In the so-called Internal Gear Pump the motor drive shaft drives the pinion, also known as the runner wheel. Its teeth mesh with the outer gear wheel, causing it to rotate. 

During rotation space becomes free between teeth, the displacement volume increases and results in fluid being drawn in (Fig. 1).

The fluid is transported to the discharge side. The sickle provides a seal between the suction- and compression space (Fig. 2).

As teeth move into teeth spaces the displacement volume is reduced and the fluid is forced into the pump outlet (Fig. 3).