Hydraulic Fixed Displacement Pump Basic Principle

Vane and piston pumps are available as fixed or variable displacement pumps. If the cam ring is fixed in position (vane pump), or the swashplate is fixed in position (piston pump), the displacement is fixed. Gear pumps, however, are available only with a fixed displacement. These pumps are less expensive, so they are widely used.

As will be seen in the next section, the leakage in some gear pumps is high, and this leakage increases as the pump wears over time. Ownership cost of fixed-clearance gear pumps is lower, but operating cost is higher over the life of the pump. Total cost (ownership + operating) for the entire design life may or may not be lower for a pump with lower initial cost.

Pressure-balanced gear pumps have volumetric efficiencies that exceed, or at least, rival those of piston pumps. Some of these gear pumps are rated for pressures in excess of 4000 psi.

An external gear pump is shown in Fig. 4.1. One gear is driven with the input shaft, and it drives the second gear. Fluid is captured by the teeth as they pass the inlet, and this oil travels around the housing and exits at the outlet. The design is simple, and it is inexpensive. It is also apparent that there are opportunities for leakage all along the housing as the fluid travels from the inlet to the outlet.

A gear pump design is available with a moveable side plate. When this side plate is out, the fluid is not confined, and no pressure is built. When it is pressed in against the rotating gear, the fluid is confined, and pressure builds. These pumps are used on over-the-road trucks. They are driven by an auxiliary shaft from the engine referred to as the power-takeoff (PTO) shaft. There is no clutch, so the PTO) shaft turns whenever the engine turns. The pump is engaged (pumps fluid) when the side plate is pressed in against the rotating gear.


A gerotor (gear) pump has an inner drive gear and an outer driven gear (Fig. 4.2). As the inner gear rotates, it drives the outer gear. The inner gear has one less tooth than the outer gear, and this feature creates chambers of decreasing volume, and thus the “pumping action.” A port plate ensures that fluid enters the chamber when it is largest and exits when it is smallest.


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