Usage:950G II AXR
The gas charged accumulator is the accumulator type most commonly used in implement hydraulic systems on mobile equipment. There are two different types of gas charged accumulators shown in illustration 1, the bladder type (shown on the left), and the piston type (shown on the right). Both types separate the gas from the oil in order to keep the gas contained.
When the oil pressure is higher than the gas pressure, the volume of gas will become smaller, and allow more oil into the accumulator. The volume of gas continues to decrease until the gas is compressed enough to equalize the gas pressure and the oil pressure.
When the gas pressure is higher than the oil pressure, the volume of gas will expand. This expansion will push oil out of the accumulator into the hydraulic system, until the gas and the oil pressure are once again equal.
Bladder type accumulators used on mobile equipment vary in size from 0.5 L (0.13 US gal) to 57 L (15 US gal). Piston type accumulators used on mobile equipment vary in size from 1.06 L (0.28 US gal) to 43 L (11 US gal).
Uses of Accumulators
The combined ability to store both pressure and a volume of oil allows many uses of accumulators in hydraulic systems. The following items are uses of accumulators:
- Permits usage of smaller pumps - The storage of a volume of oil under pressure provides makeup oil for the steering or pilot system when demand exceeds pump flow.
- Provides emergency steering and braking - The volume of oil under pressure can provide enough input to the steering and/or brake systems to control the machine, for a short period of time, in the event of pump or engine failure.
- Maintains constant pressure - The expansion and contraction ability of the gas to change volume with minimal changes in pressure is used in pilot systems to hold the controls steady even with variations in supply to the system.
- Absorbs shock loads - Accumulators are used on mobile equipment to improve roadability. This improved roadability is accomplished by absorbing the rush of oil in the system on rough terrain that would otherwise create pressure spikes and machine bounce. Examples of this usage are scraper cushion-hitch systems and wheel loader ride control systems.
Most accumulator failures would be caused by one of the following:
- Improper installation - Wrong size or wrong design accumulator being used.
- Over/under charge - The gas precharge must be at least 20% or greater of the maximum operating pressure of the hydraulic system.
- Piston seal failure - Either too smooth or too rough a finish on the accumulator cylinder wall will cause premature piston seal failure. Contamination can also cause piston seal failures by sticking in the seal and scoring the cylinder wall.
- Charging valve failure - Gas will exhaust to the atmosphere allowing 100% fill of the accumulator with oil.
- Material cracking/fatigue - Temperature below the bladder or seal lower design limit, normally -20° F to -40°F will cause the bladder or seal to become brittle and crack. Gas precharge below 20% of system maximum operating pressure will cause fatigue failures from excess flexing of the bladder. (See illustration 3).