Pneumatic Piston Cylinder Actuators
Where a diaphragm actuator is limited by its effective area and a practical air pressure of roughly 1–4 bar, a piston cylinder runs on full plant air — commonly 5.5 to 7 bar — against a sealed piston. The result is several times the thrust from the same yoke footprint, which is why cylinders dominate high-shutoff-class, high-pressure-drop and rotary valves.
The thrust arithmetic
Available thrust is simply piston area times air pressure, minus spring force where a spring is fitted:
For a 200 mm bore piston at 6 bar gauge, area ≈ 0.0314 m², so raw thrust ≈ 1880 N per bar — about 19 kN at full supply before subtracting spring load and friction. The sizing rule: required seat load for the leakage class you specified (per the valve maker's data) × a margin, typically 1.25–1.5, must be less than available thrust at the minimum guaranteed air pressure, not the normal one.
Spring vs springless, single vs double acting
- Spring-return (single-acting) — air one side, spring the other; the spring defines the fail action. Simple and self-contained, but the spring eats part of the thrust budget.
- Double-acting, springless — air both sides; maximum thrust both directions. Fail action must come from elsewhere: a solenoid plus a fail-safe volume tank, or a hydraulic/pneumatic spring return module.
Selection notes
- Check stroke length — cylinders suit long-stroke rotary and large globe trims where diaphragms run out of travel.
- Stroking speed is set by air volume and supply capacity; big cylinders usually need a volume booster or quick-exhaust to trip fast.
- Friction: piston seals add stick-slip relative to diaphragms — in throttling service a positioner is mandatory, not optional.
- Bench set and spring range must match the positioner's output span and the valve's required seat load.