Two parallel shafts hang four rings respectively. The one
shaft is fixed on a table and another shaft is fixed on a robot
arm. The robot arm forces two shafts to come closer. When the
distance between the two shafts is not close enough, the centers
of rings have some displacements in tangential direction because
of gravitational force. When the shafts come close enough, the
displacement of each ring comes to zero, and then the center
point of the rings, the center of shafts, and their contact points
lie in a straight line.