These are 2 simple dual cycloidal gear train models to get familiar with this gear type. They present a 39:1 to 40:1 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 reduces the max contact and shear forces experienced by the parts connected to the load.
Although a dual 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 consumer 3D printed plastic parts that have comparatively poor tolerances, poor repeatability, high elasticity, and low threshold plasticity.
update a load test indeed confirmed that a robust version of this model, with bearings, skipped teeth before reaching 16Nm of load torque. See the Test Results section below. So I am now designing a version with abnormal cycloidal gears that sport much longer teeth
The Fusion360 CAD model (messy), STEP and 3MF files are on Github and MakerWorld
Feel free to copy, remix, whatever (non-commercially)
Happy printing to all !
______________________________________________
Sections:
- CAD
- Printing
- Assembly
- Test Results
- Resources
CAD
Diagram for the traditional stator ring configuration (internal output):
Output ring diagram (external output):
Since the cycloidal discs have 39 teeth, the internal output version has a 39:1 reduction ratio. And the external one is 40:1 as the external ring that has 40 teeth is no longer fixed and now rotates.
Turning the input shaft ClockWise (CW) on the internal output version turns the output Counter-CW, but turns it CW on the external output version.
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 ensure everything printed properly with working tolerances, and 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 making sure to align the 3 dots while doing so. These dots just facilitate counting the numbers of turns.
- Glue the 2 input shaft parts together
- Insert the shaft into the back holder
- Install the cycloidal discs
- Glue in the pins-holding hexapus (well-know cousin of the octopus)
- Glue the output wheel and its hub together and insert on the shaft
- Slide the front holder in
The instructions are similar for the traditional cycloidal model.
Test Results
update Test results were as expected: due to the very small eccentricity of 1.215mm the gear train failed the 16Nm (40Kg / 87lbs) test. The discs were just skipping as the forces were too high, tolerances too loose and PLA's plasticity threshold too low, for that small eccentricity value. However it did work well, and much more smoothly and silently than the Sunless Wolfrom train, up to 13Nm (32Kg / 70lbs).
The model tested has a 108mm pitch diameter, was printed with PLA and included bearings. Nylon filament and a more rigid design would likely have fared better. However, this test shows what would irremediably happen anyway after just a bit of wear, no matter the plastic type used.
So the next stop in this journey will be exploring abnormal cycloidal profiles, as they allow for much higher eccentricity values and, I suspect, much lower average pressure angle than pure cycloidal profiles. Will post the link when done.
Resources
The cycloidal gear train is a 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 anytime soon.
Supposedly 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
Comments
Post a Comment