A Hydraulic Cylinder Pulls Weaker Than It Pushes
A hydraulic cylinder does not produce the same force in both directions, and the reason is geometric rather than hydraulic: on the way out, fluid pushes on the whole piston face; on the way back, the rod is in the way.
The arithmetic
Force is pressure times the area it acts on. Extending, that area is the full bore. Retracting, it is the annulus — bore area minus rod area. The hydraulic cylinder calculator:
| Cylinder | Pressure | Bore area | Annulus | Extend | Retract |
|---|---|---|---|---|---|
| 80 mm bore, 40 mm rod | 160 bar | 5,026.55 mm² | 3,769.91 mm² | 80.42 kN | 60.32 kN |
| 63 mm bore, 36 mm rod | 200 bar | 3,117.25 mm² | 2,099.37 mm² | 62.34 kN | 41.99 kN |
| 100 mm bore, 56 mm rod | 250 bar | 7,853.98 mm² | 5,390.97 mm² | 196.35 kN | 134.77 kN |
The first cylinder retracts at 75% of its extend force. The second, with a proportionally fatter rod, manages only 67%. That ratio is set entirely by the rod-to-bore ratio and is worth knowing before you design a circuit around the pull stroke.
Designing around it
Two consequences that catch people out.
Size on the weaker stroke. If the demanding motion is the pull — a clamp that draws closed, a tipping mechanism that lowers under load — sizing on the extend figure oversells the cylinder by a third. The specification should be met on the stroke that has to do the work.
The ratio is adjustable at design time: a slimmer rod gives more retract force. What it costs is buckling resistance and rod strength, which is why the rod is the size it is, so this is a trade rather than a free choice.
Speed goes the other way. The same annulus that reduces retract force also reduces the volume to fill, so at a fixed flow rate a cylinder retracts faster than it extends. A circuit sized for a comfortable extend speed can snap back quicker than expected, which matters both for cycle time and for what the load does at the end of travel.
Where the estimate stops
The model is pressure times area, and it omits things that reduce real force: seal friction, back-pressure on the return side of the circuit, pressure drop across valves and hoses, and any load-holding valve in the line. Real delivered force is lower than the calculation, sometimes appreciably.
It also assumes the pressure you enter is the pressure at the cylinder, which is not the same as the pump setting or the relief valve setting.
The part that is not arithmetic
Hydraulic systems store a great deal of energy, and the hazards are not obvious from a force figure. Pressurised fluid can inject through skin from a pinhole leak in a hose — an injury that looks minor and is a surgical emergency, and the reason you never search for a leak by hand. Accumulators hold pressure after the pump is off. Loads suspended on hydraulics can fall if a line fails or a valve is opened in the wrong order.
Servicing, hose replacement, pressure setting and any work on a live system belong with someone trained in hydraulics, following the equipment manufacturer's procedures — including proper depressurisation and mechanical support of raised loads before anyone goes near. This calculator sizes a force. It has no opinion about the circuit, and nothing on this site is a substitute for the equipment's own documentation or for competent supervision.
Speed, and the flow it needs
The same annulus geometry that reduces retract force also reduces the volume to fill. An 80 mm bore with a 40 mm rod has a bore area of 5,026.55 mm² and an annulus of 3,769.91 — so at a fixed flow rate the retract stroke completes in about three-quarters of the time.
That has two practical consequences. Cycle time is not symmetrical, and a circuit sized for a comfortable extend speed will retract faster than expected — which matters for what the load does when it arrives at the end of travel.
Regeneration, and why it is a special case
Some circuits feed the rod-side oil back into the bore side on extension, which increases speed at the cost of force — effectively the cylinder acts on the rod area alone. It is a useful trick for a fast approach stroke before a slow working stroke.
It is also a circuit design decision rather than a cylinder property, and the force available in regeneration is much lower than the plain extend figure. Sizing on the standard extend force and then running a regenerative circuit is a way to be surprised.
The number is not the delivered force
Worth restating: seal friction, back-pressure on the return line, pressure drop across valves and hoses, and any load-holding valve all reduce what actually reaches the load. The calculation assumes the entered pressure is present at the cylinder, which is not the pump setting and not the relief valve setting.
For sizing, allow margin. For anything holding a load above a person, the margin and the safety devices are matters for the equipment's own design, not for an estimate.