The modelling of positive displacement machines has to deal with the complexity of solving the flow through the unit in presence of simultaneous macro and micro-motions of the moving parts. In this paper, the main phenomena characterizing the operation of the spur external gear units are successfully analyzed by means of a numerical model developed by the authors. The model, here referred as HYdraulic GEar machines Simulator (HYGESim), consists of different modules: a fluid dynamic model, a model for the evaluation of the movements of the gears’ axes of rotation and a geometrical model. When performing a HYGESim simulation, these models are executed in a co-operative simulation. Starting directly from the CAD drawing of the unit as input, the simulation tool permits to describe the main features of the flow though the machine and to evaluate the possible wear of the casing wear accounting for a detailed description of the geometry of the internal components (i.e. teeth’s profile, design of lateral bushes). The paper describes the modelling approach and the main potentials of the simulation tool, pointing out how it can be utilized for design purposes. As a matter of fact, HYGESim permits to analyse the effects of the main design parameters on important aspects like efficiency, internal pressure peaks, local cavitation and fluctuations of flow at unit’s ports (associated with fluid borne noise). The presented tool also allows the evaluation of the radial movements of the gears’ axes of rotation resulting from the forces exerted on both gears, thus permitting to study the balancing feature related to a certain design. The paper also presents several comparisons between simulation results and experimental data coming from an experimental activity specifically performed for validation of the presented tool.