Catapult Ratchet Mechanism


#1

Hi Forum,

So I know that many people are planning to use a slip gear mechanism for their catapult this year, and I myself am also planning on doing this for now. I have made many different prototypes of it, but one thing that I am having problems with is getting the platform to stay in the down position. My intake system is a simple tray style intake, as I have seen that many other teams are using. Essentially for those who don’t understand the predicament is that the resting position for a slip gear catapult is in the up position, and in order to pick up objects it needs to be in the down position.

One idea that I have had is to put a ratchet (as I know that many teams have had) on some the part of the mechanism that only ever has to spin one way. Basically any gear between the motors and the slip gear. This way, when the mechanism pushes the platform down, it can just stop at the bottom and the ratchet will keep it in place as the ratchet will prevent the rubber bands from pulling the catapult back up. So this idea works in a perfect world but unfortunately our world is awful and nothing ever works as it should. Here are some issues that I’m having:

  1. The strength of the rubber bands causes the mechanism to break. Not necessarily snapping, but gears skipping or the ratchet fails or something of that nature.

  2. Precision. I need the platform to “lock” as close to the ground as possible for obvious reasons. The looseness in the mechanism makes it really difficult to get it right. I figured that if you put the ratchet as further towards the slip gear (not physically closer, but closer in the path of the mechanism), than you eliminate some of the looseness in the rest of the mechanism from becoming an issue.

Finding the best type of ratchet has also been an issue. I’ve tried a couple iterations of a ratchet based off of an 84 tooth gear as the ratchet gear so its really precise in where it locks, but it’s really difficult to get whatever you use as a pawl to grip the teeth of the gear, and as a result the rubber bands force the ratchet to skip and the platform goes back up. I’ve also tried a 30 tooth sprocket as the ratchet gear so its really easy to get the pawl to grip the teeth, but precision becomes an issue as it only has 30 teeth.

My question is, what do you guys think/ what are you doing to solve these problems?

P.S. I’ve tried googling different Vex ratchet mechanisms but all you get are the stupid ones from the flywheels from Nothing but Net that won’t work here.


#2

For the 84 tooth gear, did you try a 12 tooth gear as a pawl (12 tooth gear connected to drive shaft bar lock)?
You could also try making 4 pawls for each ratchet.


#3

I’ve tried multiple pawls, doesn’t quite work well enough.

I have not yet tried to use a 12 tooth gear, thank you for the idea. Although I do have some worries about how easy it is to turn in the forwards direction. Could you elaborate on your idea?


#4

I am also interested to see how to use a 12 tooth as a pawl.


#5


I don’t know if that will be strong enough for catapults though.


#6

It would probably work with tension pulling down into the gear but I am not sure how much would be needed


#7

Simple and ingenious. Can’t believe I haven’t heard of it before. I’m sure maybe if there are multiple pawls it would totally work for a chu chu.


#8

I would just run 10-20 motor power into the catapult motors to hold it down. This is what I have done for my NBN catapult and for my Starstruck prototypes.


#9

@Josef K

#how2tripPTC101
#moddedmotorsconfirmed


#10

It doesn’t actually trip the PTCs… if you do it correctly.


#11

What’s the difference between one done correctly and one that’s not?


#12

It can be a fine line. If you look at jpearman’s smart motor library you can see the estimated current used by the motor and how far away you are from a PTC trip. motor powers of 10-20 constantly holding it down should probably not trip the motor. At 50+ hold you may start to worry over time.

https://vexforum.com/t/smart-motor-library/22282/1

Now you can graph variables right in robot C too!


#13

You don’t really need ratcheting gears for the choo-choo. When it’s ‘down’ it’s in a pretty stable position


#14

@OverClocker Yeah I realized that. But for a catapult, especially this year where a lot more force is required to launch anything, I think a pawl would be necessary.


#15

We did build a choo-choo catapult this year, and it’s fine when it’s down. Anyway, the amount of force required will probably wreck your pawl.


#16

I am dealing with a very similar gear slipping problem for the vex turning point game. Every time I stop at the bottom to load a ball, the catapult simply slips back up. Did this 12 tooth pawl fix it?


#17

you could use a gear ratio so that your ratchet spins faster then the catapult, this way you both get more precision, and the ratchet has much less strain on it. you can pm me if you want more info on how I did mine.


#18

It’s not that that ratchet slips, but it’s the gears slipping. So like at the bottom, the ratchet stops the driven gear from moving, but the catapult simply slips back up.


#19

there’s something wrong with either where you placed the ratchet or your build.

Well meshed gears should never slip past each other

A picture would make your problem much easier to diagnose


#20

Here are some pictures of my ratcheting system and gear box:
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