The invention relates to a device for coupling an inner shaft and an outer shaft sliding in the direction of a common axis thereof, with balls provided between the two shafts. The balls are arranged in axial grooves of the inner shaft and in axial grooves of the outer shaft. Each row of balls is maintained by a double elastic member that bears in the groove of the inner shaft and urges the balls along two rolling tracks so that they come into contact with the groove of the outer shaft, wherein each of the two rolling tracks is hinged through a pivot surface and a bearing surface of the corresponding shaft, and the difference between the radii defines two bearing areas defining a hinge axis parallel to the common axis.