cylinders is provided with a fitting for securing the actuating cylinder to some structure. For obvious reasons, this end cap is referred to as the anchor end cap.

The piston rod may extend through either or both ends of the cylinder. The extended end of the rod is normally threaded for the attachment of some type of mechanical connector, such as an eyebolt or a clevis, and a locknut. This threaded connection of the rod and mechanical connector provides for adjustment between the rod and the unit to be actuated. After correct adjustment is obtained, the locknut is tightened against the connector to prevent the connector from turning. The other end of the eyebolt or clevis is connected, either directly or through additional mechanical linkage, to the unit to be actuated.

To satisfy the many requirements of fluid power systems, you may get piston type cylinders in various designs. Two of the more common designs (fig. 10-13) are described in the paragraphs below.

Single-Acting Piston. - The single-acting piston-type cylinder (fig. 10-13, view A) is similar in design and operation to the single-acting ram-type cylinder previously covered. The single-acting piston-type cylinder uses fluid pressure to apply force in only one direction. In some designs of this type, the force of gravity moves the piston in the opposite direction; however, most cylinders of this type apply force in both directions. Fluid pressure provides the force in one direction, and spring tension provides the force in the opposite direction. In some single-acting cylinders, compressed air or nitrogen is used instead of a spring for movement in the direction opposite that achieved with fluid pressure.

The end of the cylinder opposite the fluid port is vented to the atmosphere. This prevents air from being trapped in this area. Any trapped air would compress during the extension stroke, creating excess pressure on the rod side of the piston. This would cause sluggish movement of the piston and could eventually cause a complete lock, preventing the fluid pressure from moving the piston.

You should note that the air vent ports are normally equipped with an air filtering attachment to prevent ingestion of contaminates when the piston retracts into the cylinder.

A three-way directional control valve is normally used to control the operation of this type of cylinder. To extend the piston rod, fluid under pressure is directed through the port and into the cylinder. This pressure acts on the surface area of the blank side of the piston and forces the piston to the right. This action, of course, extends the rod to the right through the end of the cylinder. This moves the actuated unit in one direction. During this action, the spring is compressed between the rod side of the piston and the end of the cylinder. Within limits of the cylinder, the length of the stroke depends upon the desired movement of the actuated unit.

Double-Acting Piston. - Most piston type actuating cylinders are double-acting, which means that fluid under pressure can be applied to either side of the piston to provide movement and apply force in the corre- sponding direction.

One design of the double-acting piston type actuating cylinder is illustrated in figure 10-13, view B. This cylinder contains one piston and piston rod assembly. The stroke of the piston and piston rod assembly in either direction is produced by fluid pressure. The two fluid ports, one near each end of the cylinder, alternate as inlet and outlet, depending upon the direction of flow from the directional control valve.

This is referred to as an unbalanced actuating cylinder; that is, there is a difference in the effective working areas on the two sides of the piston. Assume that the cross-sectional area of the piston is 3 square inches and the cross-sectional area of the rod is 1 square

Figure 10-13.-Example of piston type of cylinder.