OPEN CENTER HYDRAULIC SYSTEM

DEFINITION

An open center hydraulic system is one having no pressure in the system when the fluid flows but actuating mechanisms are idle.

The pump circulates the fluid from the reservoir, through the selector valves, and back to the reservoir. Such a system is mainly without an accumulator.

PRINCIPLE OF OPERATION

System connection: Series

The selector valves of the open center system are always connected in series with each other. In this arrangement, the system pressure line goes through each selector valve.

Operation of the system following actuation of the component depends on the type of selector valve being used.

WORKING

Fluid is always allowed free passage through each selector valve and back to the reservoir until one of the selector valves is positioned to operate a mechanism. When one of the selector valves is positioned to operate an actuating device, fluid is directed from the pump through one of the working lines to the actuator. With the selector valve in this position, the flow of fluid through the valve to the reservoir is blocked.

The pressure builds up in the system to overcome the resistance and moves the piston of the actuating cylinder; fluid from the opposite end of the actuator returns to the selector valve and flows back to the reservoir.

ASSOCIATED VALVES AND WORKING

Several types of selector valves are used in conjunction with the open center system.

One type is both manually engaged and manually disengaged. First, the valve is manually moved to an operating position. Then, the actuating mechanism reaches the end of its operating cycle, and the pump output continues until the system relief valve relieves the pressure.

The relief valve unseats and allows the fluid to flow back to the reservoir. The system pressure remains at the relief valve set pressure until the selector valve is manually returned to the neutral position.

This action reopens the open center flow and allows the system pressure to drop to line resistance pressure.

The manually engaged and pressure disengaged type of selector valve is similar to the valve previously discussed. When the actuating mechanism reaches the end of its cycle, the pressure continues to rise to a predetermined pressure. The valve automatically returns to the neutral position and to open center flow.

ADVANTAGES & DISADVANTAGES

An advantage of the open-center system over the closed-center system is that the continuous pressurization of the system is eliminated.

Since the pressure is built up gradually after the selector valve is moved to an operating position, there is very little shock from pressure surges. This action provides a smoother operation of the actuating mechanisms.

The operation is slower than the closed-center system, in which the pressure is available the moment the selector valve is positioned.