Expandable shaft for supporting bobbins, of the type comprising a hollow cylinder (1), housing inside it a plurality of locking modules (Mi) and having, corresponding with each of these modules (Mi), a plurality of short openings (3) distributed circumferentially over its surface, through which retractable blocks (2) are made to emerge, each module (Mi) having an axially slidable thrusting element (5) which engages with said blocks (2) by means of surfaces (6) inclined with respect to the longitudinal axis of the shaft, a spline (4) which extends longitudinally substantially over the whole length of the shaft being fixed externally onto each plurality of blocks (2) of the modules (Mi) corresponding to one another in an axial alignment.