As a CM1, you will be responsible for the
maintenance, repair, and troubleshooting of hydraulic
systems. You must be able to analyze the malfunctions
of these systems and supervise your personnel in the
required corrective action. To be able to do this, you
must thoroughly understand the basic system, the
operational principles, and the components of the
NOTE: Before you continue with this chapter, you
should review the appropriate chapters of the CM 3&2,
The first part of this chapter briefly covers some of
the basic principles associated with hydraulics, followed
by coverage of various system components. The
purpose of this information is to give you an analytical
understanding of the interrelationships of principles and
components in an operating system. When you
understand the operation of a system, it is much easier
to analyze a malfunction.
BASIC PRINCIPLES OF HYDRAULICS
In automotive and construction equipment, the
terms hydraulic or pneumatic describe a method of
transmitting power from one place to another through
the use of a liquid or a gas. Several kinds of gases are
used in the various hydraulic systems; however, certain
physical laws or principles apply to all liquids and gases.
As a CM, you should be aware of this. You should also
be familiar with the following terms as they are
associated with hydraulic and pneumatic systems.
. HYDRAULICS is that branch of science that
deals with the study and use of liquids, as related to the
mechanical aspects of physics.
. PNEUMATICS is that branch of science that
deals with the study and use of air and other gases, as
related to the mechanical aspects of physics.
. FORCE is the push or pull on an object. In
hydraulics and pneumatics, force is usually expressed in
. PRESSURE is the amount of force distributed
over each unit on the area of an object. In
hydraulics/pneumatics, pressure is expressed in pounds
per square inch (psi).
A FLUID is defined as any substance made up of
small particles or molecules that have the ability to flow
or move easily (conforms to the outline of its container);
this includes both liquid and gas. The terms liquids and
fluids are often used interchangeably; however, fluids
have a much broader meaning. All liquids are fluids, but
not all fluids are liquid; fluids can be liquid, but they can
also be air and other gases that are not liquid. In support
equipment, hydraulics mean liquid and pneumatics
mean air or other gases.
INCOMPRESSIBILITY AND EXPANSION
For all practical purposes, fluids are incom-
pressible. Under extremely high pressures, the volume
of a fluid can be decreased somewhat, though the
decrease is so slight that it is considered to be negligible
except by design engineers.
Liquids expand and contract because of temperature
changes. When liquid in a closed container is subjected
to high temperatures, it expands; this exerts a pressure
on the walls of the container; therefore, it is necessary
that pressure-relief mechanisms and expansion
chambers be incorporated into hydraulic systems.
Without these precautionary measures, the expanding
fluid might exert enough pressure to rupture the system.
COMPRESSIBILITY AND EXPANSION OF
A gas is a substance in which the molecules are
separated by relatively large spaces. The two major
differences between liquids and gases are their
compressibility and expansion. While liquids are
incompressible, gases are highly compressible because
of these large spaces between the molecules.
Gases, like liquids, expand and contract because of
temperature change; but unlike liquids, a gas expands to
till completely any closed container in which it is
contained; a liquid tills the container only to the extent
of its normal volume.