Choose hydraulics when a load must be held still, positioned precisely or pushed with tonnes of force, and choose pneumatics when speed, a free working fluid and tolerance of leaks matter more, because oil is incompressible and air is not. In a 50 mm bore, oil at 20 MPa pushes with 39.3 kN and air at 6 bar gauge with 1.18 kN, yet a litre of air stores thousands of times more energy than a litre of oil at the same pressure.
Part of Pneumatic Systems
Fluid power moves force through a pipe. Hydraulics does it with a liquid that cannot be squeezed, pneumatics with a gas that can, and that single difference decides which one to build.
The Fluid Decides the Behaviour
Both systems obey the same rule: force equals pressure times piston area. What differs is what the fluid does when the load changes.
- Oil transmits position as well as force. Push 1 mL of oil in and the piston moves exactly the volume of 1 mL. Close the valve and the load stays where it is. Response is immediate.
- Air transmits force through a spring. Push in 1 mL of free air and the piston moves by an amount that depends on the load. Close the valve and the load sags if it grows. The air stores energy as it compresses.
The pneumatic principles article works the springiness in numbers: a 53 mm sag for air against 0.034 mm for oil under the same extra 500 N.
The Same Cylinder, Two Fluids
A cylinder with a 50 mm bore has a piston area of 0.0019635 m².
- Air at 6 bar gauge (600,000 Pa): 600,000 x 0.0019635 = 1,178 N, or 1.18 kN
- Oil at 20 MPa (20,000,000 Pa): 20,000,000 x 0.0019635 = 39,270 N, or 39.3 kN
The force ratio is simply the pressure ratio, 20,000,000 / 600,000 = 33.3. This is why the carrier article’s rule of thumb holds: hydraulics for heavy pressing and lifting, pneumatics for tools and clamping, with the corpus giving moderate pneumatic force as 1-5 tonnes and hydraulic force as 100+ tonnes.
Stored Energy Runs the Other Way
At 8 bar gauge, 1 litre of oil stores about 0.21 J and 1 litre of air about 1,063 J (adiabatic) to 2,004 J (isothermal); the working is in Pneumatic Principles. A burst air vessel is therefore a far larger release of energy than a burst oil line at the same pressure. The oil line’s danger is different: a pinhole jet is thin and fast enough to inject the fluid into tissue.
| Property | Hydraulic (oil) | Pneumatic (air) |
|---|---|---|
| Compressibility | Almost none, so the system is rigid | High, so the system is a spring |
| Holds a load with the valve shut | Yes, until a seal or valve leaks | No, it sags and drifts |
| Typical working pressure in this corpus | Megapascals (tens in a press) | Single-digit bar |
| Force from a given bore | Very high | Moderate |
| Speed | Slow and controllable | Fast, hard to control precisely |
| Energy stored in the fluid | Very small | Large, and released if the vessel fails |
| Main hazard from a leak | A pinhole jet injects oil through skin; oil is slippery and burns | A high-speed jet injures skin, eyes and ears; a vessel burst is violent |
| Fluid cost | Oil must be made or salvaged and kept clean | Free, from the air |
| Fits and finish | Tight bores, good seals | Looser tolerances work |
Which One to Build
- A shop press, jack, log splitter or anything that must stay put at full load: hydraulic. See Hydraulic Principles.
- A hammer, drill, grinder, spray gun or quick clamp: pneumatic. Air tools run fast and do not need the oil kept clean.
- A hand-driven force multiplier with no power source: hydraulic, because the energy can be stored in the operator’s pumping and the load holds between strokes.
- A job that needs both speed and a firm hold: use air to move and a mechanical lock or a small oil circuit to hold.
Common Mistakes
- Choosing air to hold a position. Air cannot hold one under changing load; add a mechanical stop.
- Treating oil as the safer fluid because it stores little energy. The pinhole injection hazard is severe at pressures far below a press’s working pressure.
- Treating air as the safer fluid because a leak is “just noise”. A burst receiver or a hose whipping off a coupler is a stored-energy release.
Safety
Oil under pressure injects through skin
A pinhole leak in a pressurised hydraulic line throws an invisible jet that can penetrate skin and inject oil into the tissue. The entry wound can look trivial, yet this is a surgical emergency that can cost the limb. Never search for a leak with a hand or finger. Pass a piece of card or wood across the suspect fitting and look for the oil spray on it, with the system at the lowest pressure that shows the leak and everyone clear of the line of the jet. A person with any suspected injection injury needs urgent surgical care; do not wait for pain or swelling and do not try to squeeze the oil out.
Stored air bursts and blows
A pressurised air vessel can burst, and compressed air aimed at skin can force air into tissue and the bloodstream. Do not blow air at people and do not use air to clean clothes or benches. See Receiver Tank Construction.
What’s Next
- Learn next: Gas Laws in Practice — the arithmetic behind the spring
- Combine with: Hydraulic Principles — the incompressible half of the pair