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 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
- 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
- Gravel, sand, cement
- Wood Saw
- Hack Saw
- Power Saw
- Wooden Mallet
- Power Drill
- Soldering iron
- Sewing machine
Step 3: The A-frames
Step 4: Arm
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
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
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
Step 8: Carriage
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.
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
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