How does this catapult work?

I was watching a video by this robotics youtuber and came across this video, I can’t tell how it works. Here is the video

Videos to low quality to see but most likely a slip gear and some rubber bands.

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If it is a slip gear (i.e. a gear with some teeth sanded/shaved smooth), that would be prohibited by R13.

(unless they built a slip gear without modifying any parts, which would be a pretty impressive achievement)


I saw many people on YouTube using the same type of catapult system, if it is then how is it moving fast? Here is another video from the same youtuber but with a full bot.


Here is a crude drawing but this is my guess. A pin / standoff rotates between two gears and there is a L hook that grabs on to it until it rotates around the axle and falls away from the hook causing it to release.

It would take some testing to identify where the pin is placed and the size and placement of the hook. You would also need to prevent the hook from hitting the axle. You could place pins at 180 degrees and it would fire every 1/2 turn.


Same mechanism deployed in Bankshot chineese robots…and it did use a mechanism as sketched…and rubber bands….do some bankshot robot research……


If you look at the current herobot here Fling – you can see the internals of the catapult.

There is a set of pins that run along the arms, they pull it down and when they fall off the end of the arm, the rubber bands pull the lever forward.


The video shows an interesting advantage. Fling is a choo-choo mechanism which takes one full rotation to fire where the robot in the video only required half a turn.

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I don’t understand what makes sure the L hook engages with the pin. Is the L hook only secured in one place to the catapult arm or is it pinned to the gear as well?

there is a design that can do this :slight_smile:

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This type of catapult is really hard, either is goes down way too much or it just does not want to move, I am wondering is the catapult in these video using the same system?

I imagine it would need to be a standoff, not a pin.

The hook is only connected to the catapult arm and its swing is limited. Now imagine two gears with a standoff separating them - this will be the cam. The cam comes around and grabs the hook to pull it down. As the cam rotates out of the reach of the hook it releases.


Now I understand, I thought you have to connect the L shape on the gear, I can show everyone a demo once I am finished. Thanks a lot, really appreciated.


Great explanation, thank you.

I have been having troubles on keeping the swing limited, when the hook touches the gear it slips and won’t go down. Whats your thoughts on this?

One option would be to make it fixed. Another would be to add pieces adjacent to limit the movement. In the example pic below the left grey piece would allow some movement and the right piece would stop it at 90 degrees. (Assuming they are all in the same plane against the catapult arm)



After blocking some swing movement the motor can’t take the catapult down.

how does this catapult

I think this is the Choo Choo Mechanism

Here is a prototype video
Google Video