"The Insensible" - a Counterweight Trebuchet

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About: I study Aerospace Engineering at RMIT, in the hope that one day I will be able to bring about the return of intercontinental airship cruises. Until then I like to make stuff in the shed, and surf instructables

Intro: "The Insensible" - a Counterweight Trebuchet

Lindsey over the road has a big back yard. My family as been coveting his land for some time now. Finally I decided enough is enough, and began to plot the annexation of number 5. The first hurdle to overcome was getting past the formidable defences - a 6-foot high wooden fence with reinforced gate. Thus I began to design a weapon capable of reducing them to rubble. The "Insensible" is a 5.85m* high counterweight trebuchet including an added 2m of sling.

Note: I've written this instructible in the first person, because it is far from an authoritative set of instructions, but merely an account of how we went about building a trebuchet, compiled to help others create their own.

Step 1: Theory

The trebuchet is a siege engine that was employed in the Middle Ages, both by the Muslims and the Christians, and did not become obsolete until the 13th century - well after the intruduction of gunpowder.

The trebuchet consists of a base and a swinging arm. On one end of the arm (the much longer end, to gain mechanical advantage) is a sling in which the projectile to be fired is loaded. On the other end is a heavy counterweight.

The sling is a very clever edition to the trebuchet. It acts as an extention to the arm, without the added bulk of extra timber. When ready to fire, the sling is attached at both ends to the arm, but as the arm swings around, one end of the sling detaches (when it detaches is adjustable, to account for differently weighted missiles) and opens out to allow the projectile to be thrown free.

Most of the "Insensible" is made of recycled wood, with a steel crossbar (the axle for the arm), and concrete counterweight - 120kg.

Step 2: Materials and Tools

Materials

- Recycled Wood
- Lots of screws, bolts and nuts
- PVA wood glue
- Gang nails
- Steel pipe + bars
- PVC pipe
- Lead-lighting solder
- Wheels from an old pram
- Rope, Chain
- Cotton
- Gravel, sand, cement

Tools

- Wood Saw
- Hack Saw
- Power Saw
- Hammer
- Wooden Mallet
- Ratchet
- Power Drill
- Soldering iron
- Sewing machine
- Chisel

Step 3: The A-frames

The base was made up of two A-frames, each 1.85m tall, which would lean towards each other when assembled.

Step 4: Arm

The arm was originaly 3 metres long. One metre below the axel and two above. (Later an extra metre was added to the long end).
Medieval French trebuchets could have arms with the long side up to 10 times as long as the short side. This provides the ability to hurl objects unbelievable distances, but puts huge strain on the materials, and requires and intimidatingly heavy counterweight.

A three metre plank of timber was selected, cut to size, and two more one-metre planks were screwed to each side of the to-be-weighted end for extra strength. A hole was drilled at the one metre mark - for the axel, and two more near the bottom for pins on which the counterweight would hang.

I went to Mitre 10 to buy a steel rod for the axel. but they didn't have any thick enough. I ended up buying a thin solid rod, and two tubes which slipped over the rod, and over each other, then soldered them together to make a thick bar of solid steel.

Step 5: X Braces

X braces

To stop the frame from skewing side to side, we needed to brace each A-frame against the other. Considerations were taken to keep the brace out of the way of the swinging counterweight.

We drilled holes inthe bottom of the frames, then glued bolts backwards into those holes, onto which x braces could be attached, and removed for disassembly.

Step 6: Counterweight

120kg was deemed the correct counterweight to still be manageable, while maintaining awesomeness. Note: now that the weights have actually been made, I have altered my definition of 'manageable'. Still, 120kg in one weight would have been impossible so what we did is this:

Made four moulds from bricks. 20cm x 30cm x 10cm should fit about 30kg of concrete. Our thanks go out to Riviera park, for supplying the gravel. We collected the sand from the beach, and washed (with tank water of course!) the gravel and sand to remove organic matter from the gravel, and salt from the sand - both of which would compromise the integrity of the concrete.

We mixed up the concrete mix, sand and gravel, the poured each mould 1/3 full, placed chicken wire reinforcement inside, filled 2/3 full, put in another layer of chicken wire and filled to the top.

PVC pipe offcuts were used to make holes where the pins would attach the weights to the arm.

Then came the most important part - writing our names in wet concrete!

Step 7: Sling

Peggy used her amazing sewing skills to sew a square of fabric around a length af rope so that it could be slid up and down the rope as adjusted the sling length - for firing different distances.

Step 8: Carriage

After a test run, and horrifying results, it was decided that the trebuchet needed wheels to reduce the stress on the structure, and keep it from completely flipping.

A pram was repurposed (destroyed) and the wheels from a little kid's bike were removed, and Kieran whipped up a carriage for the trebuchet to sit on.

It consisted of 6 wheels in two rows of three, three metres apart, joined by crossbeams, and the trebuchet sat on another set of diagonally cut crossbeams. These beams were cut to match the angles of the ends of the A-frames. See the pictures for what I mean.

Step 9: Firing Mechanism.

It was very dificult to design a firing mechanism that could hold 120kg, release quickly, and be able to be released by a human under the weight of 120kg, all without getting in the way of the sling which would come whipping around.

The result was, however, a very simple design. I soldered a chain to som leftover steel bar, which we wrapped around the arm and the carriage, and poked the other end of the bar into the other end of the chain. It was now a circle around the arm and the carriage, and when a rope tied to the bar was pulled, the bar would slip out of the chain and come free.

Differently weighted projectiles would cause the sling to release with different timing. So to make an adjustable release, we attached a screw (plus sheath so it was smooth - the thread was not exposed) halfway into an L-shaped piece of wood. This was then bolted into the end of the arm. The bolt could be loosened and the angle of the pin changed, to alter when the sling would release for maximum distance with all weights.

Step 10: The Result

Here is the finished product!

 
This is my first instructible - I hope you like it, and I welcome comments on how I can make future instuructibles even better!

Also, it is an entry in the Launch It! challenge, so if you liked it, pretty please vote for it and i'll be your best friend.

You can check out more of my projects (most of which I got the ideas for on instructables anyway) on www.patricksprojects.posterous.com

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    29 Discussions

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    scraprageous

    5 years ago on Introduction

    Thanks for the instructable - it helped a lot. A bit of editing to use the materials we had on hand and we used to send pumpkins flying for a church festival. I forgot to try for a good photo the day we were using it. Counterweights were 475 pounds - mainly old barbell sets and some chain.

    pumpkinfest 017.JPG006.JPG
    3 replies

    the one photo above is just a picture while under construction - the final product was square and had more boards (in x shapes) for support. We left the sides (base 4x4's) wide on purpose for carrying.

    Forgot the video - old phone did take a video (very short and not very good), but the trebuchet worked well this year too http://www.youtube.com/watch?v=9PtVMy1SbaM we are making some adjustments for next year - fires ok. Now to get distance better. Short sling this year due to forgetting the actual one at home.

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    squiggy2scraprageous

    Reply 5 years ago on Introduction

    Wow good job - it looks like it would have shaped up to be an awesome piece of equipment! I'm glad I could help. Definitely get some photos and /or video next time you take it out, I'd love to see it in action!

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    davtheman223

    5 years ago on Introduction

    People in asia used it too but that was because the Macedonians and other parts of greece developed it

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    MegaNerd

    6 years ago on Step 4

    How big does the axle and the 2 other bars have to be? (Thanks a lot for this instructables it was very useful and awesome) :)
    Live long and prosper.

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    squiggy2MegaNerd

    Reply 6 years ago on Step 4

    The axle we have is about 3cm diameter and we used the same bar for the counterweight pins. Originally it was a hollow tube but that bent on the first use. We couldn't find 3cm solid bar anywhere so we ended up putting a number of consecutively smaller tubes inside each other to make a solid bar.
    Thanks for the compliment and well wishes!

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    laxap

    6 years ago on Introduction

    "...one end of the sling detaches (when it detaches is adjustable, to account for differently weighted missiles) and opens out to allow the projectile to be thrown free."

    Can you please explain how it works?

    I'm making a trebuchet, and my fear is the the projectile does not free itself and stays tangled into the sling.

    Thanks in advance!

    3 replies
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    squiggy2laxap

    Reply 6 years ago on Introduction

    --setup--
    - one end of the sling is attached to the arm of the trebuchet, and the other to hangs free and is attached to a ring.
    - In step 9 you see the the release mechanism. It's pin, an you can adjust its angle with respect to the arm. The ring goes over this pin, so that the sling is now attached to the arm at both ends and the projectile won't fall out

    --firing--
    - When the arm rotates around, the sling moves with it, but at a higher speed.
    - As the two bodies (arm and sling) rotate, due to their different velocities, the angle between them will change. In the beginning, when the arm is at rest, they're at ~120° to each other.
    - Thanks to centrifugal force, as the sling rotates around, the projectile imparts a force on the sling outwards at 90° to its direction of motion like swinging a bucket round an the water stays in.
    - When the sling moves to be roughly inline with the arm, at 0°, the projectile's centrifugal force will be acting at ~0° to the arm too, and it pulls the ring off the pin.

    --adjustment--
    - What keeps the ring on the pin is friction. The projectile is always trying to pull the ring off, with its F*cos(a) where F is the centrifugal force and a is the angle between the sling and the pin.
    - at 90°, the cos(90) is 0. So it is physically impossible to pull the ring off.
    - at 1°, it would never happen, but if theoretically you had a ridiculously heavy weight moving extremely quickly, F*cos(1) would be enough to overcome the friction between ring and pin.
    - at 0°, cos(o) = 1, so ALL the centrifugal force is acting to pull the ring off the pin

    - SOOOO, the heavier the projectile, the earlier it will release, because it will have more centrifugal force to overcome friction at larger angles. This means that you have to adjust the angle of the pin so that is points more upwards (when arm is ready to fire) so that when the sling releases at 0° to the arm, not earlier.

    You asked for an answer, you got it!

    There's a good video here showing a slow mo of the sling releasing if you need a visual

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    squiggy2squiggy2

    Reply 6 years ago on Introduction

    *edit: I meant at 89° it would never happen, not 1°.
    At 1° it would almost certainly release with anything weighing more than about 1 gram (the weight of the empty sling would be enough)

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    jldiguiseppi

    6 years ago on Introduction

    we made a trebuchet for a scout project. We lobbed water balloons into the campsite of another troop, to our great amusement.
    The best projectile was to tape sparklers onto softballs. DIY meteors; very cool

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    rogiewan

    6 years ago on Introduction

    Hey now this is EXACTLY what we need over here in Ireland! Maybe a tad BIGGER though. Thanks for the instructions and helping me to realize that I am not alone in my madness and that stuff NEEDS to get launched! Amen

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    nehi

    7 years ago on Step 10

    I take it you play rome total war. Awesome !!

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    itri45

    7 years ago on Introduction

    good job with the trebuchet. few changes and suggestions first off all trebuchets use counter weights. second i would hinge the counterweight in between the main axle and itself creating greater efficiency. next try a more advancd design like a floating axle trebuchet or king arthur trebuchet or my favorite the floating axle king arthur trebuchet (FAKA).
    good work

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    squiggy2itri45

    Reply 7 years ago on Introduction

    Thanks for the suggestions!
    I've been thinking about a floating arm trebuchet for a while now, but I'd never heard of a King Arthur - I just googled it and am very excited :P

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    itri45squiggy2

    Reply 7 years ago on Introduction

    yes, the king arthur is a little more advanced especially with the trigger release system. i am proobaably going to make an 'ible on the FAKA im building. great job.
    oh and is the massive counterweight really necessary

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    squiggy2itri45

    Reply 7 years ago on Introduction

    No not at all, most of the time we're actually just running off 2 of 4 weights (so 60kg) but with a missile of any more than about 1.5kg there's a noticeable improvement in distance using the full 120. Also, I just came across an article on "traction trebuchets", which used teams of slaves and a big rope to pull the arm around, so there :P