Hi guys,
I need help to build a catapult but I don’t know where to start. I understand how it works but can someone explain design and execution thanks for help.
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A basic catapult will need a motor, gears, drive axles, rubber bands, standoffs, and c-channels for the structure. You’ll need to design a slip gear (I made mine using a Dremel) so that once the catapult is pulled back all the way, the rubber bands can pull it up without any resistance.
This video seems to have a good explanation of how they designed theirs!
Link
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(This is the proto-CAD {used for planning the general idea, not meant to be hole-counted, untested} of a “slapper”, which works the same as a catapult except the catapult arm “slaps” a triball that was placed on a stationary platform, whereas a cata involves a triball being loaded into the catapult arm while it’s moving). However, here is a rundown of the design:
- Motors spin slip gears (pictured below)
1a. When the teeth contact the gears in (2), the catapult pulls back. When the part with no teeth reaches the gears in (2), there is nothing holding the catapult in the retracted stage, and the rubber bands (3) pull the catapult forward really quickly, lanching the triball
1b. If that seems a bit confusing, think of the medieval catapults. The part where the people are spinning and pulling the “arm” back is similar to when the slip gear’s teeth contacts the gear to the catapult arm; the part where they pull the pin and the catapult fires is when the part of the slip gear with no teeth spins to contact the cata gear. - Gears connected to the catapult
- Lots of rubber bands powering the catapult
- The triball contact-er (we are using a slapper, which I would recommend, but for a catapult you should make the arm longer with a triball “bucket” on the end of it to put triballs in)
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I notice you have 2 motors pulling back the arm of the slapper. Is that necessary due to the amount of rubber bands needed to get over the center barrier?
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No, we just wanted the extra power to make it really fast (we’ll keep changing the gear ratios, jence why it’s only a proto-CAD).
We compete in SkillsUSA (basically just VRC Skills challenges), so technically we can have a 44w cata if we wanted to.
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@Gigahertz.t @Rilaye Thanks for the help. About the slip gear is it ok to use 36t gears or should I print a 12t as a slip gear?
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Unless you are in VexU, 3D printing is illegal for competitions. I shaved off 27 teeth from a 36 tooth gear (9 teeth remaining) and that seems to be good for us, but depending on your design you may need to do a bit more or less!
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I only do this because I am in SkillsUSA (modified challenge), which permits 3d printing. VRC does not permit this.
For compactness I would try a 100rpm motor 12:36 ratio (12 tooth being the slip gear), but either way for 12:36 or 36:72 as long as you shave the right amount of teeth you should be fine (however, it is a lot more difficult to shave the metal 12 tooth gears, shaving 36 tooth ones are easier. Plus you get to choose from 10 degree (36 tooth) increments versus 30 degree (for 12 tooth) increments).
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Oh I didn’t know it was illegal thanks for help. I will be building tomorrow.
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@Gigahertz.t is there a unlimited amount of motors for skillsUSA?
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No we are limited to 88w.
It’s just that sonce we only do skills there is no oppsoing robots to justify anything more than 44w drivetrains
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Hello,
What state are you competing from and will you be competing in nationals this year?
Sincerely,
Louisiana 2023-2024 National Team.
Hello!
I’m from California Region 3. We have yet to have our state competition (it’s in April 4-7), so we have yet to see who goes. (However, it’s only my first year competing, so I’ll probably be going as a delegate. But we’ll drop by the convention center to peek at the bots. 
)
Also, just curious, how’s the catapult coming @Emcq ?
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keep us updated on what’s going on with you guys as we get closer and closer to nationals
sincerely 2023-2024 louisiana team
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@Andrewdrewas there is a topic dedicated to SkillsUSA: SkillsUSA Mobile Robotics Technology (MRoT)
This is a thread for helping the OP with their cata (so we should stop hijacking it)
Moderators could we split this and the last 5 posts to the topic above? Thank you!
@Gigahertz.t
Sorry to forgot inform you all. We builded our catapult succesfully and worked efficiently at national finals at my country but in my country there are to many teams uses blocks we couldn’t acomplish much but I’m happy how it worked out. Between this was our first year. Thanks to everyone who helped
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I had a quick question at the picture you shared in the beginning. Tip of the puncher there is someting black. We don’t have that may I ask do you know what is it? If its legal where can I buy it?
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Its called a rubber bumper:
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If you only have one motor left but want to keep a good speed. I would recommend doing a double slip gear. Which means that when you shave the gear instead of shaving off 27 teeth you take of nine leave nine take off nine leave nine which basically doubles your speed.
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I’ll have a look thanks for idea
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