A gear pump moves fluid by trapping it between rotating gear teeth and the pump housing. As the gears rotate, fluid is carried around the outside of the gears from the inlet to the outlet — not through the center where the teeth mesh. The meshing point forms a moving seal that prevents the high-pressure outlet from leaking back to the low-pressure inlet. Every revolution displaces a fixed volume, which is why gear pumps are the workhorse of hydraulic systems, lubrication circuits, and chemical metering.

Two main configurations:

• External gear pumps — two identical spur gears mesh externally. Simple, cheap, tolerant of high pressures (up to 3000+ psi), but noisy due to trapped fluid between teeth at the mesh point. Used in hydraulic power units, machine tool lube systems, and fuel injection.
• Internal gear pumps — a small external gear (pinion) rotates inside a larger internal-toothed gear (rotor), separated by a crescent-shaped divider. Quieter, smoother flow, better at handling viscous fluids like heavy oils, asphalt, and chocolate. Used in lubrication systems and food/chemical processing.

The gerotor is a special internal gear pump where the inner rotor has one fewer tooth than the outer. No crescent needed — the teeth themselves form the sealing chambers. Compact and common in automotive oil pumps and small hydraulics.

Real-world example: Your car's engine oil pump is almost certainly a gerotor or external gear pump driven directly off the crankshaft. At 3000 RPM, a small gear pump with 5 cm³ displacement delivers about 15 L/min of oil — enough to pressurize every bearing in the engine.

Flow rule of thumb:

Q (L/min) = Displacement (cm³/rev) × Speed (RPM) × Volumetric Efficiency / 1000

For a 10 cm³/rev pump at 1800 RPM with 92% efficiency: Q = 10 × 1800 × 0.92 / 1000 ≈ 16.6 L/min. Volumetric efficiency drops at higher pressures because internal leakage past the gear faces and tips increases — this is the dominant loss mechanism.

Design constraints:

• Viscosity matters — too thin (water-like) leaks past clearances; too thick (cold grease) starves the inlet. Optimum is roughly 10–500 cSt.
• No solids — gear teeth have tight clearances (microns). Particles cause rapid wear or seizure. Filter to 25 µm minimum.
• Relief valve required — positive displacement means dead-heading a gear pump will burst the housing or shear the drive shaft within seconds.
• Self-priming — gear pumps can pull a modest suction lift (3–5 m), unlike centrifugals which need flooded suction.