Closed Center Hydraulic Systems




A closed centered hydraulic system is generally associated with a variable displacement piston hydraulic pump, the spool valve is blocking the flow of hydraulic oil at the control valve so no oil is being pumped into the return to the tank. The systems can be made to run more efficiently than the open centered systems.


Axial Piston Variable Displacement Pump-Internal Parts

The variable displacement pump in orange is different from the gear pumps in the way they look physically. The gear pump looks like a rectangular block and the piston pump is that with the addition of a stroke control valve shown on top side of the pump housing.



Simply there are more bolts and things that make them a heavier pump than an equivalent displacement gear pump. They look more complicated and shown in the center picture are the pistons and rotating members of the axial piston hydraulic pump.

The picture in the far right shows the internal pistons and how they are positioned parallel to the rotating pump shaft. These variable displacement pumps need a valve to regulate pump output, at full stroke the pump outputs maximum gallons per minute and at minimum stroke produce the minimum gallons per minute as the pump shaft is rotated by an engine or motor. 

In the above schematical diagram of the Closed Centered Hydraulic System you can clearly view the spool valve in the center has no cut cavity to allow fluid to flow past the control valve housing. All ports are blocked off from flowing oil. The Stroke Control valve allows oil to flow into it and push a piston out of a tube as the back pressure goes higher in the fluid conduit system. As fluid flows within a conduit system with restrictions the back pressure rises or lowers in proportion to the resistance at the loads and combined conduit change in directions. Each time a fluid has to flow through a fitting or coupler it adds restriction to the system fluid flow and back pressure becomes measureable. When a fluid flow is blocked completely the system backpressure rises until the physical parts and hoses cannot contain the back pressure and they crack or rupture at that point.



So the variable displacement pump differs from the gear pump because the output volume in oil flow is adjusted by changing the stroke angle of the swash plate within the housing of the pump.



On a closed center hydraulic system there is essentially no system relief of full flow, the pressure is simply reduced by reducing the stroke angle of the swash plate. On some closed centered systems they do install a pilot operated pressure relief but it is not designed for full flow, only small amounts of oil volume. This seems dangerous but it's just how they are designed. The designers are counting on the system being de-stroked as back pressure oil resulting from a work load contacts the piston in the destroke valve on top of the pump (orange picture 1).



As the pump shaft is rotated the oil lines fill up with oil and are blocked at the control spool in the hydraulic control valve. The back pressure develops instantly after the fluid is full in the conduits without the presence of air. The back pressured oil travels in a conduit back into the de-stroke piston and as pressure builds the stroke angle is decreased until a specific pressure is maintained within the conduit of the system. Some manufacturers call that conduit a signal line, a sensing line, and so on. It simply sends the back pressured oil into the axial piston pump stroke control valve.



As the pump starts flowing oil it's stroke angle is greater than after back pressured oil reacts on the piston in the destroke valve on top of this pump. Pumps can be rotated in any degree of angle so this one for example is said to be on the top side of the pump.

Radial Piston Variable Displacement Pump - Internal Parts

The variable displacement pump in green is different than the orange variable pump. Color makes no difference however in these example pumps the orange is the axial piston pump and the green is the radial piston pump. The internal differences are distinct between the two in comparison. The radial piston pump pistons are shorter and larger diameter, there are only eight instead of nine pistons and the radial piston pump pistons do not have slipper shoes on the end that swivel, they are a solid piston. The pistons in the radial pump move perpendicular to the axis of rotation of the input shaft of the pump and there are no rotating members in the radial pump. The radial piston pump was designed and patented by John Deere Company and they are by far the most durable and reliable compared to any other hydraulic pump made. I have seen these pumps last for 20 years of service with little to no wear on internal parts.

Each variable displacement pump no matter who makes it or what type of pump either axial piston or radial piston has one common feature which is the ability to pump a minimum volume of oil in gallons per minute (GPM) and increase flow of oil through a system to a maximum volume output. This is accomplished by limiting the travel of the pistons within the confined bore Length x Diameter = volume. Any piston that moves or oscillates from one end of the internal bore to the opposing end within a tube has the movement termed the stroke of the piston. To change the stroke on a piston you need a way to limit the stroke and this is called a stroke control valve on the radial piston pump above (green). On the axial pumps mostly associated with hydrostatic drives it is called a swash plate that changes the stroke of the pistons which proportionally changes the volumetric output of the pump itself and is modulated by a flow pressure compensator.