Simple Pinion Epicyclic

By Spencer Connor

Somewhat reminiscent of an insect crawling around, the simplest incarnation of an epicyclic gear train involves a fixed gear with a coaxially mounted output gear. These must be of identical size and close to the same tooth count, thus very slightly different modules. Thus when overlaid there are specific locations around the perimeter where the teeth appear to align, but elsewhere they do not. Most often, the tooth count difference is just 1 (as it conveys the maximum gear ratio) such that there is only one such location on the perimeter. As the output gear rotates forward one tooth, this location of alignment makes a full revolution. To enforce this, a pinion on the input arm (also coaxial to the fixed and output gears) engages with both the fixed and output gears. There is necessarily a module mismatch somewhere between the pinion, the fixed gear, and the output gear, but it can be kept sufficiently small to not be consequential.

Note that this is a similar effect to Vernier scales.

Fun fact: you can also reverse the direction of the output by having more teeth on the output gear than the fixed gear.