For novices in the fluid power industry, a lack of understanding between fixed flow and variable flow pump concepts is common. One of the tasks of a hydraulic pump is to convert the input mechanical energy on its shaft into the hydraulic energy of an actuator that can transmit the force somewhere downstream. This force transfer is common for both fixed and variable pumps, but the method transfer is quite different.


The displacement of a pump is determined by the theoretical volume of the displacement that a gear, vane or piston will make in one revolution. If the pump is 30 cc, it would theoretically push 30 ml of fluid in a single revolution, about 1.8 in.3. For a fixed displacement pump, this 30 cm3 will not change no matter how the pump is controlled or what happens downstream. Actually, actual flow will vary based on efficiency, speed and pressure, but this is another matter. If the required flow is less than the rated flow of the pump, the excess flow must be diverted or released into the tank.

Variable displacement pumps have methods to increase or decrease displacement manually, hydraulically, or electronically. The method of changing the displacement depends on the structure of the pump, and between the piston pump and the vane pump, and between the two, the iteration is still different.

The maximum displacement of an axial piston pump is determined by the number of pistons and hole area times the stroke length. Although the stroke length can be fixed, such as most radial and curved shaft piston motors, the stroke can also be varied. Variable displacement axial piston pumps use a swash plate to guide the piston to reciprocate while rotating about the axis of the shaft. The angle of the swash plate relative to the piston determines the length of the piston stroke. In the case of variable pumps, the swash plate is supported by bearings or bushings.

Place an offset piston (and spring) and a control piston on opposite sides of the swash plate. The variable displacement piston pump is designed to be "on-stroke", which means that it wants to pump at full displacement as much as possible. The control piston is operated by an overflow valve in essence. If the downstream pressure rises above the set value of the pressure compensator, it will push out the control piston to reduce the angle of the swash plate. As the swashplate angle decreases, the piston can now move a fraction of its stroke potential. Because the displacement is determined by the area, number and stroke of the pistons, the volume of the pump can now be reduced. If the downstream pressure is still higher than the compensator setting, the stroke will decrease until the swashplate angle is close to zero, at which point the swashplate angle can only pump a sufficient amount to maintain lubrication.


The swashplate angle can be mechanically changed by lever or wheel, but in advanced applications, the electric proportional valve can control the control piston to adjust the pump flow as required. This is an advanced concept used in closed-loop electronic control applications. The proportional pressure valve will adjust the pump's control piston under the guidance of the PLC, thereby providing the precise flow required by the machine under changing conditions.


The working principle of variable displacement pressure compensated vane pump is completely different from that of piston pump. Instead of the reciprocating piston, the rotary set of the vane pump is exposed to a suction chamber and a pressure chamber inside the casing. The blades move outwardly and inwardly within a cam ring that is offset relative to the axis of the rotation group. There is a control piston in the variable vane pump, but this time it pushes towards the cam ring. When the downstream pressure rises, the control piston pushes the cam ring toward the center of the housing, thereby reducing the misalignment between the cam ring and the rotation group. This reduces effective displacement and reduces flow to reduce downstream pressure.


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