The top weight on the motor shaft rotates in a plane close to the center of the mass of assembly. Rotation of the top eccentric weights creates vibration on the horizontal plane which causes material to move across the screen cloth to periphery increasing the horizontal throw, causing oversize material to discharge at a faster rate.
The bottom eccentric weight rotates below the center of mass creating tilt on the screen giving vibration in verticals and tangential plane. Increasing the verticals component of motion, this promotes turnover of material on the screen surface helping maximum quantity of undersize material to pass through the screen. Rugged springs placed over the circular motor base amplifies the vibration.