passage. Finned baffle-like passages direct this flow
around the motor through the hollow-walled motor
case, after which it is directed by an external line into
the case of the piston pump. This constant flow
through the low-pressure chamber of the main pump
cools and lubricates all of its moving parts. It also
picks up “blow-by” oil that escapes past the
high-pressure pump pistons, and is discharged
through a coarse-screen filter cartridge installed in the
case drain port. The pump’s coolant flow is routed
through the aircraft’s heat exchanger and back to the
reservoir.
The second delivery point from the integral
centrifugal pump is directed from the centrifugal
pump scroll at positive pressure to the intake port of
the high-pressure pump.
As you can see in figure
7-22, the Vickers motor-driven variable displacement
design is similar to other engine-driven designs. The
rotating assembly consists of a baseplate, to which
nine piston rods are joined. The assembly turns in a
fixed plane. Also turning with it is a cylindrical
nine-piston block fitted inside a nonrotating yoke.
The yoke is pivot-mounted to the pump case, and has
an offset attachment for a compensator piston rod that
controls the yoke’s attitude. If the yoke is not
deflected, the cylinder block containing the pistons
will rotate in a plane parallel to the baseplate, thus
producing no stroke.
The yoke can be tilted to
displace the pistons, reaching maximum stroke when
the yoke is tilted 30 degrees from the plane of rotation
of the baseplate.
The pump compensating mechanism receives a
feedback signal of system pressure, and adjusts the
pump output by tilting the yoke a prescribed amount
to provide more or less flow. Whereas engine-driven
pumps are generally rated to produce a given pressure
and flow at a nominal drive speed, the electric
motor-driven pump has a fixed rotational speed and a
special compensating mechanism that enables the
pump to provide 6 gpm (gallons per minute) at 2,950
to 3,000 psi. It will provide more flow as system
pressure drops, reaching a maximum flow of 8 gpm at
2,200 psi. The accelerated flow enables the system to
maintain normal speed of many actuators in use
simultaneously.
Figure 7-22.—Motor-driven variable displacement piston pump schematic.
7-21
