Reciprocal Rack - Educational model

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 set

This model shows a reciprocal rack gear system, which consists of two trimmed rack segments on the upper and lower sides and a partial spur gear that acts as the pinion.

Brief description

This reciprocal rack gear system consists of two trimmed rack segments on the upper and lower sides and a partial spur gear that acts as the pinion. Since the pinion has only seven teeth out of what would normally be sixteen, it can mesh with only one of the two racks at a time.

When the pinion rotates clockwise, it pushes the upper rack to the right and the lower rack to the left. Near the point where the rack’s motion switches from one direction to the other, the teeth of the pinion that are not engaged move in the opposite direction relative to the rack teeth. To prevent the teeth from colliding, the end sections of both racks are trimmed, giving them a lower tooth height than normal.

At the switching moment between the upper and lower racks, there is a brief period in which the pinion is not fully engaged with either rack. During this interval, the rack is free to move, and the synchronization between the rack and pinion may be lost. To prevent the mechanism from locking up in such cases, the leading tooth of the pinion is trimmed so that it does not interfere with the rack when re-engaging, while the trailing tooth retains its full height to ensure the maximum stroke of the rack motion.

In this design, the mechanism functions correctly only when the pinion is rotated clockwise. Rotating it in the opposite direction causes the system to lock up. Except for the brief transition near the turning points, the rack moves at a uniform speed.

Reference

• https://507movements.com/mm_114.html 

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, in theory. 

Just snap assemble the parts.

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.

License (2026-03-13 updated)

• The 3D model(s) are licensed under Creative Commons Attribution 4.0 International. (unchanged)
• However, the text and images on this page are copyright reserved. (added on 2026-03-13)