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Figure 5: The second and third gear reductions. The final gear, the large lawnmower wheel, serves as link two for the parallelogram four-bar.
FOUR-BAR MECHANISM
The last lawnmower gear drives a parallelogram four-bar made from PVC pipes (Fig. 5).
Figure 5: A four-bar is driven by the lawnmower wheel. L4 rotates a shaft that drives the fan gears.
FAN MECHANISM
The rotation created by the four-bar now needs to speed up again to drive a fan. This speed increase is accomplished via three Roomba gears of decreasing diameter (Fig. 6). The tiny fan represents the output rotation because I was already worried about putting the Roomba back together and I didn't want to take apart a real fan.
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Figure 6: Roomba gears are driven by the four-bar and increase the speed to drive a fan.