The purpose of hydraulic actuators is to transform
fluid pressure into mechanical energy. They are used
where linear motion or rotary motion is required.
Actuators are generally of the cylinder or motor design.
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
Figure 10-12.-Example of ram type of cylinders.
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.
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 paragraphs below.
RAM TYPE OF CYLINDER. The ram type of
cylinder (fig. 10-12) is used primarily for push
functions rather than pull. Some applications simply
require 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. Some
of these various designs are discussed in the paragraphs
Single-Acting Ram. The single-acting ram (fig.
10-12, view A) applies force in only one direction. Fluid
directed into the cylinder displaces the ram and forces
it outward. Since there is no provision for retracting the
ram by the use of fluid power, the retracting force can
be gravity or some mechanical means, such as a spring.
This type of actuating cylinder is often used in the
Double-Acting Ram. A double-acting ram type of
cylinder is illustrated in figure 10-12, view B. In this
cylinder, both strokes of the ram are produced by
pressurized fluid. There are two fluid portsone at or
near each end of the cylinder. To extend the ram and
apply force, fluid under pressure is directed to the closed
end of the cylinder through port A. To retract the ram
and reduce force, fluid is directed to the opposite end of
the cylinder through port B.
PISTON TYPE OF CYLINDER. This type of
cylinder is normally used for applications that require
both push and pull functions. Thus, the piston type
serves many more requirements than the ram type;
therefore, it is the most common type used in fluid power
The housing consists of a cylindrical barrel that
usually contains either external or internal threads on
both ends. End caps with mating threads are attached to
the ends of the barrel. These end caps usually contain
the fluid ports. The end cap on the rod end contains a
hole for the piston rod to pass through. Suitable packing
must be used between the hole and the piston rod to
prevent external leakage of fluid and the entrance of dirt
and other contaminants. The opposite end cap of most