Almost all modern hubs use sealed cartridge bearings – only Shimano and the higher end Campy hubs use loose ball bearings.
Despite these hubs being common-place the tools required to correctly remove and replace the bearings are not. To correctly install the new bearings they need to be gently pressed into place by their outside race – this prevents the force required to install the bearing from being transmitted though the balls and cages and damaging the new bearing before they’re even fitted. Most bike shops try to accomplish this by using a socket which ‘closely’ matches the outer bearing race – I’ve seen a lot of high end hubs damaged by botched installations (and I have to confess to damaging a hub myself in my younger days!)
This tool is something I built to make sure cartridge bearings are installed correctly every time without any damage to the hub. I machine special press adaptors to suit each type of bearing on a lathe, and the center pin guides these adaptors and keeps them (and the bearings) square to the hub shell. After applying a thin coat of Loctite 641 to help lubricate the bearing while installing I use a soft-faced hammer to gently tap the bearings into place. Because the adaptors and bearings are held square to the hub there is no risk of damage to the new bearings or the hub.
I have bearing adapters to fit most of the common hubs I work on and if I encounter something a bit strange then I machine a new adapter to suit.
Some pics showing a Tune Mag180 rear hub. The plastic box in the background keeps all the bearing adapters organized.