A check valve is the simplest valve in hydraulics: a ball or poppet, a seat and a light spring. Oil pushing one way lifts it and flows. Oil pushing the other way presses it harder onto its seat and stops. Add a small pilot piston that can push the poppet open on purpose and you have the pilot-operated check, the valve that holds hatch covers, outriggers, davit arms and press rams where they are. In the animation the valve sits in the line under a loaded cylinder: raise, hold, then lower, and watch the pilot piston do its job.
In simple terms
Raising. The pump pushes oil towards the cylinder. The oil lifts the poppet off its seat against a light spring and goes straight through. This part is an ordinary check valve: it needs only a couple of bar to open.
Holding. When the lever goes to Hold, the oil under the piston wants to flow back out. That pressure pushes the poppet onto its seat, and the heavier the load, the tighter it seals. Because it is metal on metal, a clean poppet holds with practically no leakage, which a spool valve can’t do.
Lowering. Now the pump feeds the top of the cylinder. That same pressure also travels down a thin pilot line to a small piston behind the poppet. The piston has a bigger area than the poppet’s seat (four times bigger in the default setting), so it only needs a quarter of the load pressure to push a pin into the poppet and open it. The oil under the piston can then flow back to tank and the load comes down.
The two faults are the two problems we find most. A particle trapped on the seat stops the poppet closing, so the load sinks slowly. A blocked or crushed pilot line means the pilot piston never gets pressure, so the load won’t come down at all.
The engineer's explanation, the deep dive, key numbers and the quiz open with a free account.
For engineers
Pilot ratio. The ratio R is the pilot piston area divided by the poppet seat area, typically 3:1 to 10:1. The poppet opens when the pilot force beats the load force on the seat:
| Relationship | Formula |
|---|---|
| Opening condition | p(pilot) × A(pilot) ≥ p(cylinder) × A(seat) |
| Cylinder pressure while lowering | p(cylinder) = p(load) + p(pilot) × A(rod side) ÷ A(bore) |
| Pilot pressure needed | p(pilot) = p(load) ÷ (R − A(rod side) ÷ A(bore)) |
Worked example from the default setting: 6 t on a 100 mm bore makes 75 bar. With a 56 mm rod, A(rod side) ÷ A(bore) = 0.69. With R = 4, the pilot needs 75 ÷ (4 − 0.69) = 23 bar. With R = 2 it would need 57 bar. The rod-side pressure also pushes down on the piston, which raises the cylinder pressure the pilot has to beat; that is why the area ratio appears in the formula.
The control valve’s centre matters. In Hold the pilot line must be vented to tank so the check closes fully. A tandem or closed-centre valve can trap pressure in the B line and pilot line, holding the check slightly open so the load creeps. The usual choice is a float centre (A and B to T), as in the animation.
Where they are fitted. On cylinders that hold loads: outriggers and stabilisers, hatch cover and ramp cylinders, davit luffing cylinders, press rams, lift tables. Ideally the valve is flanged straight onto the cylinder port, so a burst hose between the valve and the control valve cannot drop the load. Double pilot-operated checks (one in each line, cross-piloted) lock a cylinder in both directions.
Deep dive
Why pilot checks misbehave on overrunning loads. Once the check opens while lowering, the load pushes the oil out faster than the pump fills the rod side. The rod-side pressure, which is also the pilot pressure, drops. The check closes. Pressure rises again, it opens again. The result is a jerky, chattering descent that can shake a structure. For loads that pull the cylinder (a crane boom, a winch lowering, a davit swinging out), a counterbalance valve is the correct choice: it meters the flow out instead of snapping open. That is lesson 11. A restrictor in the line between the check and the control valve can calm a pilot check, but it is a compromise.
Thermal pressure. A check valve that holds perfectly also traps oil perfectly. If a cylinder sits in the sun with its oil locked in, the oil expands as it warms, and with nowhere to go the pressure can climb by 8 to 10 bar per °C in a rigid steel system. On UAE decks, where steel in the sun passes 60 °C, this is a real cause of burst hoses and leaking seals on idle equipment. Some circuits add a small thermal relief valve for this reason.
Finding a leaking check. Lower the load onto a support, cap the cylinder ports, then reload it with the hoses still capped. If it still drifts, the piston seal is bypassing. If it holds with the ports capped but drifts with the valve in circuit, the valve is leaking. Our cylinder drift guide covers the full test. On the bench, we pressure test checks at working pressure and look for zero drops at the outlet over a set time.
Cracking pressure as a tool. A check with a stiff spring (3 to 5 bar) placed in a return line creates a small back pressure, used to keep pilot circuits supplied or to stop a cylinder running away on light loads. A check in a pump outlet stops the system draining back through the pump when it stops, and protects the pump from pressure spikes.
Key numbers
| What | Typical value |
|---|---|
| Cracking pressure of a plain check | 0.5 to 5 bar |
| Pilot ratio of a pilot-operated check | 3 : 1 to 10 : 1 |
| Leakage of a clean poppet check | practically zero (drops per minute) |
| Pressure rise of trapped oil when heated | about 8 to 10 bar per °C (rigid system) |
| Pilot pressure for 75 bar load, R = 4, rod ratio 0.69 | about 23 bar |
Work out your own figures with the cylinder force and speed calculator.
