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
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. 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.
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