic components can be divided into two
categories:
1. Components that produce and transport compressed air.
2. Components that consume compressed air.
• All main pneumatic components can be represented by
simple pneumatic symbols. Each symbol shows only the
function of the component it represents.
• Pneumatic symbols can be combined to form pneumatic
diagrams.
• A pneumatic diagram describes the relations between
each pneumatic component, that is, the design of the
system.

1. Production and Transportation of
compressed air
• Examples of components that produce and
transport compressed air include
• compressors and
• pressure regulating components.

(a) Compressors
• A compressor can compress air to the required
pressures.
• It can convert the mechanical energy from motors
and engines into the potential energy in compressed
air .
• A single central compressor can supply various
pneumatic components with compressed air, which
is transported through pipes from the cylinder to the
pneumatic components.
• Compressors can be divided into two classes:
reciprocating and rotary.

Pneumatic Symbol for Compressor
Reciprocating
Compressor

Rotary
Compressor

(b) Pressure Regulating Component
• Pressure regulating components are formed by
various components, each of which has its own
pneumatic symbol:
• (i) Filter – can remove impurities from compressed
air before it is fed to the pneumatic components.
• (ii) Pressure regulator – to stabilise the pressure
and regulate the operation of pneumatic
components
• (iii) Lubricator – To provide lubrication for
pneumatic components

2. The consumption of compressed air
• Examples of components that consume
compressed air include;

• Execution components (cylinders)

• Directional control valves (DCVs) and

• Assistant valves.

(a) Execution Component
• Pneumatic execution components provide
rectilinear or rotary movement.
• Examples of pneumatic execution components
include cylinder pistons, pneumatic motors, etc.
• Rectilinear motion is produced by cylinder
pistons, while pneumatic motors provide
continuous rotations.
• There are many kinds of cylinders, such as single
acting cylinders and double acting cylinders.

(a.1) Single Acting Cylinder
• A single acting cylinder has only one entrance
that allows compressed air to flow through.
• Therefore, it can only produce thrust in one
direction.
• The piston rod is propelled in the opposite
direction by an internal spring, or by the external
force provided by mechanical movement or
weight of a load.

(a.2) Double Acting Cylinder
• In a double acting cylinder, air pressure is applied
alternately to the relative surface of the
piston, producing a propelling force and a
retracting force.
• 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.
• The working surfaces are also polished and coated
with chromium to reduce friction.

(b) Directional Control Valve (DCV)
• Directional control valves ensure the flow of air
between air ports by opening, closing and switching
their internal connections.
• Their classification is determined by the number of
ports, the number of switching positions, the normal
position of the valve and its method of operation.
• Common types of directional control valves include
2/2, 3/2, 5/2, etc.
• The first number represents the number of ports; the
second number represents the number of positions.

(b) Directional Control Valve (DCV)
• A directional control valve that has two ports
and five positions can be represented by the
drawing in Fig., as well as its own unique
pneumatic symbol.

(b.1) 2/2 Directional control valve
• The structure of a 2/2 directional
control valve is very simple.
• It uses the thrust from the spring to
open and close the valve, stopping
compressed air from flowing towards
working tube ‘A’ from air inlet ‘P’.
When a force is applied to the control
axis, the valve will be pushed
open, connecting ‘P’ with ‘A’ .
• The control valve can be driven
manually or mechanically, and restored
to its original position by the spring.

• A 3/2 directional control valve can be used to control
a single acting cylinder.
• The open valves in the middle will close until ‘P’ and
‘A’ are connected together. Then another valve will
open the sealed base between ‘A’ and ‘R’ (exhaust).
• The valves can be driven
manually, mechanically, electrically or pneumatically.
3/2 directional control valves can further be divided
into two classes: Normally open type (N.O.) and
normally closed type (N.C.).

• When a pressure pulse is
input into the pressure
control port ‘P’, the spool will
move to the left, connecting
inlet ‘P’ and work passage ‘B’.
• Work passage ‘A’ will then
make a release of air through
‘R1’ and ‘R2’.
• The directional valves will
remain in this operational
position until signals of the
contrary are received.
• Therefore, this type of
directional control valves is
said to have the function of
‘memory’.

(c) Control Valve
• A control valve is a valve that controls the flow
of air.
• Examples include
 Non-return valves,
 Flow control valves,
 Shuttle valves, etc.

(c.1) Non Return Valve
• A non-return valve allows air to flow in one
direction only.
• When air flows in the opposite direction, the
valve will close.
• Another name for non-return valve is poppet
valve

(c.2) Flow Control Valve
• A flow control valve is formed by a non-return
valve and a variable throttle.

(c.3) Shuttle Valve
• Shuttle valves are also known as
double control or single control
non-return valves.
• A shuttle valve has two air inlets
‘P1’ and ‘P2’ and one air outlet ‘A’.
• When compressed air enters
through ‘P1’, the sphere will seal
and block the other inlet ‘P2’.
• Air can then flow from ‘P1’ to ‘A’.
• When the contrary happens, the
sphere will block inlet
‘P1’, allowing air to flow from ‘P2’
to ‘A’ only.

Principles of pneumatic
control

(a) Pneumatic circuit
• Pneumatic control systems can be designed in
the form of pneumatic circuits.
• A pneumatic circuit is formed by various
pneumatic components, such as
cylinders, directional control valves, flow control
valves, etc.
• Pneumatic circuits have the following functions:
1. To control the injection and release of
compressed air in the cylinders.
2. To use one valve to control another valve.

(b) Pneumatic circuit diagram
• A pneumatic circuit diagram uses pneumatic
symbols to describe its design.

• Some basic rules must be followed when
drawing pneumatic diagrams.

Basic Rules
1. A pneumatic circuit diagram represents the circuit in
static form and assumes there is no supply of pressure.
2. The pneumatic symbol of a directional control valve is
formed by one or more squares. The inlet and exhaust
are drawn underneath the square, while the outlet is
drawn on the top. Each function of the valve (the
position of the valve) shall be represented by a square.

Basic Rules
3. Arrows "↓↖" are used to indicate the flow direction of air current.
If the external port is not connected to the internal parts, the
symbol “┬” is used. The symbol “⊙” underneath the square
represents the air input, while the symbol “▽” represents the
exhaust.
4. The pneumatic symbols of operational components should be
drawn on the outside of the squares. They can be divided into two
classes: mechanical and manual.

Different kinds of basic circuits

Fluid power introduction

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