Great question — answering as the UphillTech team:
If it is a cup-and-cone bearing, you began with the right thing: you tightened one side and noticed that the play is gone. In fact it was not necessary to go further. With this kind of hub, tightening on either side will preload bearings on both sides as the axle will pull on each side with equal force. In the hub shown below the axle (7) is a threaded rod, and you see that its horizontal position is indefinite. (Image from Shimano Manuals site)
Full answer by g.kertesz: https://bicycles.stackexchange.com/a/98733 (CC BY-SA 4.0).
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