Having made an 4 year excursion from IT into the world of making gears, I can attest that what seems like something fairly simple from the outside really isn't.
Just like a bearing, gears (ideally) have a rolling contact. The smoother and harder the faces the better, until you get to the point where fractures and spalling occur. The best gears are cut, sent out to be hardened via heat treat, then ground (with "superfinishing") to exact size. As with bearings, you have to get the size just right for long life.
Setting away Mosquitos (the real Wunderwaffe of that war) for freight runs (and for stuffing the occasional Nobel Price physicist in the bomb bay) tells a rather different story about the importance of swedish-made ball bearings for UK arms production:
Swedish Exports of Ball Bearings
(old.reddit.com)39 points by zeristor 6 hours ago | 5 comments
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Just like a bearing, gears (ideally) have a rolling contact. The smoother and harder the faces the better, until you get to the point where fractures and spalling occur. The best gears are cut, sent out to be hardened via heat treat, then ground (with "superfinishing") to exact size. As with bearings, you have to get the size just right for long life.
See for example this video, how to choose your ball bearing
https://m.youtube.com/watch?v=NO-BgMiC2yg
https://news.ycombinator.com/item?id=37884749
Now challenged by a few look-alike Chinese companies.