Making machines move with compressed air — components and symbols, cylinders, 3/2 and 5/2 valves, pneumatic logic, and pressure–force–area calculations.
Pneumatics in engineering Exam
Pneumatics is the use of compressed air to move and control machines. Air is compressed by a compressor, stored in a reservoir, and when a valve is operated the compressed air pushes a piston inside a cylinder — producing a force or movement.
| Sector | Examples |
|---|---|
| Industrial | production-line cylinders, automatic tools, robotic grippers |
| Transport | bus and train brake actuators (compressed-air assisted) |
| Commercial | sliding doors, packaging machines, dental drills |
| Workshop | air-powered nail gun, paint sprayer, impact wrench |
| Advantages | Disadvantages |
|---|---|
| Air is freely available; safe if a leak occurs (no pollution) | Compressed air is noisy when released |
| Cylinders are simple and reliable | Air is springy — less precise than hydraulic systems |
| Safe in damp or flammable areas (no electrical sparks at the cylinder) | Forces are limited compared to hydraulics |
| Speed is easy to control with restrictors | A compressor uses electrical energy and creates wasted heat |
Safety — pneumatic safety rules
- Always wear safety goggles when working with compressed air.
- Never point a hose or air jet at yourself or another person.
- Check all hose connections are secure before turning on the supply.
- Use the correct working pressure (about 4–6 bar at school level).
- Do not look directly down a cylinder when air is supplied.
- When finished, isolate the air supply and let the pressure release safely.
Pneumatic components Exam
Every pneumatic system starts with the same three parts: the compressor compresses air to a higher pressure, the reservoir stores it ready to be used, and the regulator sets a safe, constant working pressure.
Learn the data-booklet symbol for each component — engineers use standard symbols so a circuit means the same thing to everyone.
See every component: its symbol, the kit and key words
Cylinders Exam
In a single-acting cylinder, compressed air pushes the piston out (the outstroke); a spring pushes it back (the instroke). In a double-acting cylinder, air can be supplied to either side of the piston — powered outstroke and powered instroke.
| Single-acting (SAC) | Double-acting (DAC) | |
|---|---|---|
| Outstroke | compressed air | compressed air |
| Instroke | spring | compressed air (other side) |
| Controlled by | 3/2 valve | 5/2 valve |
| Examples | simple clamp, conveyor ejector | pneumatic door, two-way clamp, robot gripper |
Marked practice question — describe the operation (model answer)
Q. A push-button 3/2 valve controls a single-acting cylinder. Using the correct terminology, describe what happens when the button is pressed and then released. 2 marks
Model answer: When the button is pressed the 3/2 valve actuates, so compressed air is supplied to the cylinder and the piston outstrokes (1). When the button is released the 3/2 valve returns and the air is exhausted, so the spring makes the piston instroke (1).
Single-acting cylinder + 3/2 valve
- Build: Air Supply → 3/2 button valve → single-acting cylinder. Switch to Symbol Mode and compare with the data-booklet symbols.
- Predict: what happens when you press the button? When you release it? Run and check — which part makes the piston return?
- Change one thing: switch the 3/2 valve from normally-closed (NC) to normally-open (NO). Predict, then run. What is different, and why?
Double-acting cylinder + 5/2 valve
- Build: Air Supply → 5/2 pilot valve → double-acting cylinder.
- Predict which way the piston moves in each valve state, then run and switch the valve. Why does a double-acting cylinder need a 5/2 and not a 3/2?
- Describe the action: "the 5/2 valve changes state, the piston outstrokes / instrokes." Export the image (press I) for your notes.
Valves, time delays and pneumatic logic ExamAssignment
A valve must be actuated (switched) to change which ports are connected. A 3/2 valve (3 ports, 2 positions) controls a single-acting cylinder; a 5/2 valve (5 ports, 2 positions) moves the air supply from one side of a double-acting cylinder's piston to the other.
| Valve | Ports | Port functions | Controls |
|---|---|---|---|
| 3/2 | 3 ports, 2 positions | 1 = air supply in · 2 = to cylinder · 3 = exhaust | single-acting cylinder |
| 5/2 | 5 ports, 2 positions | 1 = air supply in · 2 & 4 = to each side of the cylinder · 3 & 5 = exhausts | double-acting cylinder |
Ways of actuating a valve
| Method | How it works | Typical use |
|---|---|---|
| Manual — push-button, lever, pedal | a person operates the valve directly | operator start/stop control |
| Mechanical — roller or plunger | a moving part of the machine trips the valve | detecting the end of a piston stroke |
| Pilot-air | a small air signal on a control port moves the valve | linking valves for automatic sequences |
| Solenoid (electrical) | an electromagnetic coil moves the valve | connecting pneumatics to a microcontroller |
| Diaphragm | air pressure on a flexible diaphragm switches the valve | pressure sensing |
Restrictors and the time delay
A restrictor slows the air flow in both directions. A unidirectional restrictor restricts one direction only — air passes freely the other way through a non-return valve — so an engineer can slow the outstroke without slowing the instroke. Combine a unidirectional restrictor with a small reservoir and you get a time delay: air slowly fills the reservoir through the restricted side, and once the pressure reaches the switching value it operates a pilot-air valve. The greater the restriction, the longer the delay.
Pneumatic logic — AND and OR
| Logic function | How it is built | Meaning |
|---|---|---|
| AND | two 3/2 valves in series | output only when both valves are operated — used for two-hand safety controls |
| OR | a shuttle valve | output when either input is operated — start a cylinder from two different buttons |
Speed control with a restrictor
- Add a restrictor between the 5/2 valve and the double-acting cylinder. Set restriction to 0%, then run.
- Predict, then test at 50%, then 90%. Record the outstroke speed (fast / medium / slow) at each setting.
- Change one thing: use a unidirectional restrictor so only the outstroke is slowed. How could an engineer get a slow outstroke but a fast instroke?
Time delay with a reservoir
- Build a pilot circuit where a restrictor feeds a reservoir that pilots a valve to fire the cylinder.
- Predict: with a high restriction, does the cylinder fire sooner or later? Run and time it.
- Change one thing: lower the restriction %. Predict the new delay, then test. State the rule linking restriction to delay.
OR logic (shuttle valve) and AND logic (two-hand safety)
- OR: build two 3/2 button valves → shuttle valve → cylinder. Predict: will the cylinder fire from button A only? B only? Both? Test each. Why is this called OR?
- AND: build two 3/2 valves in series → cylinder. Does it fire with one button? With both? Why are two valves in series an AND?
- Link the AND circuit to a two-hand press, where both hands must press for the machine to operate — a safety control.
Pressure, force and area Exam
The output force from a cylinder depends on the air pressure and the piston area. Pistons are circular, so when a question gives the diameter, find the area first with A = πd²/4.
Units — read this first
- Work in newtons (N) and square millimetres (mm²) — this gives pressure in N mm⁻² (the same as N/mm²). Keep the diameter in mm and there is no need to convert to metres.
- 1 N mm⁻² = 1 000 000 Pa. If a question gives pressure in Pa, convert before mixing units — mixing Pa with mm² is the most common mistake.
Worked example — force from area (substitute & solve)
Worked example — force from diameter (two steps)
Step 1 — find the area:
Step 2 — find the force:
Worked example — rearrange (find the area needed)
Cylinder force calculator
Change the piston diameter and the working pressure. The cylinder, the arrows and both steps of the working follow the numbers.
Predict what happens to the outstroke force when the piston diameter doubles from 50 mm to 100 mm, at the same pressure.
Air pushes on the whole piston face. The shaded face is the area A.
Circuits, design & investigate Assignment
A pneumatic circuit diagram shows how the components connect, drawn with the standard data-booklet symbols. Engineers draw the circuit before building — it checks the design works and lets anyone else read and build it.
The three reference circuits
| Circuit | Built from | What it does |
|---|---|---|
| 1 — Basic single-acting | supply → 3/2 push-button valve → SAC | press = outstroke; release = spring instroke |
| 2 — Basic double-acting | supply → 5/2 valve → both ports of a DAC | valve state chooses outstroke or instroke |
| 3 — Speed control | circuit 2 + a unidirectional restrictor on each side | independent control of extension and retraction speed |
Build each one — on the school kit (safety check before applying air!) or in NoPressureSim — draw the diagram first, predict the behaviour, then test.
Automatic sequence with a limit switch
- Build a double-acting cylinder controlled by a 5/2 pilot valve. Add a roller limit valve at the end of the stroke, labelled "A+" to match the cylinder "A".
- Predict: what does the cylinder do when it reaches full extension? Run it — you have built a semi-automatic sequence: the machine detects its own position.
- Link this to solenoid control: a microcontroller output can energise a solenoid, which actuates the valve.
Design briefs — there is more than one good answer
Automatic sliding door
- An automatic sliding door must open and close under air, closing slowly for safety. Design and build the circuit, then run it.
Hint
Two-hand press, two stations
- A press must operate only when the operator uses both hands, and must be startable from either of two work stations. Design and build the circuit.
Hint
Automatic retract
- Make the cylinder retract automatically when it reaches the end of its stroke. Design and build the circuit.
Hint
Common mistakes — watch out for these in the exam
- Mixing units — a pressure in Pa with an area in mm². Work in N and mm² (N mm⁻²), or convert everything; never mix the two.
- Forgetting to find the piston area first when only the diameter is given (A = πd²/4).
- Writing "the valve goes" or "the piston moves" instead of actuates, changes state, instrokes, outstrokes.
- Confusing the logic — AND is two valves in series; OR is a shuttle valve.
- Choosing the wrong valve — a 3/2 controls a single-acting cylinder, a 5/2 controls a double-acting cylinder.
Check your booklet work
Try each task in your booklet first, then open the matching answer. Round calculated answers sensibly and always show working and units.
Try This — final answers
Try This — Pressure and force §5
- F = P × A = 0.6 × 1000 = 600 N
- A = F ÷ P = 1500 ÷ 0.5 = 3000 mm²
- P = F ÷ A = 1000 ÷ 2500 = 0.4 N mm⁻²
Try This — Piston area and force §6
- A = πd²/4 = π × 40² ÷ 4 = 1257 mm²
- A = π × 25² ÷ 4 = 491 mm²; F = 0.4 × 491 = 196 N
- A = π × 60² ÷ 4 = 2827 mm²; F = 0.5 × 2827 = 1414 N
Section practice — final answers
Practice — Pneumatics in Engineering §1
- Using compressed air to move and control machines.
- Any two: production-line cylinders, automatic tools, robotic grippers.
- Any one: sliding doors, packaging machines, dental drills.
- Air is clean/safe if it leaks; simple and reliable; safe near damp or flammable areas.
- Noisy when released; springy (less precise); limited force; compressor wastes energy.
- Any two safety rules, e.g. wear goggles; never point a hose at anyone; check connections.
- No electrical spark at the cylinder, so no ignition risk in flammable/damp areas.
- About 4–6 bar at school level.
- Liquids are (almost) incompressible, so hydraulics transmit larger forces without "give".
- Any two: air-powered nail gun, paint sprayer, impact wrench, air drill.
Practice — Pneumatic Components §2
- Compresses air to supply the system.
- Stores compressed air at the working pressure.
- Sets a safe, constant working pressure.
- See the symbol grid above (reservoir).
- See the symbol grid above (regulator).
- Regulator.
- Restrictor.
- The reservoir stores air and smooths the supply, so the system keeps working while the compressor refills.
- Passes air from either of two inputs to one output (OR logic).
- Standard symbols are understood by all engineers and make circuits quick to read and build.
Practice — Cylinders §3
- Air pushes the piston out (outstroke); a spring returns it (instroke).
- Air drives the piston both ways — powered outstroke and powered instroke.
- Double-acting cylinder.
- Single-acting cylinder.
- See the symbol grid above (single-acting).
- See the symbol grid above (double-acting).
- It uses one air connection and a spring return, so it is simpler and needs less.
- Any powered push-and-pull task, e.g. a robot gripper or a two-way clamp.
- Any push-and-return task, e.g. a stamping/clamping tool with a spring return.
- A gripper must be powered both to close and to open (hold and release), which needs a double-acting cylinder.
Practice — Valves §4
- Controls a single-acting cylinder (air on / exhaust).
- Controls a double-acting cylinder (air to each side in turn).
- 3 ports, 2 positions.
- 5 ports, 2 positions.
- It restricts air flow one way only, slowing the piston in that direction to control speed.
- Two inputs, one output; passes air from either input (OR logic).
- A small pilot-air signal on the control port moves the spool, switching the valve without a hand — used to link valves for automatic sequences.
- A restrictor slowly fills a reservoir; when the pressure is high enough it pilots the next valve — more restriction gives a longer delay.
- 5/2 valve.
- A shuttle valve — it passes air from either button to the cylinder (OR).
Practice — Pneumatic Calculations §5–6
- P = F ÷ A.
- A = πd² ÷ 4.
- F = 0.6 × 2000 = 1200 N
- A = π × 60² ÷ 4 = 2827 mm²; F = 0.5 × 2827 = 1414 N
- A = π × 80² ÷ 4 = 5027 mm²; F = 0.4 × 5027 = 2011 N
- A = F ÷ P = 2000 ÷ 0.5 = 4000 mm²
- P = F ÷ A = 900 ÷ 1500 = 0.6 N mm⁻²
- The rod takes up part of the piston area on the instroke, so the effective area is smaller; F = P × A, so a smaller area gives a smaller force at the same pressure.
- A = π × 100² ÷ 4 = 7854 mm²
- A = π × 35² ÷ 4 = 962 mm²; F = 0.6 × 962 = 577 N
Practice — Pneumatic Circuits §7
- 5/2 valve.
- 3/2 valve.
- Fit a unidirectional restrictor on the outstroke side, so air is restricted only as the piston extends.
- Supply → 3/2 push-button valve → single-acting cylinder (see reference circuit 1).
- Supply → 5/2 valve → both ports of a double-acting cylinder (see reference circuit 2).
- A shuttle valve feeds the cylinder from either button, so either one starts it (OR).
- To make sure connections are secure and no hose can whip or leak — for safety.
- Unidirectional restrictor on the outstroke side (slow out); air flows freely on the instroke (fast in).
- It checks the design works and lets others read/build it using standard symbols.
- The button pilots the 5/2 valve to change state, sending air to extend the cylinder automatically.
Check yourself
Sources & credits: The Topic 6 booklet © R Stewart, 2026. NoPressureSim is a free simulator by R Stewart. The Past Paper Finder is compiled by Mr McDonald, 2024; past-paper questions © Qualifications Scotland (SQA). The N4/N5 data booklet is reproduced for educational use, © Qualifications Scotland (SQA). The Review Questions PDFs reproduce past-paper questions and marking instructions for educational use, © Qualifications Scotland (SQA), each listed in its file; the exam-style questions, example answers and diagrams in them are by R Stewart, November 2026.