centers (fig, 4-1). External pumps may use spur
gears, herringbone gears, or helical gears to move
the fluid. In an internal gear pump, the teeth of
one gear project outward, but the teeth of the
other gear project inward toward the center of the
pump (fig. 4-2, view A). Internal gear pumps may
be either centered or off-centered.
Spur Gear Pump
The spur gear pump (fig. 4-1) consists of two
meshed gears which revolve in a housing. The
drive gear in the illustration is turned by a drive
shaft which is attached to the power source. The
clearances between the gear teeth as they mesh and
between the teeth and the pump housing are very
The inlet port is connected to the fluid supply
line, and the outlet port is connected to the
pressure line. In figure 4-1 the drive gear is turning
in a counterclockwise direction, and the driven
(idle) gear is turning in a clockwise direction. As
Figure 4-2.Off-centered internal gear pump.
the teeth pass the inlet port, liquid is trapped
between the teeth and the housing. This liquid is
carried around the housing to the outlet port. As
the teeth mesh again, the liquid between the teeth
is pushed into the outlet port. This action
produces a positive flow of liquid into the system.
A shearpin or shear section is incorporated in the
drive shaft. This is to protect the power source
Figure 4-1.Gear-type rotary pump.