Split Ring Planetary Gearbox

Whenever you make a spinning mechanism, there’s a pretty high chance that you’ll be using gears. Gears allow you to change the speed, torque, and direction of an input, allowing you to accomplish a variety of tasks. Gearboxes, combinations of multiple gears, can reduce or multiply the torque of a gearbox by factors of hundreds to one! In this instructable, I will cover how to create a split ring planetary gearbox, a type of high reduction gearbox, as well as explain how it functions. This gearbox is modeled using Onshape, and can be printed almost entirely in place, meaning there is minimal hassle with assembly!

You can access the CAD file that I demonstrated with through this link: https://cad.onshape.com/documents/25a9b2372c63117b806a478e/w/fb1d97e2e4a64580056ec2e3/e/af69241a23656420de8cde20

This will allow you to access it or make a copy of it to modify for your own purposes! You will need an onshape account to access it, but it is free to make. You can also download the STL files below:

Required materials and software are:

- Onshape (or other CAD softwares like Solidworks. Note that using a different CAD software will change how the gears are generated, although the numbers should work the same)

- Bambu Studio (or other slicing softwares like Prusaslicer)

- PLA (or your filament of choice! Preferably not something flexible like TPU though)

- 4 M3 6mm screws

A split ring planetary gearbox is a more complicated version of a planetary gearbox. With a planetary gearbox, there is a ring gear, planet gears, and a sun gear. When the sun gear rotates and the ring gear is fixed, the planets orbit slowly around the ring gear. In a split ring planetary gearbox, another ring gear is meshed with the planet gears. The second ring gear has a different number of teeth than the original ring gear, causing it to revolve even more slowly as it aligns itself with the planet gears. This link is a very useful way to visualize how split ring planetary gears work, as well as allowing you to easily find the required modules and resulting ratio that you want: https://saugstad.net/gear-animation/index.html

The key problem in designing the second ring gear is that ideally, the module and pitch diameter of the second ring gear and its associated planets must be different from the module of the first ring gear to create the difference in the number of teeth. The design this Instructable covers changes the pitch diameter and the module of the second ring gear, but not the module of the associated planetary gears. While not ideal, the amount of backlash from this aberration is not substantial thanks to large amounts of testing.

This section will cover how to design a split ring planetary gearbox in Onshape, a free CAD software. To get to printing the associated CAD file and readying it for use, skip to step 9!

In order to create the spur gears, you will need to download a feature script from Onshape. To do that, go to this link: https://cad.onshape.com/documents/5742c8cde4b06c68b362d748/v/1db29081376c095cf10e2a3d/e/c72760543a0d4412e72f6d38

From there, you are able to click the plus sign at the top of the page to add it to your onshape files.

To start, make a sketch of the pitch circles of the sun and planet gears, as well as the overall profile of the gearbox. It can also be helpful to create a sketch for the center axis of the sun gear, which can be used later for making circular patterns. The numbers that are used here are specific to this gear set.

Using the spur gear feature, create gears with the following parameters as shown in the first two images. Then, use the circular pattern geature to make two more copies of the planet gear. Lastly, take the top faces of the four gears, then extrude them upwards by 7.8mm.

The ring gears are made by creating a spur gear, then using it to remove material from another solid part. First, create the spur gear with the dimensions in the first image. Then, create a 48mm x 48mm x 8mm rectangular prism using a sketch and extrude, and use the boolean feature’s subtract option to remove the necessary material. This will form the base of the gear box.

Next, create the spur gear with the dimensions in the first image. Note that this time, the gear is initialized on the top face of the blue base part rather than on the profile sketch. Then, create a 48mm x 48mm x 10mm cylinder using a sketch, initialized on the top face of the base. Use the boolean feature’s subtract option again to remove the necessary material. This is shown in the second picture as a section view:

The basic structure of the gearbox is now done! It’s time to add retaining structures to prevent the gears from falling out or getting pressed too much. First, let’s retain the output ring gear by making the following sketch on the top face of the base, then extruding by 10.2mm. The retainer is offset by 0.1 millimeters from the output ring gear, allowing it to move freely. The sketch for this is shown in the first image.

We can create good places for mounting holes by making a 40mm square on the topmost face, then creating holes using the hole feature. The holes are designed to fit M3 hardware, but are 2.6 mm in diameter rather than 3 mm in order to create an interference fit, which will allow the screw to thread into the plastic rather than fit loosely in it. This is shown in the second image.

We also want to be able to interface with the input. We can do this by creating a hexagonal hole that can fit standard 1/4 inch drill bits, as shown in the third image.

Additionally, the output ring is not constrained and can fall out right now. We need to make another part, called the covering, that will prevent that from happening. Notably, the gears can’t fall out the bottom of the gearbox. This is because the first planetary set has a herringbone, which locks the spur gears in place with each other. However, it is possible to just print another cap and attach it to the bottom to be extra safe.

Lastly, it’s best practice to create a slight chamfer on the bottom of parts, and fillet sharp corners in order to improve the quality of the print. I used a radius of 1mm for my fillet, and a 0.4mm, 45 degree chamfer.

Thanks to several quirks in the design and manufacturing process, this gearbox can be made as one piece! This is a challenging feat because in order to work, the gears can’t fuse together, but must be as close as possible to touching in order to mesh correctly.

As stated earlier, the gearbox can be printed almost entirely in place. The covering and output ring gear both need to be printed separately, but the sun, planets, and base can all be printed as one part.

In order to prevent problems, the following settings need to be changed:

1. The seam position must be set to random. The seam position is where the printer starts and stops each layer. The area where a seam is placed usually has some slight overflow, so the slicing software will try to place the seam in internal corners. In our case, the seam will be placed all along the inside of the teeth of where the gears meet, clogging the teeth and preventing it from moving correctly. By forcing random seam placement, the seams won’t collect along where the gears meet, fixing this problem.
2. The slice gap closing radius must be set to 0.02 mm. If this setting is too high, the slicing software will ignore the small seam we have between meshed gears, causing them to fuse together.
3. Optionally, you can increase the number of outer perimeter layers or increase the infill to make the gears stronger. I used 3 perimeter layers with 15% infill.

You’re all set now! Once taking it off, if the parts printed in place don’t move freely, this may be because of binding. In order to fix this, spin the input hex with a drill using as low torque as possible to grind away excess material. Note that this type of gearbox is not backdriveable: You won’t be able to spin the output ring gear, so it’s totally normal if that happens! To assemble, simply insert the output ring gear into the cavity, then place the cover on top of it and use 4 M3 6mm screws to seal it.

This gearbox has a ratio of around 40:1, making it good for high torque applications. In order to actually use the gearbox, you would add whatever parts you wanted to the circular face of the output ring gear. It may also be helpful to add more attachment points to the gearbox itself. Above is an example of this gearbox used as part of a manipulator. This piece was made to be modular, so you can make or copy the design yourself to use!