Device for varying the useful width of a tillage implement, comprising the tillage implement (1) a central frame (1a) and two lateral supports (1b) coupled to the central frame (1a) by means of transverse guides (2) which are perpendicular to the direction of travel of a tractor vehicle in which the tillage implement (1) is engaged； simultaneously moving the two lateral supports (1b) in opposition along the transverse guides (2) by means of a power element； having provided two opposed racks (7) which are coupled by one of their ends to the side supports (1b) by means of articulated connections (9)； said opposed racks (7) being guided in a tubular structure of the central frame (1a) which is arranged in a direction parallel to the transverse guides (2)； Characterized because: - the opposing racks (7) are linked by at least two freewheeling pinions (8) located inside the tubular structure (10)； where in all the relative positions of the opposed racks (7) along the tubular guide structure (10), the pinions (8) mesh with the teeth of the opposed racks (7). (Machine-translation by Google Translate, not legally binding)1. Dispositivo para variar el ancho útil de un apero de labranza, que comprende el apero de labranza (1) un bastidor central (1a) y dos soportes laterales (1b) acoplados al bastidor central (1a) mediante unas guías transversales (2) que son perpendiculares a la dirección de desplazamiento de un vehículo tractor en el que se engancha el apero de labranza (1)； desplazándose simultáneamente los dos soportes laterales (1b) en contraposición a lo largo de las guías transversales (2) mediante un elemento de potencia； habiéndose previsto dos cremalleras contrapuestas (7) que están acopladas por uno de sus extremos a los soportes laterales (1b) mediante conexiones articuladas (9)； estando guiadas esas cremalleras contrapuestas (7) en una estructura tubular del bastidor central (1a) que está dispuesta en una dirección paralela a las guías transversales (2)； caracter
