Community Harvest Project: Floating Arm Trebuchet

In the summer of 2019 my supervisor at Waters Corporation approached me to ask if I wanted to join a fundraiser that my coworkers did each year for Community Harvest Project. They would build a trebuchet to launch pumpkins at a harvest festival in early November, and this year they had the idea to field not only their 20’ normal trebuchet, but a second floating arm trebuchet. I had heard of a trebuchet before, but wasn’t really sure what I was getting into. He gave me the height and weight limits rules, showed me a youtube video, and left the design to me.

build

After designing the trebuchet in SolidWorks, I ran it by my team of coworkers to be approved. I then ordered the necessary lumber and adhesives. We constructed 4 quarter frames over the course of a week after work.These frames were constructed out of pressure treated 2x6’s glued and screwed together for extra strength. We also had to somehow attach up to 1000lbs of counterweight to the arm and lift it, so we poured concrete into a barrel with a PVC pipe in it to create flat disks that weighed just under 100lbs each. To lift the arm we utilized a block and tackle pulley system on each side of the trebuchet.

results

On the day of the festival, the trebuchet was loaded and fired for the first time. The firing mechanism for trebuchets is relatively simple; it’s just an angled nail, but it is difficult to pin down the correct angle with simulations. The first couple of throws were not so great as we were testing and tweaking the firing angle, one throw even went backwards! After a couple of trials, the trebuchet was loaded to 400lbs, and was able to fire about 200 feet. To put more weight on, the raising mechanism needs to be changed, as the blocks and tackle that we used were very cheap and were not sufficient for hoisting more than the 400 lbs put on the system. In the video below, you can tell that I’m disappointed in the result, as I had projected the trebuchet to throw pumpkins further, but I believe that the design was still successful and has the potential to fire much further with more weight and tweaks to the firing mechanism.