Gear pump
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A Gear pump uses the meshing of gears to pump fluid by displacement. They are one of the most common types of pumps for hydraulic fluid power applications. There are two main variations; external gear pumps which use two external spur gears, and internal gear pumps which use an external and an internal spur gear. Gear pumps are 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 pump.
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[edit] Theory of operation
As the gears rotate they separate on the intake side of the pump, creating a void and suction which is filled by fluid. The fluid is carried by the gears to the discharge side of the pump, where the meshing of the gears displace the fluid. The mechanical clearances are small—on the order of a thousandth of an inch (micrometres). The tight clearances, along with the speed of rotation, effectively prevent the fluid from leaking backwards.
The rigid design of the gears and housing allow for very high pressures and the ability to pump highly viscous fluids.
Many 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 that allow the stacking of pumps. The most common variations are shown below (the drive gear is shown blue and the idler is shown purple).
Suction and pressure ports need to interface where the gears mesh (shown as dim gray lines in the internal pump images). Some internal gear pumps have an additional, crescent shaped seal (shown above, right).
Pump formulas:
- Flow rate in US gal/min = Displacement X rpm/231 (displacement in in3/revolution)
- Power in hp = US gal/min X (lbf/in³)/1714
[edit] Applications
External gear pumps are the most common type of pump for hydraulic power applications. This is because of their simplicity, reliability, and very high power ratings. A typical 10 hp (7 kW) gear pump may fit into a 4 x 4 x 4 inch (100 by 100 by 100 mm) envelope, a fraction of the size of a comparable electric motor. Maximum operating pressures are between 1000 and 4000 lbf/in² (7 and 28 MPa). Many commercial models are designed to operate as either a motor or pump. Some designs add load sensing modifications.
Internal gear pumps perform better at lower speeds and with higher viscosity fluids than external gear pumps. Engine oil pumps are usually internal gear designs.
Industry standards exist for gear pump mounting flanges, drive shaft sizes and splines, and ports. The suction port will usually be larger than the pressure port to reduce the possibility of cavitation.
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