My Educational Mechanical Examples Series

This model is one of my educational mechanical mechanism examples on 80mm x 80mm base plates.
You can find all models of the series in this collection => [Mechanical Mechanism Examples]

This model

This is an educational model of the Cycloidal Drive.

Brief Description

A Cycloidal Drive achieves a high reduction ratio in a compact, coaxial configuration similar to the planetary gear system but it can have even higher reduction ratio. In this model, the central knob serves as the input, and the inner pin disk is the output.

This mechanism functions as a type of differential drive. The reduction ratio is determined by the number of lobes on the lotus root shaped disk, which is called cycloidal disk, divided by the difference between the number of outer pins and lobes, which is one. In other words, when the central knob  completes one full turn, the cycloidal disk rotates by one lobe, and so does the output pin disk. In this model, there are ten lobes, resulting in a reduction ratio of 1:10.

On the back of the input knob, an eccentric disk is attached, which acts as the rotational axis of the cycloidal disk. Because of this eccentricity, the cycloidal disk is continuously pressed against the outer pins in one direction.

As the input knob rotates, the center of the cycloidal disk revolves around the main shaft, and the contact direction between the disk and outer pins rotates accordingly.

Since the number of outer pin is slightly greater — in this case by one — than thee number of lobes, the cycloidal disk gradually rotates as the contact angle shifts. After one complete revolution of the input knob, the disk rotates by one lobe.

The holes in the cycloidal disk transmit rotation smoothly to the output pin wheel. Each hole is in contact with a corresponding pin, but only some of them actually push the pin depending on the relative angle of motion.

Cycloidal drives are widely used in robot joints, precision machinery, and industrial reducers, where high reduction, compactness, and low backlash are required. In practical application, a cycloidal drive often uses two cycloidal disks, mounted 180 degrees out of phase, to provide smoother torque transmission and reduce vibration caused by eccentric motion. In addition, the cycloidal disk is often not formed with a theoretical cycloid curve, but with a slightly shifted epitrochoid curve to allow for small errors in pin size and alignment.

Case

This model is compatible with the case included in my first set.

Printing

• Use the models named ???-printable.stl for printing.
  The models named ???-assembled.stl are provided just to show how they should be assembled.
   
• Use well-dried PETG to have better dimensional accuracy.
• Use 0.1 mm or 0.08 mm layer height to have smoother surfaces.
• Use slow printing speed for overhangs.
• Select “Random” seam position to have smoother rotation.
  Randomly distributed seam should be easily worn out after some wearing.

Sanding and Filing

Sometimes, the gears suffer from the stringing effect and/or elephant foot effect, resulting in a too tight fit to the shafts (they are designed with a 0.15 mm radial clearance). 

If you see rough surface on the shafts due to stringing, sand off the roughness with a small piece of sand paper.

If you feel the gears do not rotate smoothly due to an elephant effect, widen the hole slightly by using a thin round bar file.

Without those issues, the parts should rotate very smoothly with minimal friction.

Assembly

No glue is needed. 

Just snap the retaining ring onto the shaft.

Other educational models

You may also be interested in the models in my educational mechanical mechanism examples.

Find them in this collection:
https://makerworld.com/collections/15048577-my-educational-mechanism-models

Acknowledgement

I got into gears thanks to K.$uzuki's amazing articles and YouTube videos. Many of the mechanisms shown in this series came from the introductions on his website. He also makes excellent gear models himself. This series wouldn’t have existed without his inspiration.

I learned a lot about technical detail of designing gear tooth profiles from Haguruma-No-Hanashi website. I’m truly grateful for that.