A pneumatic actuator converts compressed air pressure into mechanical movement, and the choice between ISO 6432 mini cylinders, compact cylinders and short-stroke cylinders is decided by three numbers: the force you need, the stroke you need and the space you have. Get those three right and the rest of the specification follows. Get them wrong and you either stall under load or pay for a cylinder that will not fit the machine.
This guide covers force calculation, the three body styles that cover most industrial duty, mounting, cushioning and the sensing options that matter when the cylinder has to talk to a PLC.
How much force does a pneumatic cylinder actually deliver?
Theoretical force is piston area multiplied by working pressure. Usable force is lower, because friction and the exhaust-side back pressure both take a share. Plan on 10 to 20 per cent below theoretical for a well-maintained cylinder, and more for long strokes with side load.
At 6 bar, the pressure most factory air systems settle at, the extend force of a double-acting cylinder is roughly:
| Bore | Piston area | Extend force at 6 bar | Retract force at 6 bar (12 mm rod) |
|---|---|---|---|
| 16 mm | 2.0 cm² | ≈ 120 N | ≈ 85 N |
| 25 mm | 4.9 cm² | ≈ 295 N | ≈ 250 N |
| 32 mm | 8.0 cm² | ≈ 480 N | ≈ 430 N |
| 50 mm | 19.6 cm² | ≈ 1180 N | ≈ 1110 N |
| 63 mm | 31.2 cm² | ≈ 1870 N | ≈ 1800 N |
| 100 mm | 78.5 cm² | ≈ 4710 N | ≈ 4600 N |
Two consequences engineers regularly miss.
Retract force is always lower than extend force, because the rod takes away piston area on the annulus side. On a 16 mm bore with a 6 mm rod the difference is around 30 per cent. If your load is pulled rather than pushed, size on the retract figure.
Pressure at the cylinder is not pressure at the compressor. Every metre of undersized tubing, every elbow and every throttle valve costs pressure. A cylinder specified at 6 bar and fed through 4 mm tubing over 8 metres may see 4.5 bar at the port under flow, which is a quarter of the force gone. Size the air line to the cylinder, not to the fitting that happens to be in the drawer.
Which body style fits which job?
ISO 6432 mini cylinders
The round-body standard for bores from 8 to 25 mm. ISO 6432 fixes the mounting dimensions, thread sizes and rod diameters, so a cylinder from one manufacturer drops into the bracket of another. That interchangeability is the whole point of the standard and the main reason to prefer it for machine building where spares matter.
Typical duty: part ejection, small clamps, gate and flap operation, pushers on conveyors.
Watch the stroke. Mini cylinders have slender rods, and a long stroke under side load will bend one. Past roughly 200 mm on a 16 mm bore, use a guided version or take the load off the rod with a linear guide.
Compact cylinders
Same bore, a fraction of the length. A compact cylinder trades stroke for installed depth, which is what you want when the machine frame gives you 60 mm and a standard cylinder needs 140 mm.
Typical duty: clamping and fixturing in jigs, tool changers, anywhere the cylinder sits inside a crowded assembly.
The trade-off is real: shorter cushioning length, so at full speed a compact cylinder arrives harder at end of stroke. Either accept the impact, fit external shock absorbers, or slow the approach with flow control.
Short-stroke cylinders
Built for strokes measured in millimetres, with a large bore relative to their length. When you need 2000 N over 10 mm of travel, a short-stroke cylinder does it in a body a standard cylinder could not match.
Typical duty: pressing, marking, riveting, indexing, holding.
Single-acting or double-acting?
Double-acting is the default. Single-acting is a deliberate choice for a specific reason.
A double-acting cylinder is driven by air in both directions, so it holds force in both directions and its speed can be controlled in both directions. Almost all production duty wants this.
A single-acting cylinder is driven by air one way and returned by an internal spring. Choose it when:
- you want a defined position on air failure, a spring-return cylinder parks itself,
- the return stroke does no work and air consumption matters,
- only one valve output is available.
The cost: the spring eats part of the extend force, often 10 to 25 per cent, and it limits practical stroke. Beyond about 100 mm the spring becomes the dominant design problem.
Cushioning: what happens at the end of stroke
A cylinder that reaches end of stroke at full speed transfers that energy into the end cap. At small bores this is noise. At 63 mm and above with a heavy load it is a fatigue problem for the whole machine.
| Option | How it works | When to use |
|---|---|---|
| No cushioning | Piston hits the end cap | Short strokes, light loads, low speed |
| Fixed cushioning | Elastomer bumper absorbs the last few millimetres | General duty, noise reduction |
| Adjustable pneumatic cushioning | Trapped air decelerates the piston, set with a screw | Heavy loads, high speed, most production duty |
| External shock absorber | Hydraulic damper outside the cylinder | Very high energy, compact cylinders with no room for internal cushioning |
Adjustable cushioning is set at commissioning under real load, not on the bench. Too much and the cylinder stalls short of end of stroke. Too little and it hammers.
Magnetic piston and position sensing
If the cylinder has to report its position to a PLC, order the version with a magnet on the piston and clamp reed or electronic sensors to the barrel. Retrofitting is not possible. The magnet is inside the piston.
Reed sensors are cheaper and switch on contact; they wear and can be damaged by strong external fields. Electronic (Hall-effect or magnetoresistive) sensors last longer, switch faster and are the right choice on high-cycle machines.
The Hose Solutions catalogue carries mini actuators both with and without the magnet precisely because this is a decision made at order time, not later.
Mounting: where the load really goes
The cylinder body is not a structural member. Mount it so the load acts along the rod axis, and where it cannot, use a mounting that allows the cylinder to follow the load.
- Rigid mountings (foot, flange, nose thread) for pure axial load with the load guided externally.
- Pivoting mountings (clevis, trunnion) where the load swings through an arc, a clamp arm or a flap.
- Rod-end accessories (rod clevis, ball joint, floating joint) to absorb the small misalignments that otherwise become side load on the rod seal.
Side load is the most common cause of premature seal failure in pneumatic actuators. A floating joint costs a fraction of the cylinder and removes the problem.
A specification checklist
Before ordering, fix these:
- Force required, at the worst case, and whether it is on extend or retract.
- Working pressure at the cylinder port, not at the compressor.
- Bore, from force and pressure, with margin.
- Stroke, plus a check on rod buckling if the stroke is long.
- Body style: ISO 6432 for interchangeability, compact for depth, short-stroke for force in a small package.
- Single or double acting, and what position the machine must fail to.
- Cushioning, sized to the kinetic energy at end of stroke.
- Magnet, if the PLC needs to know where the piston is.
- Mounting and rod-end accessory, chosen to keep side load off the rod.
- Port thread and tubing size, checked against the flow the cylinder needs to reach its speed.
Where the air comes from matters as much as the cylinder
A correctly sized actuator fed with poorly prepared air will still fail early. Water carries rust into the cylinder, oil-free air strips the factory grease from seals that need it, and particulate scores the bore.
Filtration, regulation and, where the cylinder calls for it, lubrication belong in the specification alongside the cylinder itself. So do the fittings and tubing, which decide whether the pressure you specified actually arrives.