What is the difference between an open center and a closed center hydraulic system?
In open center the oil returns to tank in neutral and no pressure builds; in closed center the line waits pressurised, usually with a variable-flow pump.
In an open center system, with the control valve in neutral the oil passes through the valve and returns to the tank; no pressure builds in the circuit until a spool is moved. In a closed center system the oil path is closed in neutral and the line waits under pressure. Most old and classic tractors are of open center design.
How open center works
The pump delivers a fixed flow. With the valve in neutral the oil returns to the tank by a free path, so the pressure stays low; when a spool is pulled that path closes and the pressure rises. The structure is simple and the cost low. Against that, since the pump runs continuously some power is spent in neutral as well and the oil heats up. The heart of the circuit is a fixed displacement pump; for the detail look at the definition of the open center hydraulic system.
How closed center works
With the valve in neutral the oil path is closed. A variable displacement (load sensing) pump is generally used: when there is no demand the pump reduces its flow, so the power wasted in neutral falls. It is better suited to several services working at the same time and independently of one another; in a closed center hydraulic system one movement slowing another is seen less often.
Comparison
| Open center | Closed center | |
|---|---|---|
| Pressure in neutral | Low | The line waits pressurised |
| Pump | Fixed flow | Generally variable flow |
| Loss in neutral | Present, the oil heats up | Less |
| Multiple services | They affect each other | More independent |
| Typical vehicle | Classic tractor | Modern earthmoving machine |
The practical way of telling which one you have is to look at the pump and the valve: which system is found on which vehicle can generally be guessed from the class of vehicle, but the definite answer comes from the valve type. The difference between a monoblock and a sectional valve comes up in that examination too.
In practice the distinction shows itself most clearly when you make two movements at the same time. On an open center circuit, when two spools are pulled together the oil escapes to the side with the lower resistance; the light load speeds up, the heavy load slows down, or even stops. On a closed center circuit the pump raises its flow to meet the demand, so this effect is smaller. The second difference is heating: in an open center system left in neutral for a long time the oil heats up even though no work is being done. As the temperature rises the viscosity falls and the lever response changes; that is why the effect of oil temperature on lever response is felt through the day.
Why it does not affect the choice of kit
This distinction does not affect the choice of kit directly. The LFT20 and the LFT54 are not connected to the hydraulic line; they pull the cable-controlled Lift-O-Matik lever mechanically. Whether your system is open or closed center therefore does not determine how the kit works. The only decisive condition is that the tractor has a cable-pulled Lift-O-Matik mechanism; what that mechanism is is the first step of the compatibility check.
What to do
If you are asking for the sake of a kit, do not research the center type; look directly at whether the lift lever is pulled by a cable. If you are asking in order to understand hydraulic behaviour, watch whether the oil heats up in neutral: in a system left in neutral for a long time, heating is the most visible sign of open center behaviour.
The full topic: Converting a tractor hydraulic lever to electronic control