systems, and appendix III provides symbols used
in nonaeronautical mechanical systems.
The remainder of chapter 1 is devoted to the
advantages and problems of fluid power appli-
cations. Included are brief sections on the history,
development, and applications of hydraulics,
the states of matter.
ADVANTAGES OF FLUID POWER
The extensive use of hydraulics and pneuma-
tics to transmit power is due to the fact that
properly constructed fluid power systems possess
a number of favorable characteristics. They
eliminate the need for complicated systems of
gears, cams, and levers. Motion can be trans-
mitted without the slack inherent in the use of
solid machine parts. The fluids used are not
subject to breakage as are mechanical parts, and
the mechanisms are not subjected to great wear.
The different parts of a fluid power system
can be conveniently located at widely separated
points, since the forces generated are rapidly
transmitted over considerable distances with small
loss. These forces can be conveyed up and down
or around corners with small loss in efficiency and
without complicated mechanisms. Very large
forces can be controlled by much smaller ones and
can be transmitted through comparatively small
lines and orifices.
If the system is well adapted to the work it is
required to perform, and if it is not misused, it
can provide smooth, flexible, uniform action
without vibration, and is unaffected by variation
of load. In case of an overload, an automatic
release of pressure can be guaranteed, so that the
system is protected against breakdown or strain.
Fluid power systems can provide widely variable
motions in both rotary and straight-line trans-
mission of power. The need for control by hand
can be minimized. In addition, fluid power
systems are economical to operate.
The question may arise as to why hydraulics
is used in some applications and pneumatics in
others. Many factors are considered by the user
and/or the manufacturer when determining which
type of system to use in a specific application.
There are no hard and fast rules to follow;
however, past experience has provided some
sound ideas that are usually considered when such
decisions are made. If the application requires
speed, a medium amount of pressure, and only
fairly accurate control, a pneumatic system may
be used. If the application requires only a medium
amount of pressure and a more accurate control,
a combination of hydraulics and pneumatics may
be used. If the application requires a great amount
of pressure and/or extremely accurate control, a
hydraulic system should be used.
The extreme flexibility of fluid power elements
presents a number of problems. Since fluids have
no shape of their own, they must be positively
confined throughout the entire system. Special
consideration must be given to the structural
integrity of the parts of a fluid power system.
Strong pipes and containers must be provided.
Leaks must be prevented. This is a serious
problem with the high pressure obtained in many
fluid power installations.
The operation of the system involves constant
movement of the fluid within the lines and
components. This movement causes friction
within the fluid itself and against the containing
surfaces which, if excessive, can lead to serious
losses in efficiency. Foreign matter must not be
allowed to accumulate in the system, where it will
clog small passages or score closely fitted parts.
Chemical action may cause corrosion. Anyone
working with fluid power systems must know how
a fluid power system and its components operate,
both in terms of the general principles common
to all physical mechanisms and of the peculiarities
of the particular arrangement at hand.
The word hydraulics is based on the Greek
word for water, and originally covered the study
of the physical behavior of water at rest and in
motion. Use has broadened its meaning to include
the behavior of all liquids, although it is primarily
concerned with the motion of liquids.
Hydraulics includes the manner in which
liquids act in tanks and pipes, deals with their
properties, and explores ways to take advantage
of these properties.
DEVELOPMENT OF HYDRAULICS
Although the modern development of
hydraulics is comparatively recent, the ancients
were familiar with many hydraulic principles and
their applications. The Egyptians and the ancient
people of Persia, India, and China conveyed water