A variable displacement pump is a device that converts mechanical energy to hydraulic (fluid) energy. Some of these devices can also be reversible, meaning that they can act as a hydraulic motor and generate mechanical energy from fluid energy. The displacement can be adjusted to increase or decrease the amount of fluid pumped. A common variable displacement pump used in vehicle technology is the axial piston pump. This pump has several pistons in cylinders arranged parallel to each other and rotating around a central shaft. A swash plate at one end is connected to the pistons. As the pistons rotate, the angle of the plate causes them to move in and out of their cylinders. A rotary valve at the opposite end from the swash plate alternately connects each cylinder to the fluid supply and delivery lines. By changing the angle of the swash plate, the stroke of the pistons can be varied continuously. If the swash plate is perpendicular to the axis of rotation, no fluid will flow. If it is at a sharp angle, a large volume of fluid will be pumped. Some pumps allow the swash plate to be moved in both directions from the zero position, pumping fluid in either direction without reversing the rotation of the pump.
Another very efficient design is called the bent axis pump. Bending the axis is done to get rid of problems associated with side loads on the pistons.
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