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What are the basics of hydraulic systems
Release Time :
2025-02-05
Hydraulic systems can be either open center or closed center systems. An open center system is one in which there is fluid flow but no pressure in the system when the actuator is idle. The pump circulates the fluid from the reservoir through the selector valve and back to the reservoir. An open center system can employ any number of subsystems, each with a selector valve. The selector valves of an open center system are always connected in series with each other. In this arrangement, the system pressure line passes through each selector valve. Fluid is always free to pass through each selector valve and return to the reservoir until one of the selector valves is positioned to operate the mechanism. When one of the selector valves is positioned to operate the actuator, fluid is directed from the pump through one of the work lines to the actuator. When the selector valve is in this position, the flow of fluid through the valve to the reservoir is blocked. Pressure in the system builds up to overcome resistance and move the piston of the actuator cylinder; fluid from the other end of the actuator returns to the selector valve and flows back to the reservoir. The operation of the system after the components are activated depends on the type of selector valve used.
In a closed center system, the fluid is under pressure as long as the power pump is running. There are multiple actuators arranged in parallel, with several actuator units operating simultaneously while some others are not. This system differs from the open center system in that the selectors or directional control valves are arranged in parallel rather than in series. In the closed center system, the way the pump pressure is controlled varies. If a fixed displacement pump is used, the system pressure is regulated by a pressure regulator. If the regulator fails, a relief valve serves as a backup safety device. If a variable displacement pump is used, the system pressure is controlled by the pump's integrated pressure mechanism compensator system. The compensator automatically varies the volume output. When the pressure approaches the normal system pressure, the compensator begins to reduce the pump's flow output. When the normal system pressure is reached, the pump is fully compensated (close to zero flow). When the pump is in this fully compensated state, its internal bypass mechanism provides fluid circulation through the pump for cooling and lubrication. A relief valve is installed in the system as a safety backup.
The advantage of the open center system over the closed center system is that the system does not need to be continuously pressurized. Since the pressure is gradually built up after the selector valve is moved to the working position, the impact of pressure fluctuations is small. This action makes the operation of the actuator smoother. The operation is slower than a closed center system where pressure is available as soon as the selector valve is positioned.
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