Published: April 12, 2019
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By: Grace Timken, Cal Poly San Luis Obispo
Category: Mechanical Engineering
Hashtags: #CAD #Design #Engineering #MechanicalEngineering #Project #Teamwork
The lab project for the "Design for Strength and Stiffness" class during my Junior year at Cal Poly was to create a scooter similar to a Razor scooter, made entirely out of PVC except for the fasteners. There were bonus point awarded for least deflection, highest jump, and its ability to fold up.
This project was initially very challenging for our group - PVC isn't necessarily the best material when shear forces are applied to it, and we needed to ensure that it could withstand heavy loads of someone standing and scooting around on it.
We decided to search for PVC sheets - very strong, durable, and high-strength material that can be an inch or more thick. One downside is that it is expensive, and would have put us over our budget. However, we searched on Craigslist and were able to find it for a heavily discounted price. This bypassed our first hurdle, and greatly assisted us in the build.
We based the design of our scooter off a Razor scooter, since that's a reliable product on the market that has great success. We had to make modifications for some components because we were constrained to use PVC, while most of the Razor scooter is made out of metal. We enlarged pipe sizing and cut smaller pieces of PVC and inserted them into the pipe to strengthen critical members.
One of my teammates primarily worked on the CAD and was able to tell us where the highest stress concentrations would be for our design. We continuously modified our design throughout the building process in order to decrease these stress concentrations and increase the factor of safety.
The second largest hurdle we encountered was the actual build itself. We had a great design on paper, but we quickly discovered that it was very difficult to get this exact design built. Holes were drilled wrong, members didn't stay together, and it was difficult to attach some components. To overcome this, I put in many extra hours in the machine shop with one other teammate, and we fixed all of the mistakes we had made and learned to not make them again. After a few late nights in the shop, our design had been strengthened and modified to be ergonomic, strong, and durable.
The ultimate test of our scooter was at the end of the quarter, where we had a "Scoot-Off". This was where we raced our scooters around a track that our professor had made. That was the timed test, but we had additional tests for deflection, overall dimensions, weight, and aesthetics. Our scooter not only won the first race, but it scored best in the weight and aesthetics category, and fulfilled the requirements for deflection and overall dimensions. We painted our whole scooter black, and purchased light-up wheels to add to the aesthetics.
Seen in the video is an additional race, where the scooter can be seen coming in second. The light-up wheels make it easily visible. The scooter is so durable that even after going off of jumps 2+ feet high, it did not scrape the ground or break. This project truly encompassed the design process, and helped me learn a great deal about the separation between modeling on a computer, and building in the shop. This learning process has helped me in every project since then, as I go into the shops to start fitting things together to make sure it will work before going too far into the CAD process.