2) Design Process - Group 6

To start, our project objectives included the following:

• Create a link system with proper mechanical advantage
• Provide enough torque to move the link systems
• Mimic the rowing of a boat using an oar and pivot system
• Keep the oar in the correct orientation when in the water

At the beginning of our design process, we decided that we would utilize the 3D printers in the TIW and the laser cutting machine to produce our mechanism. For the most part, 3D printing allows for flexibility to print any light and durable shape. On the other hand, laser cutting wood allows the creation of quick prototypes and provides a study platform. Our initial idea for the design was to buy a plastic floatable boat and mount the oar mechanism on top; however, due to feasibility, we decided on creating a flat platform to build our mechanism on (Cad model of the platform below).

Figure 1. Wooden platform design for the boat base.

The first step in our design process was determining the oar trajectory that would pass through the surface of the water at a slow speed (high mechanical advantage) and then quickly reset when out of the water (low mechanical advantage). To find the link system we searched mainly for striding mechanisms since the link trajectory and mechanical advantage (Ma) are closely related to what we required. Below is an image of a 6-bar link system that we based our mechanism on (website).

Figure 2. Striding mechanism with a greater force below and a quick recovery above.

Using the 6-bar link system, we modified the link sizes till we ended up with the desired trajectory and scaled the mechanism down to an appropriate size. Next, we determined the proper spacing needed between the links for effective implementation and to reduce play in the joints. The image below shows the links (rectangles with link numbers) and the spacing determined between each connection. We initially allotted 3mm space to add retainer rings on the axle. However, since we did not have access to them, we decided to make the axles modular and CAD the axles with end stoppers (more on this in the manufacturing section).

Figure 3. Link spacing design for optimized implementation.

Our next design discussion involved the transfer of power from one motor to two link mechanisms. We utilized the motor provided in class (3500 r/min) since it seemed powerful enough to move the miniature link systems. We decided to utilize crown gears to transfer the motor power perpendicular across a main axle, and to increase the torque in the system. The gear ratio we decided upon was 1:2 where the pinion had 12 teeth and the big gear had 24 teeth (cad model shown below).

Figure 4. Two crown gears: 12-tooth pinion (left) and Big 24-tooth gear (right).

Lastly, one of the major problems we faced was how we would implement the oar and pivot system as typically seen on a canoe. In order to maintain a pivot mechanism, the oar needed to connect to the linkage system, but still keeps some pivotal motion, hence we determined we needed to use a ball and socket joint. However, by using a ball and socket joint, we lose the ability to keep a fixed oar orientation so that it may hit the water with the greatest surface area. Initially, we decided to change a portion of the oar’s cross-section to a rectangle so that while the oar rotated, it may be guided to the correct orientation by a wood slot (first image below). However, this idea seemed overly complicated, hence we agreed upon fixing the oar in a rectangular pivot hole instead. This way, the pivot will restrict the oar from rotating along its axis while it is pushing water. Our group chose to create a 1:3 ratio with the oar and pivot so that the oar trajectory is three times the size of the link trajectory. Another caveat to using a pivot system is that the oar will be traversing through the pivot hole as it moves since it is taking in motion from a 2D link system. To account for this added motion, we extended the rectangular cross-section region on the pivot. Additionally, the pivot hole needed to be filleted and widened to provide enough allowable motion for the oar to move along with the link system. Below are the pictures of the modular oar and pivot system.

Figure 5. Design-idea for a wooden slotted system that would guide the oar in the correct orientation.

Figure 6. Modular CAD of the oar with a 2cm large rectangle cross-section extrusion.

Figure 7. Modular CAD of the pivot hole.