CHAPTER 3
HYDRAULIC FLUIDS
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 tempera-
ture 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 charac-
teristics 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 com-
ponents 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 instru-
ments 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
3-1
