Intermittent Gear catapult

It seems like everyone is creating separate post for their catapult questions, so sorry for creating another one.

I’m looking to teach our kids a new method for launching the catapult (previous was choo choo), so I wanted to build out a demo myself to get one working before explaining things to them. I’ve found several user created videos that showed creating the mechanism and have a test robot created and working. The problem I’m having is it takes 8 rubber bands to fire with good strength, that seems like alot in general and in the other users videos I’ve seen, they don’t appear to be using that many either, so curious as to where I may have went off course.

Pictures are below. I’m using planetary rings around the gear box (ignore the drive shaft length on the picture, that was more for modeling purposes), I have two motors powering the catapult (100% power and torque). I would prefer the rubber bands be in the front, some samples I have seen have the rubber bands in the front and work, yet I couldn’t get any real power off that way, so I have the rubber bands powering from the rear.

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Have you considered using longer arms to give you some more speed? If you look at medieval catapults they have long arms as leverage.

When you are launching the balls from the catapult to get the balls a good rainbow motion you want to change to a little 25 degree angle, that may change the angle and most likely the balls will get in with less rubber bands.

I’m using 1x18 beams and at the current configuration, when the gears turn, the catapult arm is literally about 1/4" ish off the ground prior to release and launch. The robot measures 7 1/2" high in current state. I’m sure I could move everything up one notch and try to sneak in a little bit longer beam maybe we but I think that result would be minimal. But if you think it’s the answer I’m happy to try modifying that to see.

I have played with my stopper drive shafts so that the arm hits them right about a 90 degree angle to get the best arch.

The motors seem to really be pulling hard with 8 bands, it’s not a fast reload. Dropping down to 6 bands and it seems to reload a lot quicker like it should. But it’s not launching high enough.

It really feels like it’s a matter of placing the bands in the right place/angle and I think that’s the question. Also questioning if I have the best planetary gear configuration as well I guess.

Here is the sample I’m mostly modeling after.

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I also tried to make this design, but it cannot throw 2 balls. I wonder where we go wrong, when the rubber bands increase, the motors are very difficult.

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Are you referring to the angle of the rubber bands? I have tried the rubber bands at different places and angles, the placement in the picture was the only thing I could get to work, but be it with 8 bands.

Have you tried putting a rubber band in the middle, possible that could add more power without adding tension on the robot.

Also I just realized this is not the lever, that is where I go wrong.

Yes, 8 rubber bands is a lot. But I wouldn’t worry too much about it in this situation because of the reason why you need so many. It’s because you are attaching them super close to the fulcrum of the catapult. Since you have less leverage for each rubber band, you’re going to need more of them.

You can fix this by extending the pieces out the front to attach the rubber bands further away from the robot. However, I wouldn’t recommend doing this because this means the rubber bands will be at an angle, something that can be detrimental to your catapult. Instead if you are really worried about it, you can try this. Since you are only “single wrapping” your rubber bands in that you aren’t “doubling them up” they just go from the top axle to the bottom 2x beam. If you them made them go back up to the shaft though, you wouldn’t need as many because you are stretching the rubber bands more and storing more potential energy.


This is an easy way to get a code of conduct violation if the students use this robot to build their own. There is a team I know that lost their worlds spot because of this.

You could show them the idea online, but even telling your students “hey look at this” can influence their design decisions.


My first year as a coach I built a couple of model components but found that the kids would just instinctively copy them, so I stopped doing so.

At the end of the day, just be sure to let them drive the competition decisions. If you are teaching a lesson on different types of slip gears, that’s one thing. If you are leading your students in a certain way to change the design of their competition robot, that’s definitely another.


@sankeydd This is just a teaching the teacher thing here, so no worries. There was a lot of good videos on VEX choo choos that explains how they work and I was able to pick up on that to learn and explain. There aren’t any VEX tutorials on intermittent gearing and how to apply, so the easiest way to learn is just to do it yourself (aka a demo). Before I go teach them, I’m just making sure what I teach would actually works and not fall flat on my face with a dud, lol

Also, it took kids 4 months to get a robot put together, the season would be over before they could build another robot, LOL.


@FRC973 Thanks Joe, I’ll try stretching them out further and see what happens. I’ll also try the smaller rubber bands since they are not as long. This may “cut back” the number of bands I use, but the mystery will remain will the motors be able to turn the planetary gearing for the catapult any more efferently than it does in the current state of 8 rubber bands. I’ll try and upload a video of the best I can get it and see if that truly is as good as it gets or if there are any other suggestions to learn,

I think this reinforces @sankeydd 's point. You are at the point where you have successfully built the mechanism to use as an example to teach a concept. I don’t see any issue building an example of a mechanism i.e. a simple arm choo choo, planetary, 4-bar arms, DR4B, etc.

What it appears to be happening now is taking this generic example and not only applying it to this year’s game, but fine tuning it to work ideally with this year’s game. That is major red areas for me.

Take Fling for example. Can it score balls? Yes. Can it score them in the high goal? Theoretically. Does it fit under the bar? Not really. Does it fit in the starting zone? Not in the real world. It is up to the teams to take this example and make it work for them. That is the example set by RECF and anything beyond that would cause concern for me.


@SCFarrell @sankeydd - Fellas, appreciate the concern, I’ve been around VEX for over 3 years, well aware of the rules. I tried to be clear that I was asking questions to help me learn so that I can teach the concept to the kids and have an understanding of it. There is no robot being built for them to copy or step by step diagram for them to follow based on some experimenting I’m doing. I would hope my kids math teachers could answer the hard questions when asked, thus I’m trying to take the extra time to learn the in’s and out of someone’s idea so that I can give direction when they need pointers. Feel free to DM me and happy to have a virtual adult beverage chat if there is anything else. Thanks.


I always ask this and I’ll do this for the 2021-22 season. What exactly is the difference between a team building the VEX “Hero bot” and any other bot they find on the web? (I don’t want to see the hero bots be discontinued since they are pretty cool designs, ) If my teams do cool coding and up the herobot is that enough of a difference?


I was reading this thread from the sidelines, but Foster’s last post brought something up so, I’ll take the bait so to speak :slight_smile:

Let me know if this should be created as a separate thread, or if I’m not in the right thread.

I like how Fling was introduced this year as the hero bot and it was a more complex bot. However, the issue I see is that it was just introduced like all other hero bots without any accompanied learnings on the why’s and how’s and concepts. It was introduced with just the regular build instructions. What was lacking is all the great learnings that should’ve come with it. I know Brandon posted a 2 part Youtube video later on about Fling, but at the time of the Fling release it was just the build instructions. Sorry, if there was actually accompanied learning materials or concepts to be taught at the time of the release. I didn’t see any.

The problem I have is that something like Fling is perfect opportunity where Vex should include materials for teachers and coaches to teach about concepts, mechanisms, etc. Such as why a Choo-choo was chosen. Why was the gearning for the Choo-choo done (I believe it’s 1:5)? Why not compound? Why not 1:2? Why were the 2 60 tooth on the Choo-Choo pinned together with 1x stand-offs? And why were there beams in between the 2 60 tooth gears? Why can’t the shaft extend past the gear where it’s a single shaft? Why the roller? And using a catapult is an awesome opportunity to teach about levers and mechanical advantage. When our team went through our build process, we started with Fling and started breaking it down and had our kids learn the concepts of the catapult, the pivot/fulcrum, angles of shooting basketballs and field goal kicks, etc. We also had the ah-ha moments as to why the choo-choo was built the way it was, and why Brandon made the decisions he made (or at least we think we understand).

BTW, @mizdared you may want to look up class 1, class 2, and class 3 levers and teach that to your kids. A catapult is nothing more than a lever.

I’m hoping if the next hero bot is something that is like Fling and more complex than normal, there would be accompanied materials for coaches and teachers to bring out those concepts. Otherwise just giving instructions on building fling is pretty much the same as have a team watch a Youtube video and copy that design as Foster brings up. Sorry, as I said I’m taking a bite from Foster’s bait and adding to this thread. :slight_smile:


Hero bots are given an explicit carveout.


There is a difference, especially for younger kids, between seeing a bunch of ideas on YouTube and your coach providing an example. If they see an idea on YouTube, they will have to think about if they can do it. If their coach has built an example then they know they will be able to get help building that robot from their coach. That will have a lot more sway over the team’s design decisions.

As far as what is available online, there are plenty of high preforming designs online, some very early in the season. We have to be agnostic about this, because it simply is what it is. You cannot put the genie back in the bottle. However, it is also an even playing field. Once the design is out, then all teams have access to those materials, not just a select few. As long as teams are limited to what they all collectively can find, at least that’s fair.

RECF does not have to agnostic about it. There are event partners that violate these policies, and the REC Foundation could have some sway over them. I’ve sent in a few COC violations, and while I don’t expect to hear much back I know from what I can see that they don’t like to act on the violations.


Honestly, because giving an answer and saying “trust me” isn’t a great way to learn.
As a mentor for over 10 years now, I know that a conversation with a student could go like this:

Mentor: “Hey kids, why did we use a choo-choo catapult that had two sets of gears per side and beams between them?”

Student: “I don’t know. That weird looking guy on the internet that talks out of the side of his mouth said it worked and to trust him. We just followed the instructions.”

I bet those ah-ha moments will stick in everyone’s memory better than if it were to have been explained to them once.

This is just my opinion though, It should be noted that I do not work with the curriculum side of things and we may supply some sort of curriculum in the future rendering this arguement moot. I may be different than most, but I certainly learn best from experience. So I try to teach with the same method.


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