External gear pumps and motors are robust and low cost positive displacement machines and are widely used in industrial and mobile applications. Nowadays however, optimal global efficiency represents a more crucial aspect to be considered when designing a hydraulic machine. For this reason, it becomes a primary necessity to investigate the phenomena which determine and affect the hydraulic machine total efficiency. In this work, the volumetric efficiency dependence on the operating speed and delivery pressure of external gear pumps is investigated by means of a mathematical model already presented in a previous paper and the results obtained are compared with experimental data. First of all, the mathematical model is briefly presented; then the predicted results are discussed considering the influence of the pump operating conditions. Moreover, the influence on the volumetric efficiency of the behavior of the hydraulically balanced bearing blocks, which are responsible of the pump axial balance, is exposed. In particular, it is shown that the tilted position of the bearing blocks and the height of the lateral clearances can greatly affect the volumetric efficiency. The data coming from an experimental investigation carried out on commercial external gear pump units are then presented; both the volumetric and the mechanical efficiency have been measured. The comparison between the experimental data and the numerical results allows discussing the bearing blocks balancing behavior during pump operation.
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