the required state of cleanliness of all the units and their
fluids. Regular operation of hydraulic equipment
prevents the accumulation of sludge and the freezing of
adjacent parts. Regular use also aids in preventing
corrosion. The necessity for proper lubrication and
cleanliness cannot be too strongly emphasized
Detailed instructions on the maintenance of a
specific unit should be obtained from the appropriate
manufacturers technical manual, but the following
general information will also be useful.
The Fluid System
If an inspection of an oil sample drawn from a
hydraulic system reveals evidence of water, sludge, or
acidity, the system must be drained, then cleaned with
prescribed acid-free cleaning fluid (flushing oil), and
filled with clean hydraulic oil. A hydraulic system may
be drained and cleaned as follows:
1. Remove the permanent filters and wash them in
flushing oil. Then use low-pressure air for drying
purposes. If the filters have replaceable elements, install
2. Drain the system of old hydraulic oils as
completely as possible.
3. Close all connections, and fill the system with
acid-free cleaning fluid.
4. Start andoperate the unit under idling conditions
to fill the system thoroughly with cleaning fluid.
5. Secure the unit and allow it to stand idle for the
prescribed period (usually about an hour). This period
of idleness permits the cleaning fluid to dissolve any
6. Start and operate the unit with a light load for 3
to 5 minutes, unless otherwise specified. Allow the
equipment to stand idle for about 15 minutes, then repeat
the whole cleaning process. Do this two or three times.
Never operate a hydraulic unit with a full load when
it is filled with cleaning fluid. Keep the operating
pressure as low as possible.
7. If time permits, allow the system to stand idle
for an additional hour following the series of short
8. Drain the system of cleaning fluid. Reclean
permanent filters or, if necessary, install new
replaceable filters. Close the system, and fill it with the
proper hydraulic oil.
As the system is filled, strain the hydraulic oil
through a fine wire screen of 180 or 200 mesh. If the oil
is not clean, run it through a centrifuge. You should
provide adequate protection against dust and moisture
entering the system. Moisture should be expelled from
the oil before it is poured into a system. Oil with
noticeable water content should be rejected or
When a hydraulic system is being filled, sufficient
hydraulic fluid should be used to completely fill the
active parts of the mechanism, leaving no air pockets.
Air valves should he opened during the filling process,
so that air can escape to the oil expansion box. Be sure
the valves are closed tightly after the system has been
filled. For more information on hydraulic fluid filtration,
read NSTM, Chapter 556, Hydraulic Equipment
(Power Transmission and Control). For additional
information on hydraulic fluids, refer to NSTM, Chapter
262, Lubricating Oils, Greases, and Hydraulic Fluids
and Lubricating Systems.
Pumps and Motors
Whether the pumps and motors of hydraulic
transmission are of the axial or radial piston type, the
maintenance procedures, as well as the operating
principles, are relatively the same. In general,
maintenance information on other types of pumps also
applies to hydraulic pumps and motors. For more
information concerning hydraulic pumps and motors,
read section 2 of NSTM, Chapter 556, Hydraulic
Equipment (Power Transmission and Control).
Neoprene is the most commonly used seal around
the shafts of most modern hydraulic pumps and motors,
but other types of shaft packing are also used.
On some modern hydraulic transmissions, shaft
stuffing box packing is of the square-braided pure
asbestos type. This packing is easily removed, but you
must take care to be sure that it is not replaced too tightly.
If properly installed, this packing makes a tight joint
when you apply light pressure. If packing wears quickly,
the shaft should be inspected for roughness. If a lathe is
available, you may be able to eliminate the roughness
from the shaft by a finishing cut to smooth the surface.
If a lathe is not available, it may be necessary to replace
the shaft. Packing should be renewed at prescribed
intervals to eliminate the possibility of the packing
becoming hard and scoring the shaft. When packing is
being replaced, make certain there is a uniform
thickness around the shaft. An excess of packing on one
side of the shaft will cause breakage. Stuffing boxes