Hydraulic accumulator

A hydraulic accumulator is an energy storage device. It is a pressure storage reservoir in which a non-compressible hydraulic fluid is held under pressure by an external source. That external source can be a spring, a raised weight, or a compressed gas. The main reasons that an accumulator is used in a hydraulic system are so that the pump doesn't need to be so large to cope with extremes of demand, so that the supply circuit can respond more quickly to any temporary demand and to smooth pulsations.
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