The vibration signature of a gear depends upon the torque load on the gear. The amplitude and the
relative harmonic content or signal shape are both known to change with changing load. To properly
model the vibration, these signal amplitude and shape effects must be included in the model. In the
current study, two discrete torque levels, 80% and 100%, were included in the one empirical model.
An alternative empirical model could be made by creating a distinct model for each torque level,
utilizing measurements from the gear in good condition to train a model for each torque level.
Individual models for each torque level may work well for the test rig environment; however, in
flight, the torque range is broad and continuous. A modeling method that incorporates the nonlinear
dependence of the vibration signal on torque, and possibly other operating parameters, is needed. A
nonlinear extension of Principal Component Analysis [44] (an alias for SVD) might accomplish this.