back-and-forth motion of pistons inside of
cylinders that provides the flow of fluid. Recipro-
cating pumps, like rotary pumps, operate on
the positive principlethat 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 recipro-
cating inside sets of cylinders.
HAND PUMPS
There are two types of manually operated
reciprocating pumpsthe 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 reservior 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 reservior, 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.
4-9
