Gear Pump Types

Gear pumps are the most common type of positive displacement pump. They are ideal for transferring high viscosity fluids such as automotive oils, plastics, paint, adhesives, and soaps with a steady and pulseless flow and offer self-priming capabilities. This pump type cannot run dry or handle abrasive-laden fluids, and places high shear on the fluid.

They operate by creating suction at the inlet with a rotating assembly of two gears –a drive gear and an idler. Pump flow is determined by the size of the cavity (volume) between gear teeth, the amount of slippage (reverse flow), and the speed of rotation (rpm) of the gears.

Gear pumps can be either external or internal and vary in design and operation.

Internal Gear Pumps

Internal gear pumps operate by drawing fluid into the pump inlet between the rotor (large exterior gear) and idler (small interior gear) teeth, operating with a “gear-within-a-gear” principle. This pump type is incredibly versatile and can efficiently handle high viscosity fluids at moderate speeds.

Other features include:

  • High temperature, moderate pressure, low capacity pumping
  • Spur gear design (bidirectional)
  • Large and bulky footprint
  • Rotors supported by one or two bearings
  • Constant and even discharge with any pressure
  • Low net pressure suction head (NPSH) required
  • Can run dry for short periods

External Gear Pumps

External gear pumps tend to be less expensive and easier to operate than internal gear pumps, but they also require four bearings supporting the rotor shafts (versus one or two for internal gear pumps). They are most commonly used lubrication pumps in machine tools, in fluid power transfer units, and as oil pumps in engines. External gear pumps operate using two gears with external-cut teeth to generate flow.

Other features include:

  • Medium/low temperature and high pressure pumping
  • Spur or herringbone gear design
  • Compact and efficient design
  • Close tolerances and shaft support on both sides of the gears
  • No overhung bearing loads
  • Large outlet sizes and high capacities