Oil squeezing power losses in gears: a CFD analysis

Abstract

Efficiency is becoming more and more of a main concern in the design of power transmissions and the demand for high efficiency gearboxes is continuously increasing; also the euro standards for the reduction of pollutant emissions from light vehicles imposed to improve the efficiency of the engines and gear transmissions are becoming more and more restrictive. 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 quite high compared to the efficiency of the engines and appropriate models to predict the power losses due to gear meshing, to bearings and to seals that already exist, in order to have 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 CFD (computational fluid dynamic) 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). This report is focused on the oil squeezing power loss. This kind of loss is associated with the compressionexpansion 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 efficiency. In this work this phenomenon has been studied by means of some CFD simulations. The influence of some operating conditions, such as the lubricant properties, rotational speed and temperature, have been studied.