Pumpkin Trebuchet
During a first year engineering class we were placed in teams of six and tasked with designing, constructing, and testing a device capable of launching a 3-5lb pumpkin a distance of 60 yards. We opted for a floating arm trebuchet as it best fit the criteria we had decided on as a team. After testing and troubleshooting issues, we were ready for competition. Our team was one of only two teams capable of throwing 60+ yards out of 25+ teams total.
**Please excuse the lack of progress pictures- we didn't think to take any at the time.**
Pumpkin Chunking Project Overview - 2191.pdfRules and Regulations
This is the document that outlines the aims and goals of the project. It's attached here for those curious individuals that wish to read it.
Overall, it just explains some of the history about the project followed by the goal of throwing a pumpkin 60 yards. The professor also explained that many of the rules are written because of safety incidents in prior years, but to be diligent so as to not be the cause of more.
Each team was given a small tool kit and a stack of materials to build with. A sheet of plywood, some 2x4's, a box of screws, etc. Most if not all teams purchased additional tools and materials on their own.
Timeline
Having elected a team leader (myself) and a safety officer, we set to work creating a timeline for the project in the form of a Gantt chart. Looking back, it's a small miracle we actually had everything done in time between classes and other obligations.
The team certainly didn't follow the chart explicitly, but we did reference it often to get a sense as to how much faster we need to get things done.
Decision Matrix and Design Finalization
Each group member came up with a design or two, then as a team we roughly analyzed each one for our chosen design criteria. The design with the highest score became our basic design. The floating arm trebuchet narrowly came out on top, so that's what we began designing and drawing in CAD.
Trebuchet Drawings w BOM.PDFFinal Design
Since this was only our first semester of college, we didn't have the knowledge or skills to calculate the strength of different materials, so it was very much a "this might work, if it breaks we'll fix it" mentality. As one might expect, things did indeed break- those parts will be discussed in a following section.
We wanted to buy as few materials as possible, so the construction was just 2x4's and screws. Add a couple wheels, a little PVC, and a few $4 cinder blocks and it's all we really need. It's far from a perfect or optimal design, but we believed it to be capable of getting the job done and easy enough to modify if things went wrong.
Since we wouldn't be following drawings explicitly for the project, we didn't spend the time and effort to make any other than for the BOM and important details.
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Parts, Pieces, and Financials
The team was given a lot of materials and some tools to use, but it certainly wasn't enough. We frequently visited the local hardware store to buy additional lumber, screws, and other supplies. Our original sling (soon to be safety flag) was picked up from the thrift store while our plywood was traded to another team for more screws. To the right is the total list of bought materials and the prices we had paid for them.
Our final financial document even laid out who paid for what and calculated exactly who needed to pay who so everyone eventually paid the same. For example: if someone had spent $40 on materials, that individual would receive money from the others until they were only out $21.52.
Testing, Breaking, and Fixing
Every group had planned to test their launcher prior to the event to make sure it worked properly. Unfortunately, not every team had managed their time well enough to do so before the deadline. We were one of only a few teams that actually managed to test.
One of the first things to go was the main axle- the 0.75" diameter steel rod from earlier. The falling weight was strong enough to completely bend the bar, so we bought a pipe and filled it with concrete. The original bar was recycled as the axle for the wheels on the throwing arm, since that bar had bent as well.
The sling flew off and was replaced with a piece of tarp- the original sling then being used as a safety flag. Then the adjustment pin bent and some clamps broke. Slowly but surely, we solved every problem and were ready for competition.
Lime Time 2: Pumpkin Spice Edition
At the end of the day we were all extremely proud of the work we had put in and the results of our labor. We celebrated by picking up bits of pumpkin, disassembling the trebuchet (it couldn't stay there after all), and starting work on the final presentation.
Naturally, since our trebuchet was one of only two that could launch 60+ yards, we had one of the highest grades in the class. Definitely a great start to a hopefully long and prosperous engineering career for all of us.