Trebuchet

Intro

We are students from National Park College’s Physics class. The class has an engineering focus because all of us are Engineering majors. We were allowed to pick something to create and our group decided on a trebuchet because we thought it would be interesting to build and some of us had prior knowledge of designing smaller trebuchets in the past.

General Notes

Trebuchets are medieval siege devices meant to use the momentum of a swinging counterweight to launch an object from a long arm. A sling is fashioned on the end of the long arm to sling it forward when the momentum of the long arm reaches its apex.

1. 3 -2x12in Boards
2. 4 ft Dowel Rod ¾”
3. 10 Screws
4. 12x12 Plywood
5. Marbles
6. 2 2.5lb Weights
7. Koozie
8. String or Very Thin Rope
9. 2 1 7/8 Bushings with ¾” Inner Radius
10. 1 Nail
11. Duct Tape

First you will need to use a saw to cut the unassembled parts of the trebuchet. Cut the 4 ft dowel rod into an 8in piece, a 6in piece, and an 18in piece. These will act as the arms to the trebuchet. Set them aside for now.

Cut two 4x8in pieces of plywood to create the brace for the arms as shown in the images. Set them to the side until later steps.

Use one of the two 2x12 to create 2 scalene triangles with 12 in bases and 18in to the peak. The ones shown in the image do not have tips to make them less hazardous but the flat top sits at 18in high.

Using the second 2x12 as the base, use some screws to position the freshly cut triangles 8in apart from each other from their outside edges.

Drill a hole using a hole saw to drill holes in both parallel triangles you just placed upright. These holes need to be parallel as well so take care this structure will be the base of the trebuchet.

Take the two bushings that you have and place them into one of the holes on the base. If you need to make the fit snugger then duct tape the outer edges to add a bit of radius to the bushings. Do the same on the other hole.

Now to attach all of the arms together start by taking the 6in and 18in dowel rod and drilling out a half circle on one end of each rod ¾”.

Put some wood glue and tightly fashion the dowel rod directly in the center of the 8in rod (4in), making a “t” shape. The attached pictures show the correct way it is attached.

Place the pieces of plywood from earlier directly on the front and back of the face of the “t” shape to where the plywood is distributed longwise across the longest axil of the wooden joint. Use screws to evenly screw the front and back plywood to the rods on both sides. We used 4 on one side and 6 on the other side.

Now use a pair of strong wire cutters to cut the head of the nail off and fashion it at the end of the longest arm of the “t” shape away from the joint.

Prepare the rod for the 2.5lb weights by drilling a hole adjacent to the face of the “t” shape on the 6in section and the longest section. This is for the string, so it doesn’t have to be that large but consider the strings thickness.

Now Tie the 2.5 to the hole on the 6in section with a small amount of slack for it to swing freely using the newly drilled hole creating a loop through the weights. If the 2.5lb weights you have don’t contain a gap on their outer edge like ours then lengthen the string and tie it through the barbell hole instead. Enough slack for the weight to swing from the rod is essential but take care that there is not too much that it inhibits the swing when you attach the “t” structure to the base.

Attach a string to the hole of the other arm using the hole to make a loop and leave sufficient string to make another loop in later steps. This string will be used as the sling and will act as a release for the load of the trebuchet

Now take the koozie and use scissors to cut a rectangle out of it using the folded edge of the koozie. Images shown.

Take either a hole puncher or some scissors to create four holes on the corners of the koozie.

Put the string on the long arm of the trebuchet through the holes in the koozie going from the exterior twice and leaving the interior twice so that the koozie shapes into more of a cup shape.

Take the protruding end of the string from the koozie and make a small loose loop that will hang on the nail. This loop is meant to slide off the nail at the apex of the trebuchet and launch the load (In our case a marble).