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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 small.
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 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 4-2.—Off-centered internal gear pump.
Figure 4-1.—Gear-type rotary pump.
or reduction gears if the pump fails because of is pumped in the same manner
as in the spur gear excessive load
or jamming of parts. pump. However, in the herringbone pump, each set
of teeth begins its fluid discharge phase before the
previous set of teeth has completed its
Herringbone Gear Pump discharge
phase. This overlapping and the relatively
larger space at the center of the gears The
herringbone gear pump (fig. 4-3) is a tend to minimize pulsations and give a
steadier modification of the spur
gear pump. The liquid flow than the spur gear pump.
Figure 4-3.—Herringbone gear pump.
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.
Centered Internal Gear Pump
Another design of internal gear pump is illustrated
in figures 4-5 and 4-6. This pump consists
of a pair of gear-shaped elements, one within
the other, located in the pump chamber. The
inner gear is connected to the drive shaft of the
power source.
The operation of this type of internal gear pump
is illustrated in figure 4-6. To simplify the explanation,
the teeth of the inner gear and the spaces
between the teeth of the outer gear are numbered.
Note that the inner gear has one less tooth
than the outer gear. The tooth form of each gear
is related to that of the other in such a way that
each tooth of the inner gear is always in sliding
contact with the surface of the outer gear. Each
tooth of the inner gear meshes with the outer gear
at just one point during each revolution. In the
illustration, this point is at the X. In view A, tooth
1 of the inner gear is meshed with space 1 of
the outer gear. As the gears continue to rotate in
a clockwise direction and the teeth approach point
X, tooth 6 of the inner gear will mesh with space
7 of the outer gear, tooth 5 with space 6, and
so on. During this revolution, tooth 1 will mesh
with space 2; and during the following revolution,
tooth 1 will mesh with space 3. As a result,
the outer gear will rotate at just six-sevenths the
speed of the inner gear. At one
side of the point of mesh, pockets of increasing
size are formed as the gears rotate, while
on the other side the pockets decrease in size. In
figure 4-6, the pockets on the right-hand side of
the drawings are increasing in size toward the bottom
of the illustration, while those on the left-hand
side are decreasing in size toward the top
of the illustration. The intake side of the
pump would therefore be on the right and the discharge
side on the left. In figure 4-5, since the right-hand
side of the drawing was turned over to
show the ports, the intake and discharge appear reversed. Actually, A in one drawing
covers A in the other.
Figure 4-5.—Centered internal gear pump.
Figure 4-6.—Principles of operation of the internal gear pump.
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