The Cochrane Rotary Pump is a positive-displacement water pump that uses two intermeshing lobed rotors turning in opposite directions inside a close-fitting casing to move liquid without valves. Patented by John Cochrane in the mid-19th century, it carries fluid trapped between the rotor lobes and the casing wall from the inlet port to the outlet port on every revolution. Engineers used it as a marine bilge pump, mill feedwater pump, and general circulation pump because it self-primes, handles entrained air, and delivers a near-steady flow at modest pressure — typically 20 to 60 psi at 100 to 400 RPM in working installations.
Cochrane Rotary Pump Interactive Calculator
Vary displacement, speed, efficiency, and pressure to see delivered flow, slip loss, and hydraulic power in a twin-lobe rotary pump.
Equation Used
The Cochrane rotary pump is treated as a positive-displacement machine. Delivered flow equals displacement per revolution times shaft speed times volumetric efficiency. The efficiency term accounts for slip past rotor tips and casing clearances. Hydraulic power is then estimated from delivered flow and pressure rise.
- Pump behaves as a positive-displacement lobe pump.
- Volumetric efficiency represents slip from clearance, wear, and leakage.
- Hydraulic power ignores bearing, seal, gear, and drive losses.
- Liquid is water-like and incompressible.
Operating Principle of the Cochrane Rotary Pump
Two lobed rotors sit on parallel shafts, geared together externally so they never actually touch each other inside the chamber. As they spin in opposite directions, each lobe sweeps a crescent-shaped pocket of water from the inlet side around the casing wall to the outlet side. There are no valves, no springs, no flapper plates — the close clearance between rotor tip and casing (typically 0.10 to 0.25 mm on a working installation) is what seals one stroke from the next. That clearance is what defines a valveless rotary pump as a positive displacement pump rather than a centrifugal one.
If you let that tip clearance grow past about 0.4 mm, slip flow climbs fast and the volumetric efficiency drops from a healthy 90% down toward 60%. You will feel it as reduced delivery on a calibrated bucket test, and you will hear it as a hollow gulping noise when the pump tries to lift water on first prime. Run the rotors too fast and you get the opposite problem — cavitation on the suction side because water cannot fill the expanding pocket quickly enough. On a typical 150 mm rotor body, that ceiling is around 450 RPM with cold fresh water and a flooded suction.
Why was it designed without valves? Because John Cochrane was solving a specific 1850s problem — pumps on coal-fired ships and mill engines kept chewing up flap valves on grit-laden water. A valveless rotor pair had nothing to wear out except the rotor faces themselves, and those could be re-machined on a lathe. Common failure modes today are still the same: rotor tip wear from sand and scale, gear backlash letting the rotors clash, and shaft seal weeping on the drive side once the gland packing dries out.
Key Components
- Twin Lobed Rotors: Two cast iron or bronze rotors with 2 or 3 lobes each, mounted on parallel shafts. The lobes intermesh without contact thanks to external timing gears. Tip-to-casing clearance is held between 0.10 and 0.25 mm — slack that out and slip flow ruins your output.
- Casing: A figure-eight-shaped iron housing machined to match the rotor swept profile within ±0.05 mm. Inlet and outlet ports sit at the pinch points where the rotors mesh. Casing wear is the main long-term failure mode in gritty service.
- Timing Gears: External spur gears on the shaft ends keep the rotors phased 90° apart on a 2-lobe build. Backlash above 0.15 mm lets the lobes clash inside the chamber — you will hear it as a rhythmic ticking that gets worse with load.
- Shaft Seals: Traditional gland packing or modern lip seals where the shafts exit the casing. On heritage builds the gland is hand-tightened until weeping just stops; over-tighten and you score the shaft.
- Drive Pulley or Coupling: Belt pulley on Victorian installations, direct gearmotor coupling on modern restorations. Drives the larger of the two timing gears, with the second rotor following through the gear pair.
Industries That Rely on the Cochrane Rotary Pump
Cochrane Rotary Pumps showed up wherever engineers needed steady, self-priming water transfer without the maintenance headache of valves. They worked equally well in salty marine bilges and freshwater mill feed lines because the design has nothing to clog. You still find them in service in heritage installations and in modern equivalents from manufacturers like Viking Pump and Roper Industries that descend directly from this lobe-rotor lineage.
- Marine: Auxiliary bilge pump on the SS Great Britain and similar mid-19th-century iron-hulled steamers, lifting bilge water up to 4 m at roughly 120 RPM.
- Textile Mills: Feedwater supply for Lancashire boilers at mills along the River Irwell, delivering 40 to 80 GPM into the boiler drum at 60 psi.
- Heritage Restoration: Restored Cochrane unit driving the cooling-water circuit on the working beam engine at Kew Bridge Steam Museum, London.
- Agriculture: Belt-driven irrigation pump on Welsh hill farms, taking suction from a stream and lifting to a 2000 L header tank for sheep wash and stock troughs.
- Process Industry: Modern lobe-pump descendants from Viking Pump moving thin lubricating oils and process water in food and pulp plants at 200 to 400 RPM.
- Fire Service: Hand-cranked Cochrane variants used as portable fire pumps on Victorian railway estates, capable of 25 GPM at 30 psi from two trained operators.
The Formula Behind the Cochrane Rotary Pump
Volumetric output of a Cochrane-style lobe pump is the swept volume per revolution multiplied by speed, minus slip. The reason this formula matters is that the practitioner gets to pick where on the curve to run. At the low end of the typical range — 50 to 100 RPM — you get near-perfect volumetric efficiency but modest flow. At the high end — 350 to 450 RPM on a 150 mm rotor — you push more water but cavitation and slip eat into your gains. The sweet spot for most heritage installations sits around 150 to 250 RPM where you get strong delivery and the rotors are not yet straining the suction.
Variables
| Symbol | Meaning | Unit (SI) | Unit (Imperial) |
|---|---|---|---|
| Q | Delivered flow rate | m³/s | GPM |
| Vd | Displacement volume per revolution (sum of both rotor pockets) | m³/rev | in³/rev |
| N | Rotor speed | rev/s | RPM |
| ηv | Volumetric efficiency (accounts for slip past tip clearance) | dimensionless | dimensionless |
Worked Example: Cochrane Rotary Pump in a restored Cochrane pump on a heritage tannery effluent line
You are sizing a restored Cochrane Rotary Pump to lift dilute lime-bate effluent from a 3 m sump up to a settling tank in a working heritage tannery on the River Eden in Cumbria. The rotor pair displaces 0.45 L per revolution. You have a vintage 3 hp line-shaft drive available giving rotor speeds adjustable between 80 and 380 RPM via stepped pulleys. You want to know what flow rate you can actually expect across that range, and where to set the belt for steady duty.
Given
- Vd = 0.45 L/rev (0.00045 m³/rev)
- Nlow = 80 RPM
- Nnom = 200 RPM
- Nhigh = 380 RPM
- ηv (good condition) = 0.90 dimensionless
Solution
Step 1 — convert nominal speed to revs per second:
Step 2 — compute nominal flow at 90% volumetric efficiency:
That is the design point — 21 GPM at 200 RPM is enough to clear a 3 m³ sump in roughly 4 minutes, well within a reasonable shift cadence for the tannery.
Step 3 — check the low end of the operating range at 80 RPM:
At 80 RPM the rotors are barely turning — you can count the lobes by eye — and slip is minimal so ηv creeps up to about 0.92. 8.7 GPM is fine for trickle duty but the sump will take 11 minutes to clear, which is too slow if effluent is arriving faster than that.
Step 4 — check the high end at 380 RPM, where slip rises and ηv drops to about 0.78 because suction starvation begins:
The pump theoretically delivers more, but you will hear it — cavitation chatter on the suction port and a noticeable drop in delivered head. Pulleys above about 320 RPM are not worth the wear on a 150-year-old casing.
Result
Set the belt for the 200 RPM step and you get a steady 21. 4 GPM with the rotors running quiet and the volumetric efficiency at 90%. At 80 RPM the pump trickles at 8.7 GPM — useful only if effluent inflow is genuinely light. At 380 RPM the theoretical 35.2 GPM is real but you pay for it with cavitation noise and accelerated rotor-tip erosion, so the sweet spot sits firmly in the 180 to 240 RPM band. If you measure 15 GPM instead of the predicted 21, the most likely causes are: (1) tip clearance opened beyond 0.4 mm from sand wear letting slip flow climb, (2) gland packing leaking on the suction shaft so the pump cannot hold prime, or (3) timing-gear backlash above 0.15 mm letting the rotors lose phase and lose a portion of each pocket back through the mesh.
Cochrane Rotary Pump vs Alternatives
Choosing a Cochrane-style lobe pump versus its modern alternatives comes down to what you value: serviceability and self-priming behaviour, or peak efficiency and pressure capability. Here is how it stacks up against the two pumps a practitioner would realistically compare it against on a water-transfer duty.
| Property | Cochrane Rotary Pump | Centrifugal Pump | Gear Pump |
|---|---|---|---|
| Typical operating speed | 80–400 RPM | 1450–3500 RPM | 500–1750 RPM |
| Pressure capability | 20–60 psi | 20–150 psi (multi-stage higher) | 100–3000 psi |
| Self-priming | Yes — handles air slugs | No — needs flooded suction or foot valve | Yes |
| Volumetric efficiency | 75–92% depending on tip clearance | N/A (kinetic, varies with head) | 85–95% |
| Solids tolerance | Tolerates fine grit, no hard solids | Open-impeller variants handle solids | Poor — gear teeth chip |
| Maintenance interval | Re-machine rotors every 5–10 years in clean service | Impeller and seal every 3–7 years | Gear set every 2–5 years on thin fluids |
| Typical installed cost | High (heritage rebuild) or moderate (modern lobe equivalent) | Low to moderate | Moderate to high |
| Best application fit | Steady low-pressure transfer, dirty water, heritage drives | High-flow low-viscosity bulk water | High-pressure oils and viscous fluids |
Frequently Asked Questions About Cochrane Rotary Pump
That is slip multiplying with discharge pressure. On an open bucket test the pump sees zero back-pressure, so even sloppy tip clearance still passes most of the swept volume. Once you pipe it to a real head, the pressure drives water back through the tip-to-casing gap on the discharge side, and slip flow scales roughly linearly with that pressure differential.
Quick diagnostic: cap the discharge with a pressure gauge and a throttling valve. Crack the valve to simulate increasing head and watch the gauge versus the delivered flow. If flow falls off a cliff above 30 psi on a pump rated for 60 psi, your tip clearance is gone and the rotors need re-machining or replacement.
A 2-lobe rotor gives you larger displacement per revolution and better solids tolerance because each pocket is bigger, but the flow pulses are noticeable — you will feel the discharge line tick at twice rotor speed. A 3-lobe rotor smooths the pulsation by 50% but cuts the per-revolution displacement by about 25% for the same casing size, and the smaller pockets clog faster on gritty water.
Rule of thumb: pick 2-lobe for tannery effluent, mill feed, and any water that has visible particulate. Pick 3-lobe for clean process water and food-grade duty where pulsation upsets downstream instruments.
Thermal expansion is opening the tip clearance. Cast iron expands roughly 11 µm per metre per °C — a 150 mm rotor warming from 10°C to 40°C grows about 50 µm in radius. If the casing was machined for tight cold clearance and the rotors heat faster than the casing (which they do, sitting in the hot fluid), you can actually get the opposite problem: the rotors briefly bind and then the gland weeps, which lets air in.
Check the gland packing first. If it is wet and blowing bubbles when you wipe it dry, that is your air leak. Re-pack with graphite-impregnated rope and tighten just until weeping stops with the pump running warm.
Yes — and volumetric efficiency actually improves because slip flow scales inversely with viscosity. A pump that runs at 88% efficiency on water often hits 95%+ on a 100 cSt oil. Two warnings though.
First, the drive torque rises sharply with viscosity. A 3 hp drive that comfortably ran cold water may stall on cold glycerine at startup. Second, the gland packing chosen for water will not seal oil well — you need oil-rated packing or a mechanical seal upgrade. The lobe geometry itself is happy on either fluid.
That is timing-gear backlash letting the rotor lobes contact each other inside the casing. The external gears are supposed to keep the rotors phased so they intermesh without touching — typical backlash spec is below 0.15 mm at the gear pitch line. Once that opens up from gear-tooth wear, the rotors clash on every revolution, and load makes it worse because torque reversal takes up the slack with a snap.
Pull the gear cover and rock one shaft against the other with the rotors at the mesh point. If you can feel more than a hair's movement at the gear, replace the gear pair before the rotors chip.
Honest answer: for a clean off-grid job with a flooded suction, a small centrifugal will out-cost it 3 to 1 and run more efficiently. The Cochrane wins in three specific situations — when you need self-priming from a dry suction lift over 3 m, when the water carries grit that would chew up a centrifugal impeller, or when you want a slow direct drive from a wind or water wheel running at 100–200 RPM where a centrifugal would never reach its design point.
If none of those three apply, use a centrifugal. If even one applies, the lobe pump earns its keep.
References & Further Reading
- Wikipedia contributors. Rotary vane pump. Wikipedia
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