The operating cycle of planetary gear transmissions is composed by the run up
regime, steady state regime with fixed speed and the run down regime. Run up and
run down are one of the main non stationary excitations for planetary gearboxes.
This paper investigates, in these two regimes, the dynamic behaviour of a
special configuration of two stages planetary gear which are mounted
back-to-back. A dynamic model is developed. The variability of speed in the run
up and run down regimes is included in the model. The computation of the dynamic
response shows that additional sidebands instability zones are generated. Using
the Short Time Fourier Transform, an amplitude-frequency modulation is observed
in the non stationary condition. An experimental study done on a back-to-back
planetary gear test rig is conducted for validation of the mathematic model.