Gear pumps are positive displacement rotary pumps that transport liquids using rotating gears. They function through the use of two or more internal gears that create vacuum pressure, propelling the fluid media. Gear pumps are compact, high pressure pumps which provide a steady and pulseless fluid flow comparable to double diaphragm pumps and peristaltic pumps. They are best suited for high viscosity pumping applications such as oils, plastics, paint, adhesives, or soaps.

Gear Pump Operation

Gear pumps are positive displacement pumps, meaning they use expanding and contracting chambers to move fluids at a fixed rate. Specifically, they are rotary positive displacement pumps, which utilize a rotating mechanism or assembly to cause this contraction and expansion. To learn more about selecting different types of positive displacement pumps, visit the Positive Displacement Pumps Selection Guide page on GlobalSpec.

Gear pumps are the most common type of positive displacement pump used. Typically, a rotating assembly of two gears (a drive gear and an idler) moves to create suction at the pump inlet and draw in fluid. The liquid is then directed between the teeth of the gears and the walls of the casing to the discharge point. Volume decreases as the liquid travels from inlet to outlet, causing a buildup of pressure. Pressure relief valves are typically built-in to the pump to protect the pumping system from a closed valve in the discharge piping. Flow in gear pumps is determined by the size of the cavity (volume) between the gear teeth, the speed of rotation (rpm) of the gears, and the amount of slippage (reverse flow). Slip increases as the pump wears.

The volumetric efficiency of gear pumps is poor at low speeds and low flow rates, meaning they should be operated close to their maximum rated speeds.

Types of Gear Pumps

Gear pumps are either external or internal based on their design and operation.

External Gear Pump

External gear pumps utilize two identical gears with external teeth to generate flow. The rotation of the gears is such that the liquid comes into the inlet port and flows into and around the outer periphery of the two rotating gears. As the liquid comes around the periphery it is discharged to the outlet port.

Image Credit: Pump School

External gear pumps have close tolerances and shaft support on both sides of the gears. This allows them to run to pressures beyond 3,000 psi / 200 bar and also provide greater flow control and a more reliable measure of liquid passing through a pump. It also means they are not well suited to handling abrasive or extreme high temperature fluids. Slippage, which reduces efficiency and flow, increases as viscosity decreases and approaches zero at 5000 SSU (Saybolt Universal Seconds).

External gear pumps are typically less expensive and simpler to maintain than internal types, with moderate efficiencies. They are most commonly used to transfer fuel oils such as gasoline, diesel, and kerosene. In addition, they are used for high pressure applications such as elevators, damper controls, and other hydraulic devices. They are also popular for precise transfer and metering applications. 

Internal Gear Pump

Internal gear pumps generate flow using a gear with externally-cut teeth contained in and meshed with a gear with internally-cut teeth. As the gears come out of mesh on the inlet side, liquid is drawn into the pump. The liquid is forced out the discharge port by the meshing of the gears. Some contain a crescent shaped partition used to separate the inlet volume from the discharge volume between the two gears.

Image Credit: Pump School

These pumps excel at moving high viscosity fluids, but have a useful viscosity range of 1cP (centiPoise) to over 1,000,000cP. The single point of end clearance (the distance between the ends of the rotor gear teeth and the head of the pump) is adjustable to accommodate high temperatures, maximize efficiency for high viscosities, and to accommodate for wear.

Image Credit: Rotor assembly of the internal gear pump in operation.

Compared to external types, internal gear pumps have improved suction and delivery characteristics and are smoother in operation, but are also more expensive and are limited to small capacities and moderate pressure. They are used in numerous different types of industrial applications for handling oils and viscous chemicals, including petrochemical, marine, asphalt, chemical, and general industrial applications.

Comparison

The following table provides a comparison between internal and external gear types:

Gear Types

Gear pumps use one of two types of gears: spur gears or herringbone gears.

• Spur gears provide excellent suction lift, are ideal for water or light oils, are bi-directional (reversible), and are the most economical. To learn more about spur gears, visit the Spur Gears Selection Guide on GlobalSpec.

• Herringbone gears are ideal for viscous fluids, offer quiet operation in a non-clogging design, but are uni-directional. To learn more about herringbone gears, visit the Herringbone Gears Selection Guide on GlobalSpec.

Specifications

When selecting gear pumps, there are a few key performance specifications to consider; namely flow rate, pressure, power, efficiency, and operating temperature. GlobalSpec's Pump Flow page provides a detailed overview of these specifications.  

Media Type

Selecting the right gear pump requires an understanding of the properties of the liquid in the addressed system. These properties include viscosity and consistency.

Viscosity is a measure of the thickness of a liquid. Viscous fluids like sludges generate higher systems pressures and require more pumping power to move through the system. Internal gear pumps tend to be better suited than external gear pumps on highly viscous fluids and vice versa.

Consistency is the material makeup of the liquid solution in terms of chemicals and undissolved solids. In most cases, gear pumps are not able to pump solutions with suspended solids because of the associated wear and degradation of the drive assembly. Solutions with corrosive chemicals should be handled by pumps with materials and parts designed to withstand corrosion.

Materials

Pumps are typically designed with a number of different materials. The base materials, which constitute the parts of the pump exposed to the pumped media and the outside environment, are the most important to consider. Fluid characteristics, pressure ratings, and operating environment factors should be considered when selecting these materials.

• Cast iron provides high tensile strength, durability, and abrasion resistance corresponding to high pressure ratings.
• Plastics are inexpensive and provide extensive resistance to corrosion and chemical attack.
• Steel and stainless steel alloys provide protection against chemical and rust corrosion and have higher tensile strengths than plastics, corresponding to higher pressure ratings.

For more information on materials and other pump features, visit GlobalSpec's Pump Features page.

References

• LiquiFlo - Gear Pump Basics (pdf)
• MIT - Gear Pumps
• Pump School - Internal Gear Pump
• Pump School - External Gear Pump
• Pump Scout - Gear Pumps

Image Credit: Johnson Pump