Constant velocity universal joint

A constant velocity universal joint comprising:

• an outer joint member having three track grooves formed therein, each track groove has roller guide surfaces arranged circumferentially opposed to each other;
• a tripod member having three radially protruding trunnions;
• a roller fitted in a corresponding track groove;
• and
• a ring fitted on each trunnion for rotatably supporting said roller so that the roller is movable axially of the outer joint member along a corresponding roller guide surface,
• wherein an outer circumferential surface of said each trunnion has a straight shape, as viewed in a longitudinal section, and has, as viewed in a cross section, a shape which allows contact with an inner circumferential surface of said corresponding ring in a direction orthogonal to an axis of the outer joint member and allows creation of a gap between the outer circumferential surface of said each trunnion and the inner circumferential surface of said corresponding ring in an axial direction of the outer joint member,
• wherein said inner circumferential surface of said corresponding ring is shaped such that a diameter of said corresponding ring increases in a direction from an axial central portion of said corresponding ring to an end-face side thereof, and
• wherein the generatrix of the inner circumferential surface of said corresponding ring is composed of a central convex circular arc and a tangential line extending from an end portion of said convex circular arc to an end face of said corresponding ring.