Introduction: Floating Arm Trebuchet

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

Step 2: Building Phase: Frame and Base

Here, we followed the dimensions on the drawing and cut very precisely.
Build a good study base using 2x4 and plywood. Try to make it as flat and planar as possible, as this will minimize the need for adjustments later. Also, get the most straight lumber available for the longer pieces on the frame.

The frame is made of 4x4 held together with deck screws and carriage bolts, and our design allowed for the frame to be removed from the base with bolts only.

Build the frame in two identical halves (left and right) and then attach each half to the frame. Check the alignment of the rails and ensure they are relatively close.

Keep in mind that the rails should be unobstructed, and the gap where the weight bar will go should be small enough that your wheels can easily glide over it. I've seen some really nice f2k's that have the wheels jump the gap on a ramp, but that would require much more engineering calculations and dynamic tuning than we had time for.

Step 3: Build the Arm (engineered Beam)

Our arm was made of many pieces of layered plywood held together with resin and deck screws. This makes the arm much stronger than regular lumber, allowing for more counterweight, larger projectiles, and a stiffer, more efficient arm.

Start by cutting the pieces of plywood. The arm can be longer than your sheet of wood, but make sure the gaps between the pieces are in different spots for each layer.

Use plenty of screws as well as resin, gluing and screwing each layer to the next.

Clamp the beam and let it dry, allowing the appropriate time for curing (follow resin instructions).

When the beam has cured, trim off any excess wood to make planar surfaces, and sand off the excess resin. Your arm should look nice and smooth like the pics.

Now comes the fun part. Make matching steel plates and drill holes into them for bolts and for both the counterweight bar and the axle, allowing room for steel pipe inserts. Line them up carefully and bolt through the arm with carriage bolts to hold them together.

Weld the inserts carefully to the plates, ensuring the axis of each insert is as close to normal (90°) from the plate as possible. This is essential for an efficient machine. (Flanges would work here as an alternative)

We added a tapered cut to the arm, going from 4x8 to 4x4 at the tip.

At this point, dry-assemble your trebuchet and ensure everything fits, and that the arm will move correctly.

Step 4: Counterweights

If gym weights are not available (we arrived at the competition and found out they were, so this was basically a waste of time for us, and gym weights are more dense and easier to lift into the weight bar) then make your own counterweight using concrete and steel pipe.

Forming tube is expensive, so instead, cut a cardboard barrel into strips or tack thick cardboard onto a piece of plywood to make forms.

Cut a piece of steel pipe for an insert and place it at the center of each form, then pour the concrete. Allow about 1 week before firing your trebuchet.

Our weights were 18" diameter and 4" thick, weighing 80-90 lbs each. This was a bit too heavy as we had to load each weight manually to fire our trebuchet.

Step 5: Trigger Mechanism and Safety

Use a door release mechanism from a car for your trigger. These can be found cheap at any junkyard. (while you're there, pull some harmonic balancers from a Chevy 350 engine for your wheels) Toyota or Nissan models work well for the trigger, and ours held with around 1000 lbs of tension on it.

IMPORTANT: You should have a separate safety mechanism on your design. We drilled holes in the upright beams perpendicular to the weight bar, directly below where it would hover when the trebuchet is ready to fire, and stuck a piece of 1" steel bar though on each side. This directly blocks the weight bar from falling while loading the trebuchet. (See 4th photo, near top of the trebuchet, steel bars hanging out)

For the trigger: Cut a stepped, square slot out of a piece of lumber, as shown in the first photo, and screw your mechanism into the wood. Ensure that the tension will be applied in a direction that is similar to the way it is intended to be used in a car door. Add a little lube to the linkage and the clamp itself if needed.

Attach a 20ft + length of rope to the release mechanism on the door latch. Pulling this should make the trigger release.

On the arm, drill a hole for an eye bolt, and use a washer on both ends. attach an appropriate length of 1900lb test chain.

The chain link on the opposite end should click into the door release easily. We made ours so that the chain had a tiny bit of slack in it initially, which we took out by tightening the nut on the eye bolt each time we fired.

The trigger held up well, but it did take a pretty hard tug to get it to release with max weight.

Step 6: Wheels, Metalwork, Sling, Final Assembly

Harmonic balancers work well for wheels, and can be pulled from a V8 engine at a junkyard. Ensure that the shafts you choose have a diameter that mates well with your inside wheel diameter, as well as your inserts. Our shafts were steel bar. The weight bar was 1.5" diameter, AISI 4140 cold drawn (probably overkill) and the axles were 1.2" diameter, AISI 1020 CD.

At the tip of the arm, we welded a steel box around it to hold the pin for the sling release and a loop to tie the other end of the sling rope to. This could be done other ways, but our method looks really cool.

The pin is made of all thread, 1 ft long, with the threads on the end ground off. This allows us to swap out pins, or adjust forward and backward. The angle and length of the pin control when the sling opens. You want this to happen when your sling is at 45°, and it will vary somewhat with projectile weight and counterweight.

The sling itself and release pin are a bit of trial and error. Our sling (not pictured, there is a shot of it in the video) was made of woven nylon web straps. It shouldn't be too deep or to shallow for your projectile, and should hug the surface of your projectile well.

Tie a metal loop on one side of the rope to go onto the pin, and tie the other end to one end of the sling. On the other end of the sling, tie another rope that goes to the loop on the end of the arm. (See the last photo if this is confusing)

When loaded, your projectile should sit directly between or slightly behind the upright beams. The length of the rope will control what arm angle your sling releases at. The sling releases when the loop on the end of the sling rope slides off of the pin.

Ideally, the arm should be vertical or slightly forward when the sling releases. (Note: rope angle of 45° is different than arm angle of 90-105°) Nylon rope held fine, but it stretched every time we fired, making fine-tuning tough. A thin steel cable might work better.

Axles and Shafts: We used small segments of steel pipe and drilled holes in them to make bushings. We used these to hold the weight bar in place, with small plates welded to them to keep the weight bar from sliding back and forth. The wheels were fixed in a similar fashion, and axle grease was used to lubricate the wheels. The wheel axle on the arm was tack welded in place, so that the axle remains stationary with respect to the arm, and only the wheels spin.

Step 7: Dry Fire, Then Launch and Tune, Have Fun.

Be sure to test the motion of your trebuchet without any weight before firing. Shims may be needed to level out the device. (We crashed once during test firing due to this) Be sure to bring bubble levels. Once smooth motion is achieved, begin adding weight, but do not fire anything until a max weight test is done. We didn't have time to do a thorough dynamic stress analysis, so we slowly added more weight. As it turned out, our design was over built and held up well. We had initially planned for around 500 lbs load, but were able to use gym weights and get close to 800 lbs without damaging the shafts.

Our design required that we load each weight one at a time onto the weight bar using ladders. This was done with the safety bars in place, as well as the chain clicked into the trigger. at this point, we used a ratchet to tighten the eye bolt, until the chain was in tension and the weight bar no longer made contact with the safety bars. Then, we cleared away and removed the safety bars, and fired. The chain was then loosened again, the weights removed, and the arm cocked and locked with the safety bars again, and the cycle repeats.

We chose to paint our trebuchet and stencil our school logo onto it, but this is just for looks.

IMPORTANT: Safety is crucial when firing trebuchets. These machines are just as dangerous as guns. When firing, make sure no-one is down range, near to, or directly behind your trebuchet. Projectiles can fly backwards, so keep people and cars AWAY from this area. (I saw a car roof get crushed by a pumpkin flying backwards)
Use a long trigger rope and keep yourself at a safe distance.

Have fun. Smash many pumpkins.

Time was of the essence on this project. I have no idea if this is the most ideal design for this type of trebuchet. The radios of rail height/overall drop, arm length, counterweight/projectile weight, wheel size, etc were all guessed based off existing trebuchets from internet sources. If anyone takes the time to do a thorough dynamic analysis on an F2K trebuchet, and would like to share their results, please post them below.
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