reading for all personnel who use and maintain aircraft
are the main source of core sand entering the system.
hydraulic servicing equipment.
Even the most carefully designed and cleaned casting
occasionally frees some sand particles under the action
CLASSES OF CONTAMINATION
of hydraulic pressure. Rubber hose assemblies always
contain some loose particles, most of which can be
The two general contamination classes are as
removed by flushing; others withstand cleaning and are
follows:
freed later by the action of hydraulic pressure and heat.
Abrasive. This includes such particles as dust,
Rust or corrosion initially present in a hydraulic
dirt, core sand, weld spatter, machining chips,
system can usually be traced to improper storage of
and rust.
replacement materials and component parts. Particles
Nonabrasive. This includes things that result
can range in size from large flakes to abrasives of
microscopic dimensions (remember the discussion
from oil oxidation and soft particles worn or
earlier on the size of a single micron). Proper
shredded from seals and other organic
preservation of stored parts is helpful in eliminating
components.
corrosion.
The mechanics of the destructive action by
PARTICLES OF LINT FROM CLEANING
abrasive contaminants is clear. When the size of the
MATERIALS. These can cause abrasive damage in
particles circulating in the hydraulic system is greater
hydraulic systems, especially to closely fitted moving
than the clearance between moving parts, the clearance
parts. In addition, lint in a hydraulic system packs easily
openings act as filters and retain such particles.
into clearances between packing and contacting
Hydraulic pressure then embeds these particles into the
surfaces, leading to component leakage and decreased
softer materials. The reciprocating or rotating motion
efficiency. Lint also helps clog filters prematurely.
of component parts develops scratches on finely
finished surfaces, ultimately resulting in increased
PA RT I C L E S I N T R O D U C E D F R O M
tolerances and decreased efficiency.
OUTSIDE FORCES. Particles can be introduced into
hydraulic systems at points where either the fluid or
Oil-oxidation products, usually called sludge,
have no abrasive properties. Nevertheless, sludge may
certain working parts of the system (e.g., piston rods)
prevent proper functioning of a hydraulic system by
are at least in temporary contact with the atmosphere.
clogging valves, orifices, and filters. Frequent
The most common danger areas are at the refill and
changing of hydraulic system fluid is not a satisfactory
breather openings and at cylinder rod packing.
solution to the contamination problem. Abrasive
Contamination arising from carelessness during
particles contained in the system are not usually
servicing operations is minimized by the use of an
flushed out, and new particles are continually created
approved dispensing cart employing proper filters, and
as friction products. Furthermore, every minute
filler strainers in the filling adapters of hydraulic
remnant of sludge acts as an effective catalyst to speed
reservoirs. Hydraulic cylinder piston rods use wiper
up oxidation of the fresh fluid. (A catalyst is a
rings and dust seals to prevent the dust that settles on the
substance that, when added to another substance,
piston rod during its outward stroke from being drawn
speeds up or slows down chemical reaction, but is itself
into the system when the piston rod retracts. Similarly,
unchanged at the end of the reaction.)
single-acting actuating cylinders use an air filter in the
vent to prevent ingestion of airborne contamination
ORIGIN OF CONTAMINANTS
during the return stroke (refer back to view C of figure
8-36).
The origin of contaminants in hydraulic systems
PA RT I C L E S C R E AT E D W I T H I N T H E
can be traced to the following areas:
PARTICLES ORIGINALLY CONTAINED IN
SYSTEM DURING OPERATION. Contaminants
created during system operation are of two general
THE SYSTEM. These particles originate during
types--mechanical and chemical. Particles of a
fabrication of welded system components, especially in
mechanical nature are formed by wearing of parts in
reservoirs and pipe assemblies. The presence is
frictional contact, such as pumps, cylinders, and
minimized by proper design. For example,
packing gland components. Additionally, over-age
seam-welded overlapping joints are preferred; arc
hydraulic hose assemblies tend to break down inside
welding of open sections is usually avoided. Hidden
passages in valve bodies, inaccessible to sand blasting,
and contaminate the system. For this reason, hoses are
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