Kohler K91 Owner's Manual page 286

1.1311.
3.4297.405
An Internal Gear (Gerotor) pump uses
two
gears to provide a constant output of oil per
revolution. The inner gear is offset from the
pump's center and is driven by the input shaft. A
larger internal toothed gear or rotor surrounds
the inner gear, providing a rotating mesh point
to trap oil.
True internal gear pumps use
a
crescent seal
which provides a sealing area between both
gears
.
Gerotor pumps utilize rounded teeth
which do not require
a
crescent for sealing.
A Roller Bearing pump is
a
variation of the
internal gear pump. Instead of using two gears,
rollers are placed between the teeth of the
driven gear. This also provides
a
Constant
displacement of oil.
External Gear pumps use
two
tightly meshed
gears to produce
a
constant displacement of oil
per revolution. The input shaft drives one of the
gears which in turn drives the second gear. The
opposed gear rotation draws oil from the inlet
port and traps it between the gear teeth and the
pump body. As the teeth come together near the
outlet port, the area is reduced and
oil is forced
out
of
the pump.
Productivity Series 10