Both gears are driven and are synchronized by timing gears. Figure 6 is a tri-lobe pump. Fluid is carried from inlet to outlet in spaces between piston surfaces. Rotors must be timed by separate means, and each rotor may have one or more piston elements. See Figure 7.
Screw pumps carry fluid in the spaces between the screw threads.
The fluid is displaced axially as the screws mesh. They have a rotor with external threads and a stator with internal threads. The rotor threads are eccentric to the axis of rotation. Multiple screw pumps have multiple external screw threads. These pumps may be timed or untimed.