Hydraulics use fluid power under high pressure to lift or to support a load. Every hydraulic system takes fluid from a reservoir through a pump to a selector control valve. This fluid flows from the valve to an actuator. At the high end of the actuating cylinder there's a piston. High pressure drives the piston down, forcing fluid out of the piston's lower side before returning it through the selector valve back to the reservoir, where the cycle continues as needed.
Fluid continues to flow through an open-center system unless the actuator is idle. Idle actuating mechanisms mean no applied pressure. Fluid flows from the reservoir, pump, selector valves and back to the reservoir. Pressure in the open-center system is built up by the selector valve's position. If the actuator is idle, then the valves are in neutral position. If the actuator revs up, then the valves are out of neutral position and the pump bears a load. There's constant volume in the open system and no pressure regulator.
There are basic differences between the closed- and open-center hydraulic systems. Fluid is always stored under pressure if the pump's operating and released when it hits a certain value by a pressure regulator or a variable volume pump valve. And the selector valves are parallel, not in a series like in the open system.
Combination Support Systems
Combination hydraulic systems have elements of closed- and open-center systems. The less traditional setup features the closed system serving a hydrostatic capacity and the open system for auxiliary applications. These systems garner full performance even during cold start-up. Combination systems require more parts, which mean additional subsystems.
- hydraulic machine image by Heng kong Chen from Fotolia.com