The gear dynamics has been a research field that attracts
extensive amount of modeling efforts. The early studies, including
those reviewed in Refs. [3,4], mostly focused on the noise and
vibration aspect of gears, using discrete or finite element
approaches for the description of the dynamic behavior of single
or multi degree-of-freedom (DOF) gear systems. For simplification
purpose, these works excluded the friction along the contact sur-
faces, which was found to be important in the dynamic response