Some clever people at NASA put together a document describing what goes into making a gear. The tooth shape and material questions I was aware of from my undergraduate days, but their calculations about just how different lubricants work around complicated shapes, expected gear lifetimes and dynamic force calculations are impressive. And it's all pretty modern knowledge apparently, mostly since the 1920's.
After reading it I'm amazed that helicopters don't crash more often!
Thursday, October 28, 2010