A bladder accumulator stores hydraulic energy in a bag of compressed gas. Oil pushed in at the bottom squeezes the nitrogen-filled bladder, and when the pressure drops the gas pushes the oil back out. Follow the oil as it charges and releases the accumulator.
A bladder accumulator is a pressure vessel that stores hydraulic energy in a cushion of compressed gas. Because oil and gas are kept apart by a rubber bladder, the gas acts like a spring: it can take in oil, hold it under pressure, and give it back when the circuit asks.
From the outside it is only a sealed steel bottle with one pipe. What it does — storing and returning energy without any moving parts except the flexing rubber — is invisible, so a cutaway is the clearest way to see the bladder, the gas and the oil working together.
Before use, the bladder is filled with nitrogen through the gas valve at the top. This pre-charge sets the pressure at which the accumulator begins to work, much like inflating a balloon to a chosen pressure.
When circuit pressure rises above the pre-charge, oil flows in through the bottom port. It pushes up against the bladder, which yields and lets the oil take more of the vessel's space.
As the bladder is compressed, the nitrogen inside is squeezed into a smaller volume. Squeezed gas pushes back harder, so its pressure climbs — and that pressure is the energy now stored in the accumulator.
When the circuit needs oil faster than the pump can supply, its pressure drops below the gas pressure. The gas expands and pushes the bladder down, forcing the stored oil back out of the port to help drive the load.
If the oil were fully expelled the bladder would be pushed toward the port. The poppet valve closes just before that, trapping a little oil and keeping the bladder safely inside the vessel.
It stores pressurised oil so the circuit has a reservoir of energy. It can absorb sudden pressure spikes, hold pressure while the pump is idle, and release a quick burst of oil when a valve needs it.
The bladder holds pre-charged nitrogen gas. When oil is pushed in at the bottom the gas is squeezed, and like any compressed gas it pushes back. Because the gas is sealed off by rubber, no gas dissolves into the oil.
The pre-charge sets the pressure at which the accumulator starts to work. If it were too low the bladder would bottom out too soon; if it were too high, little oil could enter. The right pre-charge lets the accumulator give and take oil smoothly.
It closes the oil port before the bladder can be pushed out through it. Without the poppet the bladder would extrude into the pipe and be destroyed when the oil pressure dropped.
The bladder itself is a replaceable rubber part inside the vessel. Wear or a wrong gas charge damages it, so the vessel is opened and a new bladder fitted rather than the whole accumulator replaced.