In this paper a lumped parameters numerical model is reviewed to study the meshing process of external gear pumps and motors, with the aim of highlighting the influence of some geometrical design parameters and operating conditions on inter-teeth volumes pressures.
The inter-teeth space is modeled adopting a two-volume approach, properly tailored both for the pump and for the motor units behavior description. In both cases, the communications between the interconnected inter-teeth volumes and the high and low pressure ports are sketched as variable equivalent turbulent restrictors; flow areas have been determined as functions of the gears and of the meshing grooves main design parameters. The inter-teeth pressures, and the leakage flows, are calculated solving the incompressible and isothermal continuity equation, contemporarily applied to both volumes and properly combined with the classical turbulent orifice equation. Successively, for a reference operating condition, properly chosen for underlining the inter-teeth meshing cycle and corresponding to a very low pressure load, the meshing processes of an external gear pump and of an external gear motor are compared. Among others, the attention is focused on the behavior of those hydraulic variables more conditioning the meshing cycle, and the differences rising up in inter-teeth pressure peaks and in cavitation interval amplitude and angular positioning are evidenced. Then, the meshing process analysis is expanded to cover the external gear units common operating field, both in terms of rotational speed (from 500 to 3000 rpm) and in terms of the high pressure port value (from 10 to 250 bar). In all cases, the general trend shows that cavitation has a deeper influence on external gear pumps operation, while external gear motors are subjected to more significant pressure peaks, particularly marked during the low pressure part of the meshing cycle. Attention is also focused on the influence of gears inter-axial distance and axial width on the meshing process characteristics, in order to assess how the volume dimensions could differently affect pressure peaks and cavitation phenomena in pumps and motors.
Finally, the role of the balancing plates grooves in conditioning pressure peaks and cavitation is highlighted and, with particular care devoted to external gear motors, some useful guidelines for their design are outlined.