Figure 4-17.—Fluid displacement—Stratopower pump.valve when the piston advances to its foremostposition. The low-pressure area occurring in thecylinder during the piston return causes fluid toflow from the reservoir into the cylinder.The internal features of the variable-displacement Stratopower pump are illustrated infigure 4-18. This pump operates similarly to thefixed-displacement Stratopower pump; however,this pump provides the additional function ofautomatically varying the volume output.This function is controlled by the pressure inthe hydraulic system. For example, let us take apump rated at 3000 psi, and providing flow to a3000 psi system. As system pressure approaches,say 2850 psi, the pump begins to unload (deliverless flow to the system) and is fully unloaded (zeroflow) at 3000 psi.The pressure regulation and flow arecontrolled by internal bypasses that automaticallyadjust fluid delivery to system demands.The bypass system is provided to supplyself-lubrication, particularly when the pump is innonflow operation. The ring of bypass holes inthe pistons are aligned with the bypass passageeach time a piston reaches the very end of itsforward travel. This pumps a small quantity offluid out of the bypass passage back to the supplyreservoir and provides a constant changing offluid in the pump. The bypass is designed to pumpagainst a considerable back pressure for use withpressurized reservoirs.Figure 4-18.—Internal features of Stratopower variable-displacement pump.4-15
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