most of the metals used for parts fabrication and plating
hydraulic valves have been used in particle con-
are found in hydraulic fluid, the major metallic
tamination experiments. The valves are easy to control
materials found are ferrous, aluminum, and chromium
and respond rapidly to repositioning. In these experi-
particles.
ments, the valves were operated with both ultra clean
and contaminated hydraulic fluids. The experiments
Hydraulic pumps usually contribute the most con-
proved that wear is accelerated by even small amounts
tamination to the system because of their high-speed,
internal movement. Other hydraulic systems produce
erosion of the sharp spool edges and general deteriora-
tion of the spool surfaces. Because of the extremely
chipping.
close fit of spools in servo valve housings, the valves
Hydraulic actuators and valves are affected by
are particularly susceptible to damage or erratic
contamination. Large metallic or hard nonmetallic
operation when operated with contaminated hydraulic
particles collect at the seal areas. These particles may
fluid.
groove the inside wall of the actuator body due to a
scraping action. Smaller particles act as abrasives
Organic Contamination
between the seals and the actuator body, causing wear
Organic solids or semisolids are one of the
and scoring. Eventually, the fluid leaks and the seals fail
particulate contaminants found in hydraulic systems.
because the seal extrudes into the enlarged gap between
They are produced by wear, oxidation, or polymeriza-
the piston head and the bore of the actuator body. Once
tion (a chemical reaction). Organic solid contaminants
wear begins, it increases at a faster rate because wear
found in the systems include minute particles of
particles add to the abrasive material. In a similar
O-rings, seals, gaskets, and hoses. These contaminants
manner, metallic or nonmetallic parts may lodge in the
are produced by wear or chemical reaction.
poppets and poppet-seat portions of valves and cause
system malfunction by holding valves open.
Oxidation of hydraulic fluids increases with pres-
sure and temperature. Antioxidants are blended into
Inorganic Solid Contamination
hydraulic fluids to minimize such oxidation. Oxidation
products appear as organic acids, asphaltics, gums, and
The inorganic solid contaminant group includes
varnishes. These products combine with particles in the
dust, paint particles, dirt, and silicates. These and other
hydraulic fluid to form sludge. Some oxidation
materials are often drawn into hydraulic systems from
products are oil soluble and cause an increase in
external sources. The wet piston shaft of a hydraulic
hydraulic fluid viscosity, while other oxidation
actuator may draw some of these foreign materials into
products are not oil soluble and form sediment. Oil
the cylinder past the wiper and dynamic seals. The
oxidation products are not abrasive. These products
contaminant materials are then dispersed in the
cause system degradation because the sludge or varnish
hydraulic fluid. Also, contaminants may enter the
like materials collect at close-fitting, moving parts,
hydraulic fluid during maintenance when tubing,
such as the spool and sleeve on servo valves. Collection
hoses, fittings, and components are disconnected or
of oxidation products at these points causes sluggish
replaced. To avoid these problems, all exposed fluid
valve response.
ports should be sealed with approved protective
closures.
Metallic Solid Contamination
Glass particles from glass bead peening and
Metallic solid contaminants are usually found in
blasting are another contaminant. Glass particles are
particularly undesirable because glass abrades
range from microscopic particles to those you can see
synthetic rubber seals and the very fine surfaces of
with the naked eye. These particles are the result of the
critical moving parts.
wearing and scoring of bare metal parts and plating
FLUID CONTAMINATION
materials, such as silver and chromium. Wear products
and other foreign metal particles, such as steel,
Hydraulic fluid can be contaminated by air, water,
aluminum, and copper, act as metallic catalysts in the
formation of oxidation products. Fine metallic particles
their effects are discussed in the following text.
enter hydraulic fluid from within the system. Although
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