How it works
Hydraulic force is pressure acting on area. On extension, oil pushes on the full piston face. On retraction it pushes on the smaller annulus (piston minus rod), so pull force is lower than push force but speed is higher for the same flow.
Speed comes from flow divided by area: a gallon is 231 cubic inches, so speed in in/s = gpm × 231 ÷ (60 × area in²). Stroke divided by speed gives the cycle time.
Worked example
A 4-inch bore, 2-inch rod cylinder at 2,500 psi fed by an 11 gpm pump, 24-inch stroke (typical log splitter):
- Piston area = π ÷ 4 × 4² = 12.57 in²; rod-side area = 9.42 in²
- Push force = 2,500 psi × 12.57 in² = 31,415.9 lbf (15.71 tons)
- Pull force = 2,500 × 9.42 = 23,561.9 lbf
- Extend speed = 11 gpm × 231 ÷ (60 × 12.57) = 3.37 in/s; retract 4.49 in/s
| Input | Value |
|---|---|
| Bore diameter | 4 in |
| Rod diameter | 2 in |
| Pressure | 2500 psi |
| Pump flow | 11 gpm |
| Stroke length | 24 in |
| Result | Value |
|---|---|
| Push force (extend) | 31,415.9 lbf |
| Push force in US tons | 15.71 tons |
| Pull force (retract) | 23,561.9 lbf |
| Extend speed | 3.37 in/s |
| Retract speed | 4.49 in/s |
| Extend time | 7.1 s |
| Retract time | 5.3 s |
Assumptions and limits
- Ideal calculation. Seal friction and back pressure reduce real force by roughly 5–10%.
- Uses the pressure you enter at the cylinder, not the pump relief setting; hose and valve losses reduce it.
- Retract time uses the annulus area; in-line flow controls or regeneration circuits change these speeds.
Common questions
How many tons of force does a log splitter cylinder make?
Push force in tons is pressure × piston area ÷ 2,000. A 4-inch bore at 2,500 psi is about 15.7 tons; a 5-inch bore is about 24.5 tons.
Why is retract faster than extend?
On retract the rod takes up part of the piston area, so the same flow fills less volume per inch and the rod moves faster, but with less force.
What flow do I need for a target speed?
Rearrange the speed formula: gpm = speed (in/s) × 60 × area ÷ 231.
Sources
- Fluid power basics: force = pressure × area; flow = area × velocity; 231 cubic inches per US gallon.
Updated 2026-09-30