A stacking mechanism comprising: a rotatable screw with external thread of length L and pitch P1; a plurality of N traveling nuts mounted on the screw, having internal thread of pitch P1 and external thread of pitch P2, P1>P2; an arrester preventing rotation of the nuts within a length L1 of the screw, while allowing sliding; and a threaded member of pitch P2 adapted to engage the external thread of the nuts within a length L2 of the screw. In a first position of the mechanism, the nuts are arranged over the length L1. Upon rotation of the screw, the nuts slide along the screw at rate P1 per 1 turn, transit from L1 to L2, and then slide within the length L2 at a rate P2 per 1 turn, thereby achieving reversibly a second position of the mechanism where they are arranged over the length L2, L2