Plunging type constant velocity universal joint

A plunging type constant velocity universal joint, comprising:

• an inner joint member having an outer peripheral surface with a plurality of linear track grooves formed in an axial direction and being inclined with respect to an axial line;
• an outer joint member having a first axial end, a second axial end, and an inner peripheral surface with a plurality of linear track grooves formed in the axial direction and being inclined with respect to the axial line in a direction opposite to that of the track grooves of the inner joint member, and having an inside filled with a lubricant;
• balls incorporated in respective crossing portions between the track grooves of the inner joint member and the track grooves of the outer joint member;
• and
• a cage arranged between the outer peripheral surface of the inner joint member and the inner peripheral surface of the outer joint member, and being configured to retain the balls in the same plane,
• wherein a large number of minute recesses are disposed at random in a surface of each of the balls;
• an end cap is fixed to the first axial end of the outer joint member, and a sealing device is attached between the second axial end of the outer joint member and a shaft,
• the lubricant is in the large number of minute recesses in the surfaces of the balls and is interposed at contact interfaces at contact portions between the balls and the track grooves and contact portions between the balls and pockets of the cage, whereby an oil film layer is formed,
• wherein a coefficient of friction of the lubricant is 0.070 or less. and
• wherein:
• surface roughness of the surface of each of the balls in which the minute recesses are formed is Ra 0.03 to 0.6 μm;
• a parameter skewness of the distribution curve of the surface roughness of the balls is −1.0 or less;
• and
• a ratio of a total area of a large number of the minute recesses to a surface area of the balls is 10 to 40%.