A device for coupling an inner shaft and an outer shaft includes balls arranged in rows between axial grooves of the inner shaft and axial grooves of the outer shaft, enabling the shafts to slide in the direction of a common axis. Each row of balls is maintained by a double elastic member that bears in the groove of one of the inner or outer shafts and urges the balls along two rolling tracks so that they come into contact with the groove of the other of the inner and outer shafts. The two rolling tracks are hinged through a convex pivot surface and a corresponding concave support face of one of the inner and outer shafts, and a difference between the radii of the pivot surface and the support face defines two contact areas and a pivot axis parallel to the common axis.