The screening equipment vibrates about its center of mass, accomplished by eccentric weights on the upper and lower ends of the motion-generator shaft. Rotation of the top weight creates vibration in the horizontal plane, which causes material to move across the screen cloth to the periphery. The lower weight acts to tilt the machine, causing vibration in the vertical and tangential planes. The angle of lead given the lower weight with relation to the upper weight provides variable control of the spiral screening pattern. Speed and spiral pattern of material travel over the screen cloth can be set by the operator for maximum throughput and screening efficiency. Ultrasonic modulators can be added to screens to further improve screen efficiencies with fine powders.