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| Open center systems, the pump moves oil all the time. The valves pass all the oil they don't need for a cylinder or motor load and the system pressure at idle is only the pressure drop from that flow in the plumbing. Usually very low, probably 30 or 40 psi or less. A restrictive load like a loaded cylinder raises the pressure which is limited by pressure relief valves, usually in the control valves.
Closed center the valve passes oil only to a load, and returns oil to the pump coming from the load, typically the oil from the low pressure side of the cylinder or return oil from the motor. The pump and its controls hold pressure at the system rating, like 2250 psi all the time, destroking the pump when there is no flow.
Each has its advantages and disadvantages. One can argue all day about the pump efficiency in each situation and probably two different pumps for the same scheme from different makers or different conditions will have more difference in efficiency than the other scheme. For sure open center has less complexity at the pump controls. There are none, just the pressure relief valve. But for things like steering and brakes the closed center system probably gets pressure to the critical loads faster than open center that require the pump to build up pressure to force the load.
More modern tractors use a "smarter" system that acts like closed center for the flows but doesn't hold pressure high to cut down on energy lost due to leaks (one of the disadvantages of the closed center system is that with everything at high pressure, leaks get to be expensive energy losses) but has extra controls that sense a load and ramp up the pressure. But shuts down the pump when there's no need for oil so there's less energy used to move oil around the plumbing at low pressure like an open center system does. All at considerable expense and complication in pump control. I've only used open and closed center systems, your customer's tractor is probably one of the newer load sensing systems.
Baum Hydraulics used to have a pretty good section in their catalog on these hydraulic fundamentals. Probably still there if you download the chapters on hydraulic pumps, motors, fittings, and cylinders.
Gerald J. | |
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Can we talk power beyond circuits
