ACTUATORS

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.

Cylinders

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 below.

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 hydraulic jack.

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 ports - one 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 systems.

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