The present invention relates to a self-centering expanding shaft with a composite system - of the type comprising a structural cylinder, equipped with a pneumatic expanding system to block a reel and housing at least one pneumomechanical module to center the reel, said pneumomechanical module comprising a pair of opposed slidable thrust elements (3), coupled with gripping elements (4) apt to be extracted from and withdrawn into said structural cylinder (1), and two pressure chambers (8a,8b) positioned at its ends, in which said two pressure chambers (8a,8b) are connected through a single pneumatic line (10) which crosses the two thrust elements (3), the extraction of said gripping elements (4) being obtained by supplying compressed air into said compression chambers (8a,8b), and in that, at least an elastic element (7) is provided to act on said thrust elements (3) so as to reset the conditions of rest.