  Noise measurement on a population of a gearbox manufactured in large number
reveals a high variability principally due to tolerances on each design
parameter of the gear. Gearbox noise results mainly from vibration of its
housing excited by the transmission error. High dynamic mesh load, high
vibratory response of housing and high acoustic level of the gearbox can be
observed for some critical rotational speeds. These ones correspond to
excitation in a resonant manner of some particular modes which store a high
potential energy at the meshing stiffness. Variability of the associated
critical modes depend on tolerances affected to gear design parameters. It also
depends on bearings stiffnesses dispersion. The physical origins of the
bearings stiffnesses variability are preloads. Bearings stiffnesses and
tolerances affect the characteristics of the meshing stiffness and the dynamic
transfer from dynamic meshing force to the housing vibratory response. The
purpose of this paper is to study effect of the bearings stiffnesses
variability by estimating the first two moments of statistical distribution of
critical modes.
