Leonardo made two drawings of catapults in the codex Atlanticus (above), dated somewhere in the 1480’s. While gun powder had been invented well before this, he seemed to understand that it wasn’t always reliable and that catapults still had a place in warfare. The da Vinci catapult took and interesting turn (as it were), since none were made like this before that have been documented. Da Vinci was often pitching to kings and local lords (to whom he was employed), new ideas to protect their castles and improve their effectiveness in battle (note the tank and other nasty weapons he has drawn).
He made two designs, a single arm and a double arm. It looks like the tensioning arms would have used a laminated wood of some kind, but the tension on them would have been huge.
On the single arm there was a long pole (on the right) that inserted into the drum so the swing arm could be put into firing position. It used a ratchet system where as the arm was pushed into position, a ratchet would prevent it from firing. Once in position, the release mechanism was pulled (on the left of the bottom drawing). We used that type of release mechanism in our catapult model, but not the long arm to push the swing arm into place, since there is little pressure on a model and the swing arm can be easily pulled into position.
The (top) double arm catapult used a different winding mechanism, and Leonardo gave a nice illustration of it in the top right of his drawing. It essentially is a worm screw that would slowly wind a large wheel (improving leverage) that would move the swing arm into position. It would be engaged in the wheel, wound, then when in position, a release mechanism would be engaged to hold the tension.
The worm ear would be lowered so it was not engaged, and the release mechanism would be tripped and off goes the projectile! The tension must have been enormous, and it has never been discovered if either of these models were ever made or used in battle.