Pilot Operated Valve
In a pilot operated valve the main spool is driven by a small pilot pressure, not by lever force; the lever lightens and control can be carried away.
A pilot operated valve is the two-stage valve architecture in which the large main spool is driven not directly by the force of the lever but by a small pilot pressure. A small joystick valve produces a pilot pressure proportional to the opening of the lever; this pressure acts on the end of the main spool and shifts it proportionally. The operator's lever is thereby made light and the control point can be carried away from the valve.
Why two stages
The force needed to move the main spool rises as the valve grows larger and the flow increases; beyond a certain point the lever can no longer be driven by hand with a reasonable force. The pilot stage solves this problem by multiplying the force hydraulically. The valve spool goes on doing the same job, only what pushes it changes.
The difference from a directly operated valve
| Directly operated | Pilot operated | |
|---|---|---|
| What pushes the spool | Lever force | Pilot pressure |
| Lever force | Rises with the size of the valve | Low and independent of size |
| Extra hardware | None | Pilot line, filter, most often an accumulator |
| Control distance | Beside the valve | Far away, on thin pilot hoses |
- Gain: the lever force falls markedly and the ergonomics improve.
- Requirement: a separate pilot supply line, a pilot filter and most often a small accumulator.
- Fault symptom: if the pilot pressure falls the movements slow down or cannot reach full opening.
The first thing to look at in order to tell which one your machine has is the lines going to the lever. On a directly operated valve the lever sits on the valve body and is mechanically connected to the spool. In a pilot operated arrangement thin hoses run from under the control lever to the main valve; the main valve most often stands somewhere else on the machine, away from the operator's cab. The second clue is the lever itself: if the valve is large and yet the lever can be driven with finger force, there is a pilot stage in between.
What determines the cost
A pilot operated control group contains more parts than a directly operated group of the same flow: the pilot valve, the pilot line, the filter and, where needed, the accumulator. When making a price comparison it is therefore necessary to treat not the valve body on its own but all the parts needed to bring it into working order. The number of sections is decisive as well; the difference between a monoblock and a sectional valve changes both the flexibility and the number of parts.
Its effect on the choice of retrofit kit
This distinction makes the choice of kit easier: on a pilot operated machine the operator's lever is already light, so the force that has to be applied to the lever is low. On heavy valves with direct mechanical control, on the other hand, the real force of the lever has to be measured and the choice made according to the force difference between the HCT400 and the HCT402. Look not at the catalogue value but at the force you measure yourself at the lever.
Where it does not meet the expectation
Pilot control makes the lever light but it does not hold the position of the lever on its own; a load creeping in neutral is still the business of the hydraulic side and is solved with locking elements such as a pilot operated check valve. When the pilot line becomes dirty the problem is not in the lever but in the filter: the movements first slow down, then become unstable. Changing the lever or the kit without measuring the pilot pressure is a waste of time. First movements being lazy in the cold is also most often caused by the pilot circuit not having warmed up and is not a fault.
The context in which this term is used: Installation, redundancy and field uptime