a point where it might blow seals or burst or damage the
View (C) of figure 12-23 shows how the spring
load on the compensator spool is overcome by system
container in which it is installed, etc.
pressure in excess of 2,950 psi, and the displaced spool
In aircraft, relief valves are installed within
meters pressure to the "cutoff" area of the yoke control
hydraulic systems to relieve excessive pressurized
piston. This pressure will act with the "constant hp"
fluid caused from thermal expansion, pressure surges,
force on the piston, and with increasing pressure, the
and the failure of a hydraulic pump's compensator or
piston will move rapidly from the 22-degree
other regulating devices.
displacement angle at 2,950 psi to approximately 0
degrees at 3,000 (plus 150, minus 0) psi.
Main System Relief Valves
When full pressure exists, the hydraulic power
Main system relief valves are designed to operate
output will be the minimum required to replace fluid
within certain specific pressure limits and to relieve
that leaks internally. The gearbox installed between
complete pump output when in the open position.
the motor and pump contains lubricating fluid for
internal lubrication, a dipstick for checking its fluid
Relief valves are set to open and close at pressures
level, fill port to replace fluid, drain port to drain fluid
determined by the system in which they are installed.
during maintenance, and a relief valve to allow excess
In systems designed to operate at 3,000 psi normal
fluid to be relieved overboard. The pump gearbox is
pressure, the relief valve might be set to be completely
drained and reserviced with clean hydraulic fluid as
open at 3,650 psi and reseat at 3,190 psi. These
pressure ranges may vary from one aircraft to another.
When the relief valve is in the open position, it directs
1. Remove the magnetic drain plug and catch the
fluid in a suitable container.
excessive pressurized fluid to the reservoir return line.
2. Inspect the magnetic plug for foreign particles
Figure 12-24 shows a typical main system relief
that may have accumulated during periods of operation.
valve and its component parts breakdown. The relief
Particles that look and feel like "fuzz" are considered
valve consists of a cylindrical housing that contains a
acceptable; however, particles containing metal
poppet valve and piston assembly. Each end of the
"slivers" require pump overhaul.
housing is fabricated to include a wrench-holding
surface and a threaded port for installation of a
3. Remove the filler plug, and flush the gearbox
hydraulic fitting, and the housing is stamped to identify
with hydraulic fluid.
the ports as PRESS (pressure) and RET (return).
4. Clean the magnetic plug, install with a new
A coil spring at one end of the piston retains it
gasket, and lockwire after replacing.
against a stop on the valve housing; and the poppet
5. Refill gearbox with hydraulic fluid.
valve, which is located just inside the pressure port, is
spring seated over a passage through the valve. When
6. Install filler plug and dipstick, using new
fluid pressure at the pressure port reaches 3,650 psi, the
pressure forces the piston to depress the coil spring and
Also, the pump pressure line, fitting, and filter
move clear of the poppet valve. Thus, the passage
screen are removed. The filter screen is cleaned, using
through the piston is opened, and fluid flows through
Dry-Cleaning Solvent P-D-680, and reinstalled using a
the valve into the return line. When pressure at the
new gasket. The pump pressure line is reinstalled and
pressure port is reduced to 3,190 psi, the coil spring
an operational check performed.
reseats the piston against the poppet valve, and fluid
flow through the relief valve ceases. Should the
NOTE: Hydraulic pumps that are not functioning
pressure at the outlet port exceed the pressure at the
properly can represent a serious source of
inlet port, the poppet valve will unseat, and fluid from
the return line will flow through the valve into the
Relief valves are not new to most people; different
Thermal Relief Valves
types of relief valves are used in our homes and
Thermal relief valves are usually smaller as
automobiles, as well as many other places. Relief
compared to system relief valves. They are used in
va l ve s a r e p r e s s u r e l i m i t i n g o r s a f e t y d ev i c e s
systems where a check valve or selector valve prevents
commonly used to prevent pressure from building up to