A Repsold Rotary Pump is a positive-displacement twin-rotor pump that moves fluid by trapping it between two intermeshing curved rotors and the pump casing, then carrying it from inlet to outlet without valves. Johann Georg Repsold, the Hamburg instrument-maker and fire-brigade chief, developed the design in the early 1800s for fire-engine service. Each rotor revolution displaces a fixed swept volume, so flow rate scales linearly with shaft speed and stays nearly pulse-free. That made it the workhorse for fire pumps, oil transfer, and food-grade fluid handling where smooth, metered flow matters more than raw pressure.
Repsold Rotary Pump Interactive Calculator
Vary pressure and rotor-tip clearance to estimate volumetric efficiency loss and see the twin-rotor pump motion.
Equation Used
This calculator estimates Repsold rotary pump volumetric efficiency from the article's clearance example. It uses the stated cubic clearance-slip trend and is calibrated so a 0.08 mm radial gap at 5 bar gives 95% efficiency, while a 0.20 mm worn gap at the same pressure gives 75% efficiency.
- Slip loss scales with pressure differential.
- Clearance-driven slip follows the cube of radial clearance.
- Calibrated to the article comparison: 0.08 mm gives 95% and 0.20 mm gives 75% at 5 bar.
- Fluid viscosity, rotor speed, and pump geometry are treated as unchanged.
Inside the Repsold Rotary Pump
Two shaped rotors — usually two-lobe or three-lobe profiles — turn in opposite directions inside a close-fitting casing. As each lobe sweeps past the inlet port it scoops fluid into the cavity formed between the lobe flank and the casing wall. The lobes never touch each other; external timing gears on the back of the pump hold them in phase, typically within a backlash window of 0.05 to 0.10 mm. That non-contact running is the whole reason this pump can handle thin oils, milk, blood, and abrasive slurries without chewing itself apart. The trapped pocket of fluid travels around the casing and gets squeezed out the discharge port as the next lobe closes the cavity behind it.
Because it's a positive-displacement pump, flow per revolution is fixed by geometry — the swept volume between the rotor profile and the casing bore. Double the RPM and you double the flow. The pump doesn't care much about discharge pressure either, until you start losing fluid back through the running clearances. That backflow is called slip, and it scales with pressure differential and the cube of the radial clearance between rotor tip and casing. A new pump might run with 0.08 mm radial clearance and show 95% volumetric efficiency at 5 bar; let that clearance open up to 0.20 mm through wear and efficiency falls to 75% at the same pressure.
Get the timing wrong and you'll know it fast. If the timing gears slip a tooth, the lobes collide — you'll hear a hard metallic knock and the shaft will lock or shear. If the casing bore wears oval, you'll see flow drop on the worn axis and the pump will run hotter because slip generates heat. Cavitation shows up as a gravelly rattle and pitting on the rotor leading edges, usually because suction lift exceeded NPSH or the inlet strainer choked. The fix order is always the same: check timing first, clearances second, suction conditions third.
Key Components
- Driving rotor: The shaft-driven rotor coupled to the prime mover. Carries the timing-gear mounting at the rear and is typically machined from cast iron, bronze, or 316 stainless depending on fluid service. Lobe profile tolerance is held to ±0.02 mm on the flank to keep slip within spec.
- Driven rotor: The second rotor, kept in phase with the driver by the external timing gears. Never touches the driving rotor — running clearance between the two lobes is typically 0.10 to 0.15 mm. Touch-contact means timing failure.
- Pump casing: The figure-8 bore that houses both rotors. Radial clearance between rotor tip and casing wall is the single biggest factor in volumetric efficiency. Hold it to 0.05 to 0.10 mm on a new build for fluids above 50 cSt; tighter on water-thin fluids.
- Timing gears: Mounted outside the wet end on the back of each rotor shaft. They keep the lobes phased so they pass without collision. Backlash of 0.05 to 0.10 mm is normal; anything over 0.20 mm and the rotors will start clipping each other under load reversal.
- Shaft seals: Mechanical seals or packed glands at each shaft penetration. On a fire-service Repsold pump the original design used leather packing; modern food-service variants run double mechanical seals with a flush barrier fluid at 1 bar above discharge pressure.
- Inlet and discharge ports: Cast openings on opposite sides of the figure-8 bore. No valves — the rotors themselves do all the porting. Port area is sized so fluid velocity stays under 3 m/s on suction to avoid cavitation.
Real-World Applications of the Repsold Rotary Pump
The Repsold layout shows up wherever you need smooth metered flow of a viscous or shear-sensitive fluid at modest pressure, and where valveless self-priming operation is a hard requirement. It's not a high-pressure pump — typical service is 5 to 15 bar — but it'll happily move fluids from 1 cSt to 100,000 cSt and pass small solids without complaint. That's why you see this geometry on fire engines, dairy lines, breweries, biodiesel plants, and printing-ink transfer skids.
- Fire service: The original use case — Repsold's hand-and-steam-driven fire pumps for the Hamburg fire brigade in the 1820s, capable of throwing water 20 m vertically at roughly 200 L/min from a four-man crew.
- Dairy processing: Alfa Laval SRU and Waukesha Universal lobe pumps on milk, cream, and yoghurt transfer lines at facilities like the Fonterra plant in Hamilton, New Zealand — running 8 to 12 bar with stainless rotors and EPDM seals.
- Brewing: Wort and yeast transfer at craft breweries such as Sierra Nevada in Chico, California, where the rotary lobe pump's gentle action protects yeast cell viability between fermenter and bright tank.
- Biodiesel and oleochemicals: Glycerine and methyl-ester transfer at plants like ADM's biodiesel facility in Velva, North Dakota, handling fluids up to 200 cSt at 60 °C.
- Printing and ink: Ink supply pumps on Heidelberg Speedmaster offset presses, metering thixotropic inks at 50 to 200 mL/rev with no pulsation that would show up as banding on the printed sheet.
- Pharmaceutical: Sanitary lobe pumps on cosmetic creams, ointments, and shampoo bases at Unilever production lines, where the gentle low-shear action keeps emulsions from breaking.
The Formula Behind the Repsold Rotary Pump
What you actually need from a Repsold pump is the delivered flow rate, which is the swept volume per revolution times shaft speed minus slip losses through the running clearances. At the low end of typical operation — say 50 RPM on a slow-roll metering job — slip is barely visible because the fluid carries forward faster than it leaks back. At nominal speed, around 300 to 600 RPM for most industrial sizes, slip is small and predictable. Push to the high end, 1000+ RPM, and you start fighting cavitation and viscous heating, not slip. The sweet spot sits where slip is under 5% of swept volume and inlet velocity stays under 3 m/s.
Variables
| Symbol | Meaning | Unit (SI) | Unit (Imperial) |
|---|---|---|---|
| Qdelivered | Actual volumetric flow rate at the discharge port | m³/s | GPM |
| Vswept | Swept volume per shaft revolution (geometry of the rotor profile) | m³/rev | in³/rev |
| N | Shaft rotational speed | rev/s | RPM |
| Qslip | Backflow through rotor-to-casing and rotor-to-rotor clearances | m³/s | GPM |
| ηv | Volumetric efficiency = Qdelivered / (Vswept × N) | dimensionless | dimensionless |
Worked Example: Repsold Rotary Pump in a chocolate truffle filling line
Specifying a sanitary Repsold-pattern rotary lobe pump to transfer tempered chocolate ganache from a jacketed holding kettle to a depositor head on a confectionery filling line at a Lindt production facility outside Olten, Switzerland. The ganache runs at 32 °C with a measured viscosity of 4500 cSt and the depositor needs 0.85 L/s steady flow with no pulsation that would deform the truffle shells. The pump has a swept volume of 1.7 × 10⁻³ m³/rev and the discharge backpressure is 6 bar.
Given
- Vswept = 1.7 × 10⁻³ m³/rev
- Qrequired = 0.85 L/s
- Δp = 6 bar
- μ = 4500 cSt
- Qslip (estimated at 6 bar, new clearances) = 0.04 × 10⁻³ m³/s
Solution
Step 1 — convert required flow to SI and solve for nominal shaft speed at the design point. Required flow is 0.85 L/s = 0.85 × 10⁻³ m³/s. Rearranging the formula:
Step 2 — at the low end of the practical operating range, run the pump at 15 RPM (0.25 rev/s) for slow-fill or product-changeover service:
That's roughly 23 L/min — slow enough to charge the depositor without splashing and gentle enough to handle a stiffer ganache at the start of a batch when the kettle hasn't fully thermally equilibrated.
Step 3 — at the high end, push to 80 RPM (1.33 rev/s) for clean-out-of-place water flush after the production run:
Slip rises sharply at 80 RPM because the differential pressure during CIP flush at 8 bar opens the leak path more aggressively, and on water-thin fluid (1 cSt) the running clearance leaks far more than it does on 4500 cSt ganache. You'll feel the pump getting noisy and warm above 100 RPM on water — that's the cue to back off, not push further.
Result
At nominal 31. 4 RPM the pump delivers the required 0.85 L/s of ganache to the depositor with about 4.5% slip, which sits comfortably inside the design sweet spot. At 15 RPM (low end) you get 0.39 L/s — half the nominal flow, useful for slow charge or thicker product. At 80 RPM (CIP flush) you reach 2.1 L/s on water but slip jumps roughly 4× because thin fluid leaks faster through the same clearances. If you measure 0.70 L/s instead of the predicted 0.85 L/s at 31.4 RPM, check three things in order: rotor-tip-to-casing radial clearance (worn past 0.15 mm will drop volumetric efficiency to 80%), shaft-seal flush pressure (a starved double mechanical seal pulls in air and shows up as flow drop with a chuffing sound), and inlet line vacuum (anything below −0.4 bar gauge means you're cavitating and the cavitation pockets eat displacement directly).
Choosing the Repsold Rotary Pump: Pros and Cons
The Repsold-style rotary lobe pump occupies a specific corner of the pump landscape: smooth flow, gentle handling, modest pressure, and tolerance for viscous or shear-sensitive fluids. It's not the right choice for high-pressure hydraulic power or for clean low-viscosity water duty where a centrifugal pump would be cheaper and simpler. Here's how it stacks up against the two most common alternatives engineers cross-shop it against.
| Property | Repsold Rotary Pump | Centrifugal Pump | Gear Pump |
|---|---|---|---|
| Typical operating speed | 50 to 1000 RPM | 1500 to 3500 RPM | 500 to 3000 RPM |
| Maximum discharge pressure | 10 to 15 bar | 8 to 12 bar (single stage) | 200 bar + |
| Volumetric efficiency at rated point | 90 to 96% new, 75 to 85% worn | Not applicable (kinetic, not displacement) | 92 to 97% new |
| Viscosity range handled | 1 cSt to 100,000 cSt | 1 cSt to 500 cSt | 10 cSt to 50,000 cSt |
| Shear damage to fluid | Very low — gentle on emulsions, yeast, blood | High — high tip speeds break emulsions | Moderate — meshing teeth shear product |
| Pulsation at discharge | Low (2 to 4% with 3-lobe rotors) | None (continuous flow) | Moderate (12 to 20%) |
| Tolerance for solids in fluid | Good — passes soft solids up to 10 mm | Poor — impeller damage | Very poor — tooth damage |
| Initial cost (sanitary 50 mm port) | $8,000 to $20,000 | $1,500 to $4,000 | $2,000 to $6,000 |
| Service life between rebuilds | 20,000 to 40,000 hours | 30,000+ hours | 10,000 to 25,000 hours |
Frequently Asked Questions About Repsold Rotary Pump
Slip scales inversely with viscosity. When you heat the fluid, viscosity drops and the running clearances leak more product back from discharge to suction. Going from 32 °C ganache at 4500 cSt to 45 °C ganache at 1800 cSt can roughly double your slip flow, which on a tight sanitary pump might cost you 5 to 10% of delivered volume.
The fix is either to bump RPM to compensate, or to spec tighter cold-build clearances if the product always runs hot. Don't tighten clearances below the manufacturer's hot-running spec or the rotors will gall on thermal expansion.
The 3-lobe profile cuts pulsation roughly in half and shortens the squeeze event at the discharge port, which matters for fragile emulsions, live yeast, or blood products. A 2-lobe pump pulses around 8 to 10% peak-to-peak; a 3-lobe pump pulses 2 to 4%.
The trade-off is swept volume per revolution. For the same casing size, a 3-lobe rotor displaces about 25% less per turn than a 2-lobe rotor, so you need higher RPM or a bigger frame to hit the same flow. On yeast transfer at a brewery I'd take the 3-lobe every time. On thick chocolate or peanut butter where pulsation doesn't matter and throughput does, go 2-lobe.
Classic symptom of opened-up running clearances. Slip flow scales with the cube of radial clearance and roughly linearly with pressure differential. At 1 bar your slip is small even with worn rotors; at 6 bar the same wear translates to a flow cliff.
Pull the cover and feeler-gauge the rotor-tip-to-casing clearance at four positions around the bore. New is 0.05 to 0.10 mm. If you're reading 0.20 mm or more, the casing or rotors are due for replacement or skim-machining. A common cause is running the pump dry during CIP changeover — a few seconds of dry running on stainless rotors will scuff the casing bore permanently.
No — wrong pump for that job. Repsold-style lobe pumps top out around 10 to 15 bar reliably. Hydraulic power circuits typically run 100 to 350 bar, which means you need a gear pump, vane pump, or piston pump with internal lubrication-grade clearances and hardened components.
Where a Repsold-style pump fits in a hydraulic context is low-pressure auxiliary duty: charge pumps, cooler-loop circulation, or hot-oil transfer between tanks. For the main pressure circuit, spec a Bosch Rexroth A10VSO or Parker P1 piston pump and don't try to make the lobe pump do a job it wasn't designed for.
You're hearing flow-induced pressure pulsation hitting a downstream restriction or a trapped gas pocket. Each lobe passing the discharge port produces a small flow pulse — on a 2-lobe rotor at 300 RPM that's 600 pulses per minute, or 10 Hz. If a downstream check valve, elbow, or trapped air bubble resonates at that frequency, you get audible knocking.
Closing the discharge valve raises line pressure, which collapses any trapped gas pocket and shifts the resonance. The fix is usually a small pulsation dampener (a 1 to 2 L gas-charged accumulator) on the discharge line, or rerouting to eliminate the high-point air trap.
Keep suction velocity under 1 m/s for fluids above 1000 cSt, and under 3 m/s for water-thin fluids. Pressure drop in the suction line scales with viscosity and the square of velocity, so on thick product an undersized suction line will pull your inlet vacuum past −0.5 bar and the pump will starve.
Rule of thumb: suction line should be one pipe size larger than the pump inlet flange for fluids over 1000 cSt, and the line should be as short and straight as the layout allows. If you're pulling from a low tank with a long suction run, run the calculation properly — don't trust pump capacity charts that assume water service.
The two rotors run with 0.10 to 0.15 mm clearance between their lobes — they don't touch under normal load. The timing gears are what holds that clearance. Any backlash in the gears translates directly to angular slop between the rotors.
At 0.05 mm gear backlash the rotors stay clear. At 0.20 mm backlash, under a load reversal (think pump priming, or a discharge pressure spike), the driven rotor briefly lags, the lobes clip, and you get a hard metallic tick that escalates to galling and shaft failure. Check timing gear backlash every rebuild — it's a 5-minute dial-indicator job and it's the single most under-checked failure path on these pumps.
References & Further Reading
- Wikipedia contributors. Rotary vane pump. Wikipedia
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