A double-offset plunging type constant velocity universal joint includes inner and outer joint members each having six linear track grooves extending along an axial direction. The inner joint member has a coupling hole at a center portion thereof for coupling a shaft. Six torque transmission balls retained by a cage are incorporated between the linear track grooves of the inner and outer joint members. A ratio T.sub.CAGE/D.sub.BALL of a minimum thickness (T.sub.CAGE) of the cage to a diameter of the torque transmission ball (D.sub.BALL) is from 0.225 to 0.245, wherein a ratio D.sub.BALL/D.sub.S of a diameter (D.sub.BALL) of the torque transmission ball to a spline large diameter (D.sub.S) of the coupling hole of the inner joint member is from 0.79 to 0.85, and wherein a ball contact ratio (ψ) of the torque transmission ball is from 1.08 to 1.12.

A double-offset plunging type constant velocity universal joint includes inner and outer joint members each having six linear track grooves extending along an axial direction. The inner joint member has a coupling hole at a center portion thereof for coupling a shaft. Six torque transmission balls retained by a cage are incorporated between the linear track grooves of the inner and outer joint members. A ratio T.sub.CAGE/D.sub.BALL of a minimum thickness (T.sub.CAGE) of the cage to a diameter of the torque transmission ball (D.sub.BALL) is from 0.225 to 0.245, wherein a ratio D.sub.BALL/D.sub.S of a diameter (D.sub.BALL) of the torque transmission ball to a spline large diameter (D.sub.S) of the coupling hole of the inner joint member is from 0.79 to 0.85, and wherein a ball contact ratio (ψ) of the torque transmission ball is from 1.08 to 1.12.

A fixed type constant velocity universal joint (3) includes an outer joint member (31), an inner joint member (32), eight balls (33), and a cage (34). A diameter (D.sub.31c) of the outer joint member (31) at an opening end of an inner peripheral surface (31c) is set larger than an outer diameter (D.sub.34b) of the cage (34) at a center portion in a circumferential direction of pockets (34a) when the cage (34) is seen from an axial direction of the joint. A bottom surface (31f) of a mouth section (31a) of the outer joint member (31) is arranged so as to interfere with an imaginary spherical surface (Q) being an extension of the inner peripheral surface (31c) of the outer joint member (31) toward a deep-side.

A plunging type constant velocity universal joint includes an outer joint member, an inner joint member, eight balls, and a cage. A curvature center of a spherical portion of an outer peripheral surface of the cage and a curvature center of a spherical portion of an inner peripheral surface of the cage are offset to opposite sides in the axial direction with respect to a joint center by an equal distance. A ratio PCD.sub.BALL/D.sub.BALL of a pitch circle diameter PCD.sub.BALL of the balls to a diameter D.sub.BALL of each of the balls is set from 3.3 to 3.6. A ratio T.sub.I/D.sub.BALL of a radial thickness T.sub.I of the inner joint member to the diameter D.sub.BALL of each of the balls is set from 0.30 to 0.45.

A plunging type constant velocity universal joint includes an outer joint member, an inner joint member, eight balls, and a cage. A curvature center of a spherical portion of an outer peripheral surface of the cage and a curvature center of a spherical portion of an inner peripheral surface of the cage are offset to opposite sides in the axial direction with respect to a joint center by an equal distance. A ratio PCD.sub.BALL/D.sub.BALL of a pitch circle diameter PCD.sub.BALL of the balls to a diameter D.sub.BALL of each of the balls is set from 3.3 to 3.6. A ratio T.sub.I/D.sub.BALL of a radial thickness T.sub.I of the inner joint member to the diameter D.sub.BALL of each of the balls is set from 0.30 to 0.45.

A plunging shaft includes: a first shaft having an outer part defining a cylindrical bore and a through hole; a second shaft having an inner part; a ball spline structure connecting the outer part and the inner part so as to transmit rotational power while being relatively movable with respect to each other; and a sealing member that is inserted into the through hole to seal the through hole. The ball spline structure includes: a plurality of outer ball grooves; a plurality of inner ball grooves; a plurality of power transmitting balls; and a ball cage. The outer part is provided with an air discharging groove for discharging air in a space formed by a front end of the second shaft and the cylindrical bore when the first shaft and the second shaft move relatively move to become closer to each other.

A plunging shaft includes: a first shaft having an outer part defining a cylindrical bore and a through hole; a second shaft having an inner part; a ball spline structure connecting the outer part and the inner part so as to transmit rotational power while being relatively movable with respect to each other; and a sealing member that is inserted into the through hole to seal the through hole. The ball spline structure includes: a plurality of outer ball grooves; a plurality of inner ball grooves; a plurality of power transmitting balls; and a ball cage. The outer part is provided with an air discharging groove for discharging air in a space formed by a front end of the second shaft and the cylindrical bore when the first shaft and the second shaft move relatively move to become closer to each other.

A propeller shaft includes constant-velocity joints at both the end portions of a first shaft member.

A rear-wheel drive shaft (1) includes a fixed type constant velocity universal joint (3), a plunging type constant velocity universal joint (2), and an intermediate shaft (4) coupling both the constant velocity universal joints to each other. A ratio (PCD.sub.BALL(f)/D.sub.BALL(f)) of a pitch circle diameter (PCD.sub.BALL(f)) of balls (33) of the fixed type constant velocity universal joint (3) to a diameter (D.sub.BALL(f)) of each of the balls (33) is set from 3.70 to 3.87. A ratio (PCD.sub.BALL(s)/D.sub.BALL(s)) of a pitch circle diameter (PCD.sub.BALL(s)) of balls (23) of the plunging type constant velocity universal joint (2) to a diameter (D.sub.BALL(s)) of each of the balls (23) is set from 3.3 to 3.6.

A rear-wheel drive shaft (1) includes a fixed type constant velocity universal joint (3), a plunging type constant velocity universal joint (2), and an intermediate shaft (4) coupling both the constant velocity universal joints to each other. A ratio (PCD.sub.BALL(f)/D.sub.BALL(f)) of a pitch circle diameter (PCD.sub.BALL(f)) of balls (33) of the fixed type constant velocity universal joint (3) to a diameter (D.sub.BALL(f)) of each of the balls (33) is set from 3.70 to 3.87. A ratio (PCD.sub.BALL(s)/D.sub.BALL(s)) of a pitch circle diameter (PCD.sub.BALL(s)) of balls (23) of the plunging type constant velocity universal joint (2) to a diameter (D.sub.BALL(s)) of each of the balls (23) is set from 3.3 to 3.6.