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