These are 2 simple dual cycloidal gear train models to get familiar with this gear type. They both present a 39:1 reduction ratio in a ~120x120x60mm (~4.7x4.7x2.4") volume.
Typically, cycloidal gear trains use an internal output configuration (ie the pin-disc is the output) in industrial applications and robotic joints. But for some applications, like captan drives, the external output version (pin-disc is fixed) can be more practical. And most importantly, it handles the max load torque directly via the teeth on the large external output ring, instead of via the pin-disc, which roughly halves the max contact and shear forces experienced by the parts connected to the load.
Although a dual cycloid disc configuration is a bit more complex than a single disc, it offers some advantadges such as balanced centrifugal forces, and symmetrical forces on the pin-disc and on the outer ring. In my quest for a 3D printed 20Nm-capable (14.7 ft-lb) drive those aspects will likely be important.
However, to reach 39:1 in a 120mm outside diameter casing, the eccentricity is down to ~1.25mm (0.05"). Handling 20Nm in that case is no problem for metal parts made on high precision CNC / EDM machines. But it will be an issue for gear reliability with consumer 3D printed plastic parts that have comparatively poor tolerances, poor repeatability, high elasticity, and low threshold plasticity.
This said, I am now designing a robust version with bearings to run the same 85lbs / 16Nm test as with the sunless planetary compound gear train. Will post results here when done. But since I expect the small cycloidal teeth and outer pins to fair poorly, the next stop in this journey will be abnormal cycloidal gears.
The Fusion360 CAD model (very messy), STEP and 3MF files are on Github
Feel free to copy, remix, whatever (non-commercially)
Happy printing to all !
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CAD
Diagram for the traditional stator ring configuration (internal output):
Output ring diagram (external output):
Printing
Settings:
- PLA filament
- 0.4mm nozzle, 0.24 layer height
- 2 walls, 3 top, 3 bottom
- 20% Support Cubic infill
- Normal Auto support, 10° Threshold Angle (just to support the eccentric on the shaft, see pics)
Note that due to the usual Fusion360 Export bug when handling cycloidal surfaces, both discs had to be saved as STL files instead of the more desirable STEP format.
Assembly
First, do a dry run without glue. To get the hang of which parts go first, and the orientation of each disc.
Once familiar with the steps, proceed with the gluing, ideally while making sure to align the 3 dots while doing so. These dots just facilitate counting the numbers of turns.
- Glue the 2 input shafts parts together
- Mount the shaft of the back of the holder
- Install the cycloidal discs
- Glue in the pins-holding hexapus (well-know cousin of the octopus)
- Glue the output wheel and hub together and install on the shaft
- Install the front of the holder
The instructions are similar for the traditional cycloidal model.
The cycloidal gear train is a pretty clever mechanism, with minimal backlash, high reduction ratio and high load handling. Doesn't handle high speeds too well though, so we won't see it in vehicle transmissions any time soon.
Invented by Lorenz Braren and patented in 1925. Sehr schön !
Once again, shoutout to the extremely useful ME Virtuoso website where one can parametrically simulate their own planetary / wolfrom / cycloidal / harmonic / wave drives, and
then download the STEP / DXF files. Some of which he also CNC'd or 3D
printed, and tested on the ProMakina Youtube channel
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