Hydraulic Cylinder Parts and Components

The cross-section is numbered from the rear mounting to the rod eye. A welded cylinder has the same internal parts; only the tie rods (5) are replaced by a welded cap and a threaded or bolted head.

The barrel is the pressure vessel. Its bore is honed smooth and straight because the piston seal slides on it every stroke; any scoring becomes a leak path between the chambers. Wall thickness follows bore and pressure.

Fenitsa FDH barrels are honed St52BK+S steel tube, a cold-drawn, stress-relieved grade chosen for accurate bore geometry. Scoring from dirty oil or a failed wear ring shows up as drift or lost force, not as an external leak.

The piston is the disc that divides the barrel into two chambers and turns oil pressure into force. It is fixed to the rod with a nut or a locked thread and carries the piston seal between two wear rings. “Hydraulic piston” often means the whole cylinder; in engineering terms it is only this part.

A double acting piston seals both ways, a single acting one only one way, and a cushioned cylinder adds a spear or sleeve (15). Most pistons are machined from steel or cast iron. Failure signs: worn seal grooves, a loose nut that knocks at the stroke end, scoring where a wear ring failed.

The piston rod carries the force out of the cylinder and is the part most exposed to damage. It is ground and hard chrome plated so the rod seal runs on a hard, smooth, corrosion-resistant surface. Its diameter is usually set by buckling — see the sizing guide.

Fenitsa FDH rods are Ck45 steel with hard chrome ≥ 20 µm and f7 tolerance, supplied as HR (hardened and chromed), SS (stainless) or CR (Ck45 chromed). Scratches, pitting and flaking chrome cut the rod seal; a bent rod means buckling or side load.

The front head, or gland, closes the rod end. From the inside out it carries a static O-ring, the guide bush, the rod seal and the wiper. Once the bush wears, the rod moves sideways and the rod seal leaks.

The rear cap closes the other end and usually carries port A and the rear mounting. Tie rods clamp both to the barrel; on a welded cylinder the cap is welded on and the head threaded or bolted in. See also cylinder front and rear covers.

Seals are the parts that wear out: dynamic seals slide, static O-rings sit between fixed parts.

NBR suits mineral oil at normal temperatures, FKM (Viton) takes heat, PU is hard-wearing and PTFE-based compounds give low friction. Fenitsa FDH cylinders use NBR seals as standard (code NS) for −20 °C to +80 °C; Viton (code VS) is the option, and special seals cover −30 °C to +200 °C.

Cushioning slows the piston in the last part of the stroke. A spear or sleeve (15) enters a bore in the end cover and closes the main outlet, so the trapped oil escapes only through a channel set by an adjusting screw; a check valve lets full flow back in for the return.

Cushions suit heavy or fast loads that stop inside the cylinder; they do not replace a flow control valve. Fenitsa FDH cylinders are rated for piston speeds up to 0.5 m/s.

Port A at the cap end and port B at the rod end (3 and 14) let the oil in and out; a single acting cylinder has one port and a breather. Too small a port limits speed and causes heat.

Common threads are BSP (G), metric, NPT and SAE O-ring boss; large cylinders use flange ports. On a replacement the ports must also face the hoses. For Fenitsa cylinders, size, thread and position are confirmed in the quotation; how to measure a hydraulic cylinder shows how to record them.

Something must hold the head and cap on against the full pressure force — 125.7 kN on a Ø100 mm cylinder at 160 bar. Tie rods (5) clamp them with pre-tensioned nuts, so the cylinder can be taken apart completely. On a welded cylinder the cap is welded on and the head threaded or bolted in, giving a slimmer, more shock-resistant body.

The tie rod hydraulic cylinders page compares both designs; welded hydraulic cylinders covers mobile machines.

Mountings fix the cylinder to the machine; rod ends connect the rod to the load. Pinned mountings (clevis, spherical eye, trunnion) let the cylinder swing; fixed ones (flange, foot) need accurate alignment. A spherical bearing tolerates small misalignment.

FDH mounting dimensions follow CETOP RP 58 H; front and rear trunnions are also listed. See hydraulic cylinder mounting options and rod ends.