A gear pump uses a gear mesh to pump fluid by displacement.  They are one of the most common types of hydraulic fluid pumps for power applications. The gear pump was invented around 1600 by Johannes Kepler .
Gear pumps are widely used in chemical installations for pumping liquids with high viscosity. There are two main variations: external gear pumps which use two external spur gears , and internal gear pumps which use one external and one internal spur gear (the internal spur gear teeth are inward, see below). Gear pumps are positive displacement (or fixed displacement ), meaning they pump a constant amount of fluid for each revolution. Some gear pumps are designed to function as either a motor or a pump.
As the gears spin they separate on the intake side of the pump, creating a void and suction which is filled with fluid . The fluid is moved by gears to the discharge side of the pump, where a meshing of gears displaces the fluid. Mechanical clearances are small – on the order of 10 µm. The tighter clearance, along with the speed of rotation, effectively prevents the fluid from seeping back.
The rigid design of the gears and houses allows the ability to pump very high pressure and highly viscous fluids.
A number of variations exist, including helical and herringbone gear sets (instead of spur gears), lobe-shaped rotors similar to Roots blowers (commonly used as superchargers ), and mechanical designs allowing stacking of pumps. The most common variations are shown below (the drive gear is shown blue and the idler is shown purple ).
External gear pump design for hydraulic power applications.
Internal gear ( gerotor ) pump design for automotive oil pumps.
Internal gear ( crescent internal gear ) pump design for high viscosity fluids .
An external precision gear pump is typically limited to a maximum working pressure of 210 bar (21,000 kPa) and a maximum speed of 3,000 rpm. Some manufacturers produce gear pumps with higher working pressures and speeds but these types of pumps are noisy and may require special precautions. 
The suction and pressure ports need to interface where the gear meshes (shown as dim gray lines in the internal pump images). Some internal gear pumps have an additional, crescent-shaped seal (shown above, right). It serves to isolate the eddy gears and also reduces the eddy currents.
- Flow Rate in US Gallons/Minute = Pump Capacity × RPM
- Power in hp = US gal/min × (lbf/in 2 )/1714
Gear pumps are generally very efficient, especially in high pressure applications.
Factors Affecting Efficiency
- Clearance: Geometric clearance on the gear end and outside diameter allows for leakage and back flow. Although sometimes higher clearance helps to reduce hydrodynamic friction and improve efficiency.
- Gear Backlash: High backlash between gears also allows for fluid leakage. However, it does help reduce wasted energy by trapping fluid between the gear teeth (known as pressure trapping).
- Petrochemicals: Pure or filled bitumen, pitch, diesel oil, crude oil, lubricating oil, etc.
- Chemicals: sodium silicates, acids, plastics, compound chemicals, isocyanates, etc.
- Paint and ink.
- resins and adhesives.
- Pulp and paper: acid, soap, lye, black liquor, kaolin, lime, latex, sludge, etc.
- Food: Chocolate, cocoa butter, fillers, sugar, vegetable fats and oils, molasses, animal food, etc.