surrounds at a radial distance the two races 5 1 5 2 and is maintained coaxially by an excrescence of the hub 16 the plateau 19 of the cover 17 or else as shown here a ring solid 20 in the interior of the said plateau 19 constitutes second means of abutment suitable for bringing to bear against the free face of the other race 5 2 when the chuck 13 is axially brought closer to the hub 16 in order to ensure this bringing closer a tightening instrument such as a bolt 21 positioned coaxially to its head which takes support against the lower face of the hub 16 and its stem which crosses the bottom of said hub is screwed into the bottom of the chuck 13 the tightening of the bolt 21 enables the two races 5 1 5 2 to be subjected to a pre determinable pre stress in addition one or several pieces 22 are engaged in aligned bores of the support surface 11 of the hub 16 and of the chuck 13 in order to make them solid in rotation in order that the races 5 1 5 2 can be welded to one another along the interior edges of their co operating faces by means of a.