Trébuchet!

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.

Trebuchet

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.

trebuchet-counterweight

It hangs under the trébuchet, rotating freely:

trebuchet-counterweight-suspended

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

trebuchet-neutral

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)

trebuchet-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.

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s

%d bloggers like this: