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LOBE PUMP
The lobe operation as pump
uses the same principle of the
external gear pump described
Figure 4-7.—Lobe pump.
previously. The lobes are considerably larger than gear
teeth, but there are only two or three lobes on
each rotor. A three-lobe pump is illustrated in
figure 4-7. The two elements are rotated, one directly
driven by the source of power, and the other
through timing gears. As the elements rotate,
liquid is trapped between two lobes of each rotor
and the walls of the pump chamber and carried
around from the suction side to the discharge
side of the pump. As liquid leaves the suction
chamber, the pressure in the suction chamber
is lowered, and additional liquid is forced into
the chamber from the reservoir.
The lobes are constructed so there is a continuous
seal at the points where they meet at the
center of the pump. The lobes of the pump illustrated
in figure 4-7 are fitted with small vanes at
the outer edge to improve the seal of the pump. Although
these vanes are mechanically held in their
slots, they are, to some extent, free to move outward.
Centrifugal force keeps the vanes snug against
the chamber and the other rotating members.
SCREW PUMP
Screw pumps for power transmission systems are
generally used only on submarines. Although low
in efficiency and expensive, the screw pump is
suitable for high pressures (3000 psi), and delivers
fluid with little noise or pressure pulsation.
Screw pumps are available in several different designs;
however, they all operate in a similar manner.
In a fixed-displacement rotary-type screw pump
(fig. 4-8, view A), fluid is propelled axially
Figure 4-8.—Screw pumps.
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.
back-and-forth motion of pistons inside of cylinders
that provides the flow of fluid. Reciprocating pumps,
like rotary pumps, operate on the
positive principle—that is, each stroke delivers
a definite volume of liquid to the system.
The master cylinder of the automobile brake system,
which is described and illustrated in chapter
2, is an example of a simple reciprocating pump.
Several types of power-operated hydraulic pumps,
such as the radial piston and axial piston, are
also classified as reciprocating pumps. These pumps
are sometimes classified as rotary pumps, because
a rotary motion is imparted to the pumps by
the source of power. However, the actual pumping
is performed by sets of pistons reciprocating inside
sets of cylinders.
HAND PUMPS
There are two types of manually operated reciprocating
pumps—the single-action and the
double-action. The single-action pump provides
flow during every other stroke, while the double-action
provides flow during each stroke. Single-action
pumps are frequently used in hydraulic
jacks.
A double-action hand pump is illustrated in figure
4-10. This type of pump is used in some aircraft
hydraulic systems as a source of hydraulic power
for emergencies, for testing certain subsystems
during preventive maintenance inspections,
and for determining the causes of malfunctions
in these subsystems.
This pump (fig. 4-10) consists of a cylinder, a
piston containing a built-in check valve (A), a piston
rod, an operating handle, and a check valve (B)
at the inlet port. When the piston is moved
Figure 4-10.—Hydraulic hand pump.
to the left, the force of the liquid in the outlet chamber
and spring tension cause valve A to close. This
movement causes the piston to force the liquid
in the outlet chamber through the outlet port
and into the system. This same piston movement
causes a low-pressure area in the inlet chamber.
The difference in pressure between the inlet
chamber and the liquid (at atmospheric pressure)
in the reservoir acting on check valve B
causes its spring to compress; thus, opening the check
valve. This allows liquid to enter the inlet chamber.
When the piston completes this stroke to the left,
the inlet chamber is full of liquid. This eliminates
the pressure difference between the inlet chamber
and the reservoir, thereby allowing spring
tension to close check valve B. When
the piston is moved to the right, the force
of the confined liquid in the inlet chamber acts
on check valve A. This action compresses the
spring and opens check valve A which allows
the liquid to flow from the intake chamber
to the outlet chamber. Because of the area
occupied by the piston rod, the outlet chamber
cannot contain all the liquid discharged from
the inlet chamber. Since liquids do not compress,
the extra liquid is forced out of the outlet
port into the system.
PISTON PUMPS
Piston pumps are made in a variety of types
and configurations. A basic distinction is
made between axial and radial pumps. The axial
piston pump has the cylinders parallel to
each other and the drive shaft. The radial piston
design has the cylinders extending radially
outward from the drive shaft like the
spokes of a wheel. A further distinction is
made between pumps that provide a fixed delivery
and those able to vary the flow of the fluid.
Variable delivery pumps can be further divided
into those able to pump fluid from zero to
full delivery in one direction of flow and those able
to pump from zero the full delivery in either direction.
All piston pumps used in Navy shipboard systems
have the cylinders bored in a cylinder block
that is mounted on bearings within a housing.
This cylinder block assembly rotates with the
pump drive shaft.
Radial Piston Pumps
Figure 4-11 illustrates the operation of the radial
piston pump. The pump consists of a pintle, which
remains stationary and acts as a valve; a
Figure 4-11.—Principles of operation of the radial piston pump.
cylinder block, which revolves around the pintle and
contains the cylinders in which the pistons operate;
a rotor, which houses the reaction ring of
hardened steel against which the piston heads press;
and a slide block, which is used to control the
length of the piston strokes. The slide block does
not revolve but houses and supports the rotor,
which does revolve due to the friction set up
by the sliding action between the piston heads and
the reaction ring. The cylinder block is attached
to the drive shaft.
Referring to view A of figure 4-11, assume that space
X in one of the cylinders of the cylinder block
contains liquid and that the respective piston of
this cylinder is at position 1. When the cylinder block
and piston are rotated in a clockwise direction,
the piston is forced into its cylinder as it
approaches position 2. This action reduces the volumetric
size of the cylinder and forces a quantity
of liquid out of the cylinder and into the outlet
port above the pintle. This pumping action is
due to the rotor being off-center in relation to the
center of the cylinder block.
In figure 4-11 view B, the piston has
reached position 2 and has forced
the liquid out of the open end of
the cylinder through the outlet above the
pintle and into the system. While the piston moves
from position 2 to position 3, the open end of
the cylinder passes over the solid part of the pintle;
therefore, there is no intake or discharge of
liquid during this time. As the piston and cylinder
move from position 3 to position 4, centrifugal
force causes the piston to move outward
against the reaction ring of the rotor. During
this time the open end of the cylinder is open
to the intake side of the pintle and, therefore, fills
with liquid. As the piston moves from position
4 to position 1, the open end of the cylinder
is against the solid side of the pintle and no
intake or discharge of liquid takes place. After the
piston has passed the pintle and starts toward position
2, another discharge of liquid takes place. Alternate
intake and discharge continues as the rotor
revolves about its axis-intake on one side of
the pintle and discharge on the other, as the piston
slides in and out.
Notice in views A and B of figure 4-11 that the
center point of the rotor is different from the center
point of the cylinder block. The difference of
these centers produces the pumping action. If the
rotor is moved so that its center point is the same
as that of the cylinder block, as shown in figure
4-11, view C, there is no pumping action, since
the piston does not move back and forth in the
cylinder as it rotates with the cylinder block.
The flow in this pump can be reversed by moving
the slide block, and therefore the rotor, to
the right so the relation of the centers of the rotor
and the cylinder block is reversed from the position
shown in views A and B of figure 4-11. View
D shows this arrangement. Liquid enters the cylinder
as the piston travels from position 1 to position
2 and is discharged from the cylinder as the
piston travels from position 3 to 4. In
the illustrations the rotor is shown in the center,
the extreme right, or the extreme left in relation
to the cylinder block. The amount of adjustment
in distance between the two centers determines
the length of the piston stroke, which controls
the amount of liquid flow in and out of the
cylinder. Thus, this adjustment determines the displacement
of the pump; that is, the volume of liquid
the pump delivers per revolution. This adjustment
may be controlled in different ways. Manual
control by a handwheel is the simplest. The
pump illustrated in figure 4-11 is controlled in
this way. For automatic control of delivery to
accommodate varying volume requirements during
the operating cycle, a hydraulically controlled
cylinder may be used to position the slide
block. A gear-motor controlled by a push button
or a limit switch is sometimes used for this purpose.
Figure 4-11 is shown with four pistons for the sake
of simplicity. Radial pumps are actually designed
with an odd number of pistons (fig. 4-12).
This is to ensure that no more than one cylinder
is completely blocked by the pintle at any one
time. If there were an even number of pistons spaced
evenly around the cylinder block (for example,
eight), there would be occasions when two
of the cylinders would be blocked by the pintle,
while at other times none would be blocked.
This would cause three cylinders to discharge at
one time and four at one time, causing pulsations
in flow. With an odd number of pistons spaced
evenly around the cylinder block, only one cylinder
is completely blocked by the pintle at any one
time. This reduces pulsations of flow.
Figure 4-12.—Nine-piston radial piston pump.
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