Abrasive particles contained in the system are
A liquid has a definite volume but no definite
not flushed out. New particles are continually
shape. If you put a liquid into a container, it
created as friction sludge acts as an effective
assumes the shape of that container. Since liquids
catalyst to speed up oxidation of the fresh fluid.
are almost incompressible, they transmit pressure
well. Although the application of large forces will
A catalyst is a substance that, when added to
cause a small decrease in the volume, this decrease
another substance, speeds up or slows down
is negligible. For more detailed information on
chemical reaction. The catalyst itself is not
changed or consumed at the end of the reaction.
the principles of hydraulics, study the training
manual Fluid Power, NAVEDTRA 12964.
Origin of Contaminants
The contaminants in hydraulic systems can be
HYDRAULIC FLUIDS
traced to four major sources.
Petroleum-based liquids are the most widely
1. Particles originally contained in the system.
used fluids in hydraulic systems. Refined
These particles originate during fabrication of
hydraulic fluid is clear in color. Red dyes are
welded-system components, especially reservoirs
added to this fluid so that hydraulic system leaks
and pipe assemblies. Proper design and cleaning
are easier to find and identify. Special petroleum-
reduce the presence of these particles. For
based fluids are used for certain applications. For
example, parts designed using seam-welded,
example, MIL-H-83282B is the hydraulic fluid
overlapping joints reduce contamination. Parts
approved for use in, and in the servicing of, Navy
designed using arc welding of open sections
aircraft hydraulic systems. MIL-H-6083C is the
increase contamination. Parts designed with
approved hydraulic fluid for the preservation,
hidden passages, beyond the reach of sand-
packaging, and use in hydraulic test benches.
blasting, are the main source of core sand
contamination.
CONTAMINATION
2. Particles introduced from outside forces.
Particles enter hydraulic systems at points where
Experience has shown that trouble in a
the liquid or working parts of the system are in
hydraulic system occurs whenever the hydraulic
temporary contact with the atmosphere. Struts
fluid becomes contaminated. The nature of the
and piston rods are constantly exposed to the
trouble--whether a simple malfunction or the
atmosphere. The most common danger areas are
complete destruction of a component--depends
at the refill and breather openings and at cylinder
to some extent on the type of contaminant.
rod packings. Contamination results from care-
Two general classes of contaminants are
lessness during servicing and cleaning. Particles
abrasives and nonabrasives. Abrasives include
of lint from cleaning rags can cause abrasive
core sand, weld splatter, machining chips, and
damage in hydraulic systems, especially to closely
rust. Nonabrasives are contaminants resulting
fitted moving parts. Rust or corrosion present in
from oil oxidation and the soft particles that are
a hydraulic system usually can be traced to
worn or shredded from seals and other organic
improper storage of materials or parts. Proper
components. Oil-oxidation products, usually
preservation of stored parts helps to reduce
called sludge, have no abrasive properties. Never-
corrosion.
theless, sludge may prevent proper operation of
3. Particles created within the system during
a hydraulic system by clogging the valves, orifices,
operation. Contaminants created during system
and filters.
operation are of two general types--mechanical
The mechanics of the destructive action by
and chemical. Mechanical particles are formed by
abrasive contaminants is clear. When the size of
the wearing of parts in frictional contact, such as
the particles circulating in the hydraulic system
pumps, cylinders, and packing gland parts. These
is greater than the clearance between moving
worn particles can vary from large chunks of
parts, the clearance openings act as filters.
packings to steel shavings of microscopic size,
Hydraulic pressure pushes these particles into the
which system screens cannot filter.
softer materials. This results in blocked passages
or scratches on finely finished surfaces from
The chief source of chemical contaminants in
movement between parts. These scratches result
hydraulic fluids is oxidation. Chemical con-
in internal component leakage and decreased
tamination forms as a result of the high pressure
efficiency.
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