unmeshed teeth, at the suction side of the pump, is
carried by the teeth towards the sides. Then, the liquid
is trapped between the tooth pockets and the casing, and
carried through to the discharge side of the pump. The
liquid entering the discharge side cannot return to the
suction side because the meshing teeth at the center
force the liquid out of the tooth pockets.
In the vane-type pump (figs. 4-35 and 4-36), a steel
rotor and shaft, one end supported in the pump cover,
revolve in the body, the bore of which is eccentric to the
rotor. Two sliding vanes are placed 180 degrees apart in
slots in the rotor, and are pressed against the body bore
by springs in the slots. When the shaft is rotated, the
vanes pick up fuel at the inlet port and carry it around
Figure 4-34.--Typical gear fuel pump assembly.
the body to the outlet side, where the fuel is discharged.
Pressure is produced by the wedging action of the fuel
as it is forced toward the outlet port by the vane. A
spring-loaded relief valve is provided in the cover of the
pump, connecting the inlet and outlet ports. This valve
opens at a pressure of approximately 55 psi. Its purpose
The simple gear pump (fig. 4-34) has two spur
is to relieve excessive pump pressure, which will build
gears that mesh together; one is the driving gear, the
up if fuel lines or filters become clogged. When the
other the driven gear. Clearances between the gear faces
valve opens, fuel passes from the discharge side
and casing are only a few thousandths of an inch. When
(pressure side) to the suction side of the pump.
the gears turn, liquid in the spaces between the
Figure 4-35.--Cutaway view of vane-type fuel pump.