The power pump may be one used with a separatepressure regulator control. The power pump may beused with an integral pressure control valve thateliminates the need for a pressure regulator. Thissystem differs from the open center system in that theselector or directional control valves are arranged inparallel and not in series. The means of controllingpump pressure will vary in the closed center system.If a constant delivery pump is used, the systempressure will be regulated by a pressure regulator. Arelief valve acts as a backup safety device in case theregulator fails.If a variable displacement pump isused, system pressure is controlled by the pump’sintegral pressure mechanism compensator.Thecompensator automatically varies the volume output.When pressure approaches normal system pressure,the compensator begins to reduce the flow output ofthe pump. The pump is fully compensated (near zeroflow) when normal system pressure is attained. Whenthe pump is in this fully compensated condition, itsinternal bypass mechanism provides fluid circulationthrough the pump for cooling and lubrication. A reliefvalve is installed in the system as a safety backup.An advantage of the open center system over theclosed center system is that the continuouspressurization of the system is eliminated. Since thepressure is built up gradually after the selector valveis moved to an operating position, there is very littleshock from pressure surges. This action provides asmoother operation of the actuating mechanisms. Theoperation is slower than the closed center system, inwhich the pressure is available the moment theselector valve is positioned. Since most aircraftapplications require instantaneous operation, closedcenter systems are the most widely used.Power systems are designed to produce andmaintain a given pressure. The pressure output ofmost of the Navy’s high-performance aircraft is 3,000psi. The hydraulic system, shown in figure 7-2, is anexample of a representative 3,000 psi hydraulic powersystem. The aircraft has three independent hydraulicpower systems. The two primary systems are theflight hydraulic power system and the combinedhydraulic power system. These systems arepressurized by two independent engine-drivenhydraulic pumps on each engine. The auxiliarypower system also operates on 3,000 psi pressure. Itis pressurized by the hydraulic hand pump and/or theelectric motor-driven hydraulic pump. The auxiliarypower system is similar to the combined hydraulicpower system. The primary difference is that thecombined system supplies hydraulic pressure toutility hydraulic circuits and the flight controls.The hydraulic control valves and actuators thatoperate the primary flight controls are of the tandemconstruction type. This design permits operationfrom either or both of the two power systems. Withthis arrangement, either engine can fail or be shutdown without complete loss of hydraulic power toeither system. The flight system reservoir suppliesfluid to the two engine-driven flight system pumps.The combined system reservoir supplies fluid to thetwo engine-driven combined system pumps and to theauxiliary hydraulic power system. Both reservoirsare of the pressurized piston type. They arepressurized by engine bleed air during engineoperations and by an external air (nitrogen) sourceduring maintenance operations.Hydraulic system pressure is indicated on theintegrated hydraulic pressure indicator.Thisindicator displays the output pressure of the flight andcombined hydraulic power systems.The flighthydraulic power system provides power for theoperation of the rudder, stabilizer, and flaperons. Italso provides power for operation of the automaticflight control system actuators, which are an integralpart of the rudder and stabilizer control surfaceactuators. The flight hydraulic system also controlsthe automatic operation of the isolation valve. Thisvalve is a part of the combined hydraulic system.The combined hydraulic power system consists oftwo parallel circuits—one to power the primarysystems and the other to power the secondarysystems. The primary system consists of spinrecovery, rudder, stabilizer flaperon, speed brakes,and electric ram air turbine systems. The secondarysystem consists of wing slats, wing flaps, wing fold,landing gear, arresting gear, wheel brakes, nosewheelsteering, and the nose strut locking systems.The isolation valve shuts off flow to thesecondary systems during flight and limits thecombined system’s pressure requirements tooperation of the primary circuit. Operation of theisolation valve is both automatic and manual.The reservoir pressurization system provides thereservoir with a differential pressure of 40 psi toprevent engine-driven pump cavitation. The pressureis maintained at 40 psi by the air regulator. In theevent of regulator failure, the relief valve installedbetween the regulator and the reservoir preventsoverpressurization. The relief valve opens at 50 psi.7-4
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