CLOSED CENTER HYDRAULIC SYSTEM

DEFINITION

A closed center hydraulic system is one having pressure in the system whenever the power pump is operating and fluid flows.

PRINCIPLE OF OPERATION

System connection: Parallel

The means of controlling pump pressure varies in the closed-center system.

WORKING

Flow within a closed center system is created by actuating the power pump; however, only enough flow is being produced to keep the pump lubricated and to achieve a standby pressure at the directional control valve.

In a closed center system, when a spool is stroked, a passage is exposed for the flow to enter while a pressure signal is sent from the directional control valve to the pump. This pressure signal informs the pump to then produce the flow needed to complete the hydraulic work.

If a constant delivery pump is used, the system pressure is regulated by a pressure regulator. A relief valve acts as a backup safety device in case the regulator fails.

If a variable displacement pump is used, system pressure is controlled by the pump’s integral pressure mechanism compensator. The compensator automatically varies the volume output.

When pressure approaches normal system pressure, the compensator begins to reduce the flow output of the pump. The pump is fully compensated (near zero flow) when normal system pressure is attained. When the pump is in this fully compensated condition, 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 closed center directional control valve communicates with the pump so that when a work section is not being used the pump destrokes so that it stops producing a large displacement of hydraulic oil.

COST INDEX

A closed center system, while more expensive perhaps, is usually more efficient as it is not continuously sending oil through the valve when it is not being used. Consequently, less energy and less fuel is used – which results in savings on fuel costs.

CONCLUSION

Since most aircraft applications require instantaneous operation, closed-center systems are the most widely used.