210 which bears on the bottom 155 of the axial groove 152 the coupling device shown in cross section in fig1 comprises balls 3 which are arranged in three axial rows 20 the axial rows 20 are transversely located at 120 u00b0 from one another the inner shaft 1 is provided with three axial grooves 171 which are transversely located at 120 u00b0 from one another the outer shaft 2 is provided with three axial grooves 172 which are transversely located at 120 u00b0 from one another for each row 20 of balls 3 there is provided in the outer shaft 2 a concave portion having the shape of an axial groove 172 the section of which comprises two concave sides 173 174 being tilted with respect to each other which come into contact with the balls 3 for each row 20 of balls 3 there is provided in the inner shaft 1 a concave portion having the shape of an axial groove 171 the section of which comprises a bottom 175 and two flanks 176 177 the bottom 175 is substantially perpendicular to the median plan 30 going through the common axis 4 and the axis of the centres of the balls 3 in said row 20 for each row 20 of balls 3 two rolling tracks 180 are arranged on either side of the median plan 30 the rolling tracks 180 cooperate with the concave portion having the shape of an axial groove 171 of the inner shaft 1 each rolling track 180 has the shape.