Planetary Gears taking the torque

Since planetary gears mesh with the sun gear and ring gear at several locations, more teeth are engaged to drive the load, compared to a conventional gearand- pinion mesh. Therefore, for the same load, planetary gearing requires smaller gears (although in greater number) than a standard pinion-to-gear reduction. Likewise, the radial arms of the planet carrier transfer substantial moment to the output shaft – another illustration of the efficiency of a concentric arrangement.

Perhaps a less obvious but no less significant consideration is that with multiple, equally spaced planets (as is usually the case) the input and output shaft bearings are spared the radial loads resulting from separating and tangential gear reaction forces, because these reactions cancel out. Plus, since no such forces act on these bearings, there is less potential for distortion of the outer casing.

With more planets comes an increase in load capacity and torsional rigidity; the more divided the load, the less deflection and wear of gear teeth. It follows that quite a large load can be driven in a comparatively small and streamlined planetary gear unit. Three is typical for the number of planets, but there are often more, and sometimes less. Again, it is common for multiple planets to be equally spaced.

Helical gears can be used for load capacity beyond spur gears, given comparable gear sizes and numbers of planets – because helicals are angled, not straighttoothed, even more teeth mesh at once. But with helical planetary gearing there are axial reactions, and these don’t cancel with multiple planets like the tangential and separating gear reactions do, so bearings have to account for the thrust load.