Open Center Hydraulic System
A hydraulic layout in which the pump's flow returns freely to tank while the control valves are in neutral. No pressure builds up until a spool is moved, so idle energy consumption is low. The great majority of classic and mid-age tractors use this layout and generally run with a fixed-displacement gear pump.
In an open center system the valve leaves an open path from pump to tank in the neutral position. When the lever is moved this path is restricted, pressure rises and oil is directed to the cylinder. Because the flow is fixed, movement speed depends largely on engine speed: at idle the linkage lifts slowly, at higher revs it lifts quickly.
- Running two functions at the same time splits the flow and slows both down.
- Since there is no pressure in neutral, the system heats up less at idle.
- Once the lever passes the transport position this advantage is lost and the relief valve opens continuously.
Tractors with a cable-controlled quick-lift mechanism are typically in this class. Kits such as the LFT20 and LFT54 make sense in exactly this layout: they move the control lever electronically without ever entering the hydraulics. The compatibility criterion is not the make of the tractor but the presence of this cable-operated mechanism.
Context in which this term is used: Converting a tractor hydraulic lever to electronic control