return (discharge) stroke. Double-action pumps
relief valve also remains closed at this time. As the
provide output flow during each stroke of the piston.
piston moves upward, a low-pressure area (lower than
Both classes of pumps are used in hydraulic systems of
the surrounding atmosphere) is created in the lower
support equipment. For example, single-action pumps
chamber of the cylinder. At the same time, atmospheric
are common sources of power for hydraulic jacks and
pressure is acting on the surface of the fluid in the
some hydraulically operated workstands.
reservoir. This fluid, under the force of atmospheric
Double-action pumps are used in weapon loaders as an
pressure, forces the ball of the gravity check valve off
emergency source of hydraulic power. Double-action
its seat. Fluid then flows from the reservoir into the
pumps are also used in hydraulic component test
low-pressure area of the pump cylinder.
benches.
During the DOWN (power) stroke, the force
Single-Action Hand Pumps.--Two examples of
exerted on the piston forces fluid against the gravity
single-action hand pumps are shown in figure 8-13.
check valve and the spring-loaded check valve. This
These pumps operate very much like piston (1) in
force immediately overcomes the force of atmospheric
figure 8-1. However, some means must be provided to
pressure and forces the ball of the gravity check valve
trap fluid in the system and allow a new supply of fluid
into its seat. Fluid is then trapped in the lower portion
to enter the pump chamber during the return stroke.
of the pump cylinder, causing a resistant force against
This is normally accomplished through the use of
the bottom of the pump piston.
check valves (fig. 8-13). As noted earlier, pumps of this
As the piston continues the DOWN stroke, the
type are used in hydraulic jacks and hydraulically
resistant force increases the pressure of the trapped
operated workstands.
fluid. The pressure increases until it overcomes the
During the UP (suction) stroke of the pump, any
pressure of the fluid in the line to the actuator and the
pressure in the line to the actuator, plus spring tension,
tension of the spring in the check valve. This forces the
holds the ball of the spring-loaded check valve on its
ball of the check valve off its seat. Fluid then flows
seat. This traps fluid in the line to the actuator. The
from the pump cylinder into the line to the actuator.
HANDLE
The spring tension of the relief (safety) valve is
adjustable. It is set to relieve at a pressure slightly
ADJUSTS
above that required to overcome the maximum force
SCREW
RELIEF
that may be exerted on the actuator. For example,
(SAFETY)
VALVE
assume that the actuator is the ram cylinder of a 10-ton
PUMP
TO
hydraulic jack, rated at 20,000 pounds. The maximum
RESERVOIR
TO
allowable load is about 10 percent above the rated load
SEAT
ACTUATOR
or, in this case, 22,000 pounds. Thus, the relief valve is
GRAVITY
SPRING-LOADED
set to relieve the pressure required to lift a load in
CHECK
CHECK VALVE
VALVE
excess of 22,000 pounds. (The rated load, maximum
SEAT
allowable load, relief valve setting, etc., are listed in
FROM
RESERVOIR
the applicable maintenance instructions manuals.)
A
Assume that a load in excess of the maximum
allowable load is placed on the actuator. During the
OUTLET
DOWN stroke of the pump piston, the pressure in the
PORT
system will increase to the point that it overcomes the
spring tension of the relief valve. This forces the ball of
CHECK
the relief valve off its seat and allows fluid to flow from
VALVE
B
the pump cylinder back to the reservoir. In this way, the
relief valve prevents excessive pressure buildup, which
INLET
could cause damage to the system. Hence, the term
PORT
safety valve.
Double-Action Hand Pumps.--Double-action
CHECK
VALVE
hand pumps are used in some aircraft hydraulic
PISTON
ASf08013
A
B
s y s t e m s a s a s o u r c e o f h y d r a u l i c p ow e r f o r
Figure 8-13.--Single-action hydraulic hand pump.
emergencies and for testing subsystems during
8-11
