Gearwork Demonstrator

This toy's original concept was to show kids how gears work with a hands-on demonstration but it also turns out to be fun to play with.

The gear train consists of 7 stages.  Each stage has both a larger 42 tooth gear and a smaller 14 tooth gear giving a 3:1 gear ratio.  The resulting gear train ratios are 1:3:9:27:81:243:729:2187 so for each input gear revolution, the output gear spins 2187 times.  Obviously the torque required on the input gear is very high making it difficult to turn by hand but since each gear down the train has a lower torque it makes understanding the relationship between torque and speed ratios intuitive for kids to experience.

The middle gears make for an enjoyable fidget toy since the torque required is moderate and the speed of the flag spinner is pretty fast.  A remix with a whistle would be neat.

Parts

There are 5 unique gear models, three of which (compound gears A, B, and C) are printed twice for a total of 8 gears.  The following assembly picture has each gear type and position labeled.  

The Input Gear and Gear C require a spacer to center align the gear.  This eliminates the need for supports when printing the gears.

The frame has threaded holes on the right side and a countersink on the left for the M3 screws.  The screws hold in the plastic well enough, just don't over tighten the screws.

A spinner flag fits on the output gear to indicate output revolutions.  Each gear also has a small arrow indentation to simplify counting gear rotations as an exercise.

In addition to the printed parts, a screw and a nylon spacer (see note below) are required per gear for assembly (8 of each are needed in total).  The provided links are for Grainer but equivalent parts can be found on Amazon or other sellers.

• (8) M3-0.5 x 20mm flat-head Phillips (54FR62)
• (8) 15mm nylon spacer (3ZMR3)

About the nylon spacers:  I used these nylon spacers as bushings so the gears spin more smoothly.  I had a big bag of them on-hand so it was an easy choice.  It seems like the cost is much higher now than I'd paid years ago so, if you cannot find them, I included an optional 3d model for the spacer you can print.  Although, I doubt the performance will be as good since the layer lines will probably bind some on the screw threads.  Sanding may help but you'll have to experiment to see.

Printing Settings

• Material: PLA
• Layer Height: 0.2mm
• Perimeters: 3
• Infill: 15% Gyroid
• Supports: None
• Bed Adhesion: Skirt
• Printing Orientation:  See file notes

Additional Tips:

• Gear meshing may be effected by incorrect printer calibration so it would be best to test two gears on the frame to ensure the gears spin freely before printing everything else.
• The right-side frame has M3 threaded holes modeled but may require running a tap thru if the screws won't fit.