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