The invention relates to an axially sliding expanding shaft, of the type mounted in cantilever fashion on a support structure, comprising a main shaft coupled with an expanding cylinder (11) provided with extractable gripping elements designed to retain the core of a reel mounted on the expanding shaft, in which said main shaft (12), on the one hand, is locked in rotation and in translation with said expanding cylinder (11) and, on the other hand, is slidable inside a bearing member (13) mounted rotatably on said support structure, adjusting means (16) for controlling the relative sliding movement between said main shaft (12) and said bearing member (13) also being provided; preferably the adjusting means (16) remain static so as to allow adjustment also with the shaft (12) rotating.