In the operating scheme is shown how the rotations of the gears displace the product from the inlet side to the outlet side. The entire operation can be resumed in three phase: Suction, Delivery and Discharge. Into a gear pumps the two rotors, that are respectively called driving and driven rotor, are seated in a body that round them from all side. On the both sides of the mesh zone of the teeth there is a hole in the body of the pump. These holes are respectively called suction and discharge. Spinning the rotors the fluid is moved, through the chambers between the teeth of the rotors and the body, from the suction side to the discharge side. Fluid does not pass through the meshing zone of the gears due to small tolerances.  A back-flow of the fluid is blocked by the contact of the teeth of the two rotors in the meshing zone. Obviously presences of solid particles inside the media prejudice this process. These pumps cannot be used to pump fluids with abrasive proprieties or with solid particles inside without an adequated filtering system.