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A variable displacement pump is a type of mechanical-electrical motor placed in a fluid channel. Built either to generate electrical power from the natural flow of fluid past it or to apply mechanical force to make stationary fluid flow in a pipe gives the variable displacement pump a dual use — adjustable capability. Such pumps have versatile applications because they can be set to capture the maximum amount of hydraulic fluid energy as the velocity of fluid flow in a channel dynamically changes, or they can be switched to applying mechanical energy powered by electricity to increase fluid flow.
Hydraulic motors designed on the variable displacement pump axial principle have three common features regardless of their size or the complexity of their controls. These include a central axis, a cylinder of pistons surrounding it, and a circular plate to which the ends of all of the pistons are attached. They are usually built on the axis piston pump or bent axis pump design, which allows the vertical angle at which the pistons are positioned to be adjusted either by angling the axle itself, or the plate that is attached to the ends of the pistons.
The circular array of pistons in a variable displacement pump move in unison, but they displace different amounts of fluid depending on their horizontal positions in the piston shafts in which they move. As the internal retainer plate, or external swash plate, rotates at an off-center vertical angle of 25° to 40° in the pipe, the pistons alternate between pushing fluid through an exhaust pipe, and pulling it into an intake pipe. These outlet and inlet pipes are known as kidney slots and the piston chambers are referred to as cylinder barrel bores.
Another key feature of a variable displacement pump is what's known as the compensator. This is a collection of controls often attached to the external housing of the pump, which adjusts the angle of the swash plate or retainer plate. If the swash plate is set to a perfect vertical angle on the pump as it rotates, the displacement will be zero and fluid flow will stop. Compensators can be simple or complex controls, but their purpose is to change the variable settings on the pump for maximum efficiency.
Least resistance to flow is achieved either through a bypass valve or by matching pump displacement to the flow being induced. Lowering displacement by leveling the swash plate is known as inherent hydrostatic braking, which brings flow to a stop, a property that is a selling point for variable displacement axial piston pumps for mobile equipment as it nearly eliminates the risk of a rollaway incident.
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