For my mechanical prototyping course, we were assigned to design and assemble a sculpture with a small team that, from the rotation of one DC motor, produced sound.
As the theme of the project was nature, my team worked with the movie “The Little Mermaid” and its underwater setting. Specifically, we focused on the song “Under the Sea.” To play this song we created a system that uses magnets on rotating wheels to drop steel marbles onto xylophone keys. Since we were a little limited in the scope of this project, we only included the introduction to the song. We also included a cam system at the top to mimic Sebastian the Crab playing seashells as drums at the top of our sculpture, just as he does during the song in the movie.
In Depth Description:
At the beginning of the system, we have a motor attached to a set of pinion gears that transfers power to the cam shaft at a ratio of 1:2. This shaft has two identical elliptical cams set 90 degrees from each other to create an alternating motion. These cams each have one follower that leads to the arms of the crab on top of the box. The resulting motion is the arms moving up and down alternating to create the impression that the crab is raising its arms to play the clams on the top box as if they were drums.
The cam shaft has a sprocket at its end which transfers power at a 1:1 ratio through a chain to the lower shaft with an identical sprocket. Along this shaft are 5 wheels. Each wheel has 32 sockets evenly spaced along its outer diameter. These holes are spaced so that each one equals an 8th note in the song. The motor spins at 26rpm, which is geared down to 13. With 32 8th notes, each wheel includes enough space for 4 measures of the song going at 56 bpm, similar to the original 60 bpm of the song.
Magnets are placed in the sockets along the wheels to pick up marbles as they pass. A sheet metal funnel is included for each wheel. This funnel stops the marble's movement, dislodging it from the magnet and leading it to hit the xylophone key. Marbles then roll into the lower side of the funnel and are led to the bottom side of the wheel where they are once again picked up by the magnets. With this technique, marbles are picked up and dropped at the correct points to create the notes to the song: “Under the Sea.”
Reflection
As can be seen in the video, this project never fully functioned. The cam system on the top worked perfectly, but the wheel xylophone system failed to spin as expected. Our design, in theory, could have worked well. Yet it had a few major problems, discovered at the last minute, that couldn’t be solved in time. First of all, we had depended upon the fact that the sheet metal pieces could be made accurately and consistently, but in the end, it was a very complex shape that could not be bent accurately by hand and should have been 3D printed to increase consistency. Additionally, we were depending upon a set screw to mount the wheels. However, we had clearance issues and were unable to drill holes in our wheels. Sadly we had to resort to glue and attempting to press-fit the wheels with the spacers to keep them locked with the shaft. This solution ended up failing, and from it, our sculpture overall, failed.
From this, I learned the importance of planning for tolerance errors. If we had left more space in between the wheel dividers a lot of problems would have been easier to solve. This would have also allowed for easier maintenance, which is something we never even considered while designing our sculpture. There were also far superior systems for securing out wheels which we were unable to implement at the time such as clamping hubs.