Trebuchet Project

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About: I like to design and build random things.

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.

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 Adapter, 3/4”, Unthreaded & Female thread
  • PVC Adapter, 3/4”, Unthreaded & Male thread (x3)
  • 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

Launch It! Contest

Finalist in the
Launch It! Contest

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28 Discussions

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Neno Y1

Question 9 months ago on Step 14

Did u know any places in Australia sells products like this

1 answer
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mtairymdNeno Y1

Answer 9 months ago

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

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mgreen20

4 years ago

If u put it on wheels u could get up to 20% more power

5 replies
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Student DevinPmgreen20

Reply 1 year ago

Explain, please? Who is "u" and where would the wheels go?

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Student DevinPmtairymd

Reply 1 year ago

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?

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mtairymdStudent DevinP

Reply 1 year ago

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.

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Student DevinPmtairymd

Reply 1 year ago

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.

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logos16

2 years ago

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
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Student DevinPlogos16

Reply 1 year ago

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?

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mtairymdStudent DevinP

Reply 1 year ago

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.

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ecrouch

1 year ago

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.

1 reply
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mtairymdecrouch

Reply 1 year ago

Yeah, we skimped out on the latching mechanism. Also, due to liability concerns (middle school kids), we stuck with squishy balls :).

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Obey_Kendall

2 years ago

That helped me non. I have to do a school project and I don't even know how to put this monster together!

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IanM91

3 years ago

How would i go about converting this to a cup style launching system?

2 replies
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mtairymdIanM91

Reply 3 years ago

Although common for catapults, cup style launching systems are not typically used for trebuchets. The payload is generally carried in a pouch (google trebuchet components) attached by a rope. We tried this technique but it was not consistent enough for rambunctious middle school kids.

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ShiftP

3 years ago on Introduction

If i double the diameter alone of the pvc in all portions will it work? If so if i double the length of the pvc and keep the diameter the same will it work? And if i double both the diameter and the length of the pvc beefing the scale up by 100% will it work? With the first 2 questions, will either of the changes if they work will it effect the distance the projectile will travel?

2 replies
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eckenheimerShiftP

Reply 3 years ago on Introduction

As a general rule of thumb, when a 3D object is scaled up to a larger size, the size of load bearing components, such as the PVC struts, should be increased exponentially. That's why the giant insects in the old movies (see: https://www.pinterest.com/entophile/bug-movies/ ) couldn't exist. They'd need elephant-like legs just to support their body weight.

So, if a 3/4" pipe is made twice as long, the new diameter should nominally be 4 times the original, or 3". If the length is tripled, then 9 times the diameter is needed for the equivalent strength. Given that the load/stress for the trebuchet won't be anywhere near the load limit of the original 3/4" pipe, a 1" or 1.5" the pipe will be fine, and the 3/4" pipe will probably still be adequate at twice the length.

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mtairymdShiftP

Reply 3 years ago on Introduction

1st question: Yes, it will work. Not needed though - 1/2" diameter is strong enough.

2nd question: It would most likely fail due to the longer runs of 1/2" pipe (not strong enough)

3rd question: I think it would work fine if you scaled both. I would actually like to see a scaled up version with 1.5" pipe. Maybe you could launch a soccer ball or something.

4th question: I don't think the first option would help much - might even slow it down. The second option would probably break.