Hydraulic accumulators, most simply put, are energy storage devices. They are similar to rechargeable batteries within electrical systems, except that they store and discharge energy in the form of pressurized fluid. The accumulator itself is a pressure vessel that holds hydraulic fluid and a compressible gas, most commonly nitrogen. The shell or housing of the accumulator is made from materials such as steel, stainless steel, aluminum, titanium, and fiber-reinforced composites. Within the device a flexible barrier, such as a piston or rubber bladder, separating the oil from the gas.
In hydraulic accumulators, hydraulic fluids only slightly compress under pressure. Adversely, gases can be compressed into small volumes under high pressures, a benefit which engineers take advantage of in accumulator design and usage. Essentially, potential energy is stored in the compressed gas and is released as needed to force oil from the accumulator into a circuit. During operation of the device, the gas volume is pre-charged, typically to approximately 80-90% of the minimum system working pressure. This causes the gas volume to expand and fill most of the accumulator with only a small amount of oil remaining inside. During operation, the hydraulic pump raises the system pressure, forcing fluid to enter the accumulator. Its flow in and out is controlled by valves. The piston or bladder moves and compresses the gas volume, as fluid pressure exceeds the precharge pressure. This is the device’s source of stored energy.
There are three common types of hydraulic accumulators, those being bladder, piston, and diaphragm. Bladder accumulators feature large ports that allow for rapid fluid discharge and help ensure that the device is comparatively resistant to dirt and contamination. In general, bladder accumulators should be mounted vertically, although they can be mounted on their sides when used in low-cycle applications. Accumulators of this type are typically designed to have a 4:1 pressure ratio (maximum pressure to gas-charged pressure) to protect the bladder from excessive distortion and material strain.
Piston accumulators are similar to hydraulic cylinders without a rod. Like other types, piston accumulators consist of a fluid and gas section separated by a movable piston. Some piston accumulators replace the high pressure gas section with a spring or heavy weight to apply force to the piston, but this configuration is uncommon. Piston accumulators are ideal for large stored volumes up to and beyond 100 gallons. They can have high flow rates and their pressure ratio is limited only by the design. They are frequently used in rugged, heavy duty applications, but are sensitive to contamination that can damage the seals.
Diaphragm accumulators operate similarly to bladder accumulators. The difference is that they use an elastic diaphragm to separate the oil and gas volumes rather than a rubber bladder. Diaphragm accumulators are economical, compact, and lightweight devices that offer relatively small flow and volume, usually around one gallon. Because the elastic diaphragm does not distort as much as a bladder, diaphragm accumulators are able to handle higher compression ratios up to 10:1. Additionally, they have wider mounting flexibility, can better withstand contamination, and respond rapidly to changes in pressure, making them ideal for shock applications.
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