Pilot Operated Check Valve
A lock valve passing one way that also opens the other way on pilot pressure from the opposite line; closed, a cone seat holds the suspended load.
A pilot operated check valve is a locking valve that normally allows passage in one direction only, but which can also be opened in the reverse direction by pilot pressure coming from the opposite line. Because when closed the sealing is provided by a cone seating, it holds a suspended load in place for a long time. Fitted to the cylinder port, it removes the creep created by the internal leakage of the spool valve. In the field it is most often known by the name "lock valve".
How it works
The internal leakage of spool valves, of the order of microns, causes a suspended load to creep within hours; this is called hydraulic drift. A pilot operated check valve closes that path: the oil passes freely as it goes into the cylinder, while its way out is locked by the cone. When you want to lower, the pressure applied to the opposite port lifts the cone by way of the pilot piston and the way is opened. An ordinary check valve does not have this second ability; it only passes in one direction.
- Pilot ratio: a ratio of 4:1 means that a pilot pressure of a quarter of the load pressure will be enough to open the valve.
- Typical trouble: if there is back pressure in the tank line the valve does not close fully or opens and closes with a chatter.
- Fitting rule: the valve is connected as close to the cylinder as possible, preferably straight onto the port; if the hose in between bursts it loses its locking function.
The difference from a load holding valve
| Pilot operated check valve | Load holding valve | |
|---|---|---|
| Its main function | Locks or opens | Locks and controls the lowering speed |
| Lowering speed control | None | Yes |
| Behaviour under overload | Keeps the load from running away | Lowers the load under control |
That is why it is not enough on its own with suspended loads that carry a risk of free fall; the load holding and counterbalance valve comes into circuit as a separate element. Why it has to be mounted straight onto the cylinder port rests on the same reasoning for both. The hose burst safety valve, which protects the line, is a third and separate function.
Diagnosis in the field
With a load creeping slowly in neutral the distinguishing question is this: is the creep coming from the lever itself, or from the lock valve? If you take the electronic control completely out of service and bring the lever manually to neutral and the creep continues, the problem is on the hydraulic side. The detailed form of the same test is explained under the arms lowering by themselves. The next step is to separate whether the creep comes from the valve or from the cylinder: if the cylinder seal is passing, the oil escapes from one side of the piston to the other and the lock valve cannot stop it, because the valve closes only the way out of the port. That is why you should make sure the cylinder is sound before fitting a new lock valve.
Where it gives the wrong result
If there is a return filter or a narrow line creating back pressure in the tank line, the valve works unstably; in that case changing the valve does not solve the problem, the back pressure has to be brought down. A lock valve also does not take the place of a safety function: before working under a suspended linkage, lower the load to the ground or put a mechanical support in place. The duty of the valve is to hold the load in place while parked, not to protect a person.
The context in which this term is used: Installation, redundancy and field uptime