A buildable hydraulic system is a cylinder, a pump, a relief valve and a way to hold the load, and the first design choice is the working pressure: pushing 10 tonnes-force takes a 79 mm bore at 20 MPa, a 112 mm bore at 10 MPa or a 158 mm bore at 5 MPa. Lower pressure means a bigger cylinder and slower travel but easier parts, so choose the lowest pressure whose parts you can verify.
Part of Hydraulic Systems
The practical systems are the jack, the press and the splitter or lifter built from them. This article lays out how to choose the pressure, the cylinder and the layout before building one.
Pressure, Bore and Speed Trade Against Each Other
Force F = pressure x area, so the bore for a required force is d = √(4 x F / (π x P)). Ten tonnes-force is 10 x 9,810 = 98,100 N.
| Working pressure | Bore needed | Oil to move 100 mm |
|---|---|---|
| 20 MPa | 79.0 mm | 0.49 L |
| 10 MPa | 111.8 mm | 0.98 L |
| 5 MPa | 158.1 mm | 1.96 L |
The 10 MPa row gives 112 mm and the 5 MPa row 158 mm. Check the 10 MPa row: area = 98,100 / 10,000,000 = 9.81 x 10⁻³ m², so d = √(4 x 9.81 x 10⁻³ / π) = 0.1118 m. Oil to move 100 mm is the area times 0.1 m: 0.98 L. Halving the pressure doubles the area and the oil per millimetre of travel, so the pump has to deliver twice as much oil for the same speed.
The Parts’ Ratings Set the Pressure
The corpus’s pipe and hose methods do not reach megapascals: wrought-iron pipe is rated 3-8 bar and fabric hose 3-10 bar (Metal Pipes, Hoses and Tubing); 5 MPa is 50 bar. So a system at 5-20 MPa must use parts with a known rating: salvaged hydraulic cylinders, valves and hose from heavy equipment (Heavy Equipment), or custom steel parts whose strength you have calculated and proof-tested. Hand-made hose and iron pipe from the corpus are for the low-pressure end only. Any part with no known rating is taken as having none.
Power, Speed and the Source
A system’s speed is the pump’s flow divided by the cylinder’s area, and the pump’s power is pressure times flow (Control Valves). The power comes from a person (about 50-75 W on a crank, 60-100 W on a treadle per Chain Pump), a wheel or a motor. A 79 mm ram moving at 1 mm/s needs only 4.9 x 10⁻⁶ m³/s of flow, which is 98 W at 20 MPa, above what one person gives on a crank and at the top of what a treadle gives; speed is bought with power.
Design Rules for a Safe Layout
- Keep the pressurised volume short and rigid. Bolt the pump to the cylinder, as in a bottle jack, instead of running a long line.
- Fit a relief valve at the pump outlet with nothing between them (Pressure Relief Valves).
- Hold the load mechanically, with pins, stands or a screw collar, not with oil alone.
- Fit a gauge where you can see it.
- Bleed all air from the high points before loading.
- Test every new system with a safe load before trusting it, with everyone clear.
Which System
- Lifting: Hydraulic Jack, a pump and ram in one body.
- Pressing, bending, straightening: Hydraulic Press, a ram in a frame.
- Log splitting, bending or lifting at a site: a salvaged cylinder and pump, as in Heavy Equipment, with the same layout rules.
Common Mistakes
- Choosing the pressure for the pump and forgetting the hoses. The weakest part sets the limit.
- Skipping the mechanical support because the jack “holds”.
- A system with no relief valve, or one set from the pump’s maximum.
- Sizing for force and forgetting power. A fast ram on a hand pump will not move.
Safety
Oil under pressure injects through skin and loads fall
A pinhole leak in a pressurised line throws a fine, invisible jet that can inject oil through the skin; the wound looks trivial but is a surgical emergency that can cost the limb. Never search for a leak with a hand. Pass card or wood across the fitting and look for oil on it, at low pressure and with everyone clear. A raised load held by oil alone can drop if a seal or fitting leaks, so prop it first. Anyone who might have been hit needs urgent surgical care at once.
What’s Next
- Learn next: Hydraulic Jack — the simplest complete system
- Combine with: Hydraulic Press — the workshop standard