Rotary Gear Pump: How It Works

How a rotary gear pump moves fluid

A rotary gear pump is a positive-displacement machine: two external gears (one driven, one idler) rotate in a close-fitting housing. Teeth unmesh at the inlet, the cavity grows, and fluid fills the spaces between the teeth. That slug of fluid rides around the outside of each gear, against the housing wall. At the outlet the teeth mesh again, the cavity shrinks, and the fluid is pushed out.

Flow is set by gear size and shaft speed. Pressure is not self-generated. It rises only as far as the system load: pipe friction, a filter, a filled tank, or a closed valve, up to the tested 100 PSI maximum on every NAPCO PA pump. That is why flow stays near-constant as viscosity or downstream resistance changes, unlike a centrifugal pump.

External vs internal gear designs

NAPCO builds external gear pumps. Two gears mesh outside each other on parallel shafts. The housing wraps both gears. The design has few rotating parts, takes a standard repair kit, self-primes, and runs in either direction.

Internal gear pumps put a smaller gear inside a ring gear, often with a crescent divider. They can run smoother, but they are more complex to service. NAPCO does not build internal-gear units. For plant transfer of fuels, chemicals, adhesives, and emulsion, the external design is the one we stock and support.

Displacement, flow, and pressure

Displacement is the volume moved per revolution. Larger gears and a wider face move more fluid each turn. NAPCO publishes flow at the rated shaft speed, not a cubic-centimeter figure: the PA300 line reaches 158 GPM at 280 RPM; the PA200 line reaches 69 GPM at 190 RPM. Theoretical flow follows GPM = (cc/rev × RPM) ÷ 3,785. Flow scales with shaft speed up to the rated maximum (280 RPM on the 3-inch, 190 RPM on the 2-inch). Size from the published curves on how to size a rotary gear pump.

The pump makes flow. The system makes pressure. Open discharge, pressure stays low. A restriction, a tall lift on the outlet, or a thick fluid raises discharge pressure toward the 100 PSI tested maximum. On the 3-inch curve at 280 RPM on water, 100 PSI gives 140 GPM and 13.4 brake horsepower. A relief path is required; a blocked discharge will climb until something yields.

As gears, bearings, and seals wear, more fluid slips back from outlet to inlet and delivered flow falls below the published curve. That is the usual rebuild trigger: match the PK kit to the pump and replace gears, bearings, seals, and gaskets on site.

Self-priming and reversible flow

The same expanding cavities that draw liquid also pull air out of the inlet. Once the line fills, the pump moves fluid. A centrifugal impeller cannot trap and expel air that way, so it needs a flooded or pre-filled suction. NAPCO PA pumps are self-priming.

Reverse the drive and flow reverses. No re-plumbing. That covers load and unload, line clearing, and tank-to-tank transfer in either direction on one pump.

NAPCO 2-inch and 3-inch lineup

Four pumps, one external-gear architecture. Pick size by flow, then material by the fluid. See the full lineup.

• PA300S: 3-inch stainless, 158 GPM at 280 RPM, 100 PSI. Plant and surface transfer of emulsion explosives and corrosive chemicals.
• PA300C: 3-inch ductile iron, same flow and pressure. Fuels, adhesives, and non-corrosive plant transfer.
• PA200S: 2-inch stainless, 69 GPM at 190 RPM, 100 PSI. Smaller stainless lines and auxiliary transfer.
• PA200C: 2-inch ductile iron, same 2-inch rating. Compact fuel and adhesive duty.

Standard gears are nitrile; Viton® gears are optional. Housing and shafts are stainless on S models and ductile iron on C models. High- and low-pressure seals are Viton®. Bearings are roller. Rebuild with the matching PK kit from the kit finder: PK300S, PK300C, PK200S, or PK200C. Kits include both gear assemblies, bearings, seals, gaskets, and cover-bolt o-rings. The housing, end covers, wear plates, and assembly lubricant stay on the pump. A field rebuild is typically 2–5 hours and 30–50% cheaper than a new pump. Step-by-step: how to rebuild a rotary gear pump.

When a gear pump beats a centrifugal

Choose a rotary gear pump when the fluid is thick, the flow must stay predictable, the line must self-prime, or you need reverse transfer without extra valves. Centrifugal pumps win on thin, high-volume flooded service where viscosity is low and you can live with flow that falls as head rises. Full comparison: rotary gear pump vs centrifugal pump.

Typical NAPCO duties are plant and surface transfer: fuels, adhesives and sealants, chemicals, and emulsion explosives. Stainless for ammonium nitrate and corrosive fluids; ductile iron for non-corrosive oils and resins.

Related guides

FAQs