The external gear pump belongs to the class of rotating postive displacement pumps. They can manage low viscosity liquids like alcohols, solvents or liquid gases as well as medium and high viscosity liquids like polymer melts, gumbase or rubber. Liquids containing solids are not really suitable for gearpumps unless the particles are really small.
A gear pump comprises a housing with two covers. The powered gear wheel and driven gear wheel are borne in four friction bearings. The protruding drive shaft is sealed. The meshing gear wheels are enclosed by the housing. The amount of clearances between the tips of the teeth and the housing is extremely small and precisely defined There are ports in the housing. One port is at the suction side (inlet) and the other at the pressure side of the pump (discharge). One gear wheel is normally driven by a motor connected to the drive shaft, which protrudes from the housing. When the gear wheels are rotating, a chamber, which is filled with the medium being pumped, is formed between two teeth and the housing. The medium is thus conveyed from the suction side to the pressure side of the pump. The medium being pumped is displaced from between the gap between the teeth at the point at which the teeth mesh. For this reason, the gear pump is known as a displacement pump.