Open Center Hydraulic System

The Open Center Hydraulic system is a system that allows fluid to flow through a conduit (tubing & hoses) which have an internal hollow passage for fluids to flow within them.

There are several elements associated with the open center hydraulic systems.


1) A hydraulic pump and mostly a gear pump is associated with the open center hydraulic systems.


One big advantage of a gear pump is they are way less expensive to buy than a piston pump. Another advantage is they will work in dirtier conditions meaning the mechanic does not have to be like a surgeon with rubber gloves. The gear pump is a pump which will work longer than a piston pump when dirt has entered into the system by an open line or mechanic contaminating the hydraulic oil with dirt.

Hydraulic Gear Pump   - Internal Gears -      Internal Diagram



The Gear pump looks like a block of metal with 2 holes (ports) They can have threads in them or flat surface for flanges which have O-Rings. Most of the time these two ports are directly across form one another. One is a suction and the other is the outward flow port called a pressure port, the suction port is hooked to the reservoir tank and suction from the internal gears pulls hydraulic oil from the reservoir. The fluid then flows from the out flow port (P for pressure).



When an engine or electric motor are coupled to the Gear Pump input shaft and the engine or motor are operating at a specific RPM the gears will rotate in proportion to the rotational RPM of the engine or motor. This is called a positive displacement pump, for example the input shaft of the pump is rotated 100 revolutions per minute (RPM) and the gear pump will pump out flow a specific volume in gallons. There is no pressure involved at all at this point just oil volume in Gallons Per Minute. Depending on the size of the gears and quantity of them a gear pump as an example produces that volume of space between the gear teeth. Our example pump will produce 10 gallons per minute at 1000 RPM with the oil flowing directly from the out flow or pressure port P.



Increasing the RPM of the engine or motor (Source Power) proportionally increases the out flow from the gear pump. In our example our pump is now being rotated at 2000 RPM so it will now out flow 20 gallons per minute (GPM) of hydraulic oil volume.



This oil volume is of no use at zero pounds per square inch of pressure unless someone is just moving fluid from one point to another. The need exists to now build a pressure in the system and this is technically called back pressure and pressure readings are only evident after a pumps out flow is restricted from flowing through a conduit system.



Below is a schematical diagram of the basic Open Center System.

The elements that make up a hydraulic system are a supply of oil in a container called the Tank or (T). A Pump to flow the oil, a valve that directs the oil flow called the Control Valve, a way to regulate system back pressure called the Pressure Regulating Valve, and a conduit that allows fluid to flow from one element to the other and then return to the tank.



As the pump input shaft is rotated the gears create a suction and pull on the oil in the tank until it contacts the gears in the pump. The fluid then is able to flow out the pressure port of the pump housing. This oil flows freely in the system into a port in the control valve port P for pressure. It goes into a cavity within the control valve housing and the open part of the spool valve allows the free flow into the conduit leading back to the tank. At this point there is no pressure involved within the system, it is freely flowing as the pump is operated by the power source.



Below are pictures of the rest of the elements making up the open center hydraulic system.

Control Valve complete- Spool Valve - Pressure Regulating Valve



In the schematical drawing the housing is not drawn around the spool valve and does not show the other ports in the complete control valve element. They are not necessary at this point to convolute the point of the open center system.



2) Control Valve, this is the second element in the open center system

The out flow or P port of the gear pump enters into the P port of the control valve housing and into the cavity of the spool valve, the outflow of oil from the control valve housing flows out the T (tank) port and continues through the conduit system and flows into the hydraulic tank. Notice that the cavity in the spool valve is in the longitudinal center (long dimension) of the spool valve, the name open center applies. As the spool valve moves either up or down it blocks the fluid from the pump P port and directs it to any load desired. This could be a cylinder or motor working from hydraulic oil flow. In fluid flow systems called hydraulic open cavity allows oil to flow freely and closed is when the fluid flow is blocked.



On a simple one spool control valve there are 4 holes or ports, the inlet oil P, the flowing oil when the lever is not moved from center to T. As the spool is moved upward the flowing oil is directed to a working port A and when moved downward directs the fluid flow to port B. All elements that are work load have two ports A and B, one port in a hydraulic cylinder moves the cylinder piston and rod outward and the other port moves the piston and rod inward. This is called work port A. and work port B of a work load actuator. A motor also has port A and port B. As oil flows into port A the motor rotates clockwise and as oil flows into port B the motor rotates counter clockwise.



The terminology is what is important when describing to a mechanic a problem within a system. This page and all the rest of the pages on this site are dedicated for better customer communication with a mechanic they have chosen to perform the repairs needed.



3) Pressure Regulating Valve, the third component element in the open center system.



Only after a work load is applied to the open center system is it necessary to have the pressure regulating valve. This is a mechanical valve with a spring, adjustment bolt, and sealing valve pin. When your implement has no ability to work or is working slowly this is the first place to start. This valve relieves back pressure in the conduit system so the pump and other lines don't blow apart if the engine is strong enough to develop the rotational torque required to rip the things apart from over pressure. When these need repaired is when they are relieving all the oil or part of it directly back to tank.



The open center system is the least expensive to purchase, and the easiest system to maintain. Parts are common and have been used since the beginning of hydraulics on mobile or stationary equipment.



The set back today with these systems is fuel for the diesel engine and electricity for the electric motor. The open center system requires engine horsepower the entire time it is operating. The oil flow is steady the entire time the engine or motor is running using fuel to flow oil even when nothing is being used at the work load.