Constant-velocity universal joint

A constant-velocity universal joint comprising:

• a generally cup-shaped outer joint member having a plurality of outer track grooves provided in an inner circumferential surface thereof;
• an inner joint member having a plurality of inner track grooves provided in an outer circumferential surface thereof, said inner joint member being disposed inside said outer joint member;
• a plurality of balls interposed between said outer track grooves and said inner track grooves, so as to transmit a torque between said outer joint member and said inner joint member;
• and
• a generally annular-shaped cage disposed between said outer joint member and said inner joint member, and having a plurality of pockets holding therein said balls, wherein:
• said outer track grooves and said inner track grooves cooperate to define a plurality groove portions, such that each of said balls is disposed in a corresponding one of said groove portions,
• each of said balls, which is disposed in a corresponding one of said groove portions, is pinched between one of said outer track grooves and one of said inner track grooves that cooperate with each other to define said corresponding one of said groove portions, and is substantially in contact at outer and inner contact points with said one of said outer track grooves and said one of said inner track grooves, respectively,
• an outer tangent plane that is tangent to each of said balls at said outer contact point and an inner tangent plane that is tangent to each of said balls at said inner contact point cooperate with each other to define a pinch angle at an intersection of said outer tangent plane and said inner tangent plane,
• said groove portions include a first groove portion in which a radial distance between a corresponding one of said outer track grooves and a corresponding one of said inner track grooves in a radial direction of said outer joint member is generally increased in a direction toward an opening end of said outer joint member in a reference state in which centerlines of said outer joint member and said inner joint member lie in a straight line with a joint angle defined by said centerlines being zero degrees,
• said groove portions include a second groove portion in which a radial distance between a corresponding one of said outer track grooves and a corresponding one of said inner track grooves in said radial direction is generally reduced in said direction toward said opening end of said outer joint member in said reference state,
• said first groove portion and said second groove portion are alternately arranged about said centerlines,
• one of said first groove portion and said second groove portion is a larger-pinch-angle groove portion while the other of said first groove portion and said second groove portion is a smaller-pinch-angle groove portion, such that an absolute value of said pinch angle of said larger-pinch-angle groove portion is larger than an absolute value of said pinch angle of said smaller-pinch-angle groove portion in said reference state,
• said cage contacts one of said outer joint member and said inner joint member such that said cage has play in directions parallel to an axis of said cage,
• when the torque is not transmitted between said outer joint member and said inner joint member through said balls in said reference state, a first radial clearance between said smaller-pinch-angle groove portion and a corresponding one of said balls disposed in said smaller-pinch-angle groove portion is larger than a second radial clearance between said larger-pinch-angle groove portion and a corresponding one of said balls disposed in said larger-pinch-angle groove portion, and
• the first radial clearance and the second radial clearance are substantially equal when there is a load.