What's An Accumulator?
A hydraulic accumulator is a device in which potential energy is stored in the form of a compressed gas or spring, or by a raised weight to be used to exert a force against a relatively incompressible fluid.
They are used in fluid power systems to accumulate energy and to smooth out pulsations. A hydraulic system utilizing an accumulator can use a smaller fluid pump since the accumulator stores energy from the pump during low demand periods. This energy is available for instantaneous use, released upon demand at a rate many times greater than could be supplied by the pump alone.
Accumulators can also act as surge or pulsation absorbers, much as an air dome is used on pulsating piston or rotary pumps. They will cushion hydraulic hammer, reducing shocks caused by rapid operation or sudden starting and stopping of power cylinders in a hydraulic circuit.
There are four principal types of accumulators, the weight loaded piston type, diaphragm (or bladder) type, spring type and the hydropneumatic piston type. The weight loaded type was the first used but is much larger and heavier for its capacity than modern piston and bladder types. Both the weighted type, and mechanical spring type are very seldom used today. The hydro-pneumatic types use a gas as a spring cushion in conjunction with a hydraulic fluid, the gas and fluid being separated by a thin diaphragm or a piston. Tobul accumulators, having an aluminum piston of low inertia as standard equipment, are superior to other makes in absorbing either high or low frequency pulsations.
Hydro-pneumatic accumulators incorporate a gas in conjunction with a hydraulic fluid. The fluid has little dynamic power storage qualities. The fluid normally used in fluid power applications can be reduced in volume only about 1.7% under a pressure of 5000 PSI. Therefore when only 2% of the total contained volume is released, the pressure of the remaining oil in the system will drop to zero. However, the relative incompressibility of a hydraulic fluid makes it ideal for fluid power systems and provides quick response to power demand.
The gas, on the other hand, a partner to the hydraulic fluid in the accumulator, can be compressed to high pressures and low volumes. Potential energy is stored in this compressed gas to be released upon demand. This 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.
In most fluid power applications, pumps are used to generate the required power to be used or stored in a hydraulic system. Many pumps deliver this power in a pulsating flow. The piston pump, as commonly used for higher pressures, tends to produce pulsation detrimental to a high pressure system. An accumulator properly located in the system will substantially cushion these pressure variations.
Cushions Operating Shock.
In many fluid power applications the driven member of the hydraulic system stops suddenly, creating a pressure wave which is sent back through the system. This shock wave can develop peak pressures several times greater than normal working pressures and can be the source of system failure or objectionable noise. The gas cushion in an accumulator, properly placed 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 over 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 overcome by the addition of an adequate accumulator to the hydraulic system.
Supplements Pump Delivery.
An accumulator, capable of storing power, can supplement the fluid 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 utilize a much smaller pump, resulting in savings in cost and power.
Pressure changes occur in a hydraulic system when the liquid is subjected to rising or falling temperatures. Also, there may be pressure drop due to leakage of hydraulic fluid. An accumulator compensates for such pressure changes by delivering or receiving a small amount of hydraulic liquid. In the event the main power source should fail or be stopped, the accumulator would act as an auxiliary power source, maintaining pressure in the system.
An accumulator may be used to dispense fluids under pressure, such as lubricating greases and oils.