pump revolutions per minute (rpm). In a system requiring constant pressure, this type of pump must be
used with a pressure regulator. The two types of constant-delivery piston pumps used in Army aircraft
are the angular and cam.
Angular Piston Pump Construction. The basic components of an angular piston pump are shown in
Figure 1-6. They are--
(1) A rotating group consisting of a coupling shaft, universal link, connecting rods, pistons,
and cylinder block.
(2) A stationary group consisting of the valve plate and the pump case or housing.
The cylinder bores lie parallel to, and are evenly spaced around, the pump axis. For this reason, a
piston pump is often referred to as an axial piston pump.
Packings on seals are not required to control piston-to-bore leakage. This is controlled entirely by
close machining and accurate fit between piston and bore. The clearance is only enough to allow for
lubrication by the hydraulic fluid and slight expansion when the parts become heated. Pistons are
individually fitted to their bores during manufacture and must not be changed from pump to pump or
bore to bore.
Pump Operation. As the coupling shaft is turned by the pump power source, the pistons and cylinder
block turn along with it because they are interconnected. The angle that exists between the cylinder
block and coupling shaft causes the pistons to move back and forth in their respective cylinder bores as
the coupling is turned:
During the first half of a revolution of the pump, a cylinder is aligned with the inlet port in
the valve plate. At this time the piston is moving away from the valve plate and drawing
hydraulic fluid into the cylinder. During the second half of the revolution, the cylinder is
lining up with the outlet port in the valve plate. At this time, the piston is moving toward the
valve plate, thus causing fluid previously drawn into the cylinder to be forced out through the
Fluid is constantly being drawn into and expelled out of the pump as it turns. This provides a
multiple overlap of the individual spurts of fluid forced from the cylinders and results in
delivery of a smooth, nonpulsating flow of fluid from the pump.