The basic principle.
A directional valve selects connections between ports. A flow-control function meters how much fluid passes. A single assembly can combine these functions, but they answer different questions: where does fluid go, and how quickly?
Spools, poppets and other mechanisms vary the available openings. A restriction creates a pressure drop during flow. Flow through a simple opening depends on the opening size and pressure difference, not on position alone.
The same opening under a different load.
A changed pressure difference can change flow through an unchanged restriction. Compensation or electronic control can help manage this effect in suitable designs.
Follow the relationship.
- Select a pathConnect supply, actuator or return.
- Meter flowControl the passage area.
- Observe movementThe actuator responds to delivered flow.
Connect it to the lift.
In hydraulic lifts, valve functions shape starting, travel, levelling and stopping. Some circuits bypass pump flow during part of upward control and meter return flow during descent. The exact paths are installation-specific.
A closer look
A pressure-compensated flow control uses another mechanism to manage the pressure difference across a metering opening within its operating range.
Check your understanding
Does a fixed opening guarantee exactly the same flow under every pressure difference?
Reveal the explanation
No. Flow depends on both the opening and the pressure difference, among other factors.