Fixed and Variable Pumps, the Comparison
In the fluid power industry, it is not uncommon for beginners to have misconceptions about fixed flow and variable flow pumping concepts. While both types of pumps serve the primary purpose of converting mechanical energy into hydraulic energy to transfer force downstream, they differ significantly in their method of delivery.
A pump's movement is determined by the theoretical volume of gears, vanes, or pistons displaced in one revolution. For instance, a 30 cm3 pump will theoretically push 30 ml of fluid in a single rotation. This volume remains constant in fixed displacement pumps, regardless of control or downstream conditions. However, actual flow may vary based on factors such as efficiency, speed, and pressure, requiring excess flow to be diverted or returned to the tank if less flow is needed than the pump is rated for.
In contrast, a variable displacement pump can adjust its displacement manually, hydraulically, or electronically. The maximum displacement of an axial piston pump is calculated by multiplying the quantity and bore area of the pistons by the stroke length. Variable displacement axial piston pumps utilize a swashplate to guide the pistons, with the swashplate angle determining the piston stroke's length. The pump operates at full displacement whenever possible, and a control piston, managed by a relief valve, reduces the swashplate angle when downstream pressure rises above the compensator setting. This results in minimized pump volume and displacement, achieved by adjusting the piston stroke. Electro-proportional valves can also control the pump flow in closed-loop electronic control applications.
In the case of pressure-compensated vane pumps, they function differently from piston pumps. The rotating group of a vane pump is exposed to suction and pressure chambers inside the housing, and vanes move outward and inward inside the offset cam ring. A control piston in a variable vane pump pushes against the cam ring to reduce effective displacement when downstream pressure increases, thereby decreasing flow to maintain the desired downstream pressure level.
