What is a Hydraulic Accumulator?

Paul Scott

A hydraulic accumulator is a device which stores hydraulic pressure to act as a buffer in hydraulic systems during periods of peak demand, system pressure fluctuations, and to counteract hydraulic shock. When the system is subjected to extreme loads this pressurized fluid is introduced to relieve the demand on the hydraulic pump. This process is also repeated when there are unexpected drops in system pressure and when fluid hammer or hydraulic shock occurs. These devices use an external source to keep the fluid in the accumulator reservoir at approximately the same pressure as the average system pressure. There are several common types of hydraulic accumulator systems using raised weights, springs, compressed gas, and metal bellows to maintain the pressure in their reservoirs.

The cycling of an aircraft's undercarriage requires more hydraulic input than the rest of the system's collective average usage.
The cycling of an aircraft's undercarriage requires more hydraulic input than the rest of the system's collective average usage.

The use of an externally pressurized source of buffer fluid in a hydraulic system allows designers to use smaller pumps than those necessary to handle peak demands. These conditions are commonly encountered in hydraulic systems such as those which operate the undercarriage of large aircraft. The cycling of an aircraft's undercarriage requires more hydraulic input than the rest of the systems collective average usage. In cases such as these, the hydraulic pumps can be rated for average demand levels and be supplemented by accumulator pressure when needed. A hydraulic accumulator can supply a far larger flow of pressurized fluid for a short period than the pump can which makes them ideal for high demand situations.

Sudden unexpected pressure fluctuations in a hydraulic system can also be addressed by the pressure reserve in the accumulator. These fluctuations may be the result of environmental influences such as temperature changes or result from slow leaks. A hydraulic accumulator can take up the slack, so to speak, and prevent the pump from constantly maintaining system pressure. This is particularly useful during periods of low demand where system pressure needs only be maintained at a preset level.

A hydraulic accumulator also adds an element of protection against aggressive hydraulic phenomena such as fluid hammer which may damage system components. Fluid hammer or fluid shock is a condition caused by a sudden change in direction or stoppage of moving hydraulic fluid. The resultant pressure wave can cause minor problems such as shuddering or noise in the system or, if severe enough, irreversibly damaging system components. A hydraulic accumulator can form a barrier to protect the system against these pressure waves.

There are several methods used to maintain pressure in hydraulic accumulators all of which are external and don’t rely on system pressure. One of the oldest mechanisms in use is the raised weight accumulator which relies on a weighted piston to maintain the pressure in the reservoir. Other compression methods include pistons which rely on compressed gas or springs to maintain the pressure in the accumulator. One of the most efficient accumulator types is a variant utilizing a hermetically sealed metal bellows arrangement instead of a piston to compress the fluid. This type of accumulator is particularly effective because it allows maximum flexibility in the type of fluid used and maximum storage volumes in the accumulator.

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