Most of the gearboxes I have been involved with use single helical gears. Its excellent power capacity per mesh makes it the default system for many designers. Every aspect of these gears is studied and tested in great detail by standards committees and development labs around the world.
My first assignment for AGMA was to attend a Helical Gear Rating Committee gathering in early 1979, when that group was finishing up a major re-write of the American method of rating spur and helical gears...
Read the rest of this post and more on the Gear Technology blog. The four most recent posts all deal with the basics of helical gears.