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LUBRICATING POWER
If motion takes place between surfaces in
contact, friction tends to oppose the
motion.
When pressure forces the liquid of a hydraulic
system between the surfaces of moving
parts, the
liquid spreads out into a thin film which enables
the parts to move more freely. Different
liquids,
including oils, vary greatly not only in their
lubricating ability but also in film
strength. Film
strength is the capability of a liquid to resist being
wiped or squeezed out from between the surfaces
when spread out in an extremely thin layer. A
liquid will no longer lubricate if the film breaks
down, since the motion of part against part wipes
the metal clean of liquid.
Lubricating power varies with temperature
changes; therefore, the climatic and
working
conditions must enter into the determination of
the lubricating qualities of a liquid.
Unlike
viscosity, which is a physical property, the
lubricating power and film strength of a
liquid
is directly related to its chemical nature.
Lubricating qualities and film strength
can be
improved by the addition of certain chemical
agents.
CHEMICAL STABILITY
Chemical stability is another property which
is exceedingly important in the selection of a
hydraulic liquid. It is defined as the
liquid’s ability
to resist oxidation and deterioration for long
periods. All liquids tend to undergo
unfavorable
changes under severe operating conditions. This
is the case, for example, when a system
operates
for a considerable period of time at high
temperatures.
Excessive temperatures, especially extremely
high temperatures, have a great effect on the life
of a liquid. The temperature of the
liquid in the
reservoir of an operating hydraulic system does
not always indicate the operating
conditions
throughout the system. Localized hot spots occur
on bearings, gear teeth, or at other
points where
the liquid under pressure is forced through small
orifices. Continuous passage of the
liquid through
these points may produce local temperatures high
enough to carbonize the liquid or turn it
into
sludge, yet the liquid in the reservoir may not
indicate an excessively high temperature.
Liquids may break down if exposed to air,
water, salt, or other impurities,
especially if they
are in constant motion or subjected to heat. Some
metals, such as zinc, lead, brass, and
copper, have
undesirable chemical reactions with certain
liquids.
These chemical reactions result in the forma-tion
of sludge, gums, carbon, or other deposits
which clog openings, cause valves and
pistons to stick or leak, and
give poor lubrication to moving
increases more rapidly. As these deposits are
formed, certain changes in the physical
and
chemical properties of the liquid take place. The
liquid usually becomes darker, the
viscosity
increases and damaging acids are formed.
The extent to which changes occur in
different
liquids depends on the type of liquid, type of
refining, and whether it has been treated
to
provide further resistance to oxidation. The
stability of liquids can be improved by
the
addition of oxidation inhibitors. Inhibitors
selected to improve stability must be
compatible
with the other required properties of the liquid.
FREEDOM FROM ACIDITY
An ideal hydraulic liquid should be free from
acids which cause corrosion of the metals
in the
system. Most liquids cannot be expected to remain
completely noncorrosive under severe
operating
conditions. The degree of acidity of a liquid, when
new, may be satisfactory; but after use,
the liquid
may tend to become corrosive as it begins to
deteriorate.
Many systems are idle for long periods after
operating at high temperatures. This permits
moisture to condense in the system,
resulting in
rust formation.
Certain corrosion- and rust-preventive additives
are added to hydraulic liquids. Some of
these
additives are effective only for a limited period.
Therefore, the best procedure is to use
the liquid
specified for the system for the time specified by
the system manufacturer and to protect
the liquid
and the system as much as possible from
contamination by foreign matter, from
abnormal
temperatures, and from misuse.
FLASHPOINT
Flashpoint is the temperature at which a liquid
gives off vapor in sufficient quantity to
ignite
momentarily or flash when a flame is applied. A
high flashpoint is desirable for
hydraulic liquids
because it provides good resistance to combustion
and a low degree of evaporation at normal
temperatures. Required flashpoint minimums
vary from 300°F for the lightest oils to
510°F for
the heaviest oils.
FIRE POINT
Fire point is the temperature at which a
substance gives off vapor in sufficient
quantity
to ignite and continue to burn when exposed to
a spark or flame. Like flashpoint, a high
fire point
is required of desirable hydraulic liquids.
MINIMUM TOXICITY
Toxicity is defined as the quality, state, or
degree of being toxic or poisonous. Some
liquids
contain chemicals that are a serious toxic hazard.
These toxic or poisonous chemicals may
enter the
body through inhalation, by absorption through
the skin, or through the eyes or the
mouth. The
result is sickness and, in some cases, death.
Manufacturers of hydraulic liquids strive
to produce suitable liquids that
contain no toxic chemicals and, as a result, most
hydraulic liquids are free of harmful chemicals.
Some fire-resistant liquids are toxic, and suitable
protection and care in handling must be provided.
DENSITY AND COMPRESSIBILITY
A fluid with a specific gravity of less than 1.0
is desired when weight is critical, although with
proper system design, a fluid with a
specific
gravity greater than one can be tolerated. Where
avoidance of detection by military units
is desired,
a fluid which sinks rather than rises to the surface
of the water is desirable. Fluids having
a specific
gravity greater than 1.0 are desired, as leaking
fluid will sink, allowing the vessel with
the leak
to remain undetected.
Recall from chapter 2 that under extreme
pressure a fluid may be compressed up to
7
percent of its original volume. Highly com-pressible
fluids produce sluggish system operation.
This does not present a serious problem in small,
low-speed operations, but it must be
considered
in the operating instructions.
FOAMING TENDENCIES
Foam is an emulsion of gas bubbles in the
fluid. Foam in a hydraulic system results
from
compressed gases in the hydraulic fluid. A fluid
under high pressure can contain a large
volume
of air bubbles. When this fluid is depressurized,
as when it reaches the reservoir, the gas
bubbles
in the fluid expand and produce foam. Any
amount of foaming may cause pump
cavitation and produce poor
system response and spongy
added to fluids to prevent foaming. Minimizing
air in fluid systems is discussed later
in this
chapter.
CLEANLINESS
Cleanliness in hydraulic systems has received
considerable attention recently. Some
hydraulic
systems, such as aerospace hydraulic systems, are
extremely sensitive to contamination.
Fluid
cleanliness is of primary importance because
contaminants can cause component
malfunction,
prevent proper valve seating, cause wear in
components, and may increase the response
time
of servo valves. Fluid contaminants are discussed
later in this chapter.
The inside of a hydraulic system can only be
kept as clean as the fluid added to it. Initial fluid
cleanliness can be achieved by observing
stringent
cleanliness requirements (discussed later in this
chapter) or by filtering all fluid added
to the
system.
TYPES OF HYDRAULIC FLUIDS
There have been many liquids tested for use
in hydraulic systems. Currently, liquids
being used
include mineral oil, water, phosphate ester,
water-based ethylene glycol compounds,
and
silicone fluids. The three most common types of
hydraulic liquids are petroleum-based,
synthetic
fire-resistant, and water-based fire-resistant.
PETROLEUM-BASED FLUIDS
The most common hydraulic fluids used in
shipboard systems are the petroleum-based
oils.
These fluids contain additives to protect the fluid
from oxidation (antioxidant), to protect
system
metals from corrosion (anticorrosion), to reduce
tendency of the fluid to foam (foam
suppressant),
and to improve viscosity.
Petroleum-based fluids are used in surface
ships’ electrohydraulic steering and
deck
machinery systems, submarines’ hydraulic
systems, and aircraft automatic pilots,
shock
absorbers, brakes, control mechanisms, and other
hydraulic systems using seal materials
compatible
with petroleum-based fluids.
SYNTHETIC FIRE-RESISTANT FLUIDS
Petroleum-based oils contain most of the
desired properties of a hydraulic liquid.
However,
they are flammable under normal conditions and
can become explosive when subjected to
high
pressures and a source of flame or high tempera-tures.
Nonflammable synthetic liquids have been
developed for use in hydraulic systems where fire
hazards exist.
Phosphate Ester Fire-Resistant Fluid
Phosphate ester fire-resistant fluid for
shipboard use is covered by specification MIL-H-
19457. There are certain trade names
closely
associated with these fluids. However, the only
acceptable fluids conforming to
MIL-H-19457 are
the ones listed on the current Qualified Products
List (QPL) 19457. These fluids will be
delivered
in containers marked MIL-H-19457C or a later
specification revision. Phosphate ester
in
containers marked by a brand name without a
specification identification must not be
used in shipboard systems, as they may
contain toxic chemicals.
These fluids will burn if sufficient heat and
flame are applied, but they do not support
combustion. Drawbacks of phosphate ester
fluids
are that they will attack and loosen commonly
used paints and adhesives, deteriorate
many types
of insulations used in electrical cables, and
deteriorate many gasket and seal
materials.
Therefore, gaskets and seals for systems in which
phosphate ester fluids are used are
manufactured
of specific materials. Naval Ships’ Technical
Manual, chapter 262, specifies paints to be used
on exterior surfaces of hydraulic systems and
components in which phosphate ester fluid is used
and on ship structure and decks in the immediate
vicinity of this equipment. Naval Ships’ Technical
Manual, chapter 078, specifies gasket and seal
materials used. NAVAIR 01-1A-17 also contains
a list of materials resistant to phosphate ester
fluids.
Trade names for phosphate ester fluids, which
do not conform to MIL-H-19457 include
Pydraul,
Skydrol, and Fyre Safe.
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