Influence of Aerated Lubricants on Gear Churning Losses–An Engineering Model

Abstract

It is currently assumed that churning losses can be described by using only two physical parameters representative of the lubricant properties; that is, density and viscosity. To verify this hypothesis, a number of transient measurements were carried out on a specific gear test rig over a range of oil temperatures. It appears that, for high temperatures and/or high rotational speeds, the drag torque can suddenly increase with a larger Reynolds number. Based on extensive online lubricant aeration measurements, it is demonstrated that this particular behavior can be related to churning losses when the fraction of air in the lubricant reaches a certain threshold. In order to quantify the influence of oil sump aeration on churning losses, a simplified original model, based on surface tension and lubricant aeration, is proposed. This study shows that density and viscosity are not sufficient to estimate churning losses under some specific operating conditions and the need to account for other physical properties of the lubricant is emphasized.