Variable-displacement piston pumps (VDPP) offer control options based on pressure, flow, horsepower, or a combination of those parameters. Paul Badowski, from Cross Co., reviews the basic types of VDPP control schemes, and the reasons a pump user would use them.

Badowski first explains the concept of variable displacement, saying that the amount of flow that a pump can provide depends on a rotating group of pistons. Varying the stroke of the pistons alters pump displacement.

Variable-displacement piston pumps provide flow dependent on a rotating group of pistons. Variable-displacement piston pumps provide flow dependent on a rotating group of pistons. Several VDDP suppliers are highlighted in the Products & Suppliers directory of Engineering360 . Along with pump specifications are details of their available control schemes. These include:

Atos North America

Parker Hannifin, and

Continental Hydraulics

The most basic type of VDPP control is a pressure-compensated setup. Here, a heavy spring and piston are used to operate the swash plate. System pressure flows against one side of an internal piston, which is held by the heavy spring. When system pressure exceeds spring pressure, the swash-plate angle changes and pump flow is reduced. The pump will maintain the set pressure, producing very little or no flow, until the load varies. Then, the swash-plate angle changes and allows the pump to produce flow.

Badowski says that with this control setup, “you must have enough (horsepower) to take the pump to full pressure at full flow. If there is not enough HP, the prime mover will slow down or stall before the pressure begins to compensate and lower the flow.”

An example of such an application: using a hydraulic motor to operate a conveyor. The load is constant and the motor requires about 1500 PSI to handle the load. “Set the piston-pump compensator at 1600 PSI and let it run,” he says, “and set the system relief a few hundred PSI higher. If they are set too close, they can fight each other, causing the pump to go on and off stroke and/or the relief to open and close, causing inefficiency, heat and vibration.”

Another control option explained by Badowski is use of a load-sense compensator, which includes a lighter spring setting to control the swash plate. Upstream pressure, ported into a load-sense port on the pump, acts against the load-sense piston. When the pressure requirement exceeds the offset, the swash-plate angle changes and pump flow increases. Once the pressure is balanced, flow remains steady.

As an example of how such a control scheme is used, Badowski reflects back to the original conveyor application, but with a varying load. Consider, for example, that the conveyor requires 1500 PSI to move 50% of the time, and 2250-2500 PSI the rest of the time.

“With a standard pressure compensator, you would have to set the pump at 2600 PSI to accomplish the work,” he says. “When the work only requires 1500 PSI, the pump will be trying to produce 2600 PSI. …With a load-sense compensator, when the load requires 1500 PSI, the pump will actually produce about 1700-1800 PSI.”

Lastly, Badowski tackles the more complex torque-limiting or horsepower-limiting control. By adding an additional spring and piston, users can set a pump to always maximize its allowable input torque. This, he says, will maximize output flow and pressure at a defined setting.

As a sample application, he considers operating a large-bore (10 in.) cylinder with a 150-in. stroke. During most of the stroke, the cylinder does not perform much work and can operate at 80 to1200 PSI. However, during the last 20 in. of stroke, system pressure is 4500 PSI, “but we can move much slower,” Badowski says.

In this example, the pump has an output of 15 CIR and a maximum flow of 113 gal. at 1750 RPM; the 75-HP electric motor has a 1.15 service factor. “I want to keep my cylinder moving as fast as possible,” he says, “but I also want to ensure that I never exceed a power demand of 82 HP.”

At 82 HP, the pump produces 1254 PSI at full output, and113 GPM. As pressure increases, flow decreases and pressure increases--at 90 GPM, for example, the system will produce about 1560 PSI, and at 60 GPM the system will produce almost 2350 PSI. At 4500 PSI, pump flow will be reduced to about 31 GPM.

“The advantage of this pump,” Badowski says, “is that the internal controls of the pump are adjusting to maximize flow and pressure at all times without exceeding the available HP.”

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