Check valves
The simplest valve of all: it lets oil flow one way only. A ball on a seat — lift it and oil passes; press it home and the line is shut. The same idea, made switchable, locks a cylinder solid.
Source: Rabie, Fluid Power Engineering, Ch. 5.
Before you start
What you need first
- Reading valve symbols — ports, arrows, springs (Topic 25).
- A pressure difference drives flow; a spring sets a small opening pressure (Topics 19, 23).
What you'll be able to do
- Read the check-valve symbol and say which way oil can flow.
- Explain the cracking pressure and the spring vs spring-less choice.
- Say where check valves are used, and how a pilot-operated check valve locks a cylinder.
Start here · the one big idea
The check (non-return) valve
A check valve lets oil flow in one direction only. Inside is a ball on a seat:
- flow the right way (A → B) lifts the ball off its seat → oil passes;
- flow the wrong way (B → A) presses the ball onto its seat → the line is shut.
It is also called a non-return valve, because it stops oil returning the way it came.
Cracking pressure — spring or no spring
A small pressure is needed to push the ball off its seat and just start flow: the cracking pressure. For a check valve it is usually below 10 bar. Some have almost no spring at all (cracking under about 0.2 bar) for nearly free flow.
| With spring | No (light) spring | |
|---|---|---|
| Cracking pressure | A few bar | Under ~0.2 bar |
| Good for | Holding pressure; a defined opening point | Almost free flow (e.g. a return bypass) |
Where check valves are used
- Stop reverse flow — e.g. keep oil from running back into and spinning the pump when it stops, or hold pressure in part of a circuit.
- Bypass around another valve — a free return path for the oil while the main (metered) path handles the working stroke.
- Combine flows safely — let two sources join a line without one back-feeding the other.
Pilot-operated check valve
A plain check valve blocks reverse flow always. A pilot-operated check valve adds a third connection, the pilot port: a pilot pressure can force the valve open in the reverse direction when we deliberately want to allow back-flow.
So it behaves as: blocks reverse flow — until you tell it not to.
Hydraulic locking of a cylinder
Put a pilot-operated check valve on each port of a double-acting cylinder and you can lock it solid — no drift, even under a heavy load, because oil cannot leak back out through either check.
When the DCV sends oil to move the cylinder, that same supply pressure becomes the pilot that opens the opposite check, letting the other side drain — so the cylinder moves freely. Release the command and both checks reseat, locking it again.
✏️ Try it yourself
- In one line, what does a check valve do, and what is inside it?
- Roughly what is a check valve's cracking pressure, and when would you choose a spring-less one?
- How does a pilot-operated check valve differ from a plain one, and how do two of them lock a cylinder?
Common mistakes to avoid
| Mistake | Fix |
|---|---|
| Drawing the ball seating on the wrong side | The ball must seat against reverse flow: free flow lifts it off; reverse flow presses it home. |
| "A check valve needs no pressure to open" | It needs its cracking pressure (small, but not zero unless truly spring-less). |
| Expecting a plain check to ever allow reverse flow | It never does — you need a pilot-operated check for that. |
| Using a closed-centre DCV alone to hold a load | It leaks and drifts; use pilot check valves to lock it. |
Recap — the whole topic on one screen
| Idea | What you own now |
|---|---|
| The job | One-way (non-return) flow: a ball on a seat |
| Opening | Cracking pressure < ~10 bar; spring-less < ~0.2 bar for free flow |
| Uses | Stop reverse flow, bypass a valve, combine flows, hold pressure |
| Pilot-operated | A pilot pressure opens it for reverse flow |
| Locking | Two pilot checks lock a cylinder with no drift |