Page 102 - Root Cause Failure Analysis

Pumps 93

sion. For gears having fewer teeth, the space between them is greater and the capacity
increases for a given speed. However, this increases the tendency to have a pulsating
discharge.

In all simple-gear pumps, power is applied to one of the gear shafts, which transmits
power to the driven gear through their meshing teeth. No valves are in the gear pump
to cause friction losses as in the reciprocating pump. The high impeller velocities
required in centrifugal pumps, which result in friction losses, are not needed in gear
pumps. This makes gear pumps well suited for viscous fluids, such as fuel and lubri-
cating oils.
Helical The helical-gear pump is a modification of the spur-gear pump and has cer-
tain advantages. With a spur gear, the entire length of the tooth engages at the same
time. With a helical gear, the point of engagement moves along the length of the tooth
as the gear rotates. This results in a steadier discharge pressure and less pulsation than
in a spur-gear pump.

Herringbone The herringbone-gear pump is another modification of the simple-
gear pump. The principal difference in operation from the simple-gear pump is that
the pointed center section of the space between two teeth begins discharging fluid
before the divergent outer ends of the preceding space complete discharging. This
overlapping tends to provide a steadier discharge pressure. The power transmission
from the driving gear to the driven gear also is smoother and quieter.

Screw
There are many design variations for screw-type, positive-displacement rotary pumps.
The primary variations are the number of intermeshing screws, the screw pitch, and
fluid-flow direction.

The most common type of screw pump consists of two screws mounted on two paral-
lel shafts that mesh with close clearances. One screw has a right-handed thread, while
the other has a left-handed. One shaft drives the other through a set of timing gears,
which synchronize the screws and maintain clearance between them.

The screws rotate in closely fitting duplex cylinders that have overlapping bores.
While all clearances are small, no contact occurs between the two screws or between
the screws and the cylinder walls. The complete assembly and the usual flow path for
such a pump are shown in Figure 7-10.

In this type of pump, liquid is trapped at the outer end of each pair of screws. As the
first space between the screw threads rotates away from the opposite screw, a spiral-
shaped quantity of liquid is enclosed when the end of the screw again meshes with the
opposite screw. As the screw continues to rotate, the entrapped spiral of liquid slides
along the cylinder toward the center discharge space while the next slug is entrapped.
Each screw functions similarly, and each pair of screws discharges an equal quantity