Trebuchet Project

This was my first project. I was inspired by several episodes of Junkyard Wars. A trebuchet is a kind of catapult that uses a large counterwieght to rotate a throwing arm. The missile is attached to the other end and thrown when the weight is released. I built mine to use a sling at the throwing end to throw a marble. You can click on any of the pictures to see a larger image in a new window.

Here is a picture of the completed project. There is a still larger version of this picture (60K).. I started out with just the picture of a trebuchet in my head. Part of the fun of this project was that I didn't have a design. I sat down with boxes of parts and started connecting pieces together until it looked like a catapult. The descriptions (and some of the pictures) aren't real detailed. You should figure out how to build one on your own.

I hadn't played much with K'nex before, so I was learning how to use them as I went. I got a couple of good ideas about how to make some kinds of connections by looking at the manuals that came with the set.

I ended up with a model, as you can see here. Actually, the first version didn't have the outrigger frame on the left side. But when I fired it, it would flip over!.

This was OK but I wanted it to be bigger, with a longer throwing arm and I wanted to add a winch of sorts to crank the arm into position. The first version had a paper clip hook at the end of a string attached to the throwing arm. I wanted to add a sling so I could throw marbles and such.

So I scrapped that design and started over. I built the same basic structure, a kind of box frame, open at one end. A bar across the middle holds the pivot. The K'nex kit came with a set of wheels and I used some of the big ones as the counter weight.

To keep the pivot arm centered, I built a second, parallel bar. I used a couple of half circle (yellow) connectors. The inside and outside bars are connected by these connectors. A full circle (white) has the inside bar running through it. The yellow connectors hold it in place.

The throwing arm is centered on the pivot arm. Two of the end clip thingies are used to hold one of the full circle (white). The weight is attached to one side and the throwing arm on the other. At right angles to the throwing arm and weight is a small arm that is used to crank the throwing arm back. A loop of bell wire is used to hook the arm to the crank.

The sling is attached to the throwing arm with some ire. I put some triangular frame on each side of the main box. I think these made it stiffer, but I'm not sure.

I then built a second box on the back of the first. This is where the crank is attached. A line from there is tied to a bent paper clip. This is hooked to the wire loop on the throwing arm. Winding the crank pulls the arm down. The projectile is loaded into the sling while it is on the ground To fire the catapult, I added a rod that is pushed to dislodge the hook and the arm is released. Here is a top view of this structure.

Because the throwing arm is about the same length as the frame is high, it couldn't be pulled back very far before it hit the floor. To raise it up, I built a frame of Duplos. The set I had included some sections with wheels so I thought it would be fun to make the weapon mobile. I used smaller Lego blocks to wrap around the base of the frame to lock the K'nex pieces to the Duplos structure. The Legos are under, between and over the K'nex rods that make up the base of the trebuchet. The picture shows the layers removed. The difference in size between the Duplos and the Legos made this fit together pretty well and held the trebuchet frame securely to the mobile part of the structure. Here is a closeup of what it looks like in the completed project.

I also used the Legos to build holders for the projectiles. One of my kids made a Legos flag to put on the structure. The addition of a K'nex guy and some Duplos guys finished the project.

Here are a few more photos of the whole project from different angles.

It worked pretty well. It could throw a marble about 6 feet. I made a short MPEG (~560K,~3 seconds) of it in action. I'm not much for photo analysis, but given the frame rate of the movie and how long the marble is visible and the distance it covered, I estimated that it was moving about 3-3.5 MPH when it left the sling.

I didn't keep track of how long this took. It was certainly more than the 10 hours required in JunkYard Wars. Most of the time I fiddled with it while watching TV or something. Once it was done and I took these pictures, I dismantled it to re-use the parts in other models. Maybe I'll build this again someday, adding motors to crank it or making it bigger to throw heavier things. The whole point of this project is to sit with a bunch of stuff and figure out what you can build with it.