An improved drive mechanism for rotating a sweep arm in a storage structure that contains a free-flowing stored material. The inner end of the sweep arm is mounted on a turntable that is disposed in the inner end of a radially extending trough in the foundation of the storage structure. The turntable is provided with a pair of spaced, concentric, downwardly extending flanges, and a plurality of ribs connect the upper portions of the spaced flanges. The spaces between the ribs define upwardly diverging passages. A drive pin is positioned beneath each rib and extends between the spaced flanges. The teeth of a drive sprocket are engageable with the drive pins, and rotation of the sprocket acts to rotate the turntable and thus rotate the sweep arm around the structure to dislodge the stored material. As the drive sprocket is rotated, any stored material in the vicinity of the drive pins is extruded upwardly through the passages by the sprocket teeth to prevent possible jamming of the drive mechanism. In addition, the roots of the sprocket teeth are provided with relief and the sprocket teeth are tapered to aid in the removal of the stored material from the drive mechanism.