When a hydraulic pump operates, it performs two functions. First, its mechanical action creates a vacuum at the pump inlet which allows atmospheric pressure to force liquid from the reservoir into the inlet line to the pump. Second, its mechanical action delivers this liquid to the pump outlet and forces it into the hydraulic system.
The positive-displacement principle is well illustrated in the reciprocating-type pump, the most elementary positive-displacement gear pump, Figure 1. As the piston extends, the partial vacuum created in the pump chamber draws liquid from the reservoir through the inlet check valve into the chamber. The partial vacuum helps seat firmly the outlet check valve. The volume of liquid drawn into the chamber is known because of the geometry of the pump case, in this example, a cylinder.
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Figure 10. Pressure-flow curve of fixed-displacement oil hydraulic pump.
Figure 11. Pressure flow curve of variable-displacement hydraulic oil transfer pump with ideal flow and pressure compensation.
A pump's mechanical efficiency is also less than perfect, because some of the input energy is wasted in friction. Overall efficiency of a hydraulic oil pressure pump is the product of its volumetric and mechanical efficiencies.