Chapter 1 Introduction to Fluid Power 19
is further complicated by the inherent differences
of the two major divisions of the fluid power field:
hydraulics and pneumatics.
System Characteristics
Although hydraulic and pneumatic systems
share the characteristics of energy transfer by
means of fluid pressure and flow, differences
affect how and where they are applied. These dif-
ferences include:
■ Accuracy of actuator movement
■ Operating pressure
■ Actuator speed
■ Component weight
■ Cost
Accuracy of movement
Fluid compressibility is the inherent char-
acteristic that produces the difference between
hydraulic and pneumatic systems. A gas is com-
pressible, while a liquid can be compressed only
slightly. Hydraulic systems, therefore, can produce
more accurate, easily controlled movement of cyl-
inders and motors than pneumatic systems. Com-
pressibility produces a more “spongy” operation
in pneumatic systems that is not suitable where
highly accurate movement is required.
Operating pressure
Hydraulic systems can operate at much higher
pressures than pneumatic systems. Hydraulic sys-
tem operating pressure ranges from a few hundred
pounds per square inch (psi) to several thousand
psi. Pressures of more than 10,000 psi are used in
special situations. Pneumatic systems, in contrast,
normally operate between 80 to 120 psi. Extremely
high–pressure pneumatic systems normally are
not used.
Actuator speed
Pneumatic systems are commonly used when
high-speed movement is required in an applica-
tion. Rotation speeds of over 20,000 revolutions
per minute (rpm) are possible. Rapid-response
cylinder operation is also possible with pneumatic
systems. These designs are generally found in sit-
uations involving lighter loads and lower accuracy
requirements.
Component weight
System operating pressure affects the struc-
ture of components. Hydraulic systems operate
at higher pressures, requiring the use of stronger
materials and more-massive designs to withstand
the pressure. Pneumatic systems operate at much
lower pressures and, therefore, can be manufac-
tured using lightweight materials and designs that
minimize the amount of material.
Hydraulic applications tend to involve equip-
ment that handles heavier weights, requiring
both higher system operating pressure and physi-
cal strength of machine parts. Figure 1-6 shows
a front-end loader. The cylinders used to operate
the bucket must be of a construction that can with-
stand high system pressure and the heavy load of
the bucket and its contents.
Pneumatic systems tend to involve applica-
tions where ease of handling and lightweight
are critical for effective operation of the tool or
system. Figure 1-7 shows a pneumatic grinder
being used on a large metal casting. The grinder
is lightweight and very portable. The tool is easily
manipulated by an individual and constructed to
provide a long service life.
Figure 1-6. Hydraulic systems are commonly used
in applications where high system pressures are
needed to complete the required work.
(Deere & Company)