Motorized Functional Differential Gear System

by RonG, published

Motorized Functional Differential Gear System by RonG Nov 7, 2011

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After making Thing-O-Fun's functional differential gear system and playing with it for a while I realized that this thing just had to be motorized. I had a Solarbotics GM9 gear motor lying around and it was a perfect fit. All that was needed was a modified input shaft to match up with the shaft on the gear motor and a base to hold the batteries and power switch.

I included the scad file so that you can easily change the switch opening to fit your switch.

Watch it in motion at http://youtu.be/bFceF0CeGFk


  1. Make the functional differential gear system (you can skip the shaft and one of the handles, as they are not needed with the motorized version.
  2. Print the base and the input shaft.
  3. Put the Input Shaft on the Input Pinion and assemble the gear system.
  4. Line up the GM9 shaft with the Input Shaft and hole it in place.
  5. Solder wires ~3 inches long onto the GM9 terminals.
  6. Drill two holes about 2.8mm into the stand, using the holes in the GM9 body as a guide. Use the M3x22 screws to mount the motor.
  7. Solvent weld the gear system to the base.
  8. Glue in the battery holder, install the switch, and hook it all up.

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Any cheap sources for the motor?

cheapest I've found is $5.45

That's what I paid for mine. It is probably overkill for this, as the video http://www.youtube.com/watch?v=bFceF0CeGFkhttp://www.youtube.com/watch?v... shows it has plenty of torque to flip the whole model, including the batteries. It just happens to be one that I had available.


I am a newbie to all this.About the STL screen shots on the right; which software have you designed these parts ? Skethup or Blender?

Also could you please post any resources of tutorials for designing gears (bevel, differential, multistage,helical etc)..would be great. Your gear train is really amazing..

I would start with http://www.thingiverse.com/thing:6073http://www.thingiverse.com/thi... for some links to tutorials on gear design. BTW, to do gears like this you will need a tool like the free, open source OpenSCAD http://www.openSCAD.orghttp://www.openSCAD.org SketchUp is a great tool for some things, and I use it myself when appropriate, but gears need to be designed with mathematical precision, and openSCAD is the right tool for that.

Cube Gears
by emmett

Is there a free plugin to import STL files in google sketchup? Please let me know.


I used an open source cad program called openSCAD. It is available at http://www.openscad.org/http://www.openscad.org/ There are versions available for Linux, OS X, and Windows. I always include the source SCAD files with my designs, and try to comment them enough to also act as a tutorial.

Did you file the parts and sand them after they got out of the makerbot? Please let me know.

These screen shots with the green colored CAD designs are they from OpenScad or does thing verse has an online CAD rendering plugin? I am planning to use sketchup but my STL export plugin stopped working properly recently. I looked at the video of your differential gear train, looks great. I am surprised at the print quality of gears though. So does Makerbot give one enough print resolution that the gears printed are smooth enough to properly inter-lock with each other?

Thingiverse has a built-in rendering engine for STL files. Those are the green colored pictures.

I didn't do any sanding or filing of the parts after they were printed, that is how they came out of the TOM. To get this quality I have spent a lot of time tweaking and fine tuning the Skeinforge profiles. I would recommend Dave Durant's series of articles as a starting point. http://davedurant.w
http://ordpress.com/2010/11/04/configuring-skeinforge-index-to-articles/ordpress.com/2010/11/04/config... The single most important thing is getting the right amount of plastic extruded by testing with the 20mm calibration cubes at 1.0 (100%) fill.

My TOM has the HBP with aluminum build plate covered with Kapton tape, and a MK6+ extruder
with the stock 0.4mm nozzle using 3mm filament.

For this project I used a Layer Thickness of 0.16mm and a Width over Thickness of 1.5, giving a width of 0.24mm. Needless to say this required a really slow print (the big blue gear and the stand each took over 5 hours to print), and from what I hav
e read this requires a stepper extruder to get the filament Flow Rate low enough. I had a feed rate of 40mm/s and a flow rate that was way less than 1 (I won't post the exact number because it won't work on your machine, you need to do the calibration cubes).

What, no video? =-X

I just finished it this morning and got the static photo done. I'll post a video tonight.