valve (B). Thus, a pressure stroke is produced with
each stroke of the pump handle.
Hand pumps are examined frequently for leakage,
general condition, and efficiency in operation. To
check the operation of a hand pump, the following
procedure is recommended:
1. Connect a direct-reading hydraulic pressure
gauge into the emergency hand pump pressure line.
2. Insert and lock the hand pump handle in the
pump actuating socket.
3. Select an appropriate subsystem to operate, and
place its selector valve in an operating position.
4. Actuate the hand pump handle until the unit
being operated has completed its movement. Check
the pressure gauge for a drop in system pressure.
NOTE: Air in emergency systems will cause
the pump handle to spring rapidly to the
other end of the stroke.
5. If a pressure drop is indicated, check the system
for leakage before removing the pump for repair or
replacement.
6. Observe the hand pump handle for piston creep,
which indicates that the pump should be removed for
repair or replacement.
Removal, replacement, and operational check of
hand pumps should correspond to the procedures
recommended in the specific MIM.
Power-Driven Pumps
As previously mentioned, power pumps are
generally driven by the aircraft engine, but may also
be electric-motor driven. Power pumps are classified
according to the type of pumping action used, and
may be either the gear type or piston type. Power
pumps may be further classified as constant
displacement or variable displacement.
A constant displacement pump is one that
displaces or delivers a constant fluid output for any
rotational speed. For example, a pump might be
designed to deliver 3 gallons of fluid per minute at a
speed of 2,800 revolutions per minute. As long as it
runs at that speed, it will continue to deliver at that
rate, regardless of the pressure in the system. For
this reason, when the constant displacement pump is
used in a system, a pressure regulator or unloading
valve must also be incorporated. The pressure
regulator valve will maintain a set pressure in the
system by diverting excess pump flow back to the
reservoir. The unloading valve will divert all pump
flow back to the reservoir when the preset system
pressure is reached. This condition remains in effect
until further demand is placed on the system.
A variable displacement pump has a fluid output
that varies to meet the demand of the system. For
example, a pump might be designed to maintain
system pressure at 3,000 psi by varying its fluid
output from 0 to 7 gallons per minute. When this type
of pump is used, no external pressure regulator or
unloading valve is needed. This function is
incorporated in the pump and controls the pumping
action by maintaining a variable volume, at near
constant pressure, to meet the hydraulic system
demands.
GEAR-TYPE PUMP.—A gear-type pump consists
of two meshed gears that revolve in a housing (fig. 7-
14). The drive gear in the installation is turned by a
drive shaft that engages an electric motor. The
clearance between the gear teeth as they mesh and
between the teeth and pump housing is very small.
The inlet port is connected to the reservoir line,
and the outlet port is connected to the pressure line.
In the illustration, the drive gear is turning in a
counterclockwise direction, and the driven (idle) gear
is turning in a clockwise direction. As the teeth pass
the inlet port, fluid is trapped between the teeth and
the housing. This fluid is carried around the housing
to the outlet port. As the teeth mesh again, the fluid
Figure 7-14.—Gear-type power pump.
7-14
