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, com-pressed air or nitrogen is used instead of a spring for movement in the direction opposite that achieved with fluid pressure.

Figure 8-1 shows a single-acting, spring-loaded, piston-type actuating cylinder. In this cylinder the spring is located on the rod side of the piston. In some spring-loaded cylinders, the spring is located on the blank side, and the fluid port is located on the rod side of the cylinder.

Figure 8-1.Single-acting, spring-loaded, piston-type actuating 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. See figure 8-1. 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. The actuated unit is moved 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.

To retract the piston rod, the directional control valve is moved to the opposite working position, which releases the pressure in the cylinder. The spring tension forces the piston to the left, retracting the piston rod and moving the actuated unit in the opposite direction. The fluid is free to flow from the cylinder through the port, and back through the control valve to return.

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. Leakage between the cylinder wall and the piston is prevented by seals. Hydraulic components use seals or gaskets to prevent leakage between static parts (nonmoving), such as a valve body and a hydraulic line fitting. Seals also prevent leakage between dynamic (moving) parts, such as the piston and cylinder wall. The most common seal is an O-ring. Some static seals and all dynamic seals require a backup ring or rings.