Hub · Interactive
How Pneumatics Move a Machine
Most of the clamping, pressing and pushing on the machines we build is done with compressed air. This page covers where the air comes from, what the cylinders and valves do, and how to size a cylinder. There's a valve and cylinder you can operate, and a force calculator.
Start here
1Why machines run on air
A pneumatic cylinder is a tube with a piston in it. Let air into one end and the piston moves out. Let air into the other end and it comes back. That simple part does most of the clamping, pressing, pushing and ejecting on a production machine.
Fast
Air flows quickly and the moving parts are light, so a small cylinder can finish its stroke in a fraction of a second.
Cheap and simple
A cylinder, a valve, two fittings and some tubing. The parts are standard and widely stocked, so a worn seal or a dead valve is a quick swap.
Happy to stall
A cylinder pushing against a hard stop just sits there at full force. Nothing overheats. That is why air suits clamps and presses.
Clean
A leak puts air into the room, with no oil to clean up. Exhaust can be silenced and filtered for clean areas.
Air, electric or hydraulic?
| Drive | Good for | Watch out for |
|---|---|---|
| Pneumatic | Moving between two end positions quickly: clamp, press, push, lift, eject. Forces up to a few kilonewtons. | Air compresses, so stopping accurately in the middle of a stroke is hard. Compressed air uses more energy per job than an electric drive. |
| Electric actuator or servo | Many positions, controlled speed and acceleration, accurate stops, position and force feedback. | Higher part cost and more setup. Holding force against a stop has to be handled by the drive. |
| Hydraulic | Very high force from a small cylinder, stiff under load. Heavy presses and clamping. | Needs a power unit and oil, and care with leaks and heat. Usually more than a small machine needs. |
Many machines use more than one: air for the end-to-end moves, and a servo or stepper where the part has to stop at several positions.
Supply
2Getting air to the machine
A cylinder is only as good as the air it gets. Air starts at the compressor and passes through a few stages before it reaches a valve.
Compressor → Receiver → Dryer → main line at about 6 bar → Lockout valve → FRL → Valves → Cylinders
Compressor and receiver
The compressor fills a receiver tank, which evens out the demand. Plant air is commonly distributed at around 6 bar, about 90 psi.
Dryer
Compressed air carries water vapor that condenses as it cools in the pipes. A dryer, often a refrigerated type, takes most of it out. Drains on the tank and filters get rid of the rest.
FRL at the machine
Filter, regulator, lubricator. The filter catches water, pipe scale and dirt. The regulator sets the machine's working pressure. Most modern valves and cylinders are greased for life and run on dry air, so the lubricator is often left out. Once you start oiling, keep oiling, because the oil washes out the factory grease.
Regulate each circuit
Not every cylinder needs full pressure. A clamp on a delicate part might run at 3 bar while the press beside it runs at 6. A regulator on that one circuit sets its force and saves air.
Pressures on this page are gauge pressure (bar g), the number a regulator gauge shows.
Actuators
3Cylinders
Most cylinders on a machine are one of two types. Both are sized by bore (the piston diameter) and stroke (how far the rod travels).
Single-acting
Air pushes the piston one way and a spring pushes it back. It uses about half the air and needs only a 3/2 valve. The spring takes some of the force and some of the length, so strokes are short. Good for small clamps and stops.
Double-acting
A port at each end. Air into the cap end extends it, air into the rod end retracts it, and the other side vents each time. Force in both directions and any stroke length. This is the default on most machines.
Retract force is lower
On the way back, air pushes only on the ring around the rod. The same pressure on a smaller area gives less force. On a 32 mm bore with a 12 mm rod it's about 14% less.
Cushions
An adjustable air cushion traps some exhaust air near the end of the stroke, so the piston slows before it reaches the end cap. Small cylinders use rubber bumpers instead. A cylinder that bangs at the end needs its cushion set or its speed turned down.
Magnetic piston and reed switches
Order the cylinder with a magnet in the piston, and a reed switch or electronic magnetic sensor clipped into the barrel slot switches when the magnet passes under it. One at each end tells the PLC whether the cylinder is in or out. More on these in How Industrial Sensors Detect a Part.
Standard sizes
Round cylinders to ISO 6432 (up to 25 mm bore) and profile cylinders to ISO 15552 (32 mm and up) share mounting dimensions across brands, so a replacement doesn't mean a new bracket.
Valves
4Directional valves
A valve is named by its ports and positions: a 5/2 valve has five ports and two positions. The symbol draws each position as a box, and the arrows show where the air goes in that position.
| Valve | Ports | Positions | Typical use |
|---|---|---|---|
| 3/2 | 1 supply, 2 output, 3 exhaust | 2 | Single-acting cylinder, air blow-off, pilot signals. Sold normally closed or normally open. |
| 5/2 | 1 supply, 2 and 4 outputs, 3 and 5 exhausts | 2 | Double-acting cylinder: one output extends it, the other retracts it. |
| 5/3 | Same as 5/2 | 3, spring centered | Double-acting cylinder that has to stop, or go free, part way. See the center positions below. |
Port numbers follow ISO 5599: 1 is supply, 2 and 4 are outputs, 3 and 5 are exhausts. The signal marked 14 connects 1 to 4, and the signal marked 12 connects 1 to 2.
Solenoid or manual
A solenoid valve shifts when its coil gets power, usually 24 V DC from a PLC output. Most are pilot operated: the coil opens a tiny pilot passage and supply air moves the spool, so the valve needs air pressure to shift. Manual valves (push button, lever, foot pedal) do the same job by hand. Solenoid valves also have a small manual override for testing.
Single solenoid, spring return
One coil. Coil on, the spool moves to position 14. Coil off, the spring pushes it back to position 12. If the power goes, the cylinder goes back to its home position.
Double solenoid (memory valve)
Two coils and no spring. A short pulse on either coil shifts the spool, and it stays there after the coil switches off. If the power goes, the valve stays where it is and the cylinder holds or finishes its last stroke.
5/3 center positions
Closed center: every port blocked, so the cylinder stops and holds, though air leaking past the seals means it is not a mechanical lock. Exhaust center: both outputs vented, so the rod can be pushed by hand. Pressure center: both outputs pressurized. The cap side has more area, so the rod creeps out unless something stops it.
Core idea · Interactive
5Operate a valve and cylinder
A 5/2 solenoid valve driving a double-acting cylinder, with a meter-out flow control on each port and a reed switch at each end. Switch the solenoid, change the speed, then try cutting the power with each valve type.
Stroke time about 1.0 s
Retracted switch
Extended switch
Coil 14
Coil 12
Things to try
- Single solenoid: energize 14, let it extend, then cut power. The spring returns the spool and the cylinder goes home.
- Turn the flow controls down to 10% so the strokes are slow enough to catch.
- Double solenoid: pulse 14, then cut power while the rod is still moving. It carries on to the end of the stroke.
- Cut air half way through a stroke. The cylinder stops with no force behind it.
This valve is pilot operated, like most solenoid valves: with the air cut, switching the coil does not move the spool. When power comes back, the PLC outputs stay off until the machine is restarted.
Flow control
6Speed control: meter-out
Cylinder speed is set by restricting the air flow. On a double-acting cylinder the standard way is to restrict the air leaving the cylinder, which is called meter-out.
Why meter-out
The driving side gets full pressure and the exhaust side builds back pressure. The piston is held between the two, so it moves smoothly, and a load that tries to run ahead (a cylinder pushing downward, a heavy slide) can't run away.
Why not meter-in
Restrict the incoming air on a double-acting cylinder and the pressure builds until the piston breaks free, then it jumps, stalls and jumps again. Meter-in is used on single-acting cylinders, which have no exhaust side to restrict.
The fitting
A speed controller usually screws straight into the cylinder port. Inside is a needle valve with a check valve beside it: air flows in freely and is throttled on the way out. The body is marked with its flow direction. Fitted the wrong way round, it becomes meter-in.
Going faster
If the valve and tubing are holding a cylinder back, a quick exhaust valve on the cylinder port dumps the exhaust air right there instead of sending it back through the valve.
Sizing · Interactive
7Cylinder force calculator
Force is pressure times area. Since 1 bar is 0.1 N/mm², force in newtons is 0.1 × pressure in bar × area in mm².
Extend: F = P × A A = π × D² / 4 (D = bore) Retract: F = P × (A − a) a = π × d² / 4 (d = rod)
Extend
Retract
Bars are scaled to this bore at 8 bar. Forces are theoretical.
All standard bores at 6.0 bar
| Bore | Rod | Extend (N) | Retract (N) | Extend (kgf) |
|---|---|---|---|---|
| 10 mm | 4 mm | 47 | 40 | 5 |
| 16 mm | 6 mm | 121 | 104 | 12 |
| 20 mm | 8 mm | 188 | 158 | 19 |
| 25 mm | 10 mm | 295 | 247 | 30 |
| 32 mm | 12 mm | 483 | 415 | 49 |
| 40 mm | 16 mm | 754 | 633 | 77 |
| 50 mm | 20 mm | 1,178 | 990 | 120 |
| 63 mm | 20 mm | 1,870 | 1,682 | 191 |
| 80 mm | 25 mm | 3,016 | 2,721 | 308 |
| 100 mm | 25 mm | 4,712 | 4,418 | 481 |
Safety
8Stored energy and pinch points
Compressed air stays in a machine after the power is off. The risk is highest during setup, cleaning and maintenance, when someone reaches in thinking the machine is dead.
Lockout
Each machine needs a lockable shut-off valve that closes the supply and vents the air downstream. Close it, lock it, check the gauge reads zero, and try the controls to prove nothing moves before reaching in.
Trapped air
A closed-center 5/3 valve or a pilot-operated check valve can keep a cylinder pressurized after the supply is vented. Those circuits need a way to bleed the trapped air, and a label saying so.
Dump and soft start
A dump valve vents the machine's air when the e-stop is pressed or a guard opens, where the risk assessment calls for it. A soft-start valve brings the pressure up slowly at start-up, so cylinders that emptied overnight don't slam across with no back pressure to slow them.
Gravity
Venting the air removes the force holding a load up, and a vertical cylinder will drop. Where that could hurt someone, use a rod lock, a mechanical stop or a pilot-operated check valve.
Applications
9Where air shows up in our builds
Semi-Automated Production Jigs
The operator loads the part and a pneumatic press does the assembly pressing. Jigs in the same family handle grease application. Bore and pressure come from the force the part needs, with the margin described above.
Custom Automation Equipment with PLC Control
The PLC switches the solenoid valves and reads the reed switches back as inputs, so it knows each cylinder finished its stroke before it starts the next step. The PLC side is covered in How a PLC Runs a Machine.
Table-top machines
A bench machine that clamps, presses and releases a part usually needs only a few small-bore cylinders and a compact valve bank, fed from plant air through its own FRL.
Choosing the drive
The same question comes up on bigger builds like the Automated Drum Shaker and the Wet Dust and Fume Collector: which motions suit air, and which need a motor.
Check yourself
10Quick quiz
1. A double solenoid 5/2 valve has just been pulsed to extend a cylinder. The power fails mid-stroke. What happens?
2. Same cylinder, same pressure. Which stroke has more force?
3. What is the usual way to set the speed of a double-acting cylinder?
4. Roughly what force does a 50 mm bore cylinder give on extend at 6 bar?
Score: 0 / 4