What is in this article?:
- Accumulators add functionality to hydraulic circuits
- Piston-type accumulators
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Hydraulic accumulators are one of the most underutilized tools in the fluid-power chest, which is unfortunate, because they provide myriad advantages to a hydraulic system. Accumulators are often misunderstood, especially as it relates to troubleshooting, but the principles of their operation are straightforward, even if the mathematics used in their specification is not everyone’s cup of tea.
Accumulators are available in two groups, mechanical and hydropneumatic. Mechanical accumulators can be loaded with either a spring or with a mass, although both are uncommon. Spring-loaded accumulators are of the piston type and installed with a spring opposed to the oil side. Just as with compressed gas, the amount of spring compression has an effect on the level of output, as more potential energy is lost as the piston moves toward the outlet port and energy is depleted.
Weight-loaded accumulators are vertically mounted and are piston-type as well, but with a large mass using gravity to pressurize fluid rather than a compressed spring or gas. Because mechanical accumulators are rarely applied, we can leave them out of the discussion, except to say a weight-loaded accumulator is the only type able to provide undiminished pressure throughout the entire length of its stroke, as gravity remains constant the entire distance of piston travel.
Hydropneumatic accumulators contain both a dry side (containing compressed gas, usually nitrogen) and a wet side, containing hydraulic liquid. The dry side of the accumulator is filled with the gas to a prescribed pressure, known as the precharge, based on system requirements. Because hydraulic systems perform poorly when gases the hydraulic fluid, some form of separation is required in accumulators to prevent the mixing of fluids; bladders, diaphragms, and pistons are most common.
Bladder accumulators, Figure 1, are the most common form and use a synthetic rubber, such as Nitrile, to contain the nitrogen gas precharge. The bladder is installed inside a high-pressure-capacity sealed cylinder with hemispherical caps at either end. The ideal mounting position of bladder accumulators is with the liquid port downward and the gas port upward. When a bladder accumulator is charged, its bladder fills most of the void inside the shell, and a poppet valve at the oil port prevents the bladder from extruding out the bottom. Bladder accumulators are commonly available in a wide range of sizes, from 1⁄4 to 15 gal, depending on the range the manufacturer offers.
As a bladder accumulator fills with pressurized hydraulic fluid, the nitrogen-charged bladder compresses, storing hydraulic energy equal to the volume of fluid taken in factored with the pressure of the precharge. As the fluid is discharged when downstream hydraulic pressure drops, the bladder re-expands as it pushes the oil out. The vertical mounting position will help prevent uneven wear of the bladder as the accumulator cycles, and also help avoid odd occurrences of trapped fluid or early poppet closing. That being said, a bladder accumulator will survive longer in any position, including horizontally, if fluid contamination is well-controlled.
Bladder accumulators are easily repaired, and fortunately, the major players in the accumulator game offer interchangeable rebuild kits for common accumulator sizes. The most frequently chosen bladder accumulators are bottom repairable, although top repairable are offered as well. Top-repairable accumulators are useful for installation on machines with complex plumbing or difficult mounting location, as they allow the accumulator to be repaired while remaining installed on the machine. Bottom-repairable accumulators need to be removed from the machine for repairing.
Diaphragm accumulators use the same type of synthetic rubber as bladder accumulators, but instead of the balloon shape, they are just a membrane separating the top and bottom halves of the shell, Figure 2. Diaphragm accumulators are either the welded non-repairable type or the threaded repairable type. The welded type has a diaphragm bonded in the middle to separate the gas and liquid sides, and due to the nature of their construction, they are capable of medium pressure, typically not more than 5000 psi.
Threaded bladder accumulators, Figure 3, have a couple of advantages over the welded type, although they tend to cost more than the latter. Their threaded construction compresses the diaphragm between the top and bottom shell to hold it fast, enabling higher pressure capacity, some able to achieve 10,000 psi. The other advantage, of course, is that the diaphragm can be replaced by unscrewing the two halves. Both types of diaphragm accumulators are not typically large, ranging from only a few cubic inches of volume up to around 1 gal.