HYDRAULIC AND PNEUMATIC SYSTEMS
Learning Objective: Explain the operating principles
of hydraulic and pneumatic systems. Identify the
components, component functions, and maintenance
procedures of hydraulic and pneumatic systems.
In automotive and construction equipment, the
terms hydraulic and pneumatic describe a method of
transmitting power from one place to another through
the use of a liquid or a gas. Certain physical laws or
principles apply to all liquids and gases. You should be
familiar with the following terms, as they are
associated with hydraulic and pneumatic systems.
HYDRAULICS is a branch of science that deals
with the study and use of liquids as related to the
mechanical aspects of physics.
PNEUMATICS is a branch of science that deals
with the study and use of air and other gases as
related to the mechanical aspects of physics.
The chapter covers the basic principles associated
with hydraulics and pneumatics, 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 the
components in an operating system.
Learning Objective: Identify operational
characteristics, component functions, and maintenance
procedures of a hydraulic system.
The extensive use of hydraulics to transmit
power is due to the fact that a properly constructed
hydraulic system possesses a number of favorable
characteristics. These are as follows:
Eliminates the need for complicated systems
using gears, cams, and levers.
Motion can be transmitted without the slack
inherent in the use of solid machine parts.
The fluids used are not subject to breakage as are
Hydraulic system mechanisms are not subjected
to great wear.
If the system is well-adapted to the work it is
required to perform and not misused, it can provide
smooth, flexible, uniform action without vibration and
is unaffected by variation of load. Hydraulic systems
can provide widely variable motions in both rotary and
straight-line transmission of power. The need for
control by hand can be minimized. In addition, they are
economical to operate.
BASIC PRINCIPLES OF HYDRAULICS
The basic principles of hydraulics are few and
simple and are as follows:
Liquids have no shape of their own.
Liquids will NOT compress.
Liquids transmit applied pressure in all
Liquids provide great increase in work force.
Pressure and Force
The terms force and pressure are used extensively
in the study of fluid power. It is essential that we
distinguish between these terms. Force means a total
push or pull. It is push or pull exerted against the total
area of a particular surface and is expressed in pounds
or grams. Pressure means the amount of push or pull
(force) applied to each unit area of the surface and is
expressed in pounds per square inch (lb/in2) or grams
per square centimeter (gm/cm2). Pressure may be
exerted in one direction, in several directions, or in all
Computing Force, Pressure, and Area
A formula is used in computing force, pressure,
and area in hydraulic systems. In this formula, P refers
to pressure, F indicates force, and A represents area.
Force equals pressure times area. Thus, the
formula is written F = P x A
Pressure equals force divided by area. By
rearranging the above formula, this state may be
condensed into the following: P = F divided by A.