An actuating cylinder is a device that converts fluid power to linear motion, 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. The cylinder consists of a ram or piston operating within a cylindrical bore. Actuating 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.

RAM-TYPE CYLINDERS.- 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 is more than one halfofthe cross-sectional area of the movable element. In most actuating cylinders of this type, the rod and the

Figure 3-22.- Hydraulic pressure regulator.

Figure 3-23.- Spring-loaded pressure-reducing valve.

movable element have equal areas. This type of movable element is frequently referred to as a plunger. The most common ram-type cylinders are the single-( fig. 3-28) and double-acting (fig. 3-29).

The ram-type actuator is primarily used to push, rather than pull. Some applications require simply a

Figure 3-24.- Counterbalance valve.

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.

Figure 3-25.- The basic operation of a simple poppet valve.