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 a Birfield joint, which is a kind of constant-velocity (CV) universal joint and is most popularly used to transfer the engine's power to the wheels in cars. 

Note that six plastic 6mm BB bullets for toy guns are required in addition to the 3D printed parts.

Brief Description

Universal joints can be used to transmit rotational motion between two axes that are not straightly aligned to each other. However, the most simple implementation of a university joint, single Cardan joint is not a constant velocity joint. Even when the input is rotated with a constant rotational velocity, the velocity of the output fluctuates depending on the angular position of the axis. Compared to it, double Cardan joint (type A and type B) and Oldham coupling have been introduced as constant velocity joints in this series.

A Birfield joint is another type of a constant velocity joint, which has spherical inner shell and outer shell that envelops the inner shell. These two shells have six grooves on each and six spherical balls are rolling in these grooves. When one of the shells is rotated, the balls transmits the rotational power to the other shell. To support the balls in the right position, in between the two shells, a partial spherical cage with six openings is also equipped. Among other Rzeppa-type joints, Birfield joint has an improved groove shape, i.e. it uses elliptical grooves instead of circular grooves. To be honest, this model shows a Rzeppa joint, not a Birfield joint because I couldn't find exact groove shape of a Birfield joint and, anyways, the difference is not observable in this kind of educational model.

Birfield joints are commonly used to transmit engine's power to the wheels in automobiles.

Related Models

Case

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

Hardware

This model requires six plastic 6mm BB bullets for toy guns in addition to the 3D printed parts.

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.Printing

Sanding and Filing

Note that, in this model, the rotation of the bases for bearings is intentionally made not too smooth.

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

This model requires six plastic 6mm BB bullets for toy guns in addition to the 3D printed parts.

First, insert the cage into the outer shell.

Then, push the BB bullets from inside the cage to outside, i.e. in between the cage and the outer shell, through the opening on the cage. 

After putting six balls, insert the inner shell into the cage and finally mount the two shafts onto the bearings.

Other examples

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](https://makerworld.com/collections/15048577-my-educational-mechanism-models)

Happy printing!

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.

License

• The 3D model(s) are licensed under Creative Commons Attribution 4.0 International.
• However, the text and images on this page are copyright reserved.