Though this is perhaps a tiny bit off topic... I've made a couple of webpages for Westside Boiler Invasion's KIP program (Knowledge is Power!) that are related to gears, how they work, and whatnot...
Here's a link with some bare-bones definitions:
Gears 101
And here's a link with some applications, and how you go about solving problems...
Gears 102
There's also a page on bending stress...
mechanics of materials 102.
Matt