Importance

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Abstract

Relief portions 1c1, 1c2 are formed at the boundary regions between the guide groove 1a and the inner peripheral surface 1b, and are respectively positioned at the inner side and the inlet side taking a joint center position 0 as a reference. The relief portions 1c1, 1c2 are gradually increased toward the end portions of the guide groove 1a. Groove depths ( .theta.1, .theta.2) of the guide groove 1a at the left region and the right region are identical to each other in all cross-sections orthogonal to the groove bottom line L, and the groove depth is the largest (.theta.2) at the center region including the joint center position 0, and is gradually decreased from the center region toward the both ends. (.theta.1: .theta.2>.theta.1).

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First Claim