In this paper a lumped parameter model for the numerical simulation of the dynamic behaviour of external gear pumps and motors is presented. The model is developed in AMESim® environment, combining the use of AMESim® libraries of components and a self-made library developed in C++ code. The choice of a single modelling environment allows both the simulation of the machine dynamic behaviour and the analysis of the interaction between the external gear unit itself and the hydraulic circuit in which it is working. Particular attention is devoted in the model in order to consider the eccentric position of the gears inside the casing. Moreover, the relevance of an accurate definitions of the flow leakages involving the inter-teeth volumes, considering also the Couette flow contribution, is highlighted. Finally, a numerical analysis has been carried out by means of the model in order to determine the influence of load and rotational speed on the dynamic behaviour of the machine; results about the pressure ripple, flow rate at the outlet port, pressure transients on the inter-teeth volumes and gears position inside the casing are exposed and commented.