A CFD analysis of the oil squeezing power losses of a gear pair

Abstract

Efficiency is becoming more and more a main concern in the design of power transmissions and the demand for high efficiency gearboxes is continuously increasing. Also the more and more restrictive euro standards for the reduction of pollutant emissions from light vehicles impose to improve the efficiency of the engines but also of the gear transmissions. For this reason the resources dedicated to this goal are continuously increasing. The first step to improve efficiency is to have appropriate models to compare different design solutions. Even if the efficiency of transmissions is quit high if compared with the efficiency of the engines and appropriate models to predict the power losses due to gear meshing, to bearings and to seals already exist, in order to have a further improvement, some aspects like the power losses related to the oil churning, oil squeezing and windage are still to be investigated. In previous papers the authors have investigated by means of computational fluid dynamic (CFD) analysis and experimental measurements the churning losses of planetary speed reducers (in which there is a relative motion between the “planets + planet carrier” and the lubricant). Also the windage power losses have been studied by the authors in previous papers. This report is focused on the oil squeezing power losses. This kind of losses is associated with the compression-expansion process by the meshing teeth. The contraction of the volume at the gear mesh implies an overpressure that induces a fluid flow primarily in the axial direction and this, for viscous fluids, means additional power losses and a decrease of the efficiency. In this work this phenomena has been studied by means of some CFD simulations. The influence of some operating conditions like the lubricant properties, the rotational speed and the temperature has been studied.