Figure 10-9.-Example of a variable volume, stroke reduction
pump with variable cam plate.
valves. These valves are usually set at about 15 psi; this
ensures circulation of fluid in the pump.
The piston pump discussed is a constant displace-
ment type; that is, for any given rpm, the volume output
is constant. However, there is another version of the
piston pump used more extensively than the constant
volume pump; that is, the variable volume pump.
Variable Volume Piston Pump. There are many
versions of the variable volume pump; several different
control methods are used to vary the fluid flow through
the pump. Some of the pumps vary the volume by
controlling the inlet fluid; some vary it by changing the
angle between the pump drive shaft and the piston
cylinder block; others by using a system bypass within
the pump to vary volume output.
One advantage of the variable volume pump is that
it eliminates the need for a system pressure regulator. A
second advantage is that it provides a more stable
pressure, thus reducing pressure surges and the need for
a system accumulator; however, they are retained for use
during peak load occurrences.
Figure 10-10.-Variable displacement axial-piston pump.