Pneumatics

Pneumatic components are arranged in circuits - much like electronic circuits - using symbols for each component. You need to be familiar with the operation of two of most common pneumatic components - a single-acting cylinder with a three-port valve, and a double-acting cylinder with a five-port valve. You also need to understand how time-delayed pneumatic circuits and pneumatic logic circuits work.

Finally you need to know how to work out the force output from a cylinder.

Pneumatic symbols

In your exam, you will be asked to draw pneumatic circuit diagrams using symbols. The symbols for each of the common pneumatic components are listed below. You need to be able to recognse these symbols, and understand the function of the components they stand for.

You might find it helpful to print off this page and keep it for reference.

Pneumatic symbols

Remember that any of the valve operators (for example, push-button operation; plunger operation) can be used with the three-port valves or five-port valves. It is important to choose the one that matches the system requirements and add them to the top and bottom of the valve symbol.

Single-acting cylinder with three-port valve

In a single-acting cylinder compressed air is used to push a piston out, and a spring to push it in again. When compressed air is supplied to the cylinder, the piston rod extends (called the outstroke). When the air supply is switched off, the internal spring returns the piston rod to its retracted position (the instroke). The movement of a single-acting cylinder is normally controlled by a three-port valve - a type of simple switch which governs the flow of air. In the diagram below, a unidirectional-flow control valve is used to slow the speed of the piston on the outstroke. The instroke will be full speed. Press play to see how this works.

Drawing pneumatic circuits

Pneumatic circuits are drawn using the symbols from the chart on the previous page. The diagram below shows the pneumatic circuit for the single-acting cylinder, unidirectional flow control valve and three-port valve.

3/2 push button spring return valve and single acting cylinder

Exam tip:

When drawing circuit diagrams, it is common to show the connection being made in the bottom half of the three-port valve symbol, ie the line from the cylinder should be heading towards the exhaust or the main air symbols. This is because the spring is in control when the other end of the 3/2 is not activated, so that is what is drawn.

Double-acting cylinder with five-port valve

Press the play buttons in the diagram above to see how a double acting cylinder works.

Pilot signals flow from the 3/2s to the sides of the 5/2 when the 3/2 is activated. This yellow pilot signal pushes the spool inside the 5/2 and lets work air flow up to the double-acting cylinder, a pilot signal from the left makes a work air signal come out the top left port of the 5/2, similarly for the right. The work air is shown as red circles entering the double-acting cylinder. As the cylinder rod moves positive it pushes the air inside the front half of the double acting cylinder out an exhaust port on the 5/2, shown as blue circles. It is this exhaust air that can be used to control the speed of a cylinder.

The diagram shows an automatic circuit with a five-port valve. Its operation is detailed below.

Double-acting cylinder and five-port valve

1. When the piston on the double-acting cylinder is retracted (completes the instroke), the piston activates the roller control, allowing air to pass through the 3/2 roller spring return valve.
2. The air output from the three-port valve activates the left side of the 5/2 pilot valve (these are connected by the dashed line), releasing air at the top left of the 5/2 and causing the piston to outstroke. Air in front of the piston rod is pushed out the rod. This exhaust air can be used to control the speed of the outstroke by the unidirectional-flow control valve.
3. When the piston is fully extended, it operates the plunger, causing the 3/2 plunger spring return valve to send out a signal.
4. The air released by the 3/2 plunger spring return valve activates the right side of the 5/2 pilot valve (these are connected by the dashed line). This signal causes air to be released from the top right of the 5/2 causing the piston to retract, or go negative.
5. When the piston on the double acting cylinder is retracted (completes the instroke), the piston operates the roller-trip control and the cycle is then repeated.

Exam tips:

Single-acting cylinders are controlled by three-port valves and double-acting cylinders are controlled by five-port valves. To slow a cylinder a unidirectional-flow control valve is placed to slow the exhaust air.

When drawing a five-port valve, remember:

• It is divided into two separate halves - one half with three ports, one with two.
• In the half with the three ports, the middle port is always the air supply. The ports on either side are the exhaust.

Time delay circuits

A time delay in a peumatic circuit provides a delay between the operation of a valve and the movement of a piston. Time delays are achieved by connecting a unidirectional-flow control valve and a reservoir in series. A pneumatic circuit with a time delay is shown in the diagram and explained below.

Time delay

The time-delayed pneumatic circuit works like this:

1. When the push-button on the three-port valve on the left is pressed, the air flow is restricted by the unidirectional-flow control valve and slowly enters the reservoir.
2. The pressure builds in the reservoir slowly, causing the time delay.
3. Only when the pressure in the reservoir is high enough will it operate the five-point valve, triggering the release of air and causing the piston to outstroke.
4. When the push button is pressed on the second three-port valve (on the right), the five-point valve will switch, triggering the release of air and causing the piston to instroke.

Exam tips:

• The length of the time delay can be altered using an adjustable unidirectional-flow control valve.
• Do not confuse time delay with speed control. Time delay prevents the piston operating for a particular time period. Speed control reduces the speed of the piston's instroke or outstroke.

Logic circuits

Pneumatic logic circuits are more complex circuits controlled by more than one valve, either in series (AND circuits) or in parallel (OR circuits). Pneumatic logic circuits work in a similar way to logic circuits in electronics, with pneumatic valves instead of electronic switches.

Logic circuits are often used to make pneumatic systems safe. For example some pneumatic drills will not work unless a guard is in place. The drill requires two switches to be operated in order to work: the first by putting the guard in place, the second by pressing start.

AND circuits

An AND circuit has two valves in series. In the diagram below, both three-port valve A and three-port valve B have to be pressed before the piston in the single-acting cylinder will outstroke. Valve B will have no air supply unless valve A is open. Valve B controls the air supply to the single-acting cylinder.

AND circuit - two valves in series

OR circuits

An OR circuit has two valves in parallel. The single acting cylinder will go positive if valve A OR valve B OR both of them are pressed. The shuttle valve provides the OR control. A T-piece may appear to do a similar job, but a T-piece would allow air to escape out of the exhaust port of the 3/2 that was not pressed.

OR circuit - two valves in parallel

Exam tip:

It helps to think of valves as switches (as in electronics). Valves may be operated by different actuators (controls), for example push button, lever or roller trip, but they do the same thing: simply switch the output air on or off.

Calculating force output

There is only one formula you need in pneumatics - that for calculating the force output from a cylinder. You will be given the formula sheet at the front of your GCSE answer book. So all you have to do is make sure you know how to use it!

The force you get out of a cylinder depends on the air pressure inside it, and the size or area of the piston. The formula is:

Working out the pressure and the area can present some difficulties, so here are some tips...

Working out the pressure

The trick to success is making sure you use the right units for pressure. In the formula, pressure must be given in Newtons/mm² (N/mm² ). So if pressure is shown in bar you must convert it to Nmm². To convert bars to N/mm² simply divide by 10. For example:

5 bar = 5 ÷ 10 = 0.5N/mm²

Working out the area

The area is the surface area of the piston. As every piston is round, you may have to calculate the area of the circular piston from the radius. To do this, use the formula for area of a circle -

π × radius² - taking π to be 3.14.

For example: if a piston has a radius of 30mm, the surface area of the piston is:

3.14 × 30² = 3.14 × 30 × 30 = 2826mm²

Calculating the force

The force you get from a piston is measured in Newtons (N). Here's a sample problem:

A piston has a radius of 20mm and the pressure in the cylinder is 4 bar. Calculate the force output from the cylinder.

First you need to convert the units of pressure: Pressure = 4bar = 0.4N/mm²

Next, work out the area: Area = 3.14 × 20 × 20 = 1256mm²

Then you can calculate the force output from the cylinder: Force = 0.4 × 1256 = 502.4N

Now try a Test Bite