The inner surface of an outer cylinder forms an outer cylinder path with a simple circular cross-sectional configuration which has a radius of curvature. A multiplicity of grooves are arranged at a uniform spacing on the outer circumference of an inner cylinder, their radius of curvature being of the same order as the abovementioned radius of curvature. The outer circumference of the barrel-shaped rollers which are inserted between the grooves and the inner surface of the outer cylinder is of the same order as the abovementioned radius of curvature. The barrel-shaped rollers and the inner and outer paths form linear contacts, thereby improving the ability to withstand loading so as to prevent, chipping, splintering and other damage due to the heavy load exerted.
