[AustinSchuh]

Originally Posted by Peter Johnson

http://www.bostongear.com/pdf/gear_theory.pdf is a wonderful reference for gear theory. See page 6 of the PDF for the Lewis formula (Barth revision) for safe static stress on gear teeth, for example. Given the face width, diametral pitch, pitch diameter, pitch line velocity (or RPM), and material, the formulas and tables on that page will give you the max torque a gear can take (at least on the teeth).

Here's another calculator that I've been using lately. It sounds like it's an online version of what's referenced in that document.

http://www.rushgears.com/Tech_Tools/horsepower.php