"Outside-In" 775pro gearbox

nuclearnerd · January 2, 2019, 9:28pm

I’ve been tossing around an idea for a while to mount drive gearboxes to the outside of a versaframe rail. Sort of a “doubly flipped” arrangement. I’ve never designed a FIRST gearbox before, but I figured I’d give it a shot and see if I could come up with something worth prototyping in the fall.

Attached is what I came up with. It’s not perfect, but it’s got some interesting aspects:

Uses 4 775 pro motors, capped at 10V (and perhaps some other fancy current limiting), following the pioneering experience of Teams 2451 and 88
Mounting to the outside of a “West Coast” style frame means it is just as easy to remove the gearbox for maintenance as it is to remove a wheel. It also gives freedom to move the gearbox away from the center of the robot if that space is needed.
Uses steel gears and hardware from Andymark’s EVO shifter (except for two gear sizes which sadly I could only find in aluminum from VEX)
Currently geared 16.5x in high gear, 38.5x in low gear (2.3x spread), but these can be adjusted by changing the second and third stage ratios.
To fit in the wheel well, the gearbox can’t be any more than 3" thick. To save space, I put the first stage gear above the dog. Unfortunately this means we need extra long pinions for the 775 pros
Uses a Celt-X hex-bore encoder (3D printed modification of a CTRE versa planetary encoder)

There’s an incomplete CAD model here: https://drive.google.com/file/d/0B16qRDFhn0uuZF9VeURvQkRqOUE/view?usp=sharing I would love to hear any comments, suggestions or insights from people with more experience designing these.

AriMB · January 1, 2019, 4:48pm

This is a very interesting idea. I like the accessibility and the space savings. In reality, You can probably use any of the already-designed flipped motor gearboxes and just mount it on the other side of the frame rail.

With the shifting cylinder where it is, you could switch to CIMs and not lose much room between the rails, although a single-speed variant of this would have a lot of room taken by just CIMs. A miniCIM is only a little-bit longer than a 775pro, so it wouldn’t take up much more space to do a 3x miniCIM drive than a 4x 775pro drive for those who want a more sure-fire drivetrain.

nuclearnerd · January 1, 2019, 4:48pm

Thanks! I’m sure lots of flipped gearboxes could work this way, although most I’ve seen put the air cylinder on the wrong side. A single speed would be easier of course, but I wanted a challenge (and we’ve used shifters for every game except recycle rush)

GeeTwo · January 1, 2019, 4:48pm

My first thought was that you’re giving up the reliability of your main drive wheel being direct driven. Given the very low incidence of properly fitted belts breaking on the FRC time scale, that’s probably worth it for the easy-to-pull gearbox.

Chris_is_me · January 1, 2019, 4:48pm

I think you might actually have more success with the ball shifter shaft here. The extra length after the reduction is less of a problem here because the round part can tuck into the frame rail, and you’d be able to make the whole gearbox potentially thinner, at least the part that’s outside the rails. You may have to get creative with the 32DP reduction, though.

CSQ6375 · January 1, 2019, 4:48pm

Awesome! I think it will be better if you put he chain in the tube.

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Brian_M1 · January 3, 2019, 6:10am

He has the vexpro bearing blocks to tension chain which isn’t able to go inside tube sadly, although maybe one-day :rolleyes:

Edit: actually, that’s made me think, why can’t you put the sprocket inside the tube even with bearing blocks? I was looking at it and it seems like it would fit?

asid61 · January 3, 2019, 6:10am

In my experience chain-in-tube fits better with versablocks than it does without them, because you gain more space in the tube. In this case I don’t know if moving the chains into the tube will net a significant benefit, but it’s an easy change to make while assembling.

nuclearnerd · January 3, 2019, 6:10am

Ugh. Not a fan of the chain in tube idea personally. The point of the design is easy maintenance. Chain in tube seems like asking for headaches if the chain ever fails, but I’m open to hearing otherwise.

pchild · January 1, 2019, 4:48pm

I like this idea! I think we’re going to try a design for an 8 wheel drive based off of this that gear drives the two center wheels and then has chain-in-tube to the outer wheels.

With the gearbox design that you have now wouldn’t the forwardmost Versablocks have to slide enough to tension two chain runs?

Billfred · January 3, 2019, 6:10am

4901 was not the most-resourced team in 2016 (though we were probably in the upper half), and we pulled it off using 221 double sprockets so we could achieve our octagonal shape (so we couldn’t get T-boned or squared up against defenses, and to enable a triple climb we expected to happen way more than it did).

Make the spacing so the link count is divisible evenly by 2 and use a DarkSoul chain tool (or at least links that don’t have that curly wire pin, because that hosed us a few times), and your setup should be pretty hands-off for at least a couple events. Just make the chain loops and then dangle them in, then insert axles. It’s pretty cheap to experiment with, and if there was a year where the kit option didn’t suit our priorities I’d totally go right back to it.

Basel_A · January 3, 2019, 6:10am

Is chain-in-tube even compatible with the way this gearbox is mounted? I think you’d have to switch from four mounting points to two, which I’d personally prefer not to do.

Billfred · January 3, 2019, 6:10am

I didn’t even think about that here. We ran Toughbox Minis that year and I think our (custom) front gearbox plate was just held to the tube in two places (and a dose of hopes, dreams, and memes). I’m not seeing a good way to do that in this application.

Lil_Lavery · January 1, 2019, 4:48pm

It’s probably nothing, but I’m a little leery of those extra-length pinions. I feel like you’re exposing the 775 Pro output shaft to a lot of potential side loads that way.