Intro

For the final project, we were tasked with designing and constructing a mechanical model based on one or more of the movements in the book 507 Mechanical Movements. This project was intended to allow us to use all of the tools and skills we had learned up to this point. We wanted our design to be space-inspired, and after browsing through all of the different movements, we decided to base our design on movement #39, which consists of one spur gear revolving around a fixed spur gear. Conveniently called sun-and-planet motion, we had the idea of our design depicting the Earth orbiting the sun.

Drawings + Modeling

We began with a rough sketch of what our design would tentatively look like.

It consisted of a horizontal base and a vertical support, with the sun gear mounted to the vertical support and the planet gear free to rotate around the sun gear. The underlying principle of sun-and-planet motion is that it converts linear motion to rotational motion. At first, we didn’t know the best way to mount the arm that is attached to the planet gear. We did some research, finding real world examples of sun-and-planet motion; notably, the Whitbread engine, one of the first rotative steam engines ever built. We proposed the idea of having a slot that will fit the handle of the arm, restricting the handle to vertical motion. We first modeled all of our parts in Solidworks to check if we could integrate the parts easily. For the gears, we were able to use Solidworks’ gear toolbox, which easily creates gear models; all we needed to specify were the necessary gear dimensions, such as diametral pitch, number of teeth, width, etc. Our other parts (the base & support) were not geometrically complex, and those were easily modeled.

Prototyping

To begin prototyping, we started with laser cut cardboard, cut to the same dimensions as the final product. Though normally prototyping would start at a smaller scale, the low cost, ease of manufacture, and abundance of cardboard in the OEDK allowed us to use it as a medium fidelity prototype material. We started the prototyping process by compiling all of our components from SolidWorks into individual 2D drawings. The initial drawings were done via Solidworks built-in drawing functionality, and were then manually stripped of other features and converted into an .ai file for use on the laser cutter. 

As noted by the TA, at the time of this final project, the OEDK laser cutter is semi-functional — it occasionally doesn’t cut through material, and is offline for long periods of time inexplicably. As such, cutting our prototype cardboard was more of a challenge than normal. We had to make multiple passes at times, and faced issues with burned cardboard at others. Through trial and error of the settings, however, we eventually cut all components.

We then proceeded to assemble all of our cardboard components as we planned, using ¾” PVC (1.04” nominal OD) as dowels because we couldn’t find 1” diameter dowels in the OEDK. This process mostly consisted of trial and error; the assembly required lots of spacers and other small sections of cardboard not accounted for in our Solidworks assembly. We cut multiple small rings that we used for spacers throughout our assembly. We also found we needed these small rings on either end of a gear to serve as shaft collars to prevent movement of the pvc “shaft”. We played around with spacers until everything worked well, hot gluing everything together. We also had to sand slots and holes as necessary, which wasn’t a smooth process with cardboard, but did allow for smoother movement of the shafts.

 

Final Product

For our final product, we would normally shift to a higher fidelity material such as acrylic or wood. However, due to the status of the laser cutter, Dr. Wettergreen constrained the project such that only cardboard should be used. As such, we set out to remake our 1st iteration prototype using the same materials, while assembling it cleaner and making it aesthetically pleasing. We cut the materials in the same manner, except this time stacking three layers of cardboard in some places that we felt would experience high loads. All components were painted to make the assembly look nicer. We decided to spray paint the cardboard, as that seemed like the most practical post-processing option. Sticking with the sun and planet theme, we painted  the main gear yellow to look like the sun, and painted the cover of the outer gear blue to look like the Earth. We then used the vinyl cutter to cut a small green silhouette of the Earth and transferred it to the globe so it looks like the Earth rotating the sun. 

The assembly was slowly put together, ensuring that adhesive lines were minimized and all painted sections weren’t discolored. After finishing the assembly, a small amount of lubricant (baby powder) was applied to key holes and slots. The assembly was then cleaned up, and prepared to present to the class.

Conclusion and Cost Estimate

In conclusion, this project was rewarding, but much more difficult than initially expected. The complex sun-and-planet gearing isn’t common in the world, and as such, during research, we didn’t have many resources to fall back on save for a few wikipedia pages and a Youtube video or two. The most time consuming portion of this process was very clearly the trial and error process. The process of putting things together and trying to create the motion we were looking for was frustrating at times, and caused us to make many small tweaks to our design.

There were two large issues that presented themselves during the assembly and testing of our mechanism — member rigidity and drive gear momentum. First and foremost, the cardboard used in our design wasn’t as rigid as we needed it to be to transfer forces and torque. Even supporting the full weight of the mechanism caused some cardboard components to bend.  We tried our best to thicken portions that we thought would require the most forces transferred through, but not all of those efforts were successful. Using a stronger material with more rigidity, such as wood or metal, would have solved this problem. The second issue that is very apparent in our design movement is the lack of momentum of the drive gear. In a real world system, the drive gear would be attached on the same shaft to a flywheel, and non-torque carrying component which serve to store and transfer angular momentum in the system. For logistical reasons, we don’t have a flywheel in our system; as such, this causes the mechanism to “lock up” when the arm is at the top or bottom position in the slot. The flywheel would allow the gear to keep moving itself and allow the arm to keep moving, however in our system, a small side force of the arm is needed to keep the whole system moving smoothly. This isn’t an aspect we can change easily in our design, unfortunately, but is a consideration in making this system in a higher fidelity.

PVC – $3 for 10 ft

Cardboard – $5 for multiple sheets (rough price)

Baby Powder – $5 per bottle

Spray paint – $5 per can

Labor – $10 per hour for 12 hours (two people) – $240

 

Total w/o labor: $18

Total with labor: $258