Figure 4-17.Fluid displacementStratopower pump.
valve when the piston advances to its foremost
position. The low-pressure area occurring in the
cylinder during the piston return causes fluid to
flow from the reservoir into the cylinder.
The internal features of the variable-
displacement Stratopower pump are illustrated in
figure 4-18. This pump operates similarly to the
fixed-displacement Stratopower pump; however,
this pump provides the additional function of
automatically varying the volume output.
This function is controlled by the pressure in
the hydraulic system. For example, let us take a
pump rated at 3000 psi, and providing flow to a
3000 psi system. As system pressure approaches,
say 2850 psi, the pump begins to unload (deliver
less flow to the system) and is fully unloaded (zero
flow) at 3000 psi.
The pressure regulation and flow are
controlled by internal bypasses that automatically
adjust fluid delivery to system demands.
The bypass system is provided to supply
self-lubrication, particularly when the pump is in
nonflow operation. The ring of bypass holes in
the pistons are aligned with the bypass passage
each time a piston reaches the very end of its
forward travel. This pumps a small quantity of
fluid out of the bypass passage back to the supply
reservoir and provides a constant changing of
fluid in the pump. The bypass is designed to pump
against a considerable back pressure for use with
Figure 4-18.Internal features of Stratopower variable-displacement pump.