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.

PNEUMATIC ACCUMULATORS. - In the pneumatic accumulators. gas and oil occupy the same container. When the oil pressure rises, incoming oil compresses the gas. When oil pressure drops, the gas expands, forcing oil out.

In most cases, the gas is separated from the oil by a piston (fig. 3-31), a bladder (fig. 3-32), or a diaphragm (fig. 3-33). This prevents mixing of gas and oil, keeping gas out of the hydraulic system.

WEIGHT-LOADED ACCUMULATORS. - The weight-loaded accumulator uses a piston and cylinder along with heavy weights on the piston for loading or charging the oil. It is loaded by gravity and operation is very basic. The pressure oil in the hydraulic circuit is pushed into the lower section of the cylinder, raising the piston and weights. The accumulator is now charged and ready for work. When oil is needed, pressure drops in the system and gravity