Why changing planet positions matter in planetary gear stage calculations!
In planetary stages, it is essential to consider the changing circumferential positions of the planets when calculating both the gear meshes and the planet bearings. For simple planetary stages with, for example, three planets, the differences are usually minor.
However, when additional forces act on the planet carrier or the ring gear, such as in the example shown below with an superposition planetary gear stage featuring an additional external helical gearing on the ring gear and another helical gear on the sun shaft, the differences can become significant. This affects both gear safety factors and bearing life calculations.
In our example, the pitting safety SH for the sun-planet mesh varies between 1.68 and 1.78 depending on the planet position. The effect on bearing life is even more pronounced: the nominal bearing life L10h for planet bearing B28 ranges from 72,000 h to 140,000 h.
Although these values remain within an uncritical range in this specific case, the situation may differ depending on the design and application. If gear safety factors or bearing lives fall below target values in certain operating conditions, the results should be examined more closely, ideally supported by a load spectrum calculation.
The GWJ SystemManager software fully supports this through a genuinely coupled multi-shaft system calculation, enabling precise evaluation of planetary stages under realistic operating conditions.