A hydraulic cylinder is the simplest part of a hydraulic system and the one we strip most often. It has one job: turn oil pressure into a straight-line force. How much force, how fast, and how long it lasts depend on a handful of numbers you can play with above: bore, rod, pressure, flow and stroke.
In simple terms
Inside the steel tube (the barrel) is a piston with seals round it, fixed to a polished steel rod. Oil pumped into the closed end (the cap end) pushes the piston and the rod comes out. Oil pumped into the other end (the rod end) pulls it back in.
Pushing is stronger than pulling. When oil pushes from the cap end it presses on the whole face of the piston. When it pulls from the rod end, the rod covers part of that face, so the oil only presses on the ring around the rod. Try a thick rod in the animation and watch the pull force drop.
Pulling is faster than pushing. The same pump flow fills the smaller space on the rod side more quickly, so the rod comes in faster than it goes out.
Regeneration is a trick for speed. Instead of sending the rod-side oil back to the tank, the valve sends it round into the cap end. Now the cylinder is filled by the pump and by itself at the same time, and the rod shoots out. The catch: oil is pushing on both sides of the piston, so only the area of the rod itself gives force. Presses and log splitters use this for a fast approach, then switch back to normal for the hard push.
The faults show what we find on the bench. A bypassing piston seal lets oil slip from one side to the other, so the cylinder gets slow and the load creeps. A scored rod cuts the gland seals, so oil weeps out and dirt gets in. A bent rod pushes sideways on the bearings and seals and wears them out quickly.
The engineer's explanation, the deep dive, key numbers and the quiz open with a free account.
For engineers
| Quantity | Formula | 100 / 56 mm at 200 bar, 40 L/min |
|---|---|---|
| Full-bore area | A = π D² / 4 | 7,854 mm² |
| Ring (annulus) area | A − π d² / 4 | 5,391 mm² |
| Push force | p × A | 157 kN (16 t) |
| Pull force | p × A(ring) | 108 kN |
| Regeneration force | p × A(rod) | 49 kN |
| Extend speed | Q / A | 85 mm/s |
| Retract speed | Q / A(ring) | 124 mm/s |
| Regeneration speed | Q / A(rod) | 271 mm/s |
Units: 1 bar on 1 mm² is 0.1 N. 1 L/min into 1 cm² is 167 mm/s.
The rod-to-bore ratio sets the area ratio. A 2:1 area ratio (rod about 0.7 × bore) makes retract twice as fast as extend, and regeneration the same speed in both directions. That is why many standard cylinders come with a “2:1 rod” option.
Flow out is not flow in. When the cylinder retracts, the cap end pushes out A / A(ring) times the pump flow: 1.46 × for 100/56. Size the return line, the valve and the return filter for that, or you get back pressure and heat.
Pressure intensification. If the rod side is blocked while the cap side is pressurised (a closed meter-out valve, a sticking counterbalance valve), the rod side pressure becomes p × A / A(ring). On a 2:1 cylinder at 250 bar that is 500 bar behind the gland seals. It is a classic cause of blown gland seals and bulged barrels.
Buckling limits long, thin rods in push. The animation uses Euler’s formula, F = π² E I / L², with the free length taken as twice the stroke (a cylinder pinned at both ends, fully extended) and a safety factor of 3. Manufacturers’ charts use mounting-specific factors, so use this as a first check only.
Deep dive
Seals. A typical double-acting cylinder has a piston seal (often a PTFE-faced seal with an energiser, or a compact seal) and guide rings on the piston; and in the gland, a rod seal, a buffer seal on high-pressure designs, a wiper to keep dirt out, and guide bands to carry side load. Static O-rings and back-up rings seal the gland to the barrel and the piston to the rod.
How a piston seal bypass test works. With the cylinder on the bench, fully extended against its stop, pressurise the cap end to working pressure and open the rod-end port to atmosphere. Any steady oil from the rod port is crossing the piston. Do the same in the other direction. On the machine the same test is done with the load safely supported. Our cylinder drift guide covers how to separate seal bypass from valve leakage.
Rods. Rods are induction-hardened and hard-chrome plated, typically 20 to 30 microns of chrome. Scores from dirt or a damaged wiper go through the chrome and act like files on the rod seal. Salt air and seawater pit chrome on marine cylinders, especially on rods that sit extended for long periods. Small damage can be polished; deeper pits and scores mean re-chroming or a new rod.
Bent rods come from side loads (misaligned mounts, a jammed linkage, a crane boom hitting something), not usually from pure buckling. Run-out is measured between centres; anything above a few tenths of a millimetre per metre will wear the gland bush oval. Straightening is possible on some rods, but the chrome must be checked for cracks afterwards.
Cushions slow the piston for the last 20 to 40 mm of stroke by trapping oil behind a spear and letting it out through a small orifice. A cushion that bangs is usually worn or has its needle wide open. One that is too tight makes the end of stroke slow and can create high pressure peaks behind the piston.
In UAE conditions, dust is the enemy of rods and wipers, and heat softens elastomer seals. Seals in polyurethane or FKM and good wipers make a measurable difference on deck and construction cylinders. Bench tests after a reseal are normally done at 1.5 × working pressure, with a report.
Key numbers
| What | Typical value |
|---|---|
| Force on 1 cm² at 100 bar | 1 kN |
| Speed from 1 L/min into 1 cm² | 167 mm/s |
| Common “2:1” rod size | about 0.7 × bore |
| Bench test pressure after a reseal | 1.5 × working pressure |
| Chrome thickness on a rod | about 20 to 30 microns |
Work out your own figures with the cylinder force and speed calculator.
