Planetary gearboxes exhibit highly modulated vibration, which cannot be observed in fixed-axis gearboxes. From the existing research, two contradicting descriptions can be obtained. For planetary gearboxes used in aircraft engines, the modulation may be explained by a time varying transfer path. However, the literature also contains counterexamples to this theory, particularly in the case of small to medium sized industrial planetary gearboxes. An alternative hypothesis explains the modulation by the existence of a resulting force on the central members of the planetary gearbox. The amount of experimental data concerning the modulation of gear mesh vibration is however limited. Therefore, the objective of this thesis is to investigate the vibration of small commodity planetary gearboxes designed for servo drive applications.

In the first step of this investigation, an experimental setup is developed. The test rig can drive a planetary gearbox specimen at defined speed, torque and temperature. It allows for precisely aligned shafts in order to prevent any uncontrolled disturbance. In addition, the test rig uses extensive instrumentation consisting of accelerometers, force transducers, incremental encoders and auxiliary sensors. Six different gearbox designs are tested. For the most intensively investigated type, six identical specimens exist. The micro-geometry of these is measured before their initial assembly.

The experimentally recorded vibration of the gearboxes is processed using two different methods, which both rely on order tracking. At first, a model of the dynamics of each specimen is identified. Several pronounced resonances can be observed from the experimental data. All specimens yield similar natural frequencies, but the modulation varies noticeably. In addition, each resonance shows a different modulation. Therefore, a second method is developed with the aim of characterizing the modulation of a given gearbox specimen. All vibration data is aggregated to a vibration signature.

The experimental results show that the vibration signature is indeed specific for an individual specimen. In addition, three distinct features can be observed in the modulation. One component of the vibration signals is modulated synchronously with the planet carrier and a second synchronously with the sun gear. Furthermore, unmodulated vibration may also be present. Unexpectedly, no correlation between the manufacturing errors and the modulation can be shown. However, an additional experiment shows that the influence of the assembly process is evident. The geometrical deviations introduced during assembly cannot be measured using established measurement devices.

Therefore, a numerical multi-body model of the gearbox is developed. Using this model, geometrical errors are defined in the bearings and distributed randomly. Correlation analysis of the simulated vibration signatures and the geometrical deviations shows that the three features observed in the modulation may be linked to individual causes. The unmodulated part of the vibration signals is of particular interest, because it indicates a double flank meshing condition. Here, left and right flanks of the meshing teeth transmit a contact force simultaneously, which is undesirable.