Types of Hydraulic Cylinders

There are two types by action and four main types by construction, plus special functions built on top of them. Three questions classify any cylinder:

• How does it act? Single acting cylinders are powered one way and returned by the load, gravity or a spring; double acting cylinders are powered both ways.
• How is it built? Tie rod, welded, telescopic or plunger (ram). Heavy industrial designs with bolted flanges instead of tie rods are called mill type.
• What extra function does it have? Double rod, tandem, multi-position or adjustable stroke.

The axes are independent — a tie rod or telescopic cylinder can be single or double acting — so a supplier asks all three questions, plus bore, rod, stroke and mountings, before quoting.

Pressure class shows where each design is usually applied; the actual rating depends on the specific cylinder. Fenitsa’s FDH series works at 160 bar, is tested at 240 bar and is available for 250 bar; the tie rod range uses the same Ø40–Ø200 mm, 160 bar frame.

A single acting cylinder has one working port. Oil pressure extends the rod; when the valve opens the port to tank, the load, gravity or an internal spring pushes the piston back, while the other chamber breathes through a filter.

It needs one hose and a simpler valve, and a Ø100 mm single acting cylinder at 160 bar lifts 12.8 t — as much as a double acting cylinder of the same bore. But it cannot pull, its return speed depends on the load, and a spring return suits short strokes only.

Typical jobs are tipping bodies, scissor lifts, jacks and press rams that return under their own weight. See single acting hydraulic cylinders for the return options and a force table.

A double acting cylinder has a port at each end. Oil into port A acts on the full piston area and extends the rod; oil into port B acts on the annulus around the rod and retracts it. Force and speed can be controlled both ways, and the cylinder holds its position when the valve blocks both ports.

Because the rod takes up part of the piston face, the pull force is lower and the retract speed higher: a Ø80 mm cylinder with a Ø45 mm rod at 160 bar pushes 8.20 t and pulls 5.61 t. See double acting hydraulic cylinders and the hydraulic cylinder force calculator for any size.

A telescopic cylinder has several nested tubes, or stages, that extend one after another, the widest first. Its stroke is several times its closed length, which makes it the standard answer where a long stroke must fit a short space, such as under the front of a tipper body.

At constant pressure each smaller stage gives less force and moves faster, so every stage must still have enough force where it takes over — usually the smallest stage is critical.

Tipper cylinders are mostly single acting and retract under the weight of the body; double acting versions are used where the load cannot push them back. Fenitsa designs the bore and stages of each telescopic hydraulic cylinder per project — see also hydraulic cylinders for dump trucks.

In a tie rod cylinder the head and cap are clamped against the barrel by steel tie rods running along the outside. Nothing is welded, so the cylinder can be dismantled with hand tools, resealed and rebuilt part by part.

Tie rod cylinders dominate industrial hydraulics — presses, injection moulding machines, machine tools — because standardised mounting dimensions make replacement easy. They are bulkier, and their tie rods stretch under shock, so mobile machines prefer welded cylinders.

Fenitsa tie rod hydraulic cylinders cover Ø40–Ø200 mm bores at 160 bar, with double rod, tandem and adjustable-stroke versions; the FDH series has mounting dimensions to CETOP RP 58 H and strokes up to 5,000 mm. The tie rod page compares tie rod and welded designs point by point.

A welded cylinder has its rear cap welded to the barrel, usually together with the port bosses and mountings. The front head is threaded in or held by a bolted ring, so the seals can still be replaced. Without tie rods the body is slimmer, lighter and better at absorbing shock and vibration.

Welded cylinders are therefore standard on excavators, loaders, cranes, tippers and farm machinery, where space is tight. Cross tubes, clevises and trunnions are welded on to suit the machine, so most are built to the machine builder’s drawing.

Fenitsa builds welded hydraulic cylinders in two series — THS with a compact welded body and KHS for heavy duty and mobile equipment — with Ø40–Ø200 mm bores, Ø22–Ø110 mm rods and strokes up to 3,000 mm.

A plunger or ram cylinder has no separate piston: the rod itself is the working area, and the only moving seal is in the head. It is always single acting and returns by gravity or by the load.

Because the rod diameter equals the working diameter, a plunger is very stiff against buckling, and its barrel needs no honed bore. A Ø100 mm plunger at 160 bar gives 12.8 t, the same as a Ø100 mm piston. Typical uses are press rams, vehicle lifts and jacks; in British and Australian English, “hydraulic ram” is also used for any hydraulic cylinder.

Compare plunger, spring-return and load-return designs on the single acting cylinder page, and see hydraulic cylinders for press machines.

A double rod cylinder has a rod that passes through both heads. The effective area is equal on both sides of the piston, so it gives the same force and speed in both directions; the second rod end can also carry a sensor or an adjustable stop.

With a Ø80 mm bore and Ø45 mm rods the force at 160 bar is 5.61 t each way. Typical uses are steering cylinders and machine tool tables. Double rod is an option in the Fenitsa FDH and tie rod ranges.

A tandem cylinder puts two cylinders in line on one axis with their pistons joined by a common rod. Both pistons are pressurised together, so their forces add up — close to double the push force from the same outside diameter when the connecting rod is slim.

A Ø100 mm tandem with Ø56 mm rods pushes about 21.6 t at 160 bar, against 12.8 t for a single Ø100 mm cylinder, because the second piston works on the annulus around the connecting rod.

A multi-position cylinder joins two cylinders back to back with separate strokes, giving three positions, or four when the strokes differ. An adjustable-stroke cylinder instead uses a threaded stop to shorten one stroke.

Tandem and adjustable-stroke versions are FDH options; in Fenitsa’s pneumatic range the same idea appears as the two-position tandem cylinder.

Then size the bore, rod and stroke with the hydraulic cylinder sizing guide; for a replacement, start with how to measure a hydraulic cylinder.