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CHAPTER 3
During the design of equipment that requires fluid power, many factors are considered
in
selecting the type of system to be used—hydraulic,
pneumatic, or a combination of the two.
Some
of the factors are required speed and accuracy of
operation, surrounding atmospheric
conditions,
economic conditions, availability of replacement
fluid, required pressure level, operating
temperature
range, contamination possibilities, cost of
transmission lines, limitations of the
equipment,
lubricity, safety to the operators, and expected
service life of the equipment.
After the type of system has been selected,
many of these same factors must be considered
in selecting the fluid for the system.
This chapter
is devoted to hydraulic fluids. Included in it are
sections on the properties and
characteristics
desired of hydraulic fluids; types of hydraulic
fluids; hazards and safety precautions
for working
with, handling, and disposing of hydraulic
liquids; types and control of
contamination; and
sampling.
PROPERTIES
If fluidity (the physical property of a substance
that enables it to flow) and
incompressibility were
the only properties required, any liquid not too
thick might be used in a hydraulic
system.
However, a satisfactory liquid for a particular
system must possess a number of other
properties.
The most important properties and some characteristics
are discussed in the following
paragraphs.
VISCOSITY
Viscosity is one of the most important
properties of hydraulic fluids. It is a measure of
a fluid’s resistance to flow. A liquid,
such as
gasoline, which flows easily has a low viscosity;
and a liquid, such as tar, which flows
slowly has
a high viscosity. The viscosity of a liquid is
affected by changes in temperature and
pressure.
As the temperature of a liquid increases, its
viscosity decreases. That is, a liquid
flows more
easily when it is hot than when it is cold. The
viscosity of a liquid increases as the
pressure on
the liquid increases.
A satisfactory liquid for a hydraulic system
must be thick enough to give a good seal
at
pumps, motors, valves, and so on. These components
depend on close fits for creating and
maintaining pressure. Any internal leakage
through these clearances results in loss
of pressure,
instantaneous control, and pump efficiency.
Leakage losses are greater with thinner
liquids
(low viscosity). A liquid that is too thin will also
allow rapid wearing of moving parts, or
of parts
that operate under heavy loads. On the other
hand, if the liquid is too thick
(viscosity too high),
the internal friction of the liquid will cause an
increase in the liquid’s flow
resistance through
clearances of closely fitted parts, lines, and
internal passages. This results in
pressure drops
throughout the system, sluggish operation
of the equipment, and an increase in
power
consumption.
Measurement of Viscosity
Viscosity is normally determined by measuring
the time required for a fixed volume of a
fluid
(at a given temperature) to flow through a
calibrated orifice or capillary tube. The
instruments
used to measure the viscosity of a liquid
are known as viscometers or viscosimeters.
Several types of viscosimeters are in use today.
The Saybolt viscometer, shown in figure
3-1,
measures the time required, in seconds, for 60
milliliters of the tested fluid at 100°F
to pass through a standard
orifice. The time measured is used to express the fluid’s viscosity, in Saybolt
universal seconds or Saybolt furol seconds.
The glass capillary viscometers, shown in
figure 3-2, are examples of the second type of
viscometer used. These viscometers are
used to
measure kinematic viscosity. Like the Saybolt
viscometer, the glass capillary measures
the time
in seconds required for the tested fluid to flow
through the capillary. This time is
multiplied by
the temperature constant of the viscometer in use
to provide the viscosity, expressed in
centistrokes.
The following formulas may be used to
convert centistrokes (cSt units) to
approximate
Saybolt universal seconds (SUS units).
Figure 3-1.—Saybolt viscometer.
For SUS values between 32 and 100:
For SUS values greater than 100:
Although the viscometers discussed
above are used in
laboratories, there are other viscometers
in the supply system that are available for local
use. These viscometers can be used to
test the
viscosity of hydraulic fluids either prior to their
being added to a system or periodically
after they have been in an
operating system for a while.
Figure 3-2.–Various styles of glass capillary viscometers.
Additional information on the various types
of viscometers and their operation can be found
in the Physical Measurements
Training Manual,
NAVAIR 17-35QAL-2.
Viscosity Index
The viscosity index (V.I.) of an oil is a number
that indicates the effect of temperature
changes
on the viscosity of the oil. A low V.I. signifies
a relatively large change of viscosity
with changes
of temperature. In other words, the oil becomes
extremely thin at high temperatures and
extremely
thick at low temperatures. On the other hand, a
high V.I. signifies relatively little
change in
viscosity over a wide temperature range.
An ideal oil for most purposes is one that maintains a constant
viscosity throughout temperature changes. The
importance of the V.I. can be shown easily by considering
automotive lubricants. An oil having a high V.I. resists excessive thickening when the
engine is cold and, consequently, promotes rapid
starting and prompt circulation; it resists excessive
thinning when the motor is hot and thus provides
full lubrication and prevents excessive oil
consumption.
Another example of the importance of the V.I.
is the need for a high V.I. hydraulic oil for military
aircraft, since hydraulic control systems
may be
exposed to temperatures ranging from below
–65°F at high altitudes to over 100°F
on the
ground. For the proper operation of the hydraulic
control system, the hydraulic fluid must
have a
sufficiently high V.I. to perform its functions at
the extremes of the expected temperature
range. Liquids with a high viscosity have
a greater resistance to heat than low
viscosity liquids which have been derived from the same
source. The average hydraulic liquid has a
relatively low viscosity. Fortunately, there is a
wide choice of liquids available for use in the
viscosity range required of hydraulic liquids.
The V.I. of an oil may be determined if its
viscosity at any two temperatures is known.
Tables, based on a large number of tests,
are
issued by the American Society for Testing
and Materials (ASTM). These tables permit
calculation of the V.I. from known viscosities.
