How the force is calculated
A hydraulic cylinder turns pressure into force. The force is the oil pressure multiplied by the area it pushes on:
Fpush = p · π ÷ 4 · D² · η
Fpull = p · π ÷ 4 · (D² − d²) · η
- p
- pressure in N/m² (1 bar = 100,000 N/m²)
- D
- bore, the inside diameter of the barrel
- d
- rod diameter
- η
- efficiency, which accounts for seal friction
When extending, the oil acts on the full piston face. When retracting, it acts only on the ring around the rod, so the pull force is always smaller.
Worked example
A cylinder with a 63 mm bore and 36 mm rod runs at 160 bar. The piston area is 31.17 cm², the annulus area 21.00 cm². At 95 % efficiency it pushes with 47.4 kN (about 4.8 t) and pulls with 31.9 kN.
Standard bore and rod sizes
| Bore (mm) | Common rods (mm) | Push at 160 bar, 100 % (kN) |
|---|---|---|
| 40 | 22, 28 | 20.1 |
| 50 | 28, 36 | 31.4 |
| 63 | 36, 45 | 49.9 |
| 80 | 45, 56 | 80.4 |
| 100 | 56, 70 | 125.7 |
| 125 | 70, 90 | 196.3 |
| 160 | 90, 110 | 321.7 |
Sizes follow the ISO 3320 series used by most manufacturers.
What else to check
Buckling. A long, thin rod under compression can bend sideways long before the force limit is reached. Check the rod against the stroke and mounting style with the manufacturer's buckling chart.
Speed. Force depends on pressure, speed depends on flow. The extending speed is the pump flow divided by the piston area.
Pressure rating. Never run a cylinder above its rated pressure. Pressure peaks from valves closing or loads dropping can exceed the relief valve setting for a moment.