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]

The present model

This is an educational model of the Planetary Gear.

Brief Description

A planetary gear can achieve a high reduction ratio in a compact, coaxial configuration.

In this model, the small central spur gear—the sun gear—serves as the input, while the planetary carrier, the rotatable table on the backside that holds the three larger spur gears (the planet gears), serves as the output. The surrounding internal gear is fixed to the base plate.

When the sun gear rotates, the planet gears rotate on their own axes (self-rotation) while simultaneously revolving around the sun gear. This revolution corresponds to the rotation of the backside table, which becomes the output motion.

In this model, the sun gear has 9 teeth, each planet gear has 15 teeth, and the internal gear has 39 teeth with the relation 9 + 2 x 15 = 39 to have correct meshing with using the standard gear shapes without modification. With these parameters, the reduction ratio is as large as 1 : 5.33 .

To understand the operating principle, it helps to imagine the planetary carrier being fixed. In that case, for each full rotation of a planet gear, the internal gear rotates 15/39 turns, and the sun gear rotates in the opposite direction by 15/9 turns. So, from the internal gear’s frame of reference, during the 15/39 rotation of the planetary carrier, the sun gear rotates 15/39 + 15/9 turns. As a result, the overall gear ratio is: 

 (15/39)/(15/39+15/9) = 1/(1+39/9) = 9/(9+39) = 3/16 ~ 1 : 5.33

Planetary gears are widely used in machines that require compact and efficient torque transmission.
They are found in automobile transmissions, robotic joints, wind turbines, and industrial reducers, taking advantage of their coaxial layout, high torque density, and balanced load distribution. So, It has similar applications to the cycloidal drive, whose model is also available in this educational model series.

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.