8.2 | Iteration
Brainstorming
Right from the get go, we knew we wanted to make a rowing mechanism, so we started out with some concept sketches.
This was a good first start, but after considering how hard the circular tracks would be to manufacture, we decided to pivot. We looked for inspiration online and found this video, which ultimately shaped our new design with 3 sliding points.
This design seemed much more feasible, and we got to work creating some CAD models.
First CAD
The first iteration was just to get the dual slider crank working, showcasing the linear sliding rail along both ends and the sliding middle joint connecting both links.
If you analyze the video, you can see we are getting the desired eliptical motion from the middle connecting link. This means the oars we connect will have their ends moving in an ellipse, ideal for rowing.
Once we had that solidified, we went to work constructing the oars.
By pinning the middle of the oar around the edge of a boat, we force the end of the oar to pivot up and down as the middle link travels to the top and bottom of its elliptical path, giving us that rowing motion we desired.
After this stage we started to consider manufacturing and, for the purposes of the prototype, had to change what we had modeled. We found that the pins we were going to use weren’t going to work.
They would make the structure too weak and amidst fear of breaking the prototype on presentation day, we changed the CAD a small amount.
We designed new pins to connect everything, settling on the model above.
First print
After finishing the CAD, we started on assembly for the prototype showcase, printing everything out and putting it together.
After assembly we tested our model out but quickly ran into some… issues…
The biggest issue, of course, is the lack of mobility. This was largely due to the friction of plastic, especially with the forces of the pins sliding along inside the slot. But the promise was there!
Making the Trireme
This next part wasn’t strictly necessary to our mechanism; however, we wanted to make this project half mechanism, half artwork and started iterating upon a skeletal ship that we’d laser cut.
First electronics / BoM considerations
While we were considering all these things, we wanted to decide what motor we wanted to use.
After our kinematic analysis, we settled on the 12V DC motor as it provided the most torque and we knew we wanted to scale up our design
Motor | Type | Operating Voltage | Running Current | Stall Current | Stall Torque | RPM | Free Torque | Motor Controller |
|---|---|---|---|---|---|---|---|---|
Grear-tisan ZGA37RG31.6i | DC | DC12V | 0.68A - 1.1A | 2.19A | 42 kg.cm | 100 | 7 kg.cm | L298N |
Tiank-ongrc TD-8120MG | Servo | 4.8-7.4V | 140mA(4.8V), 200mA(7.4V) | 2.100A - 2.700A (at 6V-7.2V) | 0.14sec/60°(4.8V), 0.18sec/60°(7.4V) | Same as stall | Arduino |
We’re using alkaline batteries, with each being 1.5V.
To meet our necessary voltage, we needed 8 batteries for the DC motor or 4 batteries for the servo.
After settling on our motor, we adjusted our BOM for projected project needs.