than it can be removed, even with the best of
maintenance. Another lesser known but major
source of air is air that is sucked into the system
past actuator piston rod seals. This usually occurs
when the piston rod is stroked by some external
means while the actuator itself is not pressurized.
WATER CONTAMINATION. Water is a
serious contaminant of hydraulic systems.
Hydraulic fluids are adversely affected by
dissolved, emulsified, or free water. Water
contamination may result in the formation of ice,
which impedes the operation of valves, actuators,
and other moving parts. Water can also cause the
formation of oxidation products and corrosion
of metallic surfaces.
SOLVENT CONTAMINATION. Solvent
contamination is a special form of foreign fluid
contamination in which the original contami-
nating substance is a chlorinated solvent. Chlori-
nated solvents or their residues may, when
introduced into a hydraulic system, react with any
water present to form highly corrosive acids.
Chlorinated solvents, when allowed to com-
bine with minute amounts of water often found
in operating hydraulic systems, change chemically
into hydrochloric acids. These acids then attack
internal metallic surfaces in the system,
particularly those that are ferrous, and produce
a severe rust-like corrosion. NAVAIR 01-1A-17
and NSTM, chapter 556, contain tables of
solvents for use in hydraulic maintenance.
FOREIGN-FLUIDS CONTAMINATION.
Hydraulic systems can be seriously contaminated
by foreign fluids other than water and chlorinated
solvents. This type of contamination is generally
a result of lube oil, engine fuel, or incorrect
hydraulic fluid being introduced inadvertently into
the system during servicing. The effects of such
contamination depend on the contaminant, the
amount in the system, and how long it has been
present.
NOTE: It is extremely important that the
different types of hydraulic fluids are not mixed
in one system. If different type hydraulic fluids
are mixed, the characteristics of the fluid required
for a specific purpose are lost. Mixing the
different types of fluids usually will result in a
heavy, gummy deposit that will clog passages and
require a major cleaning. In addition, seals and
packing installed for use with one fluid usually
are not compatible with other fluids and damage
to the seals will result.
ORIGIN OF CONTAMINATION
Recall that contaminants are produced from
wear and chemical reactions, introduced by
improper maintenance, and inadvertently intro-
duced during servicing. These methods of con-
taminant introduction fall into one of the four
major areas of contaminant origin.
1. Particles originally contained in the system.
These particles originate during the fabrication
and storage of system components. Weld spatter
and slag may remain in welded system com-
ponents, especially in reservoirs and pipe
assemblies. The presence is minimized by proper
design. For example, seam-welded overlapping
joints are preferred, and arc welding of open
sections is usually avoided. Hidden passages in
valve bodies, inaccessible to sand blasting or other
methods of cleaning, are the main source of
introduction of core sand. Even the most carefully
designed and cleaned casting will almost invari-
ably free some sand particles under the action of
hydraulic pressure. Rubber hose assemblies always
contain some loose particles. Most of these
particles can be removed by flushing the hose
before installation; however, some particles
withstand cleaning and are freed later by the
action of hydraulic pressure.
Particles of lint from cleaning rags can
cause abrasive damage in hydraulic systems,
especially to closely fitted moving parts. In
addition, lint in a hydraulic system packs easily
into clearances between packing and contacting
surfaces, leading to component leakage and
decreased efficiency. Lint also helps clog filters
prematurely. The use of the proper wiping
materials will reduce or eliminate lint contamina-
tion. The wiping materials to be used for a given
application will be determined by
a.
b.
c.
substances being wiped or absorbed,
the amount of absorbency required,
and/or
the required degree of cleanliness.
These wiping materials are categorized for
contamination control by the degree of lint or
debris that they may deposit during use. For
internal hydraulic repairs, this factor itself
will determine the choice of wiping material.
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