AS9120B, ISO 9001:2015, and FAA AC 0056B ACCREDITED

Function and Operating Principles of Hydraulic Systems

Hydraulic systems have greatly changed the way many industries carry out operations, allowing for high amounts of force to be controlled for the means of performing work. While hydraulic systems vary in design based on their function, installation area, and other such factors, all share the same basic components that make fluid transmission and pressure control possible. There are also two major types of basic hydraulic units, those of which are open-center and closed-center hydraulic systems. 

With an open-center hydraulic system, fluid flow will be provided without pressure when actuating mechanisms are not active. During typical operation, fluids will exit the reservoir with the assistance of a pump, and they will move through various selector valves before making their way back into the reservoir. If subsystems are present in the assembly, each will have its own selector valve. Unlike other options, open-center systems will always have selector valves connected in series with one another so that the system pressure line runs through all. Furthermore, fluids will always pass through all valves before returning to the reservoir until a valve is positioned for operation.

Upon a selector valve adjusting to operate an actuating device, the path to the reservoir will be blocked so as to direct hydraulic fluids into the working line of the actuator. This results in a build-up of pressure that continues to rise until the actuating cylinder’s piston is moved. This fluid will then move from the other end of the actuator toward the select valve before returning to the reservoir. As selector valves may range from manual to automatic in terms of operation, it is important to consider such factors when making a purchasing decision.

With the closed-center hydraulic system, fluids are always under pressure whenever the power pump is activated. As compared to the open-center hydraulic system, closed-center types feature selector or directional control valves that are arranged in parallel, though not in series. The pump of such a system may either be a constant delivery or variable displacement pump, the former allowing for system pressure to be regulated with the use of a pressure regulator. In the instance that the pressure regulator fails, a backup relief valve will be available. With the variable displacement pump, pressure is managed through the use of an integral pressure mechanism compensator. As the compensator offers automatic volume output variation, it is able to reduce pump flow output when normal system pressure values are attained. For safety, closed-center hydraulic systems feature a relief valve.

When comparing the two, one can argue that the open-center type is more advantageous for its ability to eliminate constant pressurization. With gradual pressure increases occurring once the selector valve is placed in an operating position, pressure surge shocks can be avoided with ease to ensure smoother operations. That being said, closed-center systems are quicker to operate as fluids are always under pressure during pump operations, making them the most common choice for aircraft applications.

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