Patent ID: 7434673

Claim:
A segment-type wet friction material comprising: a core metal of a flat ring plate shape; and segment pieces made of friction material substrates cut into a segment piece shape along a circumference of the flat ring plate shape of the core metal, the segment pieces being joined on at least one surface of the core metal with an adhesive along the entire circumference of the core metal so that a plurality of oil grooves extending in a radial direction of the core metal are formed by a clearance between adjacent ones of the segment pieces, the plurality of the oil grooves being composed of first oil grooves and second oil grooves, each of the first oil grooves having a symmetrical broadened portion at one of an inner peripheral opening portion and a center portion thereof in the radial direction of the core metal, each of the second grooves having nearly a uniform width from an inner peripheral opening portion to an outer peripheral opening portion, the first grooves and the second grooves being mixedly disposed in a predetermined proportion, wherein the symmetrical broadened portion of the first oil groove is formed by a pair of oblique lines located at the inner peripheral opening portion so as to be symmetrically broadened and inclined at a constant angle to a center line of the first oil groove.