A double-acting hydraulic pump has a piston with a rod through one end and four check valves, so it pushes oil to the load on both the forward and the return stroke and roughly doubles the output of a single-acting pump. The rod occupies part of one side, so that side moves less oil but makes more pressure: a 25 mm bore with a 12 mm rod gives 9.8 mL one way and 7.6 mL back, at 1.50 and 1.95 MPa for the same 736 N on the piston.
Part of Hydraulic Systems
A double-acting pump works both directions of the piston, giving a steadier flow than a single-acting pump at the cost of two more valves and a rod seal.
Both Sides of the Piston Work
In a single-acting pump half the handle cycle is spent refilling. In a double-acting pump the piston has a chamber on each side. As it moves one way it pushes oil out of one chamber while drawing oil into the other, and as it returns the roles swap. It needs four check valves: an inlet and an outlet for each chamber (or two valves plus a shuttle arrangement). Valve construction is in Check Valves.
The Rod Makes the Two Strokes Unequal
On the rod side the piston face is the bore area minus the rod area. Take a bore of 25 mm and a rod of 12 mm, stroke 20 mm, and the 735.8 N (736 N) on the piston from 15 kgf on a 5:1 lever (Single-Acting Pump):
- Bore area = π / 4 x 0.025² = 4.909 cm²; rod area = π / 4 x 0.012² = 1.131 cm²
- Rod-side area = 4.909 - 1.131 = 3.778 cm², which is 77% of the bore area
- Volume: full side 4.909 x 2 = 9.82 mL (9.8 mL); rod side 3.778 x 2 = 7.56 mL (7.6 mL)
- Pressure for the same 735.8 N: full side 735.8 / 4.909 x 10⁻⁴ = 1.50 MPa; rod side 735.8 / 3.778 x 10⁻⁴ = 1.95 MPa
So the return stroke moves 23% less oil and makes 30% more pressure. If the relief valve or a hose is rated close to the higher figure, the less obvious stroke is the one that tests it. The same area difference matters on a double-acting cylinder, where retraction is weaker and faster (Hydraulic Cylinders).
What You Need
- A cylinder of 20-25 mm bore as in the single-acting pump, with a rod through one end.
- A piston sealed on both faces (two back-to-back cup seals, or an O-ring, per Seal Systems).
- A rod seal at the end cap, which keeps oil from leaking along the rod, and a wiper that keeps grit out.
- Four check valves and the passages that link them.
- A lever and a reservoir with a filtered fill.
Method
Step 1: Make and fit the piston and rod with the piston seals facing both pressure directions.
Step 2: Plumb the four valves so that each chamber has one inlet from the reservoir and one outlet to the delivery line.
Step 3: Test each stroke separately. Block the delivery line with a gauge fitted and pump slowly. Both strokes should build pressure; a stroke that does not has a leaking valve or piston seal.
Common Mistakes
- Treating the two strokes as equal. The rod side moves less oil and makes more pressure.
- A rod that is not sealed. Oil weeps along the rod and drags grit in.
- Only four valves in a design that needs a shuttle. Check each chamber’s path on paper before cutting.
Safety
The return stroke makes higher pressure
On the rod side the same hand force produces more pressure, 1.95 MPa against 1.50 MPa in the example, so test the weakest line fitting at the higher figure. A pinhole leak in a pressurised line injects oil through skin; never search for a leak with a hand, and pass card or wood across the suspect fitting instead. Relieve pressure before opening any joint.
What’s Next
- Learn next: Check Valves — the four valves the pump depends on
- Combine with: Directional Control Valves — steering the flow to a cylinder