It is currently assumed that churning losses can be described by using only two physical parameters representative of the lubricant properties, i.e., density and viscosity. To verify this hypothesis, a specific gear test rig was used and a number of transient measurements were made with various oil temperatures. It appears that, for high temperatures and/or high rotational speeds, the drag torque may suddenly increase with an increasing Reynolds number. In order to explain this behavior, on-line lubricant aeration measurements were taken which show that this observation can be related to churning losses, as the drag torque becomes larger when the fraction of air in the lubricant increases. It is concluded that lubricant density and viscosity are not sufficient in themselves to estimate churning losses and that additional properties related to the lubricant aeration should be considered.
