Development of a gearbox test rig to evaluate the impact of oil formulations on churning and friction losses

From large industrial to small high-speed transmission for electric vehicles, there is a growing interest in reducing the power losses on gearboxes. Hydrodynamic losses are especially relevant on high-speed gearboxes. On electric cars, the gearbox lubricant also works as a cooling fluid for the motor, which brings new challenges as performance under an electrified environment, thermal and electrical conductivity, corrosion protection etc. Most current test rigs use the torque difference between gearbox output and input to calculate the power losses. This work, uses the instantaneous deceleration on a small gearbox to obtain the no-load depending on power losses. Due to its simplicity, the proposed method has the potential to be applied to actual transmissions with very simple sensors and without the need for significant intervention on the actual equipment. The experimental results showed increased churning power losses with the immersion depth and oil viscosity. With the tested transmission oils, the churning losses showed to depend on rotational speed by a power of three, as predicted by the ISO model. The influence of oil viscosity was less evident. Reducing viscosity 2.5 times showed only around 25% reduction in the calculated churning losses. In opposition to the tested transmission oils, a low-viscosity oil with Friction Modifier additives reduced the power losses with the consecutive larger volumes of sump oil, such unexpected result can be explained by the beneficial effect of the oil FM additives.