One of the outstanding features of fluid power
systems is that force, generated by the power
supply, controlled and directed by suitable
valving, and transported by lines, can be con-
verted with ease to almost any kind of mechanical
motion desired at the very place it is needed.
Either linear (straight line) or rotary motion can
be obtained by using a suitable actuating device.
An actuator is a device that converts fluid
power into mechanical force and motion.
Cylinders, motors, and turbines are the most
common types of actuating devices used in fluid
This chapter describes various types of
actuating cylinders and their applications,
different types of fluid motors, and turbines used
in fluid power systems.
An actuating cylinder is a device that converts
fluid power to linear, or straight line, force and
motion. Since linear motion is a back-and-forth
motion along a straight line, this type of actuator
is sometimes referred to as a reciprocating, or
linear, motor. The cylinder consists of a ram or
piston operating within a cylindrical bore. Actuat-
ing cylinders may be installed so that the cylinder
is anchored to a stationary structure and the ram
or piston is attached to the mechanism to be
operated, or the piston or ram may be anchored
to the stationary structure and the cylinder
attached to the mechanism to be operated.
Actuating cylinders for pneumatic and
hydraulic systems are similar in design and
operation. Some of the variations of ram- and
piston-type actuating cylinders are described in
the following paragraphs.
The terms ram and piston are often used
interchangeably. However, a ram-type cylinder is
usually considered one in which the cross-sectional
area of the piston rod is more than one-half the
cross-sectional area of the movable element. In
most actuating cylinders of this type, the rod and
the movable element have equal areas. This type
of movable element is frequently referred to as
The ram-type actuator is used primarily to
push rather than to pull. Some applications
require simply a flat surface on the external part
of the ram for pushing or lifting the unit to
be operated. Other applications require some
mechanical means of attachment, such as a clevis
or eyebolt. The design of ram-type cylinders varies
in many other respects to satisfy the requirements
of different applications.
The single-acting ram (fig. 10-1) applies force
in only one direction. The fluid that is directed
into the cylinder displaces the ram and forces it
outward, lifting the object placed on it. Since there
Figure 10-1.Single-acting ram-type actuating cylinder.