Custom Two Speed Gearbox

[Nemo]

A couple more questions:

1) Are you planning to machine any of your own axles?
2) Are you willing to include a steel gear or two?

You'll have to answer yes to at least one of those.

Option 1: If machining your own axles, you can do a 2 stage using the 60:15 / 45:30 gear set. 2 stage will of course be lighter, smaller, and more efficient. You would need to make your own shifting / wheel axle with the dog gear slot and the hole bored for the shifting shaft. That gear set has a high/low ratio of 2.67, while you're looking for something like 4:1 (20 ft/s and 5 ft/s). So you'd have to settle for something like 16 ft/s and 6 ft/s with this gearing, or 20 ft/s and 7.5 ft/s.

At any rate, you only get to select two of the gears in this setup: CIM gear and 1st stage output gear.
CIM Gear: 12 or 14
1st Stg Output: 40, 45, or 50.

That gives you ratios between 4.86:1 and 3.33:1 for high gear. Those are very fast. (AndyMark has a lightened steel 40T gear that only weighs 0.17 lb, so I consider that a good option for a light gearbox).

*The option Art mentioned using 11T gears would be nice and light, but you'd have to come up with 11T gears somehow. They are not available off the shelf, so you'd have to find some 11T spur gear stock or get a company to Wire EDM them for you or something. Incidentally, does anybody know a place that sells 11 tooth stock? That would be a pretty sweet find...

Option 2: If you use this shifting shaft, you can make a custom 3 stage box with all COTS parts except the gear plates.

Stage 1: 12T or 14T CIM gears; 40T output gear is the only one that fits.

Stage 2: Basic set is 48:15 and 35:28, but you can do the optional set that flips it to 48:15 and 28:35 for the 4:1 speed ratio you want. That makes your high gear faster without changing the low gear. If you do 3 stage with 4" wheels and you want really fast speed, it will be hard to get a low enough gear ratio if you don't choose that flipped 4:1 gear set.

Stage 3 input: Only 17T and 20T input gears are available off the shelf (the shaft is 1/2" keyed). You could also buy a 3/8" hex gear, then bore it to 1/2" and broach a 1/8" keyway on your own. Requires arbor press + broach tools. If you did that, it opens up the 28T and 30T gears as input gears, which would provide faster gearing.

Stage 3 output: Can be 40, 45, 48, 50, 56, or 60. BUT, the stage 3 center distance has to be big enough to avoid interference between the wheel shaft and the 48T stage 2 output gear, which eliminates, for example, 40:20 as an option (unless you shrink part of the hex shaft...). If you bore and broach a 30T gear, you could do an all-aluminum 45:30 or 50:30.

...

The 2 stage designs seem more appealing for small wheels and fast speeds.

I don't have any experience with robots that go this fast. I must be missing something, because the numbers look bad to me on paper - it seems like one would have to be driving longer distances to make 20 ft/s theoretical gearing better than 16 ft/s theoretical gearing since the 20 ft/s bot will accelerate more slowly. Unless your robot is very light or the gearboxes have a third motor.