Transmission drive rattle with thermo-elastohydrodynamic impacts:numerical and experimental investigations

The paper studies the effect of torsional fluctuations resident on the automotive transmission input shaft upon the rattle phenomenon due to engine orders, as well as transmitted torque variations from any engaged gear pair. Speed variations interrupt the orderly meshing of the idling gears, thus causing impulsive action emanating as rattle. This is a Noise, Vibration and Harshness (NVH) phenomenon of significant concern to industry. The engaged (selected) gear pair teeth are subject to moderate to high loads, thus a thermo-mixed elastohydrodynamic regime of lubrication ensues. Unlike the idle gear pairs, where lightly loaded hydrodynamic conditions prevail, the elastohydrodynamic regime of lubrication ensures reduced teeth separation effects. Both types of lubricated conjunctions abound in a model of a 7-speed transmission system. The numerical predictions agree well and complement measured performance on an engine bed - transmission rig of a front wheel drive vehicle. The paper uses a non-dimensional parameter - impulsion ratio - which relates gear dynamics to the prevailing conditions in the lubricated conjunctions, showing the critical and often conflicting role that lubricant rheology plays in the promotion or palliation of rattle. Furthermore, it objectively demarcates the boundary between low intensity rattle of frequent teeth pair separations from high intensity double-sided teeth impacts with ensuing torque reversals.