This is a general description of how to create a floating arm (f2k) trebuchet.
A f2k trebuchet has an arm that rides on wheels, which roll forward on rails, while the weight bar running through the arm falls down in a vertical groove. (Watch plenty of YouTube videos to understand the motion of this design) The wheels start off above the rails, and the arm is initially supported by a second set of wheels (or one wide wheel) on a stationary axle fixed between the two halves of the frame near the rear end of the rails. The wheels on the arm hit the rails and immediately begin rolling forward, and the arm tip moves in an elliptical path.This is supposed to be a very smooth, quiet transition and the actual impact is very low.

Here's a YouTube video of our trebuchet:

This design was for a school ASME (American Society of Mechanical Engineers) Club competition. The parameters for the contest were as follows: Design a trebuchet that launches pumpkins (about basketball size). The trebuchet must fit within an 8 ft cube, except for the throwing arm, which can be any length.

Our design was very stout and held around 720lbs in counterweight, not including the weight bar and arm itself.
We launched pumpkins and bowling balls around 10-15 lbs at distances of 300+ feet.

Caution: This particular design is a very time intensive project (expect 60+ man hours) and requires some metal fabrication, such as cutting, welding, and drilling.

Step 1: Design Phase

SolidWorks was used here, although any CAD software or even a hand sketch would be ok. One advantage of 3D CAD was the ability to make an assembly and ensure a good fit for all the components, as well as smooth motion.

I've uploaded PDF's of the CAD drawings as well as a general schematic diagram for an f2k. Feel free to use them for educational or personal use. NOTE: all CAD modeling was done using student software, for educational purposes.

Using the dimensioned drawings, estimate the amount of materials needed. We spent around $300 in lumber and concrete, and several hundred dollars more on metal shafts, plates, etc. Consider cost and use whatever spare or donated materials you can find.

Try to have all the sizes of your shafts and designs for your trigger and sling release figured out at this stage, as well as how you plan to attach the shafts (welds or flanges, etc).

Consider how much weight you want to use, and size your shafts and frame accordingly. I haven't seen much information on dynamic tuning for the f2k, but we did an estimate of the static loads at the critical points and kept the stresses around 1/4 of the yield strength of the steel.

I'd also advise implementing a metal sheet on the rails, instead of bare wood. it's smoother, faster, and prevents grooves from wearing in the wood. We'd have done much better with something like this. Angle iron or steel sheet would work.

A few things to keep in mind are the optimum ratio for the arm length (2:1) , or L=3P in the schematic diagram. You want to keep the width relatively narrow, but wide enough to accommodate whatever projectiles you plan to use. 

-Lumber (4x4, 2x4, plywood)
-Deck Screws (lots of them)
-Carriage bolts, nuts, washers
-all thread, 1/2", 1ft sections (for sling pins)
-Harmonic Balancers ~7" dia (4X)
-Steel Shafts, various diameters
-Steel pipe, appropriate mating diameters to shafts
-Bolts for bushings
-Concrete (450+ lbs) and circular forms (OR just use gym weights)
-Steel plates
-Steel chain, 1900lb test, 3-4ft
-Door latch mechanism from a car
-Nylon rope (or other strong rope/cable)
-Nylon webbing for sling
-Axle grease
-Welding equipment
-Drill press
-Carpentry tools
-Paint (optional)
<p>Really cool project Brother, thanks for putting together a great instructable. We are building one now, it has been lots of fun for the family. Our starter model was 1/6 scale, it threw a bouncy ball 270 feet with 10 pounds of counterweight. We built a 1/3 scale model that throws a 1 pound projectiles 340 feet + with 70 pounds of counterweight. That model was equipped with a folding gantry and pulley system to raise the counterweight, with that it could be fired every 2 minutes. We added stabilizer arms as well, to keep it from tipping over. Throwing stuff with gravity power is fun...</p>
<p>Does anyone have a solid works file for this trebuchet?</p>
<p>You mentioned ~750# in counterweight. Can you describe the process for resetting it?</p>
Jeff,<br><br>Resetting with that much weight was time consuming. Notice we had our weight bar set up similar to a weightlifting set, in fact, at the competition they actually had gym weights which we used instead of the concrete ones we made. The process after firing was to remove the weights one by one, then raise the arm to set position (which still took a bit of muscle when empty), then set the safety and trigger, load the weights back on one at a time using step ladders, remove the safety, and fire. <br><br>If we'd have had more time and money, and the rules of the competition would have allowed, we'd have designed some sort of pulley system to simplify the reloading.
<p>We do a yearly pumpkin-chunking w/ a traditional counterweight trebuchet, but I was considering building a floating arm trebuchet. The thought of resetting it multiple times seems daunting.</p><p>As a second thought, I wanted to share the trigger we built to release. The kids were having a difficult time w/ pulling the pin from the overlapping eye bolts, so we built this... much easier w/ the heavier weights. Crude image, but you get the idea...</p>
A good way to estimate the range is to use conservation of energy. I'd maybe assume 50% efficiency as a conservative estimate for the f2k (it's probably higher), so the initial kinetic energy of the projectile is equal to the gravitational potential energy of the falling counterweight times the efficiency. (KE = .5*GPE, so .5mv^2=.5mgh) Solve for the velocity. I used a projectile motion program I wrote using Matlab last year, and assumed a release angle of 45&deg; and initial height of 15 ft. There's tons of assumptions involved there (what's the drag coefficient of a pumpkin?), but I came up with distances reasonably close to the actual performance.
<p>Do you still have the Matlab file? I would greatly appreciate it if i can take a look at it.</p>
<p>hey I'm a physics student trying to replicate your design. I came across a problem for the quick release. i was hoping you could tell me what the release system was rated for and if you have any recommendations on where i can acquire a quick release </p>
Thanks for the great instructable! I made my own (although on a slightly smaller scale. )
<p>therobotichand, would you mind posting any materials or tutorials you used? A friend and I are making trebuchet that seems to be about the same size.</p>
<p>The frame is made from 12mm diameter square cross section pine (I used a about 2.4m). All scaled proportionally from the larger version (the tall vertical pieces are 130mm long, throwing arm 150mm). Masses are lead (6x125g) cast from lead roof flashing. Wheels are from the rotating platform from a microwave but any small stong wheel will do. Axel for the masses is coathanger wire but ensure the axel never absobs the full impact of the falling masses as it will bend. Platform is pine.</p><p>Good luck with your project!</p>
<p>Thanks! Did you need to do anything special with the scaling?</p>
<p>On the working model simulator how do you make the sling release at 45 degree</p>
we didn't use a simulator for the motion. your best bet is to experiment with different sling lengths and pin angles.
In response to urbanmx, this was our very first trebuchet. We actually did not have motion software, we just looked at lots of YouTube videos. For a traditional trebuchet, the optimum weight ratio is 133:1. I have no clue what it is for f2k, but it is a more efficient trebuchet. I'd guess that for throwing oranges, you'd want to be around 5-6 ft high and an 8ft arm, and maybe 50-100 lbs of weight? You may even be able to get away with less. I'd look around on YouTube and compare different sizes of trebuchets. I've definitely seen some trebuchets in that size range throwing tennis balls, 6&quot; steel shot, etc very far.
<p>Hi, I looked it up, and for floating arm trebuchets the weight-projectile ratio tends to be about 1:50.</p><p>I'm using this kind of trebuchet for an assignment, and the ratio of our weight to out projectile is about 1:200. would this kind of trebuchet be better in this case than a traditional trebuchet?</p>
<p>Made this a couple months ago. Got materials from the tip and some plywood from bunnings. Total cost was around $70aud.</p><p>Ill try to get a video i can upload but we threw a 3kg rock about 90meters with 80kg counterweights, we didnt have concrete so we had to use 4 x 20L water jerrys. The jerrys take up alot of space to we could only get 80kg worth on.</p><p>Over all pretty good thanks for the plans. Im hoping to re make the arm if i can get a drill press, not having a straight axel for the wheels meant that it would sometimes derail. </p>
This is way too cool. We will be building this for our college pumpkin launch later this summer. I'll post pictures as we get underway. Thanks for writing this 'ible.
Oh man that is awesome. I've always wanted one did you ever build a smaller one to start with? Does it take a small army to transport it?
<p>We didn't build any smaller ones, we just went straight for the prize. The machine comes apart in 6 pieces; the base, four upright sections, and the throwing arm. They are bolted together with 1/2 inch hex bolts. We've contemplated building a pair of smaller ones after the fact. Something to launch tennis balls at each other.</p>
You can check out YouTube for more versions and there's a really nice one that a high school student built.
Neat. I like to play around building twisted rope driven catapults/onagers and also trebuchets. I went for tiny and built a ~2&quot; trebuchet that launched kernels of corn about...6-7 feet. I couldn't get a small enough pouch that was flexible, so I glued 4 lb test fish line into a tiny hole drilled into the corn kernel. I had tied a tiny loop in the fish line...the overall length of the fish line, loop and all was a little less than 2 inches. The loop then fit over a tiny wire hook at the end of the arm. The trebuchet used a 1 oz fishing sinker for weight. The biggest problem was losing the corn kernels{:
A few times in the year there's an Instructable that stands out above the rest. This is the first of the year. Awesome project, great craftmanship, well explained, crazy as hell! Add some wooden wheels - or mount it on the chassis of a small car - and you're done! This is gonna be a great summer project, thanx for sharing guys!
We did win, but barely. We threw exactly 300ft, and another team using a traditional trebuchet, a longer arm, and 1200 lbs of counterweight threw 294 ft. I was a bit disappointed in the organization of the competition, and most of the pumpkins were much larger than we had been told they would be. I think we only won because we picked a small pumpkin out.
One more question if you did build a smaller one to throw oranges would it throw them a distance much further than throwing them with your arm? I really want to try this at a much smaller scale but it would suck to go through all the trouble and have my buddies throw one farther by hand.
urbanmix, you can use workingmodel software to understand the motion and know how much force is needed to throw your oranges. I think this is the way they did, as there is a working model picture at the beginning. Just don't forget to include aerodynamic drag in your calculations, or you'll end up with a overestimated value. <br> <br>Great project, if I had the time and resources I really would build it!
Awesome job. <br> <br>Important question: <em>Did you win? <br></em>

About This Instructable




Bio: Mechanical Engineer in the machine design industry. I like things that go boom and sushi, preferably at the same time.
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