What type of valve is used to regulate the flow of
What are the three dimensions of any tubular
What are the three basic parts of a flexible hose?
What type of connector is used in a low-pressure
When tubing nuts are tightened what tool should
What type of seal application allows for a slight
leakage for seal lubrication?
What is the most common metal seal used in
What type of seal is ideally suited for both low-
pressure and high-pressure applications?
What kind of leakage is caused by the wearing of
seals and mated parts?
Learning Objective: Explain the operating principles of a
pneumatic system. Identify operational characteristics and
service procedures applicable to heavy-duty compressors.
The word pneumatics is a derivative of the Greek
word pneuma, which means air, wind, or breath.
Pneumatics can be defined as that branch of engineering
science that pertains to gaseous pressure and flow. As
used in this manual, pneumatics is the portion of fluid
power in which compressed air, or other gas, is used to
transmit and control power to actuating mechanisms.
This section discusses the basic principles of
pneumatics, characteristics of gases, heavy-duty air
compressors, and air compressor maintenance. It also
discusses the hazards of pneumatics, methods of
controlling contamination, and safety precautions
associated with compressed gases.
BASIC PRINCIPLES OF PNEUMATICS
Gases differ from liquids in that they have no definite
volume; that is, regardless of size or shape of the vessel, a
gas will completely fill it. Gases are highly compressible,
while liquids are only slightly so. Also, gases are lighter
than equal volumes of liquids, making gases less dense
Compressibility and Expansion of Gases
Gases can be readily compressed and are assumed
to be perfectly elastic. This combination of properties
gives gas the ability to yield to a force and return
promptly to its original condition when the force is
removed. These are the properties of air that is used in
pneumatic tires, tennis balls, and other deformable
objects whose shapes are maintained by compressed
Kinetic Theory of Gases
In an attempt to explain the compressibility of gases,
consider the container shown in figure 3-49 as containing
a gas. At any given time, some molecules are moving in
one direction, some are travelling in other directions, and
some may be in a state of rest. The average effect of the
molecules bombarding each container wall corresponds
to the pressure of the gas. As more gas is pumped into the
container, more molecules are available to bombard the
walls, thus the pressure in the container increases.
Increasing the speed with which the molecules hit the
walls can also increase the gas pressure in a container. If
the temperature of the gas is raised, the molecules move
faster, causing an increase in pressure. This can be shown
by considering the automobile tire. When you take a long
drive on a hot day, the pressure in the tires increases and a
tire that appeared to be soft in cool morning temperature
may appear normal at a higher midday temperature.
When the automotive tire is initially inflated, air that
normally occupies a specific volume is compressed into a
smaller volume inside the tire. This increases the pressure
on the inside of the tire.
Figure 3-49.Molecular bombardment creating pressure.