TM 11-7440-278-14
Ambient conditions such as heat or foreign materials will
units usually throw off any excess heat through the
contribute to problems. Dust and chemicals can be
surfaces of the plumbing and reservoir. Certain types of
drawn into system vents. If synthetic fluids are used,
circuits, duty cycles, or ambient conditions will definitely
keep familiar with and follow the maintenance
dictate the use of a heat exchanger. Hydraulic power
procedures recommended by the manufacturer of the
units should operate in the range of 140 max., 160
fluid.
absolute max.
b. If the power unit operates too hot, be sure that a
high-grade oil in the viscosity range of 100-300 SSU is
4-11. Replacement of Filters
being used and that the oil level is up in the reservoir.
a. After start-up of a new system, or after any
Check for any partially open bypass valves. Check for
extensive circuit alterations, filter and strainers need very
worn directional valves or leading cylinders. Internal-
special attention.
Chips from pipe threads, metal
system leaks at high pressure are the greatest source of
particles, and other foreign materials are introduced
heat.
during hookup or system alterations. Clean or replace
filter elements after the first hour of operation, after the
4-15. Accumulator Circuits
first shift, and after the first full week.
b. On many circuits, return-line filters or pres-sure
a. Some power units are built with accumulators
filters are provided. If return-line filters are not provided,
mounted on the assembly. Accumulators are used in
reservoir clean-out at the end of the first full shift and at
circuits that have short demands for high volume and
other periods of general maintenance is recommended.
then longer periods of little or no volume. They are also
used when the required volume varies substantially
throughout a sequence of operations.
4-12. Relief Valve Malfunction
b. If the accumulator malfunction seems evident as
The relief valve is the greatest source of pump and
indicated by loss of sequence timing or too frequent
system protection. Likewise, the relief valve is usually
unloading, the loss of gas charge seems likely. Gas
the first item to give warning of other problems. This
(nitrogen) charged accumulators are precharged to a
valve is designed to fail open or "fail-safe". Relief valve
certain pressure while the power unit is shut down. Any
malfunction is most often due to contaminants in the
excess fluid stored in the accumulator during operation is
fluid. This is a good time to clean up the system; flush
available when required within a cycle or for long holding
out, change filters, and open and clean any valve
periods.
The precharge gas pressure should be
passages if necessary. Noisy relief valves often point to
checked periodically and should be kept at or near the
air in the fluid or some degree of solid or gummy con-
pressure specified in the design or found to be most
tamination.
satisfactory.
4-13. Pump Malfunction
4-16. Resetting System Pressure
Pump noise is a definite sign of trouble. Check shaft
If the system is a typical HI-LO, the unloading valve and
alignment and condition of the coupler first. Cavitation is
system pressure may have been set at the factory. If the
another cause of pump noise. Check for a restricted
system needs resetting, use the following procedures:
suction line, undersized pipes if altered, or dirty filter or
a. Turn the adjusting knob of the unloading valve
strainer. Look for a suction line leak; air getting into the
fully counterclockwise.
suction line through the plumbing or low oil supply will
b. Turn the adjusting knob of the relief valve fully
cause pump noise as well as erratic and noisy valves.
counterclockwise.
Finally, a worn pump will become noisy. Usually this
c. Start the system and turn the adjusting knob of
pump will also feel hotter than the surface of the
the relief valve clockwise until the unloading pressure is
reservoir. System pressures will drop and cycle rates will
reached on the high-pressure side.
become slower. If pump is re-placed, be sure to change
d. Turn the adjusting knob of the unloading valve
filters and check out the entire system during downtime.
Also, check cleanliness of oil at this time.
clockwise until the unloading pressure is reached on the
low-pressure side.
e. Adjust the relief valve by turning clockwise until
4-14. Overheating
the desired system pressure is reached.
a. The amount of heat developed by hydraulic
power units depends upon the use, duty cycle, proper
NOTE
adjustment,
age,
etc.
Hydraulic
power
The unloading pressure can be
increased or decreased at any time
without disturbing the main system relief.
4-4
