When the vibrating screen works, the eccentric shaft drives the screen vibrating up and down. When the eccentric distance of eccentric shaft lifts up, the screen moves upward and generates the upward inertial force. When the eccentric distance goes downward, the screen also moves down and generates the downward inertial force which makes the screen body to vibrate. In order to keep balance of the screen, the damping weight moves downward with the eccentric distance moves upward, and vise versa. The sieve’s up and down distance equal to the eccentric distance of eccentric shaft. Meanwhile, the position of pulley in the eccentric shaft unchanged, that is self centering.