An assembly including: - a stationary support , such as an engine block, - a plurality of pulleys rotatably mounted on the support , - a belt mounted on said pulleys , - a tensioner , including: ∘ a plate , ∘ a rotating element mounted on the plate by means of a rotation axis stationary relative to that plate, the tensioner being able to rotate relative to the support , in a predefined rotation direction (R), between an idle position in which the rotating element is moved away from the belt and an operating position in which the rotating element is in contact with the belt , having specified that, in the operating position, the belt exerts a force on the rotating element , the direction of that force tending to cause the tensioner to rotate in that same predefined rotation direction (R); - means for preventing the tensioner from rotating in the predefined rotation direction (R) past its operating position; characterized in that , in order to rotate the tensioner relative to the support between the idle position and the operating position of the tensioner, the assembly comprises: - at least one closed aperture positioned on the plate of the tensioner or on the support , said closed aperture having a shape arranged such that when the tensioner rotates relative to the support in its idle position and its operating position, the rotational movement performed by the tensioner is not circular; - at least one means for cooperating with the or each closed aperture , this cooperating means being stationary relative to the support when the closed aperture is situated on the plate or stationary relative to the plate when the closed aperture is situated on the support. - a stationary support , such as an engine block, - a plurality of pulleys rotatably mounted on the support , - a belt mounted on said pulleys , - a tensioner , including: ∘ a plate , ∘ a rotating element mounted on the plate by means of a rotation axis stationary relative to that plate, the tensioner being able to rotate relative to the support , in a predefined rotation direction (R), between an idle position in which the rotating element is moved away from the belt and an operating position in which the rotating element is in contact with the belt , having specified that, in the operating position, the belt exerts a force on the rotating element , the direction of that force tending to cause the tensioner to rotate in that same predefined rotation direction (R); ∘ a plate , ∘ a rotating element mounted on the plate by means of a rotation axis stationary relative to that plate, - means for preventing the tensioner from rotating in the predefined rotation direction (R) past its operating position; - at least one closed aperture positioned on the plate of the tensioner or on the support , said closed aperture having a shape arranged such that when the tensioner rotates relative to the support in its idle position and its operating position, the rotational movement performed by the tensioner is not circular; - at least one means for cooperating with the or each closed aperture , this cooperating means being stationary relative to the support when the closed aperture is situated on the plate or stationary relative to the plate when the closed aperture is situated on the support. <EOS>