Accumulators store pressure in a reservoir in which hydraulic fluid is held under pressure by an external source. That external source can be a compressed gas, a spring, or a weight. They are installed in hydraulic systems for two main purposes: to store energy and to smooth out pulsations.
As energy storage, accumulators typically allow the hydraulic system to use a smaller pump because they amass energy from the pump during periods of low demand. This energy is available for instantaneous use, and is released on demand at a rate many times greater than what could be supplied by the pump alone. As a pulsation or surge damper, accumulators cushion the hydraulic hammer, reducing shocks caused by rapid operation or sudden starting and stopping of cylinders in a hydraulic circuit.
Two designs of accumulators are widely used in hydraulic systems — piston and bladder accumulators, Figure 1. Piston accumulators include weight-loaded piston type, spring type, and hydropneumatic piston type. The weight-loaded type was the first used, but is very heavy for its capacity and much larger than modern piston and bladder types. Both weighted and spring types are infrequently found today. Hydropneumatic piston accumulators are now the type most commonly used in industry.
Energy storage — Hydraulic accumulators incorporate a gas in conjunction with a hydraulic fluid. The fluid has little dynamic power-storage qualities; typical hydraulic fluids can be reduced in volume by only about 1.7% under a pressure of 5000 psi. (However, this relative incompressibility makes them ideal for power transmission, providing quick response to power demand.) Therefore, when only 2% of the total contained volume is released, the pressure of the remaining oil in the system drops to zero.
On the other hand, gas can be compressed into small volumes at high pressures. Potential energy is stored in the compressed gas to be released upon demand. Such energy can be compared to that of a raised pile driver ready to transfer its tremendous energy upon the pile. In the piston type accumulator, the energy in the compressed gas exerts pressure against the piston separating the gas and hydraulic fluid. The piston in turn, forces the fluid from the cylinder into the system and to the location where useful work will be accomplished.
Pulsation absorption — Pumps generate the power needed for use or storage in a hydraulic system. Many pumps deliver this power in a pulsating flow. The piston pump, commonly used for its high pressure capability, can produce pulsations detrimental to a high-pressure system. An accumulator properly located in the system will substantially cushion these pressure variations.
Shock cushioning — In many applications, the driven member of the hydraulic system stops suddenly, creating a pressure wave that travels back through the system. This shock wave can develop peak pressures several times greater than normal working pressures. It can cause objectionable noise or system failure. An accumulator’s gas cushion, properly located in the system, will minimize this shock.
An example of this application is the absorption of shock caused by suddenly stopping the loading bucket on a hydraulic front end loader. Without an accumulator, the bucket — weighing more than 2 tons — can completely lift the rear wheels of a loader off the ground. The severe shock to the tractor frame and axle, as well as operator wear and tear, is reduced by adding an accumulator to the hydraulic system.
Supplementing pump flow — An accumulator configured for storing power can supplement the hydraulic pump in delivering power to the system. The pump stores potential energy in the accumulator during idle periods of the work cycle. The accumulator transfers this reserve power back to the system when the cycle requires emergency or peak power. This enables a system to use a much smaller pump, resulting in savings in cost and power.
Maintaining pressure — Pressure changes occur in a hydraulic system when the liquid is subjected to rising or falling temperatures. There may also be pressure drop due to hydraulic fluid leakage. An accumulator compensates for such pressure changes by delivering or receiving a small amount of fluid. If the main power source should fail or be stopped, the accumulator would act as an auxiliary power source, maintaining pressure in the system.
Fluid dispensing — An accumulator may be used to dispense small volumes of fluids, such as lubricating greases and oils, on command.