A rotary gear pump is a type of positive displacement pump that uses the meshing and unmeshing of gears to move fluid. Unlike centrifugal pumps that rely on impeller velocity, gear pumps trap a fixed volume of fluid between gear teeth and the pump housing, then carry it from the inlet to the outlet. This makes them highly effective for transferring viscous, abrasive, and shear-sensitive liquids at consistent flow rates regardless of discharge pressure.
Rotary gear pumps are used across a wide range of industries including chemical processing, petroleum transfer, explosives manufacturing, adhesives production, food and beverage processing, and general industrial applications. They are valued for their simplicity, reliability, self-priming capability, and ability to handle fluids that other pump types cannot.
How Does a Rotary Gear Pump Work?
The operating principle of a rotary gear pump is straightforward. Two gears rotate inside a closely fitted housing. As the gear teeth come out of mesh on the inlet side, they create expanding cavities that generate suction and draw fluid into the pump. The fluid is then trapped between the gear teeth and the inner wall of the housing, carried around from inlet to outlet. On the discharge side, the gears mesh together again, reducing the cavity volume and pushing the fluid out under pressure.
Because the pump moves a fixed volume of fluid with each revolution, the flow rate is directly proportional to the pump speed (RPM). This positive displacement action means gear pumps deliver consistent, predictable flow rates that are largely independent of discharge pressure — a significant advantage over centrifugal pumps, which lose flow as pressure increases.
Key Operating Characteristics
Positive Displacement
Fixed volume per revolution provides consistent, predictable flow
Self-Priming
Tight gear-to-housing clearances create suction to draw fluid in without a flooded inlet
Bi-Directional
Can run in either direction (CW or CCW), reversing the flow path
Viscosity-Tolerant
Performance improves with viscous fluids as internal leakage decreases
Types of Gear Pumps
There are two primary types of gear pumps, each with different designs suited to different applications:
External Gear Pumps
External gear pumps use two identical gears (a drive gear and an idler gear) that rotate in opposite directions. The fluid is carried in the spaces between the gear teeth and the pump housing. External gear pumps are the most common type for heavy industrial applications.
Best for: High-viscosity fluids, abrasive slurries, high-pressure applications, chemical transfer, petroleum products, adhesives, and explosives.
Internal Gear Pumps
Internal gear pumps use a larger outer gear (rotor) and a smaller inner gear (idler) that mesh on one side. A crescent-shaped divider separates the gears, creating cavities that move fluid. Internal gear pumps tend to run more quietly.
Best for: Lower-viscosity fluids, thin oils, fuel transfer, and applications requiring quiet operation or precise metering.
NAPCO manufactures external rotary gear pumps with rubber gears (nitrile, Viton, EPDM, or urethane). The rubber gear design is specifically engineered for handling abrasive and viscous fluids while maintaining a seal against the pump housing. This differs from metal-gear pumps, which require tighter manufacturing tolerances and are more susceptible to damage from abrasive particles.
Key Components of a Rotary Gear Pump
Understanding the components helps with maintenance, troubleshooting, and repair kit selection:
| Component | Function | Common Materials |
|---|---|---|
| Housing / Body | Contains the gears and forms the pumping chamber | Ductile iron (carbon steel) or stainless steel |
| Drive Gear | Connected to the motor shaft; drives the rotation | Nitrile rubber, Viton, EPDM, or urethane |
| Idler Gear | Meshes with the drive gear; rotates freely | Same material as the drive gear |
| Shaft | Transmits motor torque to the drive gear | Alloy steel (ductile iron pumps) or stainless steel |
| Bearings | Support the shaft and gears; reduce friction | Chrome needle roller bearings |
| Seals | Prevent fluid leakage at the shaft penetration | Viton (high and low pressure seals) |
| Gaskets | Seal the housing halves together | Asbestos-free paper gaskets |
Rotary Gear Pumps vs. Other Pump Types
Engineers often evaluate gear pumps against centrifugal pumps and progressive cavity pumps. Here is how they compare:
| Characteristic | Rotary Gear Pump | Centrifugal Pump | Progressive Cavity Pump |
|---|---|---|---|
| Flow Type | Positive displacement | Dynamic / velocity | Positive displacement |
| Best Viscosity Range | Medium to very high | Low to medium | Medium to very high |
| Self-Priming | Yes | No (requires flooded suction) | Yes |
| Flow Consistency | Steady, non-pulsating | Varies with pressure | Steady but slightly pulsating |
| Bi-Directional | Yes | No | No |
| Field Serviceability | Excellent — gears and seals replaceable on-site | Moderate — impeller replacement usually in-shop | Difficult — rotor/stator must be carefully matched |
| Typical Maintenance | Replace gears, seals, bearings via repair kit | Replace impeller, seals, bearings | Replace stator (rubber element); complex alignment |
For a more detailed comparison, see our resource article: Gear Pumps vs. Progressive Cavity Pumps →
Common Applications
Rotary gear pumps are used wherever reliable, consistent transfer of viscous or difficult fluids is required:
Explosives & Mining
Emulsion explosives, bulk blasting agents, sensitizer slurries
Adhesives & Sealants
Hot-melt adhesives, epoxy resins, polyurethane, silicone sealants
Chemical Processing
Solvents, acids, alkalis, corrosive chemical transfer
Petroleum & Fuel
Diesel, fuel oil, lubricants, tanker loading/unloading
Food & Beverage
Syrups, oils, chocolate, flavorings, food-grade transfers
General Industrial
Paints, coatings, inks, waste oil, asphalt, lubricants
How to Select the Right Gear Pump
Choosing the right rotary gear pump requires evaluating several factors:
1. Flow Rate Requirements
Determine the gallons per minute (GPM) needed. NAPCO 2″ pumps deliver up to 69 GPM; 3″ pumps deliver up to 158 GPM. For a detailed sizing guide, see How to Size a Rotary Gear Pump.
2. Fluid Properties
Consider viscosity (gear pumps excel with thick fluids), temperature (affects gear material choice), chemical compatibility (stainless steel for corrosives), and abrasiveness (rubber gears tolerate abrasive particles better than metal gears).
3. Housing Material
Ductile iron (carbon steel) is economical and suitable for non-corrosive fluids. Stainless steel is required for corrosive chemicals, food-grade applications, or environments where rust contamination is unacceptable. See Stainless Steel vs. Ductile Iron Pumps.
4. Gear Material
Nitrile rubber is standard (rated to 240°F). Viton is used for higher temperatures and chemical resistance. EPDM and urethane are available for specialized applications. See Nitrile vs. Viton Gears.
5. Discharge Pressure
Ensure the pump can deliver the required pressure. NAPCO pumps are rated to 100 PSI. A mechanical relief valve is mandatory to protect the pump from deadhead conditions.
6. Serviceability
Consider how easily the pump can be rebuilt in the field. Pumps with field-replaceable repair kits (gears, seals, bearings, gaskets) minimize downtime and reduce total cost of ownership compared to pumps that must be sent back to the manufacturer.
Use our Pump Sizing Calculator to find the right NAPCO pump model for your application.
Maintenance and Service Life
One of the greatest advantages of rotary gear pumps is their simplicity. With only two moving parts (the drive gear and idler gear), there are fewer components to wear out compared to more complex pump designs. Regular maintenance focuses on:
- Gear inspection and replacement — rubber gears wear over time and are the primary consumable
- Seal replacement — shaft seals prevent external leakage
- Bearing inspection — needle roller bearings support the shaft and gears
- Gasket replacement — maintains housing seal integrity
NAPCO pumps are specifically designed to be rebuilt in the field using a complete repair kit that includes all wear parts — drive gear, idler gear, bearings, seals, gaskets, and O-rings. This means there is no need to remove the pump from the pipeline or ship it back to the manufacturer for service. A qualified technician can complete a full rebuild on-site, typically in under an hour.
For detailed maintenance procedures, see our Field Maintenance Guide or watch our repair kit installation videos.
NAPCO Rotary Gear Pump Models
NAPCO manufactures external rotary gear pumps in two port sizes and two housing materials:
All NAPCO pumps come standard with nitrile rubber gears (rated to 240°F), Viton seals, chrome needle roller bearings, and bi-directional rotation capability. Matching repair kits and flow meters are also available.
Need Help Selecting the Right Pump?
NAPCO engineers can help you choose the right pump model, housing material, and gear configuration for your specific fluid and application.