Single Acting Hydraulic Cylinders

Hydraulically it is half of a double acting cylinder: oil enters one chamber only. In the usual push version that is the cap-end chamber, so pressure acts on the full piston area and the cylinder gives the highest push force its bore allows.

The rod-side chamber is open to atmosphere through a breather (item 14 in the drawing), so air moves in and out as the piston travels. On every return stroke it draws in air — and dust and moisture too, unless the vent is filtered, points downwards or is piped back to the tank.

To extend, the directional valve connects the pump to the port. Oil fills the cap-end chamber and the rod moves out at v = Q / A with a force F = p × A, where A is the piston area — or the ram area in a plunger cylinder.

To retract, the valve connects the port to tank. The pressure falls and the rod is pushed back by the weight of the load, a built-in spring or the machine itself, while the oil returns through the same line. A 3/2 (three-way) valve is enough.

Return speed can only be controlled by throttling the return flow. Loads that lower under gravity need a flow control or counterbalance valve so they cannot run away; a spring return gives only the spring force, which pressure cannot increase.

A plunger cylinder has no piston: the ram seals at the gland and its diameter sets the force, so it suits large push loads that gravity returns, as in presses and lifts. A spring return works in any position, but the spring costs length and force, so strokes stay short. Load-return and telescopic cylinders need the load on the rod at all times, so they work vertically or steeply inclined.

The last column is the oil the pump delivers per 100 mm of stroke — the whole volume per cycle, since only one chamber fills. Tubes are honed St52BK+S steel; rods are Ck45 with hard chrome of at least 20 µm and f7 tolerance, with hardened-and-chromed or stainless rods as options. Seals are NBR or Viton; special seals cover −30 °C … +200 °C. Every cylinder is tested for internal and external leakage. Series data: FDH type hydraulic cylinders.

Example: a Ø100 mm cylinder at 160 bar pushes 125.7 kN — 12.81 t or 28,250 lbf. Deduct seal friction and, on a spring return, the spring force, and leave a margin above the load. For a plunger cylinder read the ram diameter as the bore. Other sizes: hydraulic cylinder force calculator.

A single acting cylinder is powered one way and returned by the load or a spring; a double acting cylinder is powered both ways by oil.

Choose single acting when the load always returns the rod and no pull is needed: one hose and a simpler valve. Otherwise choose a double acting hydraulic cylinder.

FDH mounting dimensions follow CETOP RP 58 H. The mountings are front flange (FFM), rear flange (RFM), rear clevis (RSM), rear clevis with spherical bearing (RHM), rod-end clevis (FSM), rod-end spherical (FRM), centre trunnion (CTM) and foot (FMM); front and rear trunnions are also listed.

For a load-return cylinder, pivots at both ends let the cylinder follow the load without side load, while a flange suits a slide guided by the machine. Keep the breather where it cannot be flooded. All parts: hydraulic cylinder mounts and rod ends.

The FDH ordering code combines these as FDH-[bore]-[stroke]-[mounting]-[seal]-[rod type]. Fenitsa provides 10 years of service and spare-parts support for its hydraulic cylinders.