LD1
Epic Contributor
Open center hydraulics are for fixed displacement pumps. Closed center are for pumps that are variable flow (displacement)
So perhaps it would be best to start by explaining the difference between pumps.
Fixed displacement pumps are just that....fixed. It displaces a certain amount of fluid for each revolution (which is why you normally see cubic inch per revolution listed as a pump spec instead of flow). So...as long as the pump is turning.....that fluid HAS to go somewhere. When you are doing nothing, it is just circulating though the system, (free flowing so to speak), and not building pressure. Because pumps dont make pressure, they move fluid.....resistance to that fluid moving is what makes pressure.
With a pump like this, you need open center (or tandem center) valves. Thus making the "system" an open center system. And what that means is, when your hydraulic valves are in their neutral position, the P (pressure port) is connected to the T (tank) port and fluid can freely move.
Variable displacement pumps are different. They can change how much oil they move with an unloader valve. The pump can be spinning, and holding steady at 2500psi or whatever your spec is, but not acutally be moving any fluid. Once pressure drops (indicating a demand for fluid), the pump loads up to maintain (or try to) 2500 psi. In a system with one of these pumps, an open center valve would never allow the pump to see the pressure and unload. So in a closed system, the P and T ports on the valve are not connected when in neutral. With the P port blocked from returning to tank, pressure is built up, pump unloads.
Cannot used a closed center valve on an open system because a fixed displacement pump CANT unload. Will build pressure, and flow high pressure oil through the relief valve and build tremendous amounts of heat.
And cannot use an open center valve on a closed center system. Because a free path back to tank will prevent it from ever making pressure, and any other hydraulic function wont work due to no pressure as the oil simply returns to tank via another function.
Thats why open center hydraulics cannot have valves simply hooked in parallel. They must be in series via power beyond
So perhaps it would be best to start by explaining the difference between pumps.
Fixed displacement pumps are just that....fixed. It displaces a certain amount of fluid for each revolution (which is why you normally see cubic inch per revolution listed as a pump spec instead of flow). So...as long as the pump is turning.....that fluid HAS to go somewhere. When you are doing nothing, it is just circulating though the system, (free flowing so to speak), and not building pressure. Because pumps dont make pressure, they move fluid.....resistance to that fluid moving is what makes pressure.
With a pump like this, you need open center (or tandem center) valves. Thus making the "system" an open center system. And what that means is, when your hydraulic valves are in their neutral position, the P (pressure port) is connected to the T (tank) port and fluid can freely move.
Variable displacement pumps are different. They can change how much oil they move with an unloader valve. The pump can be spinning, and holding steady at 2500psi or whatever your spec is, but not acutally be moving any fluid. Once pressure drops (indicating a demand for fluid), the pump loads up to maintain (or try to) 2500 psi. In a system with one of these pumps, an open center valve would never allow the pump to see the pressure and unload. So in a closed system, the P and T ports on the valve are not connected when in neutral. With the P port blocked from returning to tank, pressure is built up, pump unloads.
Cannot used a closed center valve on an open system because a fixed displacement pump CANT unload. Will build pressure, and flow high pressure oil through the relief valve and build tremendous amounts of heat.
And cannot use an open center valve on a closed center system. Because a free path back to tank will prevent it from ever making pressure, and any other hydraulic function wont work due to no pressure as the oil simply returns to tank via another function.
Thats why open center hydraulics cannot have valves simply hooked in parallel. They must be in series via power beyond
