referred to as particulate contamination. Con-
tamination is always present to some degree, even
in new, unused fluid, but must be kept below a
level that will adversely affect system operation.
Hydraulic contamination control consists of
requirements, techniques, and practices necessary
to minimize and control fluid contamination.
There are many types of contaminants which
are harmful to hydraulic systems and liquids.
These contaminants may be divided into two
different classesparticulate and fluid.
This class of contaminants includes organic,
metallic solid, and inorganic solid contaminants.
These contaminants are discussed in the following
ORGANIC CONTAMINATION. Organic
solids or semisolids found in hydraulic systems
are produced by wear, oxidation, or polymeriza-
tion. Minute particles of O-rings, seals, gaskets,
and hoses are present, due to wear or chemical
reactions. Synthetic products, such as neoprene,
silicones, and hypalon, though resistant to
chemical reaction with hydraulic fluids, produce
small wear particles. Oxidation of hydraulic fluids
increases with pressure and temperature, although
antioxidants are blended into hydraulic fluids to
minimize such oxidation. The ability of a
hydraulic fluid to resist oxidation or poly-
merization in service is defined as its oxidation
stability. Oxidation products appear as organic
acids, asphaltics, gums, and varnishes. These
products combine with particles in the hydraulic
fluid to form sludge. Some oxidation products are
oil soluble and cause the hydraulic fluid to
increase in viscosity; other oxidation products are
not oil soluble and form sediment.
METALLIC SOLID CONTAMINATION.
Metallic contaminants are almost always present
in a hydraulic system and will range in size from
microscopic particles to particles readily visible
to the naked eye. These particles are the result of
wearing and scoring of bare metal parts and
plating materials, such as silver and chromium.
Although practically all metals commonly used
for parts fabrication and plating may be found
in hydraulic fluids, the major metallic materials
found are ferrous, aluminum, and chromium
particles. Because of their continuous high-speed
internal movement, hydraulic pumps usually
contribute most of the metallic particulate
contamination present in hydraulic systems. Metal
particles are also produced by other hydraulic
system components, such as valves and actuators,
due to body wear and the chipping and wearing
away of small pieces of metal plating materials.
INORGANIC SOLID CONTAMINA-
TION. This contaminant group includes dust,
paint particles, dirt, and silicates. Glass particles
from glass bead peening and blasting may also
be found as contaminants. Glass particles are very
undesirable contaminants due to their abrasive
effect on synthetic rubber seals and the very fine
surfaces of critical moving parts. Atmospheric
dust, dirt, paint particles, and other materials are
often drawn into hydraulic systems from external
sources. For example, the wet piston shaft of a
hydraulic actuator may draw some of these
foreign materials into the cylinder past the wiper
and dynamic seals, and the contaminant materials
are then dispersed in the hydraulic fluid.
Contaminants may also enter the hydraulic fluid
during maintenance when tubing, hoses, fittings,
and components are disconnected or replaced. It
is therefore important that all exposed fluid ports
be sealed with approved protective closures to
minimize such contamination.
Air, water, solvent, and other foreign fluids
are in the class of fluid contaminants.
AIR CONTAMINATION. Hydraulic fluids
are adversely affected by dissolved, entrained, or
free air. Air may be introduced through improper
maintenance or as a result of system design. Any
maintenance operation that involves breaking into
the hydraulic system, such as disconnecting or
removing a line or component will invariably
result in some air being introduced into the
system. This source of air can and must be
minimized by prebilling replacement components
with new filtered fluid prior to their installation.
Failing to prefill a filter element bowl with fluid
is a good example of how air can be introduced
into the system. Although prebilling will minimize
introduction of air, it is still important to vent the
system where venting is possible.
Most hydraulic systems have built-in sources
of air. Leaky seals in gas-pressurized accumulators
and reservoirs can feed gas into a system faster