A heavy earth-moving machine having a lower structure and an upper structure rotatably supported by the lower structure for rotation about an axis. A substantially circular lower rail is mounted on the lower structure and includes an inclined lower surface. A substantially circular upper rail is mounted on the upper structure and includes a substantially planar upper surface that is oriented substantially perpendicular to the axis. Tapered rollers support the upper rail above the lower rail for rotation of the upper rail about the axis. Each roller engages both the upper surface and the lower surface and defines and rotates about a roller axis that intersects the axis at a non-perpendicular angle.