Background

At some point I came across this viral video of a teacher demonstrating how to revive a dried out marker. She achieves the marker revival by tying a marker to a string and swinging it around like a lasso. The centrifugal force pushes ink to the tip of the marker, making it usable again.

I think gears are super cool. By meshing together differently sized toothed discs, you can manipulate speed or torque! I had been looking for an excuse to make wooden gears, so this seemed like a good opportunity to make a hand cranked gear driven centrifuge.

Design

Gear train

I used this online gear generator to design the gear train. The design involves 4 gears rotating about 3 axles. The gears combine to make a compound gear train involving 2 stages. In the first stage, the 30-tooth input gear drives the 10-tooth part of the compound gear (30:10 ratio). In the second stage, the 20-tooth part of the compound gear drives the 10-tooth output gear (20:10 ratio).

\begin{align*} ratio_{stage A}\cdot ratio_{stageB} &= ratio_{overall} \\
\frac{30}{10}\cdot \frac{20}{10} &= 6 \end{align*}

The result is a machine that has a 6:1 gear ratio, meaning the rotation speed at the output gear will be 6 times faster than the rotation at the input (conversly, the torque at the output will be 6 times smaller than the torque at the input). If you want to learn more about gears, I highly reccomend this excellent primer on gears. Additionally, here is good resource for calculating gear ratios.

Base frame

I designed a base frame to properly space the axles such that the gears meshed together and ran smoothly. The underside of the base has 3 suction cups so the assembly can be secured to a smooth surface.

Build

I downloaded the DXF files from the online generator and imported them into Autodesk Fusion 360. With the gears imported, I added holes in the center so they could rotate around PVC axles. The gears were cut out of 1/2" plywood using a Shaper Origin. The markers are held in place with two plastic clips. The axles are 3/4" inner diameter PVC pipe. The handle is a 1" wooden dowel with 1" PVC pipe on the outside.

Suction cups and PVC pipes are epoxied to the frame

With Suction Cups

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