The essential parts of a piston-type cylinder are a cylindrical barrel, a piston and rod, end caps, and suitable seals. The end caps are attached to the end of the barrel. These end caps usually contain fluid ports. The end cap on the rod end contains a hole for the piston rod to pass through. Suitable seals are used between the hole and the piston rod to keep fluid from leaking out and to keep dirt and other contaminants from entering the barrel. The opposite end cap of most cylinders is provided with a fitting for securing the actuating cylinder to some structure. This end cap is referred to as the anchor end cap.

Figure 3-26.- Operation of a rotary spool valve.

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

To satisfy the many requirements of fluid power systems, piston-type cylinders are available in various designs with the most common being the single- (fig. 3-30, view A) and double-acting (fig. 3-30, view B).

Figure 3-27.- Operation of a sliding spool valve.

Accumulators
An accumulator is a pressure storage reservoir in which hydraulic fluid is stored under pressure from an external source. Accumulators have four major uses:

1. Store energy. Accumulators that store energy are often used as boosters for systems with fixed displacement pumps. The accumulator stores pressure

Figure 3-28.- Single-acting ram-type actuating cylinder.

Figure 3-29.- Double-acting ram-type actuating cylinder.

oil during slack periods and feeds it back into the system during peak periods of oil usage.

2. Absorb shocks. Accumulators that absorb shocks take in excess oil during peak pressures and let it out again after the surge is past. This action reduces vibrations and noise in the system. It also smoothes operation during pressure delays, such as when a variable displacement pump goes into stroke.

3. Build pressure gradually. Accumulators that build pressure gradually are used to soften the working stroke of a piston against a fixed load as in a hydraulic press.

4. Maintain constant pressure. Accumulators that maintain constant pressure are always weight-loaded types that place a fixed force on the oil in a closed

Figure 3-30.-( A) Single-and (B) double-acting piston-type cylinders.

4. Maintain constant pressure. Accumulators that maintain constant pressure are always weight-loaded types that place a fixed force on the oil in a closed circuit. Whether the volume of oil changes from leakage or from heat expansion or contraction, this accumulator keeps the same gravity pressure on the system.

While most accumulators can do any of these things, their use in a system is limited to only one. The major types of accumulators are as follows: pneumatic (gas-loaded), weight-loaded, spring-loaded.