Open Center System – Idle Mode Through the “Open Center”

The open center system is a hydraulic circuit in which the hydraulic flow can not only be switched but also offer an idle mode. For this purpose, an “open center valve” is installed in the control block.

Open Center System and Its Components

Components of an Open Center System:

• Hydraulic Tank
• Hydraulic Pump with Drive
• Open Center Valve
• Actuator
• Return Lines

Operating Principle of an Open Center System

The hydraulic oil stored in the unpressurized hydraulic tank is drawn in by the pump and pressed into the lines at high pressure. The volume flow passes through the open center valve, which has three states:

1. Circuit A: Normal Flow
2. Circuit B: Flow-Through
3. Circuit C: Reverse Flow

In normal flow, the actuator moves in the desired direction. Hydraulic cylinders extend or hydraulic rotary motors move clockwise.

In reverse flow in circuit C, the actuator is moved in the opposite direction. The hydraulic cylinder (prerequisite: a double-acting hydraulic cylinder is installed) retracts or the hydraulic rotary motor moves counterclockwise. The open center system does not work with a single-acting plunger cylinder if the hydraulic cylinder cannot be returned to its initial position via external return (gravity, spring, hydropneumatic element).

In circuit C, during flow-through, the volume flow passes practically through the center of the opened valve (hence “open center valve”) and back into the tank.

Benefits of the Open Center System

The open center system is primarily used in mobile hydraulics. It is essentially a comfort circuit designed to simplify the handling of excavators, crawlers, and cranes. With the open center system, the operator always has the option to interrupt the movement of the boom, tool, or other hydraulic components without triggering the reverse movement. The alternative would be to shut down the pump, which requires either switching off the drive motor or equipping the pump drive with a separable coupling. However, this is technically feasible only with considerable effort.

Disadvantages of Open Center Systems

Even when the volume flow is pumped directly back into the tank, the system consumes energy. Construction vehicles typically use a large-volume, high-performance diesel engine. During idle mode, fuel is simply consumed without being used productively. In addition, open center systems are known for their high flow losses. While the system enables safe and comfortable handling of the construction vehicle, this comes at the cost of reduced fuel efficiency.

Alternative: Closed Center System

The alternative to the open center system is the closed center system, in which the volume flow is interrupted via controllable shut-off valves. This can be done manually or automatically through pressure sensors. While the closed center system is considerably more complex, it offers significantly higher fuel efficiency by avoiding unnecessary idle modes.

Application of Open Center Systems

The open center system is primarily used in cost-effective and simple construction vehicles, as a complex closed center system would disproportionately increase the costs for such vehicles. However, in highly modern, fuel-optimized systems, the open center system is no longer employed.