A mechanical drive mechanism of the endless belt and pulley type having an improved belt tensioning arrangement. The shaft of the machine being driven (or that of the driving member itself) is eccentrically journaled with respect to its cylindrical bearing hub and a rigid arm having a first bore therethrough is disposed with the hub of the machine centered in that first bore. Located at the opposed end of the rigid arm is a parallel bore in which the other drive or driven shaft is journaled. Because of the eccentricity between the first shaft and its hub, as the arm is rotated about the hub as a center, the distance between the centers of the driving and driven shafts is increased or decreased, depending upon the direction of rotation of the rigid arm. As such, the tension of the endless belt coupling the drive and driven pulleys can be varied. The construction of the belt and pulley combination lends itself to being completely enclosed in a protective shroud which need not be removed to make the tensioning adjustment.