Hydraulic accumulators have various tasks to fulfill in a hydraulic system: energy storage, fluid reserve, emergency operation, equalizing of forces, damping of mechanical and pressure shocks, leakage oil compensation, oscillation damping, pulsation damping, vehicle suspension, recuperation of deceleration energy, maintaining pressure
constant and flow compensation (expansion tanks).
There is always an equilibrium between pressure of the hydraulic fluid and the counter pressure generated by the weight
(weight loaded accumulator) (A),
the spring (spring accumulator) (B)
or the gas (gas pressure accumulator)(C).
Weight loaded accumulator
Weight loaded and spring loaded accumulators are only suitable for special industrial use and are therefore of limited importance. Gas pressurized accumulators are catagorised according to their separating element (between the gas and fluid) into bladder accumulators, piston accumulators and diaphragm accumulators. The weight loaded accumulator is the only hydraulic accumulator,where the oil pressure remains constant regardless of amount filled, however a large volume of space is required for the weight.
The diaphragm accumulator consists of two hollow, hemispherical metal sections bolted together at the center. Notice in Figure that one of the halves has a fitting to attach the unit to the hydraulic system; the other half is equipped with an air valve for charging the unit with compressed air or nitrogen. Mounted between the two halves is a synthetic rubber diaphragm that divides the accumulator into two sections. The accumulator is initially charged with air through the air valve to a pressure of approximately 50 percent of the hydraulic system pressure. This initial air charge forces the diaphragm upward against the inner surface of the upper section of the accumulator.
Depending on the type of separating element used between the gas and fluid, accumulators are categorized into bladder accumulators, piston accumulators and diaphragm accumulators.
Diaphragm accumulators comprise a steel container which is resistant to compression and is usually either spherical or cylindrical in shape. The separating element inside the accumulator is a diaphragm made of an elastic material (elastomer).There are two types of diaphragm accumulator available:
• In the screwed model the diaphragm is held in position by screwing the top and bottom part to clamping nuts. It is possible to exchange the diaphragm in this model. At the bottom of the diaphragm in the centre is a valve plate, which prevents the diaphragm being pulled out when fluid is connected. This could occur primarily when the accumulator is completely emptied.
• In welded accumulators the diaphragm is pressed into the lower part before circular seam welding is carried out.
By using a suitable welding process and by situating the diaphragm correctly, this ensures that the elastomer material is not damaged when the welding is carried out. It is not possible to exchange the diaphragm.
When fluid pressure increases above the initial air charge, fluid is forced into the upper chamber through the system
pressure port, pushing the diaphragm down and further compressing the air in the bottom chamber. Under peak load, the air pressure in the lower chamber forces fluid back into the hydraulic system to maintain operating pressure. Also, if the power pump fails, the compressed air forces a limited amount of pressurized fluid into the system.
The bladder accumulator operates on the same principle and for the same purpose as the diaphragm accumulator but varies in construction, as shown in the Figure . The unit is a one-piece metal sphere with a fluid pressure inlet at the top and an opening at the bottom for inserting the bladder. A large screw-type plug at the bottom of the accumulator is a retainer for the bladder that also seals the unit. A high-pressure air valve is also incorporated in the retainer plug. Fluid enters through the system pressure port. As fluid pressure increases above the initial air charge of the accumulator, it forces the bladder downward against the air charge, filling the upper chamber with fluid pressure. The broken lines in Figure 1-11 indicate the approximate position of the bladder at the time of the initial air charge..
The piston accumulator serves the same purpose and operates by the same principles as do the diaphragm and bladder accumulators. As shown in Figure , the unit consists of a cylinder and piston assembly with ports on each end. Fluid pressure from the system enters the left port, forcing the piston down against the initial air charge in the right chamber of the cylinder. A high-pressure air valve is located at the right port for charging the unit. A drilled passage from the fluid side of the piston to the outside of the piston provides lubrication between the cylinder walls and the piston.
Pulse-tone pulsation damping
(4) Pulse-tone connection
(5) Accumulator bladder
(6) Non-return valve
This type of damping device comprises in-line connection plate (4). This enables flow and pressure fluctuations to be optimally coupled to the gas displacement.
Hence good damping characteristics are obtained up to a frequency of about 500 Hz.
Mounting elements for hydraulic accumulators
(3) Rubber back-up ring
Hydraulic accumulators must be sufficiently protected and secured due to their considerable weight and in addition due to the acceleration forces created by the fluid flows into and out of the accumulators. The securing of these accumulators must be such that no additional forces or bending forces are transferred by the accumulators to the hydraulic circuit.