Influence of Transient Nonlinear Behavior of Lubricant Stiffness and Damping Upon Transmission Gear Rattle

Abstract Transmission rattle occurs in light-to-medium loaded interactions of loose gear pairs when subjected to vibration. The article provides solutions for iso-viscous and piezo-viscous hydrodynamic lubricant behavior that are an underlying cause of gear rattle. The nonlinear stiffness and damping characteristics are obtained, and their variation under transient conditions in combined rolling, and normal approach and separating contacts are discussed. Relationships for lubricant reaction under both lightly loaded iso-viscous and medium loaded piezo-viscous conditions are determined; the latter for the first time for the case of counterformal line contact geometries. The underlying reason for idle rattle, a significant concern in vehicular transmissions, is explained in a fundamental manner through unstable spectral content of lubricant film surface ripple oscillations. As transmitted load increases during vehicle launch, piezo-viscous behavior suppresses these instabilities, inducing creep rattle during the transition from idle to drive rattle. Thus, the fundamental role of lubricant behavior in influencing transmission rattle has been demonstrated, which has not hitherto been reported in the literature.