Single Motored Catapult

I have been trying to make a catapult that would need one motor. I can seem to make one that won’t go too slow using gear ratio.I would like to ask: Which mechanism is ideal for making a working, efficient, single motored catapult - Intermittent gear - or - Choo Choo mechanism?

Also, what gear ratio? That would help very much :slight_smile:

The gear ratio of your catapult will depend on the number and type of rubber-bands you use, as well as where you place them. For a one-motor catapult, both intermittent gear and choo-choo (over-center) mechanisms will work, it just depends on what you would like it to do. Generally, an intermittent gear mechanism will work faster and be lighter, however, in our experience both mechanisms took about the same time to reload. That being said, you can decrease the amount of rubber bands you use - and therefore decrease the gear ratio - by simply making a longer catapult, since angular momentum will make it faster if it is longer. Another factor that affects this is how much the motor has to fight the tension of the rubber-bands. The catapult will slow down the further down it goes because the amount of tension in the rubber-bands is much greater since they are stretched out more.

Designing a catapult will require lots of trial and error, and it is different for every robot. I suggest you use the design process and try out a few different configurations to see what works best for you. When we tried this out, we got a minimum gear ratio of 2:5 for our choo-choo, and a 1:6 on our intermittent gear catapult, though I’m sure you could make both faster or use a different mechanism altogether. Hope this helped.


Another thing to consider is how many shots your catapult can take with every revolution of the motor. If you have an intermittent gear where every revolution of the driven gear shoots the catapult four times, and the driven gear spins once every time the motor spins six times (1:6 gear ratio), the motor has to spin 2/3 of a revolution for the catapult to shoot once. If you have a choo-choo, which only shoots once with every revolution of the driven gear, you would have to put it on a 3:2 (for speed) gear ratio for the catapult to shoot once every 2/3 revolution of the motor. That is impossible.


Great to know! I tried (and failed) using the intermittent gear catapult, and from reading from what you said, I should probably make the catapult a little longer. Thanks a lot! Also, How long would the bucket (to carry the balls on the catapult) need to be from the axel of catapult?

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So overall, intermittent gear mechanisms are better! Btw, it’s also great that you’ve helped me in the arithmatic part, thx!

Not necessarily. Assuming:

  1. The intermittent gear shoots four times per rotation of the driven gear.
  2. The intermittent gear will work at a 1:6 gear ratio.
  3. The choo-choo will only work at a slower than 3:2 gear ratio (not a hard assumption).
  4. Both catapults consistently get the balls into the high goal.

Under these (unlikely) conditions, the intermittent gear would be better. However, I’m not sure you could fulfill all of them. So consider your mechanisms carefully.

No problem!


Ok, I will gladly follow your advice, only assuming that I will change the length of the catapult from the axel.

You want to try to make the bucket as far from the point of rotation as the robot allows. Because of how physics and angular momentum works, the further away the bucket is from the point it rotates on, the faster the balls will launch from the catapult. Another thing you could do is increase the weight of the catapult at the bucket, since this will also increase the angular momentum.

How Angular Momentum Works

Angular momentum is what gives your catapult the speed it needs to launch the balls into the goal. The formula for it is L = tangent velocity x radius x mass, so if you increase the radius of the circle (in this case the length of your catapult from the point it rotates around), or the mass (the weight at the end of the catapult), you can increase your total angular momentum and launch the balls faster.

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Here are some examples of catapults from Youtube. I don’t know if they would help but the design principles are there.

(213) VEX IQ Key Concepts - Speed, Torque, and Mechanical Advantage - YouTube

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In this discussion, we are actually talking about the vex iq challenge and efficient ways to launch a catapult (that require one motor but in this case, it would require two). But I am glad you are willing to help. :slight_smile:

@242EProgrammer @94568 @N1kkiisaweom3 Thank all of you for helping, and with your help, I have successfully made a working catapult with one motor.


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