An agricultural tool comprising a bearing frame (10), a rotating driven shaft (40), an engine (50) provided with a rotating drive shaft (51) and a transmission belt (60) destined to connect the driven shaft (40) with the drive shaft (51), wherein the agricultural tool further comprises at least a mobile support (20), which is slidably mounted on the bearing frame (10) in order to vary a reciprocal distance between the drive shaft (51) and the driven shaft (40), and means (30) for pushing the mobile support (20) in a direction which distances the drive shaft (51) from the driven shaft (40) in opposition to the transmission belt (60) which connects them, wherein the mobile support (20) comprises at least a guide bush (26), which is slidably inserted on a pivot (17) of the bearing frame (10), and a casing (28) destined to house a portion of the pivot (17) which projects from the guide bush (26), and wherein the means for pushing the mobile support (20) comprise at least a spring (30) which is interposed between an external abutment (19) of the projecting part of the pivot (17) and an internal abutment (29) of the casing (28).
