This comprises a horizontal entraining loader (1) on which are placed in an initial zone several piled cardboard sheets (12) to be driven by several transversal pushers (13) to a forming station (14) for cardboard boxes (23), said transversal pushers (13) being connected to a lateral chain (2) coupled to several end pinions (3) connected to several cross axles (4-4′). It is characterized in that the horizontal entrainment loader (1) can be adjusted longitudinally, thereby varying the distance between the transverse axles (4-4′), including for this purpose a vertically moveable tractive mechanism (8), that has a clutch mechanism (20) by means of which the movement is transmitted to a large cogwheel unit (17) and to an encoder (16) arranged coaxially with said large cogwheel unit (17), without ever losing the link between aforementioned encoder (16) and the respective transversal pusher (13).