Abstract: A tool for pre-stressing of two races to be welded to one another includes a hub having a cylindrical lateral hub wall and a tubular chuck having a cylindrical lateral chuck wall coaxially received and movable in the lateral hub wall. One of the lateral walls has a cylindrical face of revolution for radially supporting and centering the two races. A first axial abutment is immovable with respect to the chuck and against which a first free axial face is engageable, while a second axial abutment is immovable with respect to the hub and against which a second free axial face is engageable. A coaxial mechanism for axially moving the chuck towards the hub and hence for tightening the second axial abutment axially towards the first axial abutment is provided such that the two races therebetween are tightened against one another until a pre-determined pre-stress value is obtained.

Abstract: Composite bearing with double ball bearings (4) for the rotary support of a rotating shaft (1) of a very high precision device, wherein this bearing with double ball bearings is constituted by the juxtaposition of two bearings with single ball bearings having two respective corresponding collars (51, 52), interior or exterior, tightened axially against one another with a predetermined pre-stress, characterized in that the two collars (51, 52) maintained tightened against one another with said pre-determined pre-stress are soldered (9) to each other by their co-operating faces.