Wheels/Belts for Roller Claw

My team is designing a roller claw as an off-season project. I’m new to FRC, and I was wondering what designs y’all have come up with. At this point, I’m most interested in the pros and cons behind different wheels and belts/chains used in the designs. Thanks!

Edit: People asked what game prices were using, and we’re using milk crates for it.

Most often, people use compliant wheels for their intakes. Back when the dinosaurs ruled the earth (ca. 2017), compliant wheels were much harder to find and people used rubber-coated Banebot wheels.

I wouldn’t recommend chains for directly interacting with game pieces, as they’ll scuff them up and some referees and robot inspectors won’t like that. They’re perfectly fine as power transmission, but most people use belts for intake power transmission since the loads are low and belts weigh less.

When using belts for material handling, normal HTD 5mm belts are workable, but a lot of people also use polycord or urethane flat belts.

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What “game piece” are you looking to manipulate with the claw?

With wheel selection, you’ll want to consider where your compliance is coming from. You’ll want some squish somewhere in the system (that’s how the manipulator holds onto the piece), but that squish can come from either the piece or the wheel. The 2020 game piece has a lot of squish to it, so people were intaking with harder durometer wheels (or sometimes quite rigid tubing). Contrast that to the 2018 game piece that is quite stiff. For that one, teams used softer, spongier wheels.

In terms of chain vs belt, I’ll assume that this is about power transmission. You’ve got a few things to consider: weight and fixibility are the two biggest. Chain is a LOT heavier (and don’t forget metal sprockets weigh more than plastic pulleys). This extra weight in your mechanism might impact other aspects of your robot (if this mechanism is on an arm, you’ve got to account for that weight in stall torque). Robots get hit a lot though, so there’s a chance you’ll need to replace things quickly. Chain is both easier and harder to replace. Easier because you link it around sprockets and therefore don’t need to disassemble your mechanism, but harder because putting chain together can be a real pain (especially compared to popping a belt back on).

For a baseline design shape, I’d take a peek at the Greyt claw to get a general idea of what this thing might look like.

You need compliance somewhere.

This can be done through the actual mechanics of the mechanism via pneumatics, surgical tubing, springs.

End touch points, these can be compliant wheels or pneumatic wheels, something squishy that gives on compression.

The item you wish to manipulate such as powercells.

^^ This. Note that if programmed or sprung properly, the roller claw can provide some large-scale compliance, where compliant wheels are better at small scale compliance (<2"). Building an intake is always a matter of finding the “sweet spot” - enough compliance, enough grip, enough speed, but not TOO much in each case, so you don’t end up mangling the game pieces (or robot parts). You can get some rules of thumb based on previous games, but there will always be a bit of “dialing in” your solution, whether tweaking speed, compliance, or durometer (softness/grip of the wheel or belt if it’s used as the actual intake). This can be mitigated by having a proximity sensor which cuts off (or at least greatly reduces) the intake motors running in the intake direction once the game piece is seated. Unless your game piece is ALWAYS going to be parked securely in place when you pick it up, be sure to include “open field” pickups early in your tuning.

we’re using milk crates for it.

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Absolutely research 2018 intakes. That game piece was literally a milk crate.

2015 also had hard plastic rectangular prism game pieces, but were a different form factor.

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Spectrum has wonderful image galleries of past robots broken down by subsystem. Here’s their 2018 gallery.

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If you’re intaking milk crates, TOTALLY concur with @BryceHanson. The 2018 game piece was a milk crate wrapped in a zippered nylon pouch. The nylon pouch usually seemed to REDUCE traction, with the possible exception of on the open face.
3946 actually created our first true roller claw in 2018. We failed miserably. In post-season, we discovered that if the intake arms were made of 1" square PVC tubing from vex (about 9-12" long), we didn’t actually need to actuate the claw part to get awesome intake and placement.

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