Open Center Valve
An open center valve sends pump flow directly to tank when no work section is in use. This way the system runs at low pressure when idle, generates little heat and can be used with a cheap fixed displacement pump. It is common on tractors, truck mounted cranes and small construction machines.
In neutral the P port is connected to T through a center passage. As you push the lever this passage is throttled, pressure rises and oil starts to route to the actuator. So starting a motion is not a switching action but a gradual throttling action.
- Advantage: at idle the pressure stays at a few bar; the oil does not heat up and no fuel is wasted.
- Limit: when two motions are made at once the flow feeds the lighter load; the heavy load slows down or stops.
- Speed-rpm link: since flow rate depends on engine rpm, all motions slow down at idle.
This layout is the one most often encountered with retrofit kits. Because the first lever movement throttles the center passage, motion starts smoothly; in return a noticeable unresponsive zone is felt around center. Ramp time and minimum threshold settings on an electronic remote control exist to balance these two behaviors.
Where this term is used: How Are Manual Hydraulic Levers Controlled Remotely? A Comparison of 6 Methods