How to cad a turret plate sprocket?

Hi! I’m trying to CAD a turret for my offseason project. We decided on cading a turret like this one:

I’m wondering what tooth count the big plate sprocket should be? And also does it matter what tooth count the smaller plate sprockets are?

Also, I want to custom CAD the big plate sprocket on Onshape. How do I find what the dimensions of the tooths are?

Thank you!

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Lots of good discussion in this thread, but this post links to a nice feature script in onshape which ive used before.

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The smaller sprocket will determine your gear ratio. The idlers you see near the smaller sprocket is to increase the engagement which helps prevent slip and wear

You could also do 3D printed sections and use a gear. I’ve seen that a few times as well

Lastly, you don’t need all of the teeth if that’s hard to cut for you. You could skip every other tooth or remove 2/3rds of the teeth (evenly spaced). Take a look at the MK4 swerve steering pulley for reference

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I wrote a whole tutorial about this!

Tooth counts don’t matter all that much, but it’s nice to have it be a round number for the sake of symmetry - the gear ratio is simply the ratio of the tooth counts.

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for the step where you make the two radial lines, where should those lines be perpendicular to?

I assume you’re talking about this? Both these lines intersect the center of the main circle of the sprocket (hence the name radial).

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The smaller sprocket should be preferably larger than 17 teeth. The handbook I’m reading recommends that you do not exceed a 10:1 gear ratio with a single chain drive, placing the maximum teeth count at 170.

(You can take this advice with a grain of salt, since the Machinery’s handbook is meant for designing things with extremely long longevity, and so you can probably bend these rules for an FRC setting)

Source: Page 230 of this PDF of the Machinery’s handbook

This part of the handbook has other advice on designing chain drives, and you can look into that advice if you’d like. I’ve found the advice from the handbook to be good, but occasionally unnecessary for FRC applications.

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this concern with high ratios, and thus low wrapping, is why the OP’s pic has those two little guide bearings.

note that the OP pic does not have teeth on the big sprocket, instead the chain is just anchored to it – it’s a kind of capstan drive. imho for this application a capstan has some advantages over a chain or belt, not least of which is that you don’t have to cut a giant sprocket!

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