A Quimby screw pump is a twin-screw positive displacement pump that uses two intermeshing helical rotors — one right-hand, one left-hand — to trap fluid in sealed cavities and convey it axially from inlet to outlet. Marine engine rooms rely on it for fuel oil and lube oil transfer because the screws never touch each other or the casing. Timing gears keep the rotors phased, so the pump moves heavy oils smoothly with almost no pulsation. A typical Quimby unit handles 10 to 2,000 GPM at pressures up to 1,500 psi.
Quimby Screw Pump Interactive Calculator
Vary screw displacement, speed, volumetric efficiency, and pressure rise to see delivered flow, slip, and hydraulic power.
Equation Used
The pump's theoretical flow is screw displacement per revolution times shaft speed. Multiplying by volumetric efficiency estimates delivered flow after internal slip. Hydraulic horsepower is calculated from actual flow and pressure rise using HP = GPM x psi / 1714.
- Twin screws rotate at equal speed with timing gears preventing contact.
- Volumetric efficiency represents internal slip through clearances.
- Flow is treated as steady positive displacement flow.
- Hydraulic power excludes mechanical and motor losses.
Operating Principle of the Quimby Screw Pump
Two helical screws sit in parallel bores inside a tight casing. One has a right-hand thread, the other a left-hand thread, and external timing gears keep them turning in opposite directions at exactly the same speed. As the threads rotate they form sealed pockets between the screw flights and the casing wall. Each pocket carries a fixed volume of fluid axially down the bore — like a bucket on a conveyor — until it dumps into the discharge port. No metal-to-metal contact between the screws. That is the whole trick.
The design exists because viscous and lubricating fluids hate centrifugal pumps. A heavy fuel oil at 380 cSt will cavitate or simply slip past the impeller of a typical centrifugal. The Quimby's positive displacement action ignores viscosity — if anything, thicker fluid seals the pockets better and reduces slip flow back through the clearance gaps. Volumetric efficiency on a well-built twin screw runs 85-95% at rated conditions and falls off as fluid thins out or as wear opens up the screw-to-bore clearance beyond about 0.15 mm radial.
Get the timing gear backlash wrong, or let the screws contact, and you destroy the pump in minutes. The same goes for running it dry — the only thing lubricating the rotor bushings is the pumped fluid itself. Common failure modes are: timing gear wear from shock loading on startup against a closed discharge, screw thread galling from particulate ingestion, and end-cover seal blowout when somebody forgets the relief valve and deadheads the discharge line.
Key Components
- Driving Screw (Power Rotor): The right-hand helix screw connected to the input shaft. It receives torque from the prime mover and transmits motion to the driven screw through the timing gears, never through screw-to-screw contact. Thread profile is typically a modified Acme or asymmetric helix machined to ±0.025 mm pitch tolerance.
- Driven Screw (Idler Rotor): The left-hand helix screw running in the parallel bore. It mirrors the driver's rotation in the opposite direction so the intermeshing flights form sealed transfer cavities. Concentricity between the two bores must hold within 0.05 mm or pocket sealing degrades and slip flow spikes.
- Timing Gears: External helical gears mounted outside the wetted volume that synchronise the two rotors. They prevent thread contact and absorb torque ripple. Backlash is set to roughly 0.05-0.10 mm on rebuild — too tight and the gears bind under thermal expansion, too loose and the screws kiss under load.
- Casing and Liner: Cast iron or bronze housing with precision-bored twin chambers that form the outer wall of each fluid pocket. Liner-to-screw clearance is typically 0.075-0.150 mm radial. Open it past 0.20 mm through wear and volumetric efficiency drops below 80% on light fuels.
- Bearings and Mechanical Seals: Hydrodynamic sleeve bearings or rolling-element bearings support the rotor shafts. Single or double mechanical seals contain the pumpage. The pumped fluid itself lubricates the internal bushings — dry running for more than 30 seconds typically scores the journals.
- Relief Valve: Internal or externally mounted bypass that opens at a set pressure, usually 110-125% of rated discharge. Without it, a closed downstream valve will spike pressure to whatever the driver can deliver and blow the end cover or shear the drive shaft.
Who Uses the Quimby Screw Pump
The Quimby screw pump found its home wherever you have to move thick, lubricating, or shear-sensitive fluid at moderate-to-high pressure with minimal pulsation. Marine, oil and gas, power generation, and food processing all use it. Centrifugals struggle with viscosity above about 200 cSt and gear pumps chew up shear-sensitive product, so the twin-screw fills the gap.
- Marine Engineering: Heavy fuel oil booster pumps on MAN B&W and Wärtsilä two-stroke marine diesels, lifting HFO from settling tanks at 380 cSt up to the engine rail at 8-10 bar.
- Oil & Gas: Multiphase transfer duty on offshore wellhead platforms — Bornemann and Leistritz twin-screw units handle gas-volume fractions up to 95% in crude transfer lines.
- Hydraulic Power: Low-pulsation hydraulic supply for paper machine drive systems and large press feed circuits where pressure ripple from gear pumps would mark the product.
- Power Generation: Lube oil supply to steam turbine main bearings on GE and Siemens utility turbines, delivering ISO VG 32 oil at 25-50 GPM with the pulsation-free flow journal bearings need.
- Food and Beverage: Chocolate and molasses transfer at confectionery plants — the gentle pumping action does not shear the product, unlike a high-RPM gear pump.
- Asphalt and Bitumen: Road paving and roofing plants use Quimby-style twin-screw units to move 180 °C bitumen at viscosities of 500-1,000 cSt without cavitation.
The Formula Behind the Quimby Screw Pump
Theoretical flow rate from a twin-screw pump comes from the geometry alone — pitch, screw diameter, rotor speed — minus internal slip. At the low end of typical operation, around 300 RPM, you get smooth flow but high slip percentage on thin fluids because there is more time for fluid to leak back through the running clearances. At the nominal 1,750 RPM (4-pole motor speed) the pump hits its volumetric efficiency sweet spot. Push to 3,500 RPM on heavy oil and you risk cavitation at the suction because the fluid cannot fill the cavities fast enough. The formula tells you the displacement; experience tells you where on the curve to sit.
Variables
| Symbol | Meaning | Unit (SI) | Unit (Imperial) |
|---|---|---|---|
| Q | Actual volumetric flow rate | m³/s | GPM |
| Do | Outer diameter of the screw flight | m | in |
| Dr | Root diameter of the screw shaft | m | in |
| L | Pitch (axial advance per revolution) of the screw | m | in |
| N | Rotational speed of the driving screw | rev/s | RPM |
| ηv | Volumetric efficiency accounting for slip flow | dimensionless | dimensionless |
Worked Example: Quimby Screw Pump in a glycerine transfer pump at a biodiesel plant
Specifying a Quimby-pattern twin-screw pump to transfer crude glycerine byproduct from the wash column to the storage day-tank at a 40 million litre/year biodiesel plant outside Lethbridge, Alberta. The fluid is 85% glycerine with residual methanol and soap, viscosity around 180 cSt at the line temperature of 50 °C, target flow 60 GPM at 90 psi discharge. The selected pump has Do = 75 mm screws, Dr = 45 mm root, pitch L = 60 mm, and you need to confirm the rotor speed at nominal duty plus understand what happens at the ends of the operating range.
Given
- Do = 0.075 m
- Dr = 0.045 m
- L = 0.060 m
- ηv (at 180 cSt) = 0.92 dimensionless
- Qtarget = 60 GPM (0.00379 m³/s)
Solution
Step 1 — compute the swept cross-sectional area of one screw cavity. The pump has two screws so the effective displacement uses both, but the standard Quimby formula already bundles this into the geometry term:
Step 2 — at nominal speed, solve for the required rotor RPM that delivers 60 GPM with 92% volumetric efficiency:
That sits comfortably inside the typical 1,200-1,800 RPM band for a pump this size — close to a standard 4-pole motor on a small VFD trim. The screws are filling cleanly, slip is bounded, and bearing loading stays moderate.
Step 3 — at the low end of the operating range, drop the speed to 600 RPM (10 rev/s) for a slow tank-stripping duty:
That is a slow, quiet trickle — useful when you are draining the last few hundred litres out of the wash column without pulling vapour. Volumetric efficiency actually rises slightly here on viscous glycerine because slip flow scales with pressure, not speed.
Step 4 — at the high end, push to 3,000 RPM for an emergency transfer:
Note ηv dropped to 0.85 — at 3,000 RPM on 180 cSt fluid the suction cannot fill the cavities cleanly, NPSH margin shrinks, and you start cavitating. Above roughly 2,400 RPM on this geometry you would hear it rattle and see flow fall off the predicted line.
Result
Run the pump at 1,455 RPM nominal to deliver 60 GPM of crude glycerine. That speed feels right for a 4-pole motor with a VFD — quiet operation, predictable flow, plenty of NPSH margin. At the low end (600 RPM, 24.7 GPM) the pump strips tanks gently with no vapour ingestion; at the high end (3,000 RPM, 114 GPM theoretical) cavitation drags efficiency down to 85% and you would hear the suction rattle. If your measured flow comes in 15-20% below predicted, check three things in order: (1) screw-to-liner radial clearance — over 0.20 mm from wear and slip flow doubles, (2) suction line strainer dP — a clogged 100-mesh basket on viscous service drops NPSH below the cavitation threshold, and (3) timing gear backlash — if it has opened past 0.15 mm the screws are momentarily contacting under pressure pulses and you'll see metallic fines in the strainer.
Choosing the Quimby Screw Pump: Pros and Cons
The Quimby twin-screw is one of three common positive displacement choices for viscous service. Pick wrong and you either burn money on a pump that overperforms the duty, or you destroy a pump that cannot handle it. Compare against the external gear pump and the progressive cavity (Moyno-style) pump on the dimensions that actually decide the selection.
| Property | Quimby Twin-Screw Pump | External Gear Pump | Progressive Cavity Pump |
|---|---|---|---|
| Operating speed range | 300-3,600 RPM | 500-1,800 RPM | 100-700 RPM |
| Max discharge pressure | Up to 1,500 psi | Up to 3,500 psi | Up to 600 psi (per stage) |
| Typical flow range | 10-2,000 GPM | 1-300 GPM | 5-800 GPM |
| Viscosity sweet spot | 50-50,000 cSt | 100-10,000 cSt | 1,000-1,000,000 cSt |
| Pulsation level | Very low (<2% ripple) | High (10-15% ripple) | Low-moderate |
| Solids tolerance | Poor — under 100 µm only | Very poor — clean fluid only | Excellent — slurries OK |
| Rebuild interval (typical) | 20,000-40,000 hours | 8,000-15,000 hours | 4,000-12,000 hours (stator wear) |
| Capital cost (relative) | High (3-5×) | Low (1×) | Moderate (1.5-2×) |
Frequently Asked Questions About Quimby Screw Pump
Volumetric efficiency on a twin-screw drops fast when the fluid thins out. At higher temperature, viscosity falls — say from 180 cSt to 80 cSt — and slip flow back through the running clearances scales roughly with the inverse of viscosity. A pump that delivered 92% ηv in winter can drop to 82-85% in summer on the same fluid.
The fix is either a tighter rebuild clearance (tighten radial from 0.150 mm to 0.100 mm) or accept the seasonal flow variance and oversize the speed setpoint by 8-10%. Don't chase it with a hotter strainer or a bigger motor — those don't address the slip mechanism.
Progressive cavity wins on that duty. The Quimby's running clearances are 0.075-0.150 mm radial — anything above about 100 µm in the fluid will gall the screws and chew the liner. A PC pump's elastomer stator deflects around solids up to several mm and tolerates abrasive content far better.
The exception is if you need pulsation-free flow for downstream metering or coating — then run a Quimby with proper upstream filtration (50 µm duplex strainer) and accept the higher maintenance burden.
That is almost always timing gear backlash opening up. The gears wear gradually from torque pulsations at startup and shutdown, and once backlash exceeds about 0.12-0.15 mm the screws begin to kiss momentarily during pressure spikes. The click you hear is the screw flights touching at the intermesh.
Pull the timing gear cover and measure backlash with a dial indicator on the gear tooth. If it's past spec, replace both gears as a matched set — never just one. Catching this early saves the screws; ignoring it for another 6 months destroys them.
That is a 15% shortfall — too big to be calibration drift. The most likely cause on a twin-screw is internal recirculation past worn screw-to-liner clearance. When radial clearance opens from 0.10 mm to 0.20 mm through wear, slip flow roughly doubles at the same dP.
Quick diagnostic: increase discharge pressure by 20% with the throttle valve and watch the flow drop. If flow falls more than about 5% with that pressure bump, slip is your problem and the pump needs a rebuild. If flow holds steady, look upstream — a partially closed suction valve or air ingestion at a pipe joint.
Mechanically yes, the pump is symmetric and the screws will convey fluid the other direction. The problem is the relief valve — it is a check-style bypass that only opens one way. Run the pump backwards against a closed suction (now acting as discharge) and you have no overpressure protection. Pressure spikes to whatever the motor delivers and you blow a seal or shear the shaft.
If reverse operation is a real requirement, spec a pump with an external relief valve on a tee that protects both ports, or install crossover piping with check valves so fluid always flows the same way through the pump regardless of source and destination.
Twin-screw pumps need extra suction margin because the cavities fill very quickly — each screw rotation creates several pocket-fill events per second, and at 1,800 RPM with a 4-start screw you have around 120 fill events per second. Each one needs the fluid column to accelerate into the cavity within milliseconds.
On viscous fluids, the inertia and friction in the suction line create instantaneous pressure dips far below the steady-state NPSH-A you calculated. The 50% margin covers those transient dips. Skimp on it and the pump cavitates intermittently — you'll see flow oscillation of 5-10% even though the gauge pressures look fine.
References & Further Reading
- Wikipedia contributors. Screw pump. Wikipedia
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