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.
Figure 12-14.--Gear-type power pump.
This condition remains in effect until further demand is
placed on the system.
All gear-type pumps are constant displacement
pumps. These pumps are usually driven by a dc wound
A variable displacement pump has a fluid output
electric motor. For those aircraft using batteries, the
that varies to meet the demand of the system. For
pump may be used to build up hydraulic pressure for
example, a pump might be designed to maintain
the brake system during towing operation.
system pressure at 3,000 psi by varying its fluid output
from 0 to 7 gallons per minute. When this type of pump
Maintenance of a pump at the organizational level
is used, no external pressure regulator or unloading
consists of replacement of the complete assembly. The
valve is needed. This function is incorporated in the
motor and pump may be ordered separately; however,
pump and controls the pumping action by maintaining
this is normally done by intermediate- and depot-level
a variable volume, at near constant pressure, to meet
the hydraulic system demands.
Removal and installation procedures are found in
G E A R - T Y P E P U M P . -- A g e a r- t y p e p u m p
t h e a p p l i c a b l e M I M . T h e f o l l ow i n g r e m ova l
consists of two meshed gears that revolve in a housing
procedures are typical examples.
(fig. 12-14). The drive gear in the installation is turned
1. Relieve reservoir pressure.
by a drive shaft that engages an electric motor. The
2. Pull the pump circuit breaker and place a
clearance between the gear teeth as they mesh and
warning card, DO NOT OPERATE, on the pump
between the teeth and pump housing is very small.
The inlet port is connected to the reservoir line, and
3. D i s c o n n e c t t h e p u m p m o t o r e l e c t r i c a l
the outlet port is connected to the pressure line. In the
connection at the motor.
i l l u s t r a t i o n , t h e d r ive g e a r i s t u r n i n g i n a
counterclockwise direction, and the driven (idle) gear
4. Drain the pump reservoir or cap the reservoir
is turning in a clockwise direction. As the teeth pass
the inlet port, fluid is trapped between the teeth and the
5. Disconnect the drain line at the pump.
housing. This fluid is carried around the housing to the
6. Loosen the pressure and suction lines "B" nuts.
outlet port. As the teeth mesh again, the fluid between
the teeth is displaced into the outlet port. This action
7. Remove the mounting screws/bolts that secure
produces a positive flow of fluid under pressure into
the pump assembly to the aircraft structure.
the pressure line. A shear pin or shear section that will
8. Disconnect completely the pressure and
break under excessive loads is incorporated in the drive
reservoir suction lines at the pump.
shaft. This is to protect the engine accessory drive if
pump failure is caused by excessive load or jamming of
9. Cap all open lines, and lift the pump assembly
out of the aircraft.