Lightly loaded lubricated impacts: idle gear rattle

Idle gear rattle is associated with the characteristic noise that unselected impacting gears radiate to the environment. It is induced by engine order vibration in the presence of backlash in the unengaged gear pairs, resulting in oscillatory response within their backlash range. A tribo-dynamic model of a front wheel drive manual transmission has been developed to study idle rattle, considering the hydrodynamic contact film reaction and flank friction. The model includes the torsional motions of the idle gears and the lateral motions of the supporting output shafts. The hydrodynamic lubricant film formed between the gear teeth under light impact loads behaves as a non-linear spring-damper mechanism, whilst the inclusion of the shafts’ bearing compliances introduces additional non-linear terms, which are modelled as piecewise linear functions. The aim of the paper is to demonstrate the effect of the lubricant on the system’s response, which is eventually transferred to the gearbox case through the bearings. The results are found to conform closely to experimental measurements taken from a vehicle equipped with a manual transmission of the same type.