Trebuchet Project

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This project is currently being used for a middle school intro to engineering program. This project is pretty simple to build and just about any home improvement store will have the supplies.

The kids are given the materials and the trebuchet drawing (see link in description). It takes the average group (2 to 5 kids) about 10 minutes to assembly the trebuchet. There are two variables in the setup. The first is the launching arm – three choices are available: short, medium and long. The other variable is the height of the weight. This is controlled by the distance on the ruler attached on the board at the base of the trebuchet. Data is collected for the three arm lengths across the weight height range (see link to spreadsheet). After the data is collected, a competition is held to hit a target (bucket, 20 feet away).

Although modified for the class requirements, this is not an original design. It was reversed engineered from a combination of the versions you will see online.

See video for actual build.

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Step 1: Materials, Tools and Notes

Materials:

• PVC Pipe, 1/2” diameter x 30” Long (x2)
• PVC Pipe, 1/2” diameter x 17” Long (x4)
• PVC Pipe, 1/2” diameter x 4” Long (x3)
• PVC Pipe, 1/2” diameter x 3” Long (x4)
• PVC Pipe, 1/2” diameter x 1.5” Long (x12)
• PVC Pipe, 3/4” diameter x 17” Long
• PVC Pipe, 3/4” diameter x 8” Long
• PVC Pipe, 3/4” diameter x 6” Long
• PVC Pipe, 3/4” diameter x 4.5” Long
• PVC Pipe, 3/4” diameter x 1.5” Long
• PVC Elbow, 1/2”, 45 degree, Female Unthreaded Socket Ends (x8)
• PVC Tee, 1/2”, Female Unthreaded Socket Ends (x10)
• PVC Tee, 3/4”, Female Unthreaded Socket Ends
• PVC Cross, 3/4”, Female Unthreaded Socket Ends
• PVC Cap, 1/2”, Female Unthreaded Socket End (x4)
• PVC Cap, 3/4”, Female Unthreaded Socket End (x3)
• PVC Glue/Cement
• Pin or nail – 1/8” diameter x ¾” long
• Epoxy
• Wood – 30” x 4.75” x 5/8”
• Paper Ruler – 30” long
• Clear Packing Tape
• 5lb weight
• Rope (30”)
• Rubber squishy ball
• Fishing line - 26” long

Tools:

• Saw
• Drill
• Ruler

Notes:

Refer to the drawing for build instructions (last step). Note that Upper and Lower frame joints (items 1 and 2) are shown as assemblies on the drawing. This is an optional step which was done to speed up the build process for classes that had limited time. Both of these assemblies were glued together. The launching arm assembly must be glued together to support the weight and swinging motion.

Step 2:

Cut PVC pipe to the lengths shown in materials list

Step 3:

Assembly the lower frames assemblies

Step 4:

Connect assemblies together with 30” pipe

Step 5:

Add the 17” pipes off the 45 degree fittings

Step 6:

Attach the upper frame assemblies to the upper sides of the pipe

Step 7:

Feed the 4” pipe through the launching arm between the two upper frame assemblies

Step 8:

Tie the 5lb weight to the launcher arm

Step 9:

Cut a piece of wood 30” x 4.75”. Thickness doesn't matter (.25” to .75” will work). Find or print out a ruler and attach it to the wood. This version was printed and covered with clear packing tape.

Step 10:

Drill a hole in the ¾” end caps. Pre-bend a pin (nail) to a 60 degree angle. Place pin in the hole and use epoxy to secure the pin to the cap.

Step 11:

As mentioned above, the arm length can be adjusted by swapping out the launcher end caps (short, medium and long). Make sure pin is facing forward.

Step 12:

Cut a piece of fishing line to 26” long. Tie knots on one side of the line. Drill or poke a small hole through the diameter of the squishy ball (slightly bigger than the fishing line). Feed the straight end of the fishing line through the ball. Tie a 1” diameter loop on the other end of the line.

Step 13:

Cock trebuchet by pulling the ball back along the board. Record the cocking distance on the data sheet. The ball should fly up to 50 feet.

Step 14: Drawing and Data Sheet

Attached are the Drawing and Data Sheet

Finalist in the
Launch It! Contest

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34 Discussions

I’m not sure what you are asking. The materials list is shown in step 1.

I think alfredog13 was asking how long is the load (bottom) portion of the launch arm? I am also confused at which 3/4 pvc length I am supposed to use for the load arm (below the cross piece) vs short, medium, long launch caps. the is it 8", 6"or 4.5" .

Sorry about that. I commented under the questions section.

I am also confused at which 3/4 pvc length I am supposed to use for the load arm (below the cross piece) vs short, medium, long launch caps. the is it 8", 6"or 4.5" .

Sorry, but I don’t recall. My best guess based on the pictures is 6”. Do a fit check while building it and the correct dimension will become obvious.

You should be able to buy PVC pipe & fittings at any large hardware store. If not, Amazon or McMaster Carr.

I understood who mgreen20 was referring to, I just prefer a passive-aggressive approach to my grammar-Nazism. Also, thank you for the link! Unfortunately, the project I am currently working on expressly forbids wheels. :( Is there any way to get the benefits of a wheeled design without them?

Wheels are not necessary. What is your goal? If it's distance, you can increase the weight, use a different projectile, etc. BTW, the variables are the fun part of the project.

My goal is to launch a 15g projectile between 15 and 25 feet. There are size restrictions, so I had to cut back a bit, but I believe the trebuchet is more than adequate for the task.

I found that 4 inches for the cross beam was not enough, so I had to use a 5.5 in. Not yet certain if necessary to widen the base structure similarly.

2 replies

Thank you for the tip! We start the build tomorrow, but first I modeled it in Solidworks, which showed the same thing. Also, is there a way to mitigate the tendency of the arm to "wobble" from side to side during launch?

As you most likely know, the wobble is due to the differences in the inner and outer diameter of the pipes. You could try to make custom spacers or find bearings that could be used instead of the cross pipe.

I made this. I'm going to add a mechanism to hold the projectile in place until ready to fire. I extended the legs and laid bags of topsoil over them for stability. mtaiymd's design achieved consistent results with projectiles of differing masses: lead sinker, golf ball, and tennis ball.