Keywords: automotive transmission rattle, gear dynamics, noise, vibration and harshness.

Manual transmission gear rattle is the result of repetitive impacts of gear teeth within the limit of their backlash. This noise, vibration and harshness (NVH) phenomenon is a major concern in the automotive industry and can occur under various loaded or lightly loaded conditions. The induced vibrations manifest themselves as structure- or air-borne noises. The structure-borne noise path is through the transmission shafts to the casing, which depending on the excitation, may act as an efficient noise radiator. The generated air-borne noise, from the impact sites falls within the band of human audible reception.

The paper describes an impact dynamics model for the meshing of gear teeth-pairs under medium to heavy loads and in the presence of backlash. The model incorporates classical Hertzian impact for the torque transmitting gear, governed by the instantaneous geometry of the contact and the prevailing kinematics of the contiguous surfaces of helical teeth pairs in contact. The unselected/loose gears in constant mesh are modelled using an analytical solution of the Reynolds' hydrodynamic flow equation, where the effect of transmission oil is accounted for in the lubricated contacts. It also includes the effect of flank friction, contributed by a combination of adhesive friction due to asperity-tip interactions, as well as viscous shear of partially lubricated conjunctions. The inclusion of such contact/impact models into gear pair dynamics sheds some light on the spectral contributions noted from transmission systems, under dynamic conditions. Among these engine orders, harmonics of meshing and natural frequencies related to contact stiffness can be identified.

Typically, the results are analysed using various signal processing methods, including fast Fourier transform (FFT) plots, gear response and their forcing functions are presented in the time domain.

It is concluded that the loose gears under a hydrodynamic regime of lubrication contribute towards a more dominant rattle noise, showing a clear fluctuation in the film thickness time history and promoting gear teeth impacts. This is appreciated in the frequency spectrum by evaluating the energy contribution of the meshing frequencies relative to the engine orders. A more orderly meshing, inducing less rattle is dominated by the meshing frequency harmonics, whereas higher rattling conditions are governed by the engine orders. On the other hand, the engaged gear pair pertaining an elasto-hydrodynamic regime of lubrication shows a minimum contribution of the engine orders, proving that loaded gear wheels have almost no effect on transmission gear rattle.

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