[Tristan Lall]

I like the concept, but there are a few little things that I'd watch out for when finalizing the design.

I'm making a couple of assumptions about those gears, namely that they're 1/4" face width, and 24 pitch (the pitch is a guess, based on the face width, and the face width is an estimate based on the size of the other components). Using a spreadsheet which calculates the number of horsepower that the gears can accept, using the Lewis formula (let's all hope that my spreadsheet skills are up to par), those gears would have to have approx. 85 000 psi tensile strength. (The Lewis formula isn't perfect, but it ought to be fine for these purposes.) Fortunately, SDP/SI sells gears that exceed that spec nicely at 180 000 psi, but at around $50 (USD) each, you'd be spending vast amounts of money. (Backtracking a step, I'm only saying 180 000 psi, because SDP/SI quotes min. R_C 35 [Edit: That's the Rockwell C hardness scale] or so, which, in steel tends to exhibit strength of 180 000 psi. Actually, they quote R_C 55 for the teeth, but I'd rather not use that number, for safety's sake. Note that I've never tested this, or received verification from SDP/SI!) Are you planning to make your own gears (slices of gear stock, maybe?), and have them heat-treated?

I'm really starting to like the possibilities here, especially because of the expandability of the thing. It would be a trivial matter (except in terms of weight) to increase the face width of the gears to 3/8", if you needed to.

Make sure that you verify your gear strength calculations, and take into account the fact that the gears will be "crashing" together a little bit, due to the teeth engaging one gear after another (sideways). I don't know how durable the gears will be under that kind of loading, especially when shifting on the fly. Those through-hardened gears are rather brittle--this is definitely a good reason to test them!

The lead screw idea sounds good, and M's suggestion of the Geneva mechanism is an interesting one.

Finally, what's the purpose of the small slots in the sides of the gears? Some sort of aperture for an optical or Hall effect sensor? I don't think they'd be necessary everywhere, in that case.