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| In an open center valve that's the last or only valve on a system, the single oil return passes all oil to the return line going to the sump through a filter when no valve is operated. And when a valve is operating a double acting cylinder that line takes the return oil from the slack side of the cylinder. Because it takes they cylinder (or motor) return oil, its not suitable for other hydraulic loads. The power beyond option on that valve separates the pass through oil from the load return oil. The pass through oil can go to another valve for more actions, that power is called power beyond. In the open center system flow is constant, but pressure only rises as hydraulic loads restrict the flow.
In a closed center system, the pressure is held constant and there's no flow when no valves are operated. There is the return port for return oil from cylinders and motors. That's low pressure going to the sump usually through a filter. The valve can often support power beyond, which is usually plugged until connected to another valve. Power beyond again has constant pressure, but not cylinder or motor return oil.
These are what ordinary systems have, what tractor makers to make variations on these I don't know for red tractors.
Motors can have two returns. One for catching the leaks from shaft seals, the other for the return oil which has the full motor oil flow. Some motors have sturdy seals on the pressure side of the shaft and weak seals on the return side so they aren't reversible. The weak seals on the return side sometimes can't stand the back pressure of valve, filter, and common tractor return circuits (on a 4020 the return is to the output of the transmission charge pump, usually 200 to 250 psi) so those motors get returned directly to the sump. Trouble can be that like the 4020, the flow of the transmission pump is smaller than the main pump depending on the cooler/reservoir to keep the main pump supplied with the hydraulic loads are returned to the proper return. Returning motor oil to the sump can short change the front pump and disable most hydraulic loads, like steering and brakes.
A load sense hydraulic system keeps pressure down while there's no loads, but sometimes with a separate hose detects when a load is going to take flow and develop pressure to raise the operating pressure of the pump. This is most like a closed center system, but the load sense reduces pump shaft power when there are no hydraulic loads.
One of the catalog sections from Baum Hydraulics has several tutorials on hydraulics. www.baumhydraulics.com
Gerald J. | |
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hydraulic theory - explain all the power beyond connections to me
