Trebuchet

For the final project, I initially wanted to do the gravity car from our physics class, but I learned that I can't reuse my projects from another class, so I decided to do something interesting but achievable. From the beginning, I was not a fan of making things from other people's instruction, I wanted to create something myself, that won't rely on other people's help, so that even if I fail, I know what I'm doing and I can make changes that improve it. Trebuchet is a good project that match my expectations. Everyone can understand the mechanics of a trebuchet, but it still takes some work to build a functional one. It is going to be interesting if it works well, and I can put it in the classroom as an ornament.

I chose to 3D print the base, and one thing that might be problematic is most 3D print can't fit very big models. Some of the bigger ones will be able to fit a decent sized one, but it's still necessary to be extremely careful about the dimensions.

At first, I made my trebuchet base in Onshape and it was a simple symmetrical model with only 4 horizontal beams connecting the two sides(p1), this draft only had a hole that allows you to put an axis in it, and it's clearly not enough for a functional trebuchet. The beam will create an uneven surface under the level and block the sling from sliding through and launch the object. It will be ok if you hang it off the edge of a table, but this special scenario shouldn't be the regular setting.

After I decided to improve the base, I exported the file from Onshape to Solidworks, the software I'm more comfortable with. The modification I made was to extend the base from one end and add two hooks for rubber band that hold the lever when the mass is hanged(p2&p3). I also made a rail on the base of the trebuchet so that ideally the sling can stretch back from the end to under the beam,.

However, it's still not perfect and I modified it a second time, and the third model(p4) is the final one I used in my trebuchet. This version is attached under this step called Trebuchet Base.SLDPRT. In this version, I lifted the hole for the shaft and increased the width of the rail to make things easier to go through. The increased height of the hole was meant to increase the gap between the shorter end of the beam and the ground, so the mass can travel more distance before stopping, lifting the object for launching all the way to the top.

There are not a lot of special things on the beam, there's a hole that goes through the entire beam on each end for strings to go through, a bigger hole for the shaft, and a hook that can hook to the rubber band to stay in place even when the mass is placed. I worried about the structural integrity of this part but it turned out it's the part I had the least problem with, and it's very sturdy.

This is the part that's holding the mass, and it can be any shape as long as it holds things. I went for a bowl liked shape that hold balls, and if the masses have a flat bottom it can be a box shaped part.

I had some issue with the first version(p2), since the mass is a lot heavier than the bowl itself, the center of mass is above the rim of the bowl. That is an unstable equilibrium that will tip over with a single tap on the side. So I changed added another hole that strings go through, and even though the center of mass doesn't change, three strings will balance the net force to 0 and it won't tip over that easily.

I used silicon mold(p1) for the sling part, the picture is the mold that you pour silicon into, and after it dries, it will be a thin piece with two holes that strings can go through. The whole step is pretty straightforward.

1. Take a threaded spindle, and four nuts. Put the spindle into the hole on the lever and the two holes on the base. Use 2 nuts to prevent the lever from wobbling and the another 2 to hold the spindle in place on the base.
2. Take a single string and thread through all 3 holes on the base, tie a knot at each end. Pull the string up and adjust until you get three sections with approximately equal length. Tie them together and take another shorter string to hang it on the end closer to the spindle.
3. Do the similar thing with the silicon sling, except it has only 2 holes, just tie the string to the other end through the hole and it will be ok.
4. Put a rubber band on the two hooks on the base.

When launching, hook the rubber band with the hook on the lever, place a light object in the sling and a much heavier object in the bowl, unhook the rubber band and it will launch.