Abstract: A self-propelled combine has a power source that is connected to a hydrostatic drive for the drive wheels by means of a belt drive, that includes a pair of coaxial idler sheaves mounted on a freely swingable arm, one sheave being connected to an input sheave by a first belt and the other idler sheave being connected by a second drive belt to an output sheave that drives the pump of the hydrostatic drive. The arm that mounts the idler sheaves is articulated, and a hydraulic cylinder between the two sections of the arm controls the distance between the idler sheave axis and the arm pivot, which in turn controls the tension in the two belts.