in a constant, uniform flow through the action
of just three moving parts-a power rotor and two
idler rotors. The power rotor is the only driven
element, extending outside the pump casing for
power connections to an electrical motor. The
idler rotors are turned by the power rotor through
the action of the meshing threads. The fluid
pumped between the meshing helical threads of
the idler and power rotors provides a protective
film to prevent metal-to-metal contact. The idler
rotors perform no work; therefore, they do not
need to be connected by gears to transmit power.
The enclosures formed by the meshing of the
rotors inside the close clearance housing contain
the fluid being pumped. As the rotors turn, these
enclosures move axially, providing a continuous
flow. Effective performance is based on the
following factors:
1. The rolling action obtained with the thread
design of the rotors is responsible for the very
quiet pump operation. The symmetrical pressure
loading around the power rotor eliminates the
need for radial bearings because there are no
radial loads. The cartridge-type ball bearing in the
pump positions the power rotor for proper seal
operation. The axial loads on the rotors created
by discharge pressure are hydraulically balanced.
2. The key to screw pump performance is the
operation of the idler rotors in their housing
bores. The idler rotors generate a hydrodynamic
film to support themselves in their bores like
journal bearings. Since this film is self-generated,
it depends on three operating characteristics of
the pump—speed, discharge pressure, and fluid
viscosity. The strength of the film is increased by
increasing the operating speed, by decreasing
pressure, or by increasing the fluid viscosity. This
is why screw pump performance capabilities are
based on pump speed, discharge pressure, and
fluid viscosity.
The supply line is connected at the center of
the pump housing in some pumps (fig. 4-8, view
B). Fluid enters into the pump’s suction port,
which opens into chambers at the ends of the
screw assembly. As the screws turn, the fluid flows
between the threads at each end of the assembly.
The threads carry the fluid along within the
housing toward the center of the pump to the
discharge port.
VANE PUMP
Vane-type hydraulic pumps generally have
circularly or elliptically shaped interior and flat
end plates. (Figure 4-9 illustrates a vane pump
with a circular interior.) A slotted rotor is fixed
to a shaft that enters the housing cavity through
one of the end plates. A number of small
rectangular plates or vanes are set into the slots
of the rotor. As the rotor turns, centrifugal force
causes the outer edge of each vane to slide along
the surface of the housing cavity as the vanes slide
in and out of the rotor slots. The numerous
cavities, formed by the vanes, the end plates, the
housing, and the rotor, enlarge and shrink as the
rotor and vane assembly rotates. An inlet port is
installed in the housing so fluid may flow into the
cavities as they enlarge. An outlet port is provided
to allow the fluid to flow out of the cavities as
they become small.
The pump shown in figure 4-9 is referred to
as an unbalanced pump because all of the
pumping action takes place on one side of the
rotor. This causes a side load on the rotor. Some
vane pumps are constructed with an elliptically
shaped housing that forms two separate pumping
areas on opposite sides of the rotor. This cancels
out the side loads; such pumps are referred to as
balanced vane.
Usually vane pumps are fixed displacement
and pump only in one direction. There are,
however, some designs of vane pumps that
provide variable flow. Vane pumps are generally
restricted to service where pressure demand does
not exceed 2000 psi. Wear rates, vibration, and
noise levels increase rapidly in vane pumps as
pressure demands exceed 2000 psi.
RECIPROCATING PUMPS
The term reciprocating is defined as back-and-
forth motion. In the reciprocating pump it is this
Figure 4-9.—Vane pump.
4-8
