Potentiometer Planetary Reduction Drive Knob

thingiverse

A novel approach to the compound planetary gearbox has been devised. Split sun gears are employed, synchronizing the ring and planet gears. One sun gear is fixed to the threaded shaft of the potentiometer or tuning capacitor, while the other is secured to the knurled dial. The input is provided by the outer ring. This design may prove useful for Arduino analog inputs or ham radio tuning applications. https://youtu.be/32HHkyQ1MCM By redefining the R=S+2P constraint, numerous gear ratios up to 100:1 are achievable from a standard planetary arrangement. A sample ratio of 16:1 is demonstrated, taking into account tolerance, backlash, and Arduino sampling resolution limitations. Note: The customizer will not accept arbitrary values; several constraints must be satisfied. A compilation of possible combinations can be found in this Google sheet: https://docs.google.com/spreadsheets/d/12aYCHHGVahjbIu21Ix0mpmor-ldebmyBSsCOzph3iyE/edit?usp=sharing. A writeup of the underlying mathematics will be posted at a later stage, but it is contained within the SCAD source code. It is essential to maintain a one-layer gap between the sun gears after slicing, allowing them to be separated after printing. A default layer height of 0.2mm can be adjusted in the customizer if necessary. Otherwise, the gears should print in place fully assembled. Over-extruding slightly will help reduce backlash, ensuring the gears stick together initially but can still be cracked apart cleanly. A decorative cover plate can be glued to the ring gear if desired. Suitable potentiometers require a sufficient threaded length on the shaft to fit the gear, allowing extra for mounting. 18mm guitar pots appear to be an optimal size. My preferred configuration thus far is GearedPotDial30mm4x5x6.stl, featuring a compact 30mm knob diameter and 4/5 teeth on six small planet gears. The resulting gear ratio yields approximately 10 full turns in the same direction as the shaft. Update: Herringbone gears maintain structural integrity but are not required on both layers, as the potentiometer will hold the sun gears together. In fact, we could likely get away with a single helical layer for each sun gear, but this may introduce undesired axial force. UPDATE: Further refinement is necessary. After attempting non-helical gears for one layer, I discovered that helical gears serve another function: smoothing rotation. Straight cut gears exhibit a "lumpy" feel as the teeth pass perpendicular, whereas helical or herringbone gears maintain peak engagement at all times. We should be able to tolerate the axial load introduced by helical gears (by not using herringbone on both layers). I will keep one layer of herringbone to hold the gears in position. If we instead used only helical gears, the entire assembly could be printed separately and held together by the potentiometer, but the teeth would only align in a single arrangement and require meticulous alignment. Plus, the length of the turning shaft is usually not a limiting factor.