With so many large implements now demanding multiple oil flows from high-output hydraulic systems, every producer understands meeting those specifications will almost certainly require a tractor with a closed-centre hydraulic system.
Less efficient, open-centre systems were once the industry standard. And there are still many new tractors on the market today that rely on them, especially tractors in the utility horsepower class. Demands on their hydraulic systems are usually much lower than those put on high-horsepower field tractors. “Value” tractors are typically fitted with these systems as well, because they are less expensive and require a less sophisticated arrangement to function.
So how, exactly, do open- and closed-centre systems differ from each other? Although specific hydraulic systems can vary somewhat in their designs and be quite complex, the fundamental concept behind each type is simple to understand. Here’s a look at what makes them different.
The open-centre system
This design uses a constant displacement pump, usually a gear type, that supplies oil continuously, whether there is a demand for it or not. When no flow is required by any individual circuit, the spool inside the control valve stays in the neutral position, which allows oil to pass right through the centre of the control block and return to the reservoir. (see Figure 1 below)
In the neutral position, the valve doesn’t cause a lot of restriction to oil flow, which helps keep the oil from overheating and maintains a longer service life. Persistent high pressure caused by being forced through a restriction creates heat that would degrade oil over time.
When oil flow is needed and the operator pushes a control lever (or presses an electric switch which activates a solenoid at the control valve to accomplish the same result), the spool inside the control valve moves in relation to the openings in the valve block and diverts oil flow to the circuit that requires it. (see Figure 2 below)
When that oil is needed to control a hydraulic ram (or cylinder), high pressure oil moves into one side and displaces oil from the other. The displaced oil flows back through another opening in the control valve and eventually into the reservoir. To ensure pressure doesn’t exceed the maximum limit, open-centre systems rely on a pressure relief valve.
One fault of open-centre systems is the pump output is directly related to engine r.p.m. That means at low engine speeds pump output drops, therefore hydraulic circuits respond slowly. Another problem arises when more than one valve is activated at once. Because of the requirement that oil flow continuously, it must flow through all control valves. With valves connected in series, activating one valve can cut off or diminish oil flow to the others in a multi-valve system, so the second circuit in operation will function slowly or not at all.
To overcome that problem, open-centre systems commonly use valves connected in a series-parallel arrangement. It uses a second parallel route for oil flow that connects to the valve inlet and dead ends at the last valve in a series, allowing oil to flow to all valves all the time. But with that configuration the circuit requiring the lowest system pressure will move first, followed by the next lowest, and so on.
The closed-centre system
This one offers dramatic improvements in efficiency and circuit response. Unlike the open-centre model, oil does not flow continuously through the valves. So, the engine doesn’t waste power pumping oil for no reason. (see Figure 3 below)
To accomplish this, closed-centre systems use a variable-displacement pump. It pumps only long enough to build up pressure at the valve inlets, then it stops. When a valve is opened, the pump senses a reduction in pressure and begins pumping again to supply oil flow. That also eliminates the need to have a pressure relief valve plumbed into the system. (see Figure 4 below)
Because oil doesn’t have to pass through all the valve blocks in series on a closed-centre system, flow can be divided to multiple circuits at the same time. To provide better circuit responses the sizes of lines, valves and cylinders can be tailored to the flow requirements of each function. Components in an open-centre system, on the other hand, must all be sized to accept the pump’s full output.
And in a closed-centre system a larger pump can be used to allow for reserve oil flow at full engine speed and still provide high efficiency at low engine r.p.m.