A single-acting hydraulic hand pump is a piston in a cylinder with two check valves that pushes oil to the load on the working stroke and refills on the return, and its pressure is hand force times lever ratio divided by piston area. A 20 mm bore on a 5:1 lever at 15 kgf makes 2.3 MPa and 6.3 mL a stroke, while a 25 mm bore makes 1.5 MPa and 9.8 mL, so a smaller piston buys pressure and costs flow.
Part of Hydraulic Systems
The single-acting pump is the simplest hydraulic pressure source: one piston, one inlet valve, one outlet valve and a lever, pumping oil on the down stroke only.
One Stroke Pumps and One Stroke Refills
- On the up stroke the piston draws oil from the reservoir through the inlet check valve.
- On the down stroke the inlet valve closes and the outlet check valve opens, and the oil is forced out to the load.
- The outlet valve closes behind it, so the load holds while the piston goes back up.
Following the carrier, build it from a steel tube of 20-25 mm bore and 150-200 mm long, a steel rod turned to the bore with 0.02-0.05 mm clearance as the piston, a leather or rubber cup seal on the piston, two ball-and-spring check valves (Check Valves) and a lever arm of 5:1 to 8:1. The corpus’s well hand pumps use the same piston, cup seal and check valve at low pressure (Hand Pumps).
Pressure Is Hand Force Over Area
Pressure = (handle force x lever ratio) / piston area. Volume per stroke = piston area x stroke, and the carrier’s table uses a 20 mm stroke.
Worked check of the table, 15 kgf = 147.2 N on the handle.
| Bore | Lever | Force on piston | Piston area | Pressure | Volume per 20 mm stroke |
|---|---|---|---|---|---|
| 20 mm | 5:1 | 735.8 N | 3.142 cm² | 2.34 MPa | 6.28 mL |
| 25 mm | 5:1 | 735.8 N | 4.909 cm² | 1.50 MPa | 9.82 mL |
| 20 mm | 8:1 | 1,177 N | 3.142 cm² | 3.75 MPa | 6.28 mL |
For the first row: force = 15 x 9.81 x 5 = 735.8 N; area = π / 4 x 0.02² = 3.142 x 10⁻⁴ m²; pressure = 735.8 / 3.142 x 10⁻⁴ = 2.34 x 10⁶ Pa. The carrier’s figures (2.3, 1.5, 3.7 MPa and 6.3, 9.8 mL) match.
Strokes to move a load. To raise a 100 mm ram (area 78.5 cm²) by 10 mm needs 78.5 x 1 cm = 78.5 mL, which at 6.28 mL a stroke is 12.5 strokes.
What Limits the Pump
- Operator force. Hand effort fixes the pressure for a given piston and lever.
- Parts’ ratings. The table pressures, 1.5-3.75 MPa or 15-37.5 bar, are above the 3-10 bar the corpus gives for iron pipe and fabric hose (Metal Pipes, Hoses and Tubing). Any line from the pump must be rated for the full pressure the pump can make, and a pump bolted straight to the cylinder avoids a long line.
- Seal leakage. The cup seal must hold the full pressure; see Seal Systems.
Method: Build and Test
Step 1: Bore and hone the cylinder to a smooth finish; see Hydraulic Cylinders.
Step 2: Fit the piston with its cup seal, lip facing the pressure.
Step 3: Install both check valves, then fit a release valve (a needle valve from the load side to the reservoir) so the load can be lowered slowly.
Step 4: Fill with clean oil, bleed all air, and pump against a gauge. A pump that will not build pressure has a leaking valve or seal, or air in the oil.
Common Mistakes
- No release valve. The load cannot be lowered, or it drops when a fitting is loosened.
- Air trapped under the piston. The pump pulses without building pressure; prime and bleed it.
- A lever ratio that makes pressure beyond the rating of the line.
Safety
A hand pump can make more pressure than its fittings hold
At 3.75 MPa a pinhole in a fitting throws an invisible jet that can inject oil through skin, and a burst line sprays oil. Fit a relief valve, use fittings rated above the pressure the table gives, and relieve pressure before loosening anything. Never search for a leak with a hand; use card or wood. Support any load with a stand, not with oil alone.
What’s Next
- Learn next: Double-Acting Pump — pumping on both strokes
- Combine with: Hydraulic Jack — pump and ram in one body