The Weight-Loaded or Gravity accumulator, shown in Fig. 4–8, consists of a long, finely ground and polished vertical steel cylinder that is fitted with a long, close fitting, smooth finished piston. A sealing device of some type is fitted into the cylinder wall to prevent fluid from leaking past the piston. Weights are mounted or placed on the piston to maintain a constant fluid pressure with in the cylinder and the remainder of the system. The amount of weight depends on the system pressure. The piston is prevented from over traveling by limit switches that turn the pump off when the level is too high and turn the pump on when the level becomes low.
The fluid capacity of most weight – loaded accumulators does not exceed 250 cubic inches (slightly over one gallon). Weight – loaded accumulators are used infrequently because they are large, heavy, costly, and sluggish. Their response to changes in fluid demand is slow especially during high input surges because of the large mass of the weights and the frictional drag of the pressure seals.
Spring – Loaded Piston accumulator
The Spring-Loaded Piston accumulator, shown in Fig. 4–9, is similar to the weight –loaded accumulator in design and principle. The pressure is maintained within the system by one or more springs. As excess system fluid is admitted to the cylinder, the piston raise, compressing the springs. As the system required fluid, the springs expand and force the piston down, adding fluid to the system. The amount of fluid in the cylinder and the system is controlled by pressure switch that turns the pump on when the system pressure reaches the minimum pressure point and turns it off when the accumulator is charged and maximum pressure is reached. Most spring – loaded accumulators have a safety stop switch that limits piston travel and also shuts off the pump if the spring breaks
Like weight – loaded accumulators, the fluid capacity of spring – loaded piston accumulators is small (usually less than a gallon). Although spring – loaded piston accumulators have a low maintenance cost, they are comparatively large and expensive for their capacity. Their response is fairly fast, but their use is limited to small volume and low pressure applications.