Internal vs External Gear Pump: Working Diagram, Uses & How to Choose

Introduction
Not all gear pumps are the same. The two primary configurations—internal gear pump and external gear pump—differ fundamentally in design, performance, and application. If you are selecting between them, understanding the internal gear pump unique advantages (including its ability to handle thick, non-lubricating fluids) is critical. This article compares both types with working diagrams, uses, and a decision matrix.

External Gear Pump: Overview
The external gear pump uses two identical gears (one driven by the motor, one idler) rotating in opposite directions.

• Working Diagram: Inlet → fluid fills tooth spaces → gears carry fluid around housing → meshing at outlet forces fluid out.
• Pros: Simple, low cost, handles high pressures (up to 3,000 PSI).
• Cons: Requires tight clearances; cannot handle solids; noisy at high speed.

Internal Gear Pump: Overview
The internal gear pump uses a rotor (external gear) and an idler (internal gear) rotating in the same direction, plus a crescent-shaped separator.

• Working Diagram: The rotor drives the idler. Fluid enters between the rotor teeth and the idler. The crescent blocks off the cavity, forcing fluid to travel to the outlet. As the teeth re-mesh, fluid is displaced.
• Pros: Extremely quiet, handles high viscosity (up to 2,000,000 cP), self-priming, can run dry briefly, handles non-lubricating fluids (water, solvents).
• Cons: Lower maximum pressure (typically 200-400 PSI) than external gear pumps.

Head-to-Head Comparison Table

Working Diagram Explanation
Imagine two circles: In an internal gear pump, the smaller gear (rotor) is mounted eccentrically inside the larger hollow gear (idler). The crescent sits in the gap. As they rotate, the crescent prevents fluid from returning to the inlet—this creates a continuous, smooth flow with minimal pulsation. External gear pumps have no crescent; their seal is created purely by tooth-to-tooth contact.

Uses of Internal Gear Pumps

• Polymer Processing: Transferring hot, viscous resins and adhesives.
• Paint & Ink: Moving heavy pastes without damaging the pigment.
• Liquid Sugar & Corn Syrup: Food-grade internal gear pumps handle thick, sticky sugars.
• Lube Oil Circulation: Quiet operation makes them ideal for printing presses and machine tools.

How to Choose: A Decision Matrix
Choose an External Gear Pump if:

• You need high pressure (>500 PSI).
• Fluid is clean, lubricating (hydraulic oil, diesel).
• Budget is tight.
• Noise is not a concern.

Choose an Internal Gear Pump if:

• Fluid is thick (>10,000 cP) or non-lubricating (water, solvents, acids).
• You need quiet operation (indoor factories, labs).
• You occasionally run the pump dry.
• Fluid contains small soft solids (waste sludge, food puree).

Unique Pump’s Internal Gear Pump Range
Unique Pump manufactures heavy-duty internal gear pump models in cast iron and stainless steel. Their G-Series internal gear pumps feature:

• Replaceable hardened steel gears.
• Self-adjusting wear plates (maintains efficiency over time).
• Stuffing box or mechanical seal options.
• Available as a rotary gear pump with motor assembly.

Maintenance Tips

• Internal gear pumps have longer life because the crescent takes the wear instead of the housing.
• Change oil in the bearing housing every 2,000 hours.
• For sticky fluids, use a heated jacket to reduce viscosity at startup.

Conclusion
The internal gear pump is the superior choice for thick, abrasive, or non-lubricating fluids, while external gear pumps dominate high-pressure hydraulics. For a balanced, quiet, high-viscosity solution, Unique Pump’s internal gear pumps deliver unmatched reliability.