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Axial Piston Pumps
In axial piston pumps of the in-line type, where
the cylinders and the drive shaft are parallel (fig.
4-13), the reciprocating motion is created by a
cam plate, also known as a wobble plate, tilting plate,
or swash plate. This plate lies in a plane that
cuts across the center line of the drive shaft and
cylinder barrel and does not rotate. In a fixed-displacement
pump, the cam plate will be rigidly
mounted in a position so that it intersects the
center line of the cylinder barrel at an angle approximately
25 degrees from perpendicular. Variable-delivery
axial piston pumps are designed so
that the angle that the cam plate makes with a
perpendicular to the center line of the cylinder barrel
may be varied from zero to 20 or 25 degrees to
one or both sides. One end of each piston rod is
held in contact with the cam plate as the cylinder block
and piston assembly rotates with the drive shaft.
This causes the pistons to reciprocate within the
cyIinders. The length of the piston stroke is proportional
to the angle that the cam plate is set from
perpendicular to the center line of the cylinder
barrel.
A variation of axial piston pump is the bent-axis
type shown in figure 4-14. This type does not
have a tilting cam plate as the in-line pump does.
Instead, the cylinder block axis is varied from
the drive shaft axis. The ends of the
Figure 4-14.—Bent-axis axial piston pump.
connecting rods are retained in sockets on a disc that
turns with the drive shaft. The cylinder block is
turned with the drive shaft by a universal joint assembly
at the intersection of the drive shaft and the
cylinder block shaft. In order to vary the pump displacement,
the cylinder block and valve plate are
mounted in a yoke and the entire assembly is
swung in an are around a pair of mounting pintles
attached to the pump housing. The
pumping action of the axial piston pump is
made possible by a universal joint or link.
Figure 4-13.—In-line axial piston pump.
Figure 4-15 is a series of drawings that illustrates how
the universal joint is used in the operation of
this pump.
First, a rocker arm is installed on a horizontal shaft.
(See fig. 4-15, view A.) The arm is joined to
the shaft by a pin so that it can be swung back and
forth, as indicated in view B. Next, a ring is placed
around the shaft and secured to the rocker arm
so the ring can turn from left to right as shown
in view C. This provides two rotary motions
in different planes at the same time and in
varying proportions as may be desired. The rocker
arm can swing back and forth in one arc, and
the ring can simultaneously move from left
Figure 4-15.–Relationship of the universal joint in operation of
the axial piston pump.
to right in another arc, in a plane at right angles to
the plane in which the rocker arm turns. Next,
a tilting plate is added to the assembly. The
tilting plate is placed at a slant to the axis of
the shaft, as depicted in figure 4-15, view D. The
rocker arm is then slanted at the same angle as
the tilting plate, so that it lies parallel to the tilting
plate. The ring is also parallel to, and in contact
with, the tilting plate. The position of the ring
in relation to the rocker arm is unchanged from
that shown in figure 4-15, view C. Figure
4-15, view E, shows the assembly after the
shaft, still in a horizontal position, has been rotated
a quarter turn. The rocker arm is still in the
same position as the tilting plate and is now perpendicular
to the axis of the shaft. The ring has
turned on the rocker pins, so that it has changed
its position in relation to the rocker arm, but
it remains parallel to, and in contact with, the tilting
plate.
View F of figure 4-15 shows the assembly after the
shaft has been rotated another quarter turn. The
parts are now in the same position as shown in
view D, but with the ends of the rocker arm reversed.
The ring still bears against the tilting plate.
As the shaft continues to rotate, the rocker arm
and the ring turn about their pivots, with each changing
its relation to the other and with the ring always
bearing on the plate.
Figure 4-15, view G, shows a wheel added to the
assembly. The wheel is placed upright and fixed
to the shaft, so that it rotates with the shaft. In
addition, two rods, A and B, are loosely connected
to the tilting ring and extend through two
holes standing opposite each other in the fixed wheel.
As the shaft is rotated, the fixed wheel turns
perpendicular to the shaft at all times. The tilting
ring rotates with the shaft and always remains
tilted, since it remains in contact with the tilting
plate. Referring to view G, the distance along
rod A, from the tilting ring to the fixed wheel,
is greater than the distance along rod B. As
the assembly is rotated, however, the distance along
rod A decreases as its point of attachment to
the tilting ring moves closer to the fixed wheel, while
the distance along rod B increases. These changes
continue until after a half revolution, at which
time the initial positions of the rods have been
reversed. After another half revolution, the two
rods will again be in their original positions. As
the assembly rotates, the rods move in and out
through the holes in the fixed wheel. This is the
way the axial piston pump works. To get a pumping
action, place pistons at the ends of the rods, beyond the fixed wheel, and insert them into cylinders.
The rods must be connected to the pistons
and to the wheel by ball and socket joints. As
the assembly rotates, each piston moves back and
forth in its cylinder. Suction and discharge lines
can be arranged so that liquid enters the cylinders
while the spaces between the piston heads
and the bases of the cylinders are increasing, and
leaves the cylinders during the other half of each
revolution when the pistons are moving in the
opposite direction.
The main parts of the pump are the drive shaft,
pistons, cylinder block, and valve and swash plates.
There are two ports in the valve plate. These
ports connect directly to openings in the face
of the cylinder block. Fluid is drawn into one port
and forced out the other port by the reciprocating
action of the pistons.
IN-LINE VARIABLE-DISPLACEMENT
AXIAL
PISTON PUMP.— When the drive
shaft is rotated, it rotates the
pistons and the cylinder
block with
it. The swash plate placed at an angle causes
the pistons to move back and forth in the cylinder
block while the shaft, piston, cylinder block,
and swash plate rotate together. (The shaft, piston,
cylinder block, and swash plate together is
sometimes referred to as the rotating group or assembly.)
As the pistons reciprocate in the cylinder
block, fluid enters one port and is forced out
the other.
Figure 4-13 shows piston A at the bottom of its
stroke. When piston A has rotated to the position
held by piston B, it will have moved upward
in its cylinder, forcing fluid through the outlet
port during the entire distance. During the remainder
of the rotation back to it original position,
the piston travels downward in the cylinder.
This action creates a low-pressure area in
the cylinder. The difference in pressure between the
cylinder inlet and the reservoir causes fluid to
flow into the inlet port to the cylinder. Since each
one of the pistons performs the same operation
in succession, fluid is constantly being taken
into the cylinder bores through the inlet port and
discharged from the cylinder bores into the
system. This action provides a steady, nonpulsating
flow of fluid.
The tilt or angle of the swash plate determines the
distance the pistons move back and forth in their
cylinders; thereby, controlling the pump output.
When the swash plate is at a right angle to the shaft,
and the pump is rotating, the pistons do not
reciprocate; therefore, no pumping action takes
place. When the swash plate is tilted away from
a right angle, the pistons reciprocate and fluid
is pumped.
Since the displacement of this type of pump is
varied by changing the angle of the tilting box, some
means must be used to control the changes of
this angle. Various methods are used to control this
movement—manual, electric, pneumatic, or hydraulic.
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