Compressed air stores energy in its own squeezed volume, so a pneumatic actuator is springy, cannot hold a position under a changing load, and keeps enough stored energy to move after the supply is shut off. The force it delivers is gauge pressure times piston area: a 50 mm bore at 6 bar gauge pushes with 1,178 N.
Part of Pneumatic Systems
Pneumatics uses a compressed gas as the working fluid in cylinders, motors and tools. Everything it does well and everything it does badly follows from the fact that a gas can be squeezed.
A Gas Can Be Squeezed, So Air Power Is Springy
Hydraulic oil barely changes volume under pressure, so a hydraulic cylinder moves exactly as far as the oil put into it. Air does not behave like that. Push on trapped air and it shrinks, and as it shrinks its pressure rises: at constant temperature, pressure times volume stays constant (Boyle’s law, derived properly in the gas laws article). Every pneumatic machine is a spring made of gas, and that one fact organises everything below.
Three consequences follow, and the rest of the pneumatic tree keeps coming back to them:
- The actuator gives under load. A load that rises compresses the air behind the piston, so the piston sinks until the pressure catches up.
- The system holds energy in the fluid itself. The air in a vessel, a hose or a cylinder is energy waiting to be released, which is not true of oil.
- Speed is easy and precision is hard. Air is light and moves fast, which suits hammers and clamps, but it cannot position a load to a fraction of a millimetre.
Force Is Gauge Pressure Times Piston Area
Pressure gauges read gauge pressure, which counts from the surrounding atmosphere. The force on a piston is the difference between the pressure on its two faces, so a piston with open air on the back face feels gauge pressure times area.
- 1 bar = 100,000 Pa = 0.1 MPa. The atmosphere is about 101,325 Pa, close to 1 bar.
- Absolute pressure = gauge pressure + atmosphere. A tank reading 6 bar gauge holds about 7 bar absolute.
- Force (N) = gauge pressure (Pa) x piston area (m²).
Worked example. A cylinder with a 50 mm bore at 6 bar gauge:
- Area = π / 4 x 0.05² = 0.0019635 m² (19.6 cm²)
- Force = 600,000 Pa x 0.0019635 m² = 1,178 N, roughly the weight of a 120 kg mass
Hydraulic systems use the same formula with pressures a hundred times higher, which is why they lift tonnes. Doubling the bore quadruples the force, because area goes with the square of the diameter.
Trapped Air Sags Under a Load That Oil Would Hold
Take the cylinder above with a 200 mm column of trapped air at 6 bar gauge (7 bar absolute) holding up 1,178 N. Add 500 N of load and let the air settle slowly at constant temperature.
- Extra pressure needed = 500 N / 0.0019635 m² = 254,648 Pa (2.55 bar)
- New absolute pressure = 7 bar + 2.55 bar = 9.55 bar
- Boyle’s law: new length = 200 mm x 7 / 9.55 = 146.6 mm
- The piston sinks 53.4 mm
An oil column of the same size under the same extra pressure compresses by the extra pressure divided by oil’s bulk modulus. With a bulk modulus of about 1.5 GPa (recalled from engineering tables, not from this corpus) that is 254,648 / 1.5 x 10⁹ = 1.7 x 10⁻⁴ of the length, or 0.034 mm. The air sags about 1,600 times further. This is why a pneumatic clamp can hold a part and a pneumatic jack cannot hold a car.
Switching Off the Compressor Does Not Switch Off the Energy
Take 1 litre of air at 8 bar gauge (9 bar absolute). If it expanded slowly to the atmosphere it could do up to 2,004 J of work (the isothermal limit, 101,325 x 9 x 0.001 x ln 9 with the pressure and volume in SI units); a fast release behaves more like the adiabatic figure of about 1,063 J. One litre of oil at the same pressure stores only about 0.21 J, because it barely compresses (½ x 800,000 Pa x 5.3 x 10⁻⁷ m³). Air at the same pressure and volume holds thousands of times more energy.
So a cylinder with air still in it can move when a valve is opened, a hose is cut or a load shifts, long after the compressor stopped. A receiver tank holds the same energy and can release it all at once if it bursts; see Receiver Tank Construction.
Common Mistakes
- Reading gauge pressure as absolute. In the Boyle’s-law calculation above the pressure must be absolute, or the answer is wrong by a factor of 1.17.
- Expecting a pneumatic cylinder to stop at an intermediate position. It stops where force balances, and that point moves whenever the load or the supply pressure moves. Use a mechanical stop.
- Treating “the compressor is off” as “the system is dead”. The stored air is the hazard.
Safety
A shut-off compressor still leaves a loaded spring
Air trapped in a cylinder, hose or receiver can move a piston, whip a hose or burst a vessel with no compressor running. Before opening, adjusting or clearing anything on an air system: close the supply valve, open the receiver drain or a vent until the gauge reads zero, and treat any actuator holding a load as live until its air has been vented and its load supported mechanically. Never stand where a cylinder rod, tool or hose end would travel if it moved.
Air is not a toy and not a cleaning tool
A jet of compressed air can drive air through broken skin into tissue and the bloodstream, where an air embolism can kill, and a jet at an ear, eye or any body opening can damage it. Air used to blow dust off clothes or a bench drives swarf and grit into eyes and skin. Do not aim an air nozzle at a person, and never use it to blow down clothing. See Pneumatic Tools and Devices.
What’s Next
- Learn next: Hydraulic vs Pneumatic — the same force formula with an incompressible fluid
- Combine with: Air Compression — where the squeezed air comes from