Helical Gear Pump
The helical gear pump (fig. 4-4) is still
another modification of the spur gear pump.
Because of the helical gear design, the
overlapping of successive discharges from
spaces between the teeth is even greater than it
is in the herringbone gear pump; therefore, the
discharge flow is smoother. Since the discharge
flow is smooth in the helical pump, the gears
can be designed with a small number of large
teeth—thus allowing increased capacity without
sacrificing smoothness of flow.
The pumping gears of this type of pump are
driven by a set of timing and driving gears that
help maintain the required close clearances
without actual metallic contact of the pumping
gears. (Metallic contact between the teeth of the
pumping gears would provide a tighter seal
against slippage; however, it would cause rapid
wear of the teeth, because foreign matter in the
liquid would be present on the contact
surfaces.)
Roller bearings at both ends of the gear shafts
maintain proper alignment and minimize the
friction loss in the transmission of power. Suitable
packings are used to prevent leakage around the
shaft.
Off-centered Internal Gear Pump
This pump is illustrated in figure 4-2, view B.
The drive gear is attached directly to the drive shaft
of the pump and is placed off-center in relation to
the internal gear. The two gears mesh on one side
of the pump, between the suction (inlet) and
discharge ports. On the opposite side of the
chamber, a crescent-shaped form fitted to a close
tolerance fills the space between the two gears.
The rotation of the center gear by the drive
shaft causes the outside gear to rotate, since the
two are meshed. Everything in the chamber rotates
except the crescent. This causes liquid to be
trapped in the gear spaces as they pass the
crescent. The liquid is carried from the suction port
to the discharge port where it is forced out of the
pump by the meshing of the gears. The size of the
crescent that separates the internal and external
gears determines the volume delivery of the pump.
A small crescent allows more volume of liquid per
revolution than a larger crescent.
Figure 4-4.—Helical gear pump.
4-5
