Introduction to Pneumatic System

Introduction to pneumatic system

1. 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 clamp.

(a) The advantages of pneumatic systems
Pneumatic control systems are widely used in our society, especially in the industrial sectors for the driving of automatic machines. Pneumatic systems have a lot of advantages.

(i) High effectiveness
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.

(ii) High durability and reliability
Pneumatic components are extremely durable and can not be damaged easily. Compared to electromotive components, pneumatic components are more durable and reliable.

(iii) Simple design
The designs of pneumatic components are relatively simple. They are thus more suitable for use in simple automatic control systems.

(iv) High adaptability to harsh environment
Compared to the elements of other systems, compressed air is less affected by high temperature, dust, corrosion, etc.

(v) Safety
Pneumatic systems are safer than electromotive systems because they can 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.

(vi) Easy selection of speed and pressure
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.

(b) Limitations of pneumatic systems
Although pneumatic systems possess a lot of advantages, they are also subject to many limitations.

(i) Relatively low accuracy
As pneumatic systems are powered by the force provided by compressed air, their operation is subject to the volume of the compressed air. 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

(ii) Low loading
As the cylinders of pneumatic components are not very large, a pneumatic system cannot drive loads that are too heavy.

(iii) Processing required before use
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.

(iv) Uneven moving speed
As air can easily be compressed, the moving speeds of the pistons are relatively uneven.

(v) Noise
Noise will be produced when compressed air is released from the pneumatic components

(c) Main pneumatic components

(a) Compressor

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 (Fig. 2). 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: reciprocatory and rotary.

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

D) Actuators

(i) 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 (Fig. 4). 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 (Fig. 5).

(ii) 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 (Fig. 6). 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.

E) Directional control valve

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. A directional control valve that has two ports and five positions can be represented by the drawing in Fig. 8, as well as its own unique pneumatic symbol.

(i) 2/2 Directional control valve
(ii) 3/2 Directional control valve
(iii) 5/2 Directional control valve

(F) 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.

(i) 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 (Fig. 13).

(ii) Flow control valve

A flow control valve is formed by a non-return valve and a variable throttle (Fig. 14).

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(iii) 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.