4. Prototyping & Building

1. Introduction     2. Design Considerations     3. Kinematics    5. Results & Discussion

EARLY ITERATIONS

I originally planned to use the two sets of lawnmower gears on opposite sides of a four-bar (Fig. 1), a design that I modifed after the kinematic analysis. I also considered using a four-bar that would trace a circular pattern to spin the fan mechanism, but decided it would be easier to use a parallelogram design to spin a shaft. Finally, I considered using a belt around the lawnmower wheels instead of a rigid bar to link the trainer and fan rotations, but couldn't find appropriate materials to attach a belt that wouldn't fall off. 

TRAINER TO MECHANISM

Attaching the trainer to the mechanism was difficult. There is no clear way to attach a rotating shaft to the trainer fan (Fig. 2A). I experimented unsuccessfully with using a socket from a ratchet set to fit on the bolt in fan. Then, I designed a wood apparatus that would wedge between the blades of the trainer fan (Fig. 2B).

Figure 1: The orignal fan design featured the two sets of lawnmower gears and an apparatus to attach trainer to fan mechanism.

Figure 2: A) rotating part of trainer and B) apparatus to attach to trainer. 

GEAR REDUCTIONS

The first step in reducing the mechanism speed was to use two bike gears with a ratio of 4:9.5 (Fig. 2A & 2B). It was difficult to fit shafts into these gears. I employed hot glue for this task. I attached the bike gears onto a board, but I couldn't drill holes and attach shafts straight enough for the gears to spin evenly, so the chain kept falling off. I eventually decided to abandon that part of the mechanism and focus on the remaining gears and four-bar.

Figure 3: A) The small gear driven by the bike trainer, B) The larger bike gear driven by the small gear and driving a lanwnmower gear, and C) a schematic of the first gear reduction. 

The second two gear reductions used the lawnmower gears, which were easier to install (Fig. 4, Fig. 5).

Figure 4: Second gear reduction; the lawmower gears are driven by the larger bike gear. 

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. 

Figure 6: Roomba gears are driven by the four-bar and increase the speed to drive a fan. 

1. Introduction     2. Design Considerations     3. Kinematics    5. Results & Discussion