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
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.