PUMPS
All aircraft hydraulic systems have one or more
power-driven pumps and may have a hand pump as an
additional source of power. Power-driven pumps are
the primary source of energy, and may be either
engine-driven or electric-motor driven. As a general
rule, motor-driven pumps are installed for use in
emergencies; that is, for operation of actuating units
when the engine-driven pump is inoperative. Hand
pumps are generally installed for testing purposes as
well as for use in emergencies.
In this section, the various types of pumps used in
naval aircraft, both hand- and power-driven, are
described and illustrated.
Hand Pumps
Hand pumps are used in hydraulic systems to
supply fluid under pressure to subsystems, such as the
landing gear, flaps, canopy, and bomb-bay doors, and
to charge brake accumulators. Systems using hand
pumps are classified as emergency systems. Most of
these systems may be used effectively during
preventive maintenance.
Double-action type of hand pumps are used in
hydraulic systems. Double action means that a flow
of fluid is created on each stroke of the pump handle
instead of every other stroke, as in the single-action
type. There are several versions of the double-action
hand pump, but all use the reciprocating piston
principle, and operation is similar to the one shown in
figure 7-13.
This pump consists of a cylinder, a piston
containing a built-in check valve (A), a piston rod, an
operating handle, and a check valve (B) at the inlet
port. When the piston is moved to the left in the
illustration, check valve (A) closes and check valve
(B) opens.
Fluid from the reservoir then flows into the
cylinder through inlet port (C). When the piston is
moved to the right, check valve (B) closes. The
pressure created in the fluid then opens check valve
(A), and fluid is admitted behind the piston. Because
of the space occupied by the piston rod, there is room
for only part of the fluid; therefore, the remainder is
forced out port (D) into the pressure line. If the piston
is again moved to the left, check valve (A) again
closes.
The fluid behind the piston is then forced
through outlet port (D). At the same time, fluid from
the reservoir flows into the cylinder through check
Figure 7-13.—Double-action hydraulic hand pump.
7-13
