As stated previously, the output of the constant
volume pump is determined by pump rpm and the fixed
angle between the drive shaft and the rotating cylinder
block. If the angle was not fixed and could be varied, the
piston stroke would be changed, thus varying the pump
output. Changing the pump piston stroke is the method
used on most variable volume pumps found in support
The stroke reduction pumps (figs. 10-9 and 10- 10)
are fully automatic variable volume pumps. The
pressure compensating valves shown in both figures use
system pressure to control and vary the piston stroke of
the pump, thus changing the output.
NOTE: The piston stroke of the pump (fig. 10-10)
is determined by the angle of the cam plate. The drive
shaft passes through, but does not touch, the inclined
cam plate to rotate the cylinder block and pistons. The
hanger assembly in figure 10-10 provides this same
function as the cam plate in figure 10-9.
The pumps may also be configured to allow manual
volume control of the pump. Manual volume is
controlled by a handwheel to vary the piston stroke or
may use manual pressure compensating valves such as
those used on many hydraulic test stands.
HAND PUMPS. The hand pump normally serves
as a substitute for the main power pump on most
hydraulic systems; however, the hand pump is widely
used as the only power source in some equipment.
Examples are hydraulic jacks, hydraulically actuated
workstands, and similar equipment.
The two designs of hand pumps you will be using
are single action and double action (fig. 10- 11). The
double-action hand pump creates the flow of fluid with
each stroke of the pump handle; two strokes are required
for the single-action pump. There are several versions
of single- and double-action hand pumps but all operate
on the reciprocating piston principle. The unit shown in
figure 10-11, view A, consists of a cylinder, a piston, an
operating handle, and two check valves-check valve A
and check valve B. The inlet port is connected to the
reservoir, and the outlet port is connected to the pressure
system. As the piston is moved to the right by the pump
handle, fluid from the reservoir flows through check
valve A into the pump cylinder. As the piston is moved
to the left, check valve A closes and check valve B opens.
The fluid in the pump cylinder is forced out of the outlet
port into the pressure line. Thus, with each two strokes
of the hand, a single pressure stroke is produced.
check valve A, a large piston rod, an operating handle,
and check valve B at the inlet port.
When you move the piston to the right, check valve
A closes and check valve B opens. Fluid from the
reservoir then flows into the cylinder through the inlet
port. When you move the piston to the left, check valve
B closes. The pressure created in the fluid then opens
check valve A, admitting fluid behind the piston. (Note
that the large piston rod takes up much of the space
behind by the piston rod.) Because of the space
occupied by the piston rod, there is room for only part
of the fluid; thus, the remainder of fluid is forced through
the outlet port into the pressure line. This is one pressure
stroke. Again if you move the piston to the right, check
valve A closes. The fluid behind the piston is forced
through the outlet port. At the same time fluid from the
reservoir flows into the cylinder through check valve B.
his pump has a pressure stroke for each stroke of the
The double-action hand pump (fig. 10-11, view B)
consists of a cylinder, a piston containing a built-in
Figure 10-11-Typical hand pumps.