parts. Once a small amount of sludge or other
deposits is formed, the rate of formation generally
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
Many systems are idle for long periods after
operating at high temperatures. This permits
moisture to condense in the system, resulting in
Certain corrosion- and rust-preventive addi-
tives 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 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 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.
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.
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