Lately, a side research project asked me to devise a mechanical prototype, something I haven’t done in quite awhile. But to the difference that it asked for gears, motors, and all that. Turns out that LEGO is a good way to prototype these things.


Taking a break, I accidentally a trébuchet.

The trébuchet is a medieval siege weapon used to hurl cows, cadavers, rocks, flaming pianos, beyond a city’s wall to spread disease, break defenses or to set it ablaze. It’s a type of catapult using a sling activated by a counterweight whose effect is multiplied by a lever-type mechanism.

The counterweight is crucial, but LEGO, being made out of plastic, aren’t very heavy. So I got a block of steel with a convenient hole in the middle as the counterweight. The counterweight has to pivot, so it should be some kind of basket-like contraption.


It hangs under the trébuchet, rotating freely:


The trébuchet itself is just a lever rotating around an axle, itself stabilized by two half-A type frames. It’s mechanically really simple.


The only tricky part is the sling. Made out of string and a piece of thin cardboard, the sling is tied at one end, and only hooked at the other. The release mechanism relies on centrifugal force to unhook the sling. The hook is shown in the next picture (yes, it does look like a guitar)


* *

Yes, it does work. Kind of. Mostly. The lever and counterweight portion of the trébuchet works perfectly, despite the conspicuous imbalance of weight between the whole LEGO apparatus and the steel counterweight. I think the counterweight weighs more than the rest of the device. That’s something I’ll need to keep in mind if I scale the thing up. The sling unhooks itself correctly on launch, but the cradle (or whatever you want to call the portion that holds the object) is too rigid. It does not always hold the object in place. If everything goes well, the trébuchet hurls a (very) small object across the room, but 3 times out of 4, the sling drops the object, and the trébuchet misfires.

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