Hydraulic cylinder types: which one fits your job?

Single- or double-acting - what is the difference?

Hydraulic cylinders differ first in their working principle. A single-acting cylinder receives oil on one side only; the return stroke comes from the load, a spring or its own weight. A double-acting cylinder is pressurised on both piston faces and works actively in both directions.

The double-acting differential cylinder is the machine-building standard because it delivers controlled force both extending and retracting. Because the rod reduces the ring area, the retract force is lower than the extend force - the so-called area ratio, often 2:1.

• Single-acting: pressurised one way, returned by spring or load - cheap and compact.
• Double-acting: active in both directions with precise control - the standard for feed and clamping.
• Telescopic: several nested stages give a long stroke from a short retracted length.
• Synchronous (double-rod): through-going rod delivers equal force and speed both ways.

What designs and mountings are available?

The design sets the pressure rating, mounting and serviceability. Tie-rod cylinders to ISO 6020 are common for medium pressures up to around 210 bar, while welded round and compact cylinders to ISO 6022 handle 250 to 350 bar and beyond.

The mounting transfers the force into the machine. Foot, flange, clevis and trunnion mounts are chosen by load direction - whenever the cylinder pushes or pulls with side movement, articulating mounts (eye, clevis) are almost always the right choice.

Which seals sit inside a hydraulic cylinder?

The sealing system governs leak-tightness, friction and service life. There are piston seals (separating the pressure chambers), rod seals (keeping oil inside), guide bands, and the wiper that keeps dirt off the retracting rod.

• NBR (nitrile): standard for mineral oil up to about 100 °C, good value for money.
• PU (polyurethane): highly wear- and pressure-resistant, ideal for rod seals.
• PTFE glide rings: extremely low friction, for high speeds and stick-slip-free motion.
• FKM (Viton): for higher temperatures and aggressive fluids.

How do I size force and stroke correctly?

Force equals operating pressure times effective area: F = p × A. On the extend stroke the full piston area counts; on the retract stroke only the ring area (piston minus rod). From the required force and available pressure you determine the bore diameter.

With a long stroke the rod must be sized against buckling per Euler - the longer the extension, the larger the required rod diameter. Add a safety margin and account for the maximum piston speed so the end-of-stroke cushioning safely absorbs impacts.

Frequently asked questions