should be packed loosely and the packing gland set up
lightly to allow adequate leakage for cooling and
lubrication. See NSTM, Chapter 078, “Gaskets,
Packings, and Seals,” for more detailed discussion of
O-rings and other types of seals for hydraulic system
equipment.
There is very little likelihood of poor alignment
between the driving and driven members of a hydraulic
transmission if the wedges, shims, jacking screws, or
adjusting setscrews were properly set and secured when
the connecting units were installed. However, when a
casualty occurs or a unit is replaced, it is possible for the
unit to become misaligned enough to cause severe stress
and strain on the coupling and connected parts.
Excessive misalignment should be eliminated as soon
as possible by replacing any defective parts and by
readjusting the aligning devices. If this is not done, pins,
bushings, and bearings will wear out too fast and will
have to be replaced frequently.
Since there is no end play to either the pump shaft
or the motor shaft, flexible couplings are generally used
in hydraulic transmissions. Such couplings permit
satisfactory operation with a slight misalignment,
without requiring frequent renewal of parts.
Pipings and Fittings
If properly installed, the piping and valves of a
hydraulic system are seldom a source of a trouble,
except for leakage. Some leaks, however, can be serious
enough to cause a reduction in the efficiency of the unit.
You should make frequent inspections for leakage and
take steps to eliminate any leakage found. Guidance and
requirements for the installation, inspection, and
maintenance of piping and associated fittings are
contained in NSTM, Chapter 505, “Piping Systems,”
If leaks occur at a flanged joint in the line of a
hydraulic system, tighten the flange bolts evenly, but not
excessively. If the leaks persist, use the auxiliary gear
while the leaking flange is being refitted with copper
asbestos or O-ring packing. Be sure the flange surfaces
are cleaned carefully before the packing is applied.
CAUTION
Exposure to asbestos fibers is a recognized
health hazard. Refer to NSTM, Chapter 635,
“Thermal, Fire, and Acoustic Insulation,” for
safety requirements applicable to handling
asbestos packing and gaskets.
If certain measures are taken, operation of hydraulic
equipment may be continued while leakage repairs arc
being made in some parts of the system. When the lines
in an auxiliary system leak, they should be valved off
from the main line connection to prevent leakage
between the two systems. If leaks occur in the pumping
connections to the three-way valves of a steering gear
installation, the pump can be cut out with the valve, and
another pump cut in. If the three-way valves fail to cut
out the leaking unit, and it becomes necessary to cut out
both pumps of a steering gear installation, the valves
may be closed at the ram cylinder. Hydraulic systems
will work without pressure control. So by closing the
valves in the lines where they join the main piping,
leaking pressure control pipes or cylinders can be cut
out of the system for repairs.
Expansion lines and replenishment lines in
hydraulic systems of older ships are seldom a source of
leakage or breakage, since they are not under any
appreciable pressure. However, all hydraulic line
connections must be maintained intact. in more recent
installations, however, replenishing lines are under
pressure as much as 300 psi. In these modern
installations, the hydraulic systems should not be
operated during the repair of these lines.
Relief valves and shuttle valves of a hydraulic
system may also be a source of trouble. The seats of
relief valves that are leaking should be reground. Loss
of power is a symptom of a leaking relief valve. Shuttle
valves may stick and fail to cut off. This condition is
evidenced either by the escape of oil from the
high-pressure side of the line into the expansion tank or
by the failure of the pressure control. When a shuttle
valve fails to operate, the stop valves should be closed
and the defective valve removed and repaired.
Incorrectly adjusted needle valves can be another
source of trouble. Needle valves that are adjusted too
fine may cause the device operated by the valve to stop
short of its intended stopping point. This may happen
because the valve adjustment allows more fluid to pass
through leakage points in the system than through the
valve. NSTM, chapter 556, provides a good source of
general information concerning different types of valves
used in hydraulic systems and their maintenance.
HYDRAULIC CARGO HATCH COVERS
Cargo hatch cover opening and closing operations
are supplied by an electrohydraulic power unit. The
system consists of an electric motor-driven hydraulic
pump mounted on a hydraulic fluid reservoir tank and
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