Pneumatics

Components

• Single-acting cylinder: air outstrokes the piston; a spring returns it.
• Double-acting cylinder: air drives the piston both out and in.
• 3/2 valve: 3 ports, 2 states. Actuated by push button, lever, roller, plunger, solenoid, diaphragm (low-pressure signal from an air bleed) or pilot air; usually spring return.
• 5/2 valve: 5 ports, 2 states; controls a double-acting cylinder. Often pilot-air operated in both directions, so it stays in its last state (memory).
• Restrictor and uni-directional restrictor (UDR): control air flow. A UDR restricts flow one way only.
• Reservoir: a tank that takes time to fill; with a restrictor it gives a time delay.
• Shuttle valve: OR control. Two valves in series: AND control.

Why pneumatics?

• Advantages: air is clean, so leaks don't contaminate food or medicine production; no sparks, so it is safe in flammable or explosive areas; fast, reliable and low cost to maintain; a cylinder can stall under overload without damage.
• Disadvantages: needs a compressor; the exhaust is noisy; air compresses, so positions are less precise and forces are smaller than in hydraulics; leaks waste energy.
• Safety: wear eye protection, switch the air off before changing a circuit, and never point compressed air at anyone.

Describing pneumatic circuits

Use the correct words: 3/2 valves actuate, 5/2 valves change state, pistons outstroke and instroke. E.g. "Valve 2 is actuated, so pilot air changes the state of valve 3 and cylinders A and B outstroke. Piston B actuates valve 4; pilot air changes valve 3 back and both cylinders instroke." All three course reports (2024–2025) criticised vague terms here.

Interactive simulation: try this on the revision site, where you can change the values and see the result.

Semi-automatic and fully automatic circuits

• Semi-automatic: a push-button valve starts one cycle. Pilot air changes the state of the 5/2 valve and the piston outstrokes; at the end of its stroke it actuates a roller trip valve, which changes the 5/2 valve back, and the piston instrokes. The user must press again to repeat.
• Fully automatic: a roller trip at each end of the stroke, so the piston keeps outstroking and instroking until the air is switched off or a stop valve is used.
• A T-piece splits one air line into two, e.g. to send main air to two valves.

Speed control and time delays

• To slow a piston, restrict the exhaust air leaving the cylinder, using a UDR in the exhaust line or a restrictor on the 5/2 valve's exhaust port. Restricting exhaust gives smoother, more controlled movement.
• A time delay: a UDR feeding a reservoir before a pilot or diaphragm-operated valve. The smaller the restriction or the bigger the reservoir, the longer the delay.

Design challenge: parcel sorter

The brief. A delivery depot in Livingston needs a pneumatic pusher to move parcels onto a side conveyor. It must push slowly, wait 2 seconds, then return automatically.

1. Name the type of cylinder and main control valve.
2. Explain how the pushing speed could be controlled.
3. Explain how the 2-second delay could be created.
4. Describe the operation of the circuit using correct terms.

Force, pressure and area

• P = F ÷ A. Area of a piston A = π d² ÷ 4.
• On the instroke of a double-acting cylinder, air pushes on the piston minus the rod: effective area = piston area − rod area. Square each diameter first, then subtract.
• Work in N/mm² with areas in mm². Don't convert to m² unless asked: a 2025 error was giving N/m² without converting.
• Don't use the diameter directly in P = F/A; calculate the area first.

Pressure in bar and worked examples

• 1 bar = 0.1 N/mm², so 3 bar = 0.3 N/mm² and 0.5 N/mm² = 5 bar.
• Force: a 40 mm cylinder at 3 bar: A = π × 20² = 1257 mm², F = 0.3 × 1257 = 380 N.
• Pressure: 454 N from a 34 mm piston: A = π × 34² ÷ 4 = 907.9 mm², P = 454 ÷ 907.9 = 0.50 N/mm² = 5 bar.
• Diameter: 400 N at 5 bar: A = F ÷ P = 400 ÷ 0.5 = 800 mm²; d = √(4A ÷ π) = √(3200 ÷ π) = 32 mm.
• Instroke of a double-acting cylinder: piston 25 mm, rod 10 mm, 0.4 N/mm²: A = π × 12.5² − π × 5² = 412 mm², F = 0.4 × 412 = 165 N.
• A single-acting cylinder's instroke is driven by its return spring, so you can't change its force with air pressure.
• To increase the force: increase the pressure or use a larger piston.

Sequential and electro-pneumatic control HIGHER

Higher circuits sequence up to three cylinders: each cylinder's piston actuates a roller valve that triggers the next step, with time delays where needed. Faults to look for: a pilot valve where a diaphragm actuator is needed (air bleed), a time delay connected the wrong way round (UDR and reservoir swapped), or a missing delay between steps. In electro-pneumatic systems, solenoid-actuated valves are switched by a microcontroller, so the sequence is set in software.

Writing and describing a sequence HIGHER

Write the order using + for outstroke and − for instroke. A furnace loader: cylinder A raises the door by instroking, B pushes the block in and comes back, then A closes the door: Start, A−, B+, B−, A+, Stop.

• Each movement ends by actuating a roller trip valve that pilots the next 5/2 valve to change state.
• Describe the circuit stage by stage: "Valve A is actuated and changes the state of valve B, so cylinder A instrokes. When fully instroked it actuates valve C, which changes the state of valve D, so cylinder B outstrokes…"
• Add a time delay (restrictor and reservoir) where a step needs a pause, e.g. to let a part cool.

Key terms

Quick check