A screw pump moves fluid axially along the length of one or more intermeshing helical rotors housed inside a close-fitting bore. As the screws turn, sealed pockets of fluid form at the suction end, travel along the axis without churning or being squeezed, and are discharged at the other end. Unlike gear or lobe pumps, the flow path is nearly straight and continuous — which is why screw pumps are famously quiet, pulsation-free, and gentle on the fluid.

There are three common configurations:

• Single-screw (progressing cavity): one helical rotor turning inside an elastomer stator. Best for slurries and shear-sensitive fluids.
• Twin-screw: two intermeshing screws driven by timing gears so metal never touches metal. Handles multiphase mixtures (oil + gas + water + sand) that would destroy other pumps.
• Three-screw: one driven power rotor plus two idler rotors that seal hydraulically. The workhorse for high-pressure lubricating oil, fuel oil, and hydraulic power.

Why the industry loves them:

• Low pulsation — the axial flow means no discrete "pulses" per revolution, so downstream instruments and filters don't get hammered.
• High viscosity tolerance — efficiency actually improves with viscosity because internal slip drops. A three-screw pump on heavy fuel oil at 500 cSt can hit 85%+ volumetric efficiency.
• Self-priming and dry-run tolerant (twin-screw especially) — critical for oilfield multiphase boosting where gas slugs are routine.
• Quiet — often under 70 dB, versus 85+ for equivalent gear pumps.

Real-world example: The main lube oil pumps on large marine diesel engines and steam turbines are almost universally three-screw pumps. A container ship's main engine might use a three-screw pump delivering 200 m³/hr at 6 bar of SAE 40 oil, running continuously for 20,000+ hours between overhauls. The pulsation-free flow is what keeps journal bearings from cavitating.

Rule of thumb for sizing: Theoretical flow scales linearly with speed and with the cube of screw diameter. A quick estimate:

Q (gpm) ≈ D³ × N × K, where D is screw OD in inches, N is rpm/1000, and K ≈ 0.3–0.4 for three-screw geometry. So a 4" three-screw at 1750 rpm gives roughly 4³ × 1.75 × 0.35 ≈ 39 gpm — close enough for a first pass before catalog selection.

Watch out for: Screw pumps are not forgiving of solids that exceed the running clearance (typically 25–75 μm on metal-to-metal three-screws). One tramp weld bead can gall a rotor and end an overhaul cycle early. Strainers upstream are not optional.