What Is a Magnetic Gear Pump? Gear Pump + Magnetic Coupling Explained
In this guide
- Quick Answer
- What Is a Magnetic Gear Pump?
- How a Gear Pump Works (the gear half)
- How the Magnetic Coupling Replaces the Shaft Seal (the coupling half)
- How the Two Technologies Combine: The Power Path
- Magnetic Gear Pump vs. Sealed Gear Pump
- Gear Type Selection: External vs. Internal vs. Lobe
- Magnetic Gear Pump vs. Mag-Drive Centrifugal Pump
- Engineering Data: Efficiency, Temperature, Viscosity, Torque
- Best Applications
- How to Select a Magnetic Gear Pump (8 Steps)
- Common Engineering Mistakes
- Troubleshooting: Problem → Cause → Solution
- FAQ
- Why Choose GreenSky?
- References
Quick Answer
A magnetic gear pump is the combination of two well-known technologies: a positive-displacement gear pump (two meshing gears that trap and carry a fixed volume per revolution) and a sealless magnetic coupling (an outer magnet on the motor shaft that drags an inner magnet on the gear shaft through a stationary containment shell). The gear set does the pumping; the coupling transmits the torque with no rotating shaft seal, so there is no leak path.
The result is a leak-free pump that also meters precisely — because gear-pump flow is directly proportional to speed (Q = D × N × ηv) — which is why it dominates chemical dosing, pharmaceutical metering, fuel transfer and semiconductor fluids where zero leakage and tight flow control both matter.
What Is a Magnetic Gear Pump?
A magnetic gear pump is not a new pump type; it is two mature technologies bolted together at the torque path. Understanding that framing is the key to specifying one correctly.
- Technology A — the gear pump. A positive-displacement machine that moves fluid by trapping it in the pockets between gear teeth and the housing and carrying it from inlet to outlet. It delivers a fixed volume per revolution and flow tracks speed almost independently of pressure.
- Technology B — the magnetic coupling. A sealless torque transmitter that replaces the mechanical shaft seal. An outer permanent-magnet ring on the motor shaft and an inner ring on the gear shaft are separated by a thin, stationary, pressure-bearing containment shell. The motor’s rotating field crosses the shell and drags the inner magnet — and the gears — in lockstep, with no physical contact across the pressure boundary.
Put them together and you get the defining feature: the only seal between the process fluid and the atmosphere is a static weld or O-ring at the shell, not a dynamic rotating shaft seal. As with the related magnetic drive (centrifugal) pump, this is what makes the pump sealless. Heavy-duty sealless pumps are governed by API 685 (3rd ed., 2022); the gear-pump side itself is covered by API 676 (4th ed., 2022) for rotary positive-displacement pumps.
How a Gear Pump Works (the gear half)
The gear side is identical whether the pump is sealed or magnetic. Two gears mesh inside a tightly toleranced housing:
- Suction. As teeth disengage on the inlet side, the expanding cavity drops local pressure and draws fluid in.
- Transport. Fluid is trapped in the pockets between the teeth and the bore and carried bodily around the periphery — half on each gear, in opposite directions.
- Discharge. At the outlet the teeth re-mesh, collapsing the cavity and forcing the trapped fluid into the discharge line. The line of mesh contact seals outlet from inlet; whatever leaks past it is slip.
The governing flow equation is the heart of a gear pump:
Qtheo = D × N and Qactual = D × N × ηv
where D is displacement per revolution (ml/rev or cc/rev), N is shaft speed (RPM) and ηv is volumetric efficiency (typically 80–93% for gear pumps). Because each turn moves a fixed geometric volume, flow is nearly proportional to speed and almost independent of discharge pressure — the trait that makes gear pumps superb meters. The hydraulic shaft torque follows Thyd = Δp × Vd / (2π × ηm) (Δp in Pa, Vd in m³/rad). Volumetric efficiency falls as pressure rises (slip ∝ Δp) and as clearance grows with wear — a worn gear set can leak roughly eight times as much once clearances double.
How the Magnetic Coupling Replaces the Shaft Seal (the coupling half)
In a conventional gear pump the motor shaft must pass through the housing to reach the drive gear, which demands a mechanical seal or gland packing — the perpetual leak and maintenance point. The magnetic coupling removes that penetration entirely.
- Outer magnet assembly — mounted on the motor shaft, usually rare-earth segments (NdFeB or SmCo) in alternating poles.
- Containment shell (can / isolation sleeve) — a thin, non-magnetic, pressure-bearing cup that separates the wet inner assembly from the dry motor. This single part defines the pump.
- Inner magnet assembly — mounted on the gear shaft inside the shell, magnetically locked to the outer assembly.
In a synchronous permanent-magnet-to-permanent-magnet coupling the two rings rotate at identical speed with zero slip — like two bar magnets, one spinning in your hand pulling the other around a pane of glass. The coupling torque is not constant: it rises with pole count and scales roughly with the inverse square of the air gap (T ∝ 1/g²), so shell thickness and magnet-to-magnet spacing are the dominant design variables. Below the coupling’s pull-out torque the magnets stay locked; above it they slip and decouple, an inherent overload protector that also stops the pump if the gears jam.
How the Two Technologies Combine: The Power Path
This is the part most “pump” articles skip — the actual integration. The combined power path has five stages, and the design rules of each technology must be honoured at every stage:
Stage 1 — Motor torque
An electric motor — most often a three-phase induction or a permanent-magnet synchronous (PMSM) machine under a VFD — converts electrical energy into shaft torque T ≈ kt·I. In variable-speed metering duty the drive holds current in phase with back-EMF using field-oriented control to maximise torque per ampere and give a clean RPM-to-flow relationship.
Stage 2 — Magnetic coupling across the shell
The outer magnet spins with the motor; its rotating field penetrates the non-magnetic shell and drags the inner magnet in lockstep. There is no shaft, no bearing, no seal crossing the pressure boundary. The torque that arrives at the gear shaft equals the motor torque up to the coupling’s pull-out limit, minus a small eddy-current loss (near-zero for non-metallic shells, 5–15% for conductive metallic shells).
Stage 3 — Gear set converts torque to flow
The inner magnet is fixed to the drive gear, so gear speed equals motor speed (1:1 in a direct-coupled design). The meshing gears convert that rotation into the fixed displacement D, and Q = D × N × ηv delivers the metered flow. A fraction of discharge is internally recirculated to cool the magnets and lubricate the product-lubricated bearings — this minimum continuous flow must never be allowed to drop.
Stage 4 — Synchronous lock and decoupling
Below pull-out torque the magnets stay locked and the pump behaves exactly like a geared motor. If load torque suddenly exceeds pull-out (blockage, over-pressure, cold high-viscosity fluid), the poles slip, the coupling decouples, the gears stop while the motor keeps spinning, and the shell heats rapidly until protection trips. After a decouple event the pump must be stopped and restarted to re-synchronise — it will not self-re-engage.
Stage 5 — Closed-loop metering
Because flow is proportional to RPM and the magnetic coupling adds no slip, the pump becomes a linear flow actuator: command the VFD a frequency, get a predictable flow. That is the whole reason to combine these two technologies rather than use a sealed gear pump with a flow-control valve.
| Design rule from Technology A (gear pump) | Design rule from Technology B (magnetic coupling) | Combined consequence |
|---|---|---|
| Fixed volume per rev → flow ∝ speed | No slip in synchronous coupling | Linear, valve-free metering |
| Load torque rises with Δp and viscosity | Pull-out torque is finite (T ∝ 1/g²) | Coupling must be sized to worst-case gear torque × 1.2–1.5 |
| Bearings lubricated by fluid | Inner assembly sits inside shell, wetted | Absolutely no dry-run, no solids |
| Housing is pressure boundary | Shell is the only seal (static) | Leak path eliminated; shell = critical part |
| Inexpensive to build in many materials | Magnets/can add cost | Premium price, but lower lifecycle cost for hazardous fluids |
Magnetic Gear Pump vs. Sealed Gear Pump
| Attribute | Magnetic gear pump (sealless) | Traditional sealed gear pump |
|---|---|---|
| Shaft seal | None — magnetic coupling | Mechanical seal or gland packing (wear/leak point) |
| Leakage to atmosphere | Effectively zero (static shell only) | Possible at seal over time |
| Maintenance | Seal never replaced; bearings are the wear item | Seal replacement on schedule |
| Dry-run tolerance | None (bearings fluid-lubricated) | Poor, but slightly more forgiving |
| Solids / slurry | Not suitable | Better with flush plans |
| High viscosity | OK, but raises coupling load & heating | OK, standard strength |
| Best for | Toxic, flammable, corrosive, high-purity, metering | General oils, non-hazardous, high-pressure, abrasive |
| Capital cost | Higher | Lower |
Gear Type Selection: External vs. Internal vs. Lobe
The gear half itself comes in three geometries, and the choice is independent of the magnetic drive — but it sets the displacement, pulsation and pressure envelope.
| Gear type | Principle | Vol. efficiency | Pulsation | Typical pressure | Best use |
|---|---|---|---|---|---|
| External gear | Two identical meshing gears | 80–91% | Moderate | Up to ~250 bar | Compact, low-cost, wide chemical use, micro metering |
| Internal gear | Gear inside a ring gear with a crescent | 85–93% | Low (quiet) | Up to ~175 bar | Viscous fluids, sanitary, low-pulsation transfer |
| Lobe / circumferential piston | Non-contact lobes, timing gears | 82–92% | Very low | Up to ~20 bar | Sanitary, shear-sensitive, food & pharma |
For magnetic drive, the external gear is by far the most common — its simple, short shaft is easy to couple to an inner magnet and it scales from micro-dosing (0.1 ml/rev) to industrial flows. Internal and lobe types are chosen when pulsation or sanitary cleaning dominate.
Magnetic Gear Pump vs. Mag-Drive Centrifugal Pump
Both are sealless via magnetic coupling, but the pumping half is fundamentally different. This matters when an engineer assumes “mag-drive = centrifugal.”
| Attribute | Magnetic gear pump (PD) | Mag-drive centrifugal pump |
|---|---|---|
| Pumping principle | Positive displacement (fixed volume/rev) | Dynamic (impeller, head from velocity) |
| Flow vs. pressure | Flow ~ constant with pressure; self-regulates volume | Flow falls as pressure rises along the curve |
| Metering capability | Excellent (Q ∝ RPM) | Poor without external control valve |
| Pressure generation | High differential pressure at low flow | High flow at low differential |
| Viscosity handling | Good to very high (with right gears) | Poor — efficiency drops sharply |
| Best applications | Dosing, metering, injection, transfer of viscous fluids | High-flow circulation, cooling, bulk transfer of thin fluids |
Engineering Data: Efficiency, Temperature, Viscosity, Torque
| Parameter | Typical range / value | Notes & standard |
|---|---|---|
| Magnetic coupling transmission efficiency | 95–98% | Non-metallic shell ≈0 eddy loss; metallic shell 5–15% loss |
| Gear pump volumetric efficiency ηv | 80–93% | External 80–91%, internal 85–93% (API 676 class) |
| Combined overall efficiency | ~78–91% (coupling × gear × motor) | Recover most loss with IE4/IE5 motor + VFD |
| Metering / dosing accuracy | ±0.5% to ±1% | Repeatable, linear with RPM |
| Magnet temperature limit — NdFeB | ~80–200 °C by grade | Irreversible demagnetisation above limit |
| Magnet temperature limit — SmCo | >300 °C | Specify for hot services |
| Containment shell limit | 120–150 °C (PFA/ceramic); ~260 °C (Hastelloy/316) | Add eddy-current heating to fluid temp |
| Viscosity range | 0.3 to ~100,000 cP | Depends on gear geometry & materials; high ηv needs careful sizing |
| Coupling torque capacity | 1–75 N·m (micro) up to 1,200 N·m standard, >25,000 N·m custom | Pull-out must exceed 1.2–1.5× gear load torque |
| Speed range | 500–5,000 rpm (gear side) | FLUID-O-TECH MG series rated to 5,000 rpm |
| Pressure | Up to ~250 bar (gear), 17 bar common for mag-coupled dosing | Per gear type and shell rating |
Best Applications
Because the combination delivers both zero leakage and precise metering, magnetic gear pumps are specified wherever those two needs coincide:
- Chemical processing — dosing of acids, bases, solvents and catalysts; transfer of toxic or flammable liquids (sulfuric/nitric acid, acetone, chlorinated solvents).
- Pharmaceutical & biotech — GMP-grade metering of active ingredients, vaccines and biologics where contamination is unacceptable.
- Semiconductor — ultrapure and photoresist chemicals; particle-free, low-pulsation transfer.
- Automotive & fuel systems — fuel transfer, lubrication and coolant loops; the compact, leak-free profile suits mobile and confined packages.
- Food & beverage — sanitary lobe/internal versions for oils, syrups and flavourings with hygienic, no-contamination transfer.
- Water treatment & analytical instruments — micro-dosing of reagents, sampling and lab/analyser fluid handling.
- Oil & gas and nuclear — boron acid circulation, additive injection, hazardous-service transfer.
How to Select a Magnetic Gear Pump (8 Steps)
- Define the fluid. Chemistry (corrosion), viscosity at operating temperature, temperature, solids/abrasive content, and whether it is toxic/flammable (drives shell and magnet material choices).
- Set flow and pressure. Required Q at the worst-case Δp. Remember gear-pump flow is speed-set, so pick displacement D and RPM together.
- Pick the gear geometry. External for general/micro, internal for viscous/low-pulsation, lobe for sanitary/shear-sensitive.
- Choose shell + magnet materials. Non-metallic shell + SmCo for hot/aggressive; metallic shell + NdFeB for moderate, cost-sensitive duty.
- Size the gear load torque. From
Thyd = Δp × Vd / (2π × ηm)at max Δp and viscosity — this is the number the coupling must survive. - Size the magnetic coupling. Pull-out torque ≥ 1.2–1.5× the gear load torque; verify T ∝ 1/g² leaves adequate margin at the chosen shell thickness.
- Size the motor. Shaft power = (Δp × Q / 600) ÷ ηgear; specify the motor at ≥1.15× that, matched to real duty (continuous S1 vs intermittent S3 per IEC 60034-1), and choose an IE4/IE5 permanent-magnet motor with a VFD for flow control.
- Add protection. Flow switch or motor-current monitor (current drops the instant flow is lost), suction strainer, and — for toxic/flammable service — secondary containment with leak detection per API 685.
Common Engineering Mistakes
- Undersizing the coupling pull-out torque. Cold-start viscous fluid or a momentary pressure spike exceeds pull-out → decouple. Always apply the 1.2–1.5× margin.
- Assuming the motor RPM = flow without ηv. At high Δp, slip cuts actual flow; use
Q = D × N × ηv, notD × N. - Wrong magnet grade for temperature. NdFeB above ~150 °C risks demagnetisation; specify SmCo for hot duty.
- No dry-run protection. Bearing failure from a single dry second is the most common field failure — fit a flow switch or current monitor.
- Pumping solids or slurry. Particles lapping between inner magnet and shell destroy both; use a sealed pump with flush instead.
- Ignoring eddy-current heating on metallic shells. Add the induced heat to fluid temperature before checking the magnet limit.
- Specifying an IE2/IE3 motor in 2027+ markets. The U.S. DOE 10 CFR 431 rule makes IE4 mandatory for many mid-range motors from June 2027; an IE4/IE5 PM motor also recovers the coupling loss at system level.
Troubleshooting: Problem → Cause → Solution
| Problem | Probable cause | Solution |
|---|---|---|
| No flow / coupling decoupled | Load torque exceeded pull-out (cold viscous fluid, blockage, over-pressure) | Stop, clear blockage, warm/reduce viscosity, re-start to re-synchronise; upsize coupling margin |
| Bearing failure / seized gears | Dry run or abrasive solids | Fit flow switch + suction strainer; never run dry; switch to flushed sealed pump for slurries |
| Shell overheating | High viscosity load + eddy currents in metallic shell | Larger motor headroom, non-metallic shell, or SmCo + thicker shell; reduce Δp |
| Loss of dosing accuracy | Gear wear (clearance↑ → slip↑) or magnet partial demag | Recalibrate, replace gears/bearings; verify magnet grade vs temperature |
| Declining flow at fixed RPM | Rising pressure or falling viscosity (hot thin fluid) | Check system Δp; note slip rises with Δp, falls with viscosity |
| Excessive noise / vibration | Cavitation (NPSH margin low) or coupling near pull-out | Improve inlet head, check NPSH; reduce load or upsize coupling |
FAQ
What is a magnetic gear pump and how does it differ from a standard gear pump?
Both are positive-displacement pumps built around two meshing gears. The difference is the drive: a standard gear pump uses a shaft through the housing and needs a dynamic mechanical seal (the leak/wear point), while a magnetic gear pump replaces that seal with a magnetic coupling — outer magnet on the motor, inner magnet on the gear shaft, separated by a stationary sealed shell. The gear action is identical; only torque transmission changes, and the pump becomes sealless.
How does the magnetic coupling transmit torque without a shaft seal?
The outer magnet spins with the motor and its rotating field penetrates the non-magnetic shell, dragging the inner magnet and gears in lockstep. In a synchronous PM coupling there is zero slip, so gear speed = motor speed. The shell is a static pressure boundary, not a moving seal, so there is no leak path. Pull-out torque scales roughly with 1/g² and must exceed load torque by 1.2–1.5× or the magnets decouple.
Why is flow proportional to speed in a magnetic gear pump?
Because the gear pump is positive displacement, each revolution moves a fixed volume. Actual flow is Q = D × N × ηv and barely depends on discharge pressure — unlike a centrifugal pump. That linear flow-to-speed relationship is exactly why these pumps dominate precise dosing and metering.
Can a magnetic gear pump run dry or handle solids?
No to both. The internal bearings are lubricated and cooled by the fluid; dry running destroys them in seconds, and abrasive particles (≈20–100 µm) lap the inner magnet against the shell. They are built for clean, corrosive, toxic, flammable or high-purity liquids. Always fit a flow switch or current monitor and a suction strainer.
What temperature and viscosity limits apply?
NdFeB magnets risk demagnetisation around 80–150 °C; SmCo stays stable above 300 °C. Non-metallic shells top out near 120–150 °C, metallic near 260 °C. Viscosity can span 0.3–100,000 cP by geometry and materials, but high viscosity raises load torque and eddy-current heating — so pick SmCo + non-metallic shell for hot, viscous, aggressive fluids.
How do I size the motor and magnetic coupling?
Start from Phyd = Δp × Q / 600, add gear and bearing losses, and size the motor at ≥1.15× that. Use an IE4/IE5 PM motor with a VFD. The magnetic coupling pull-out torque must exceed 1.2–1.5× the worst-case gear load torque, and the magnet grade must survive shell temperature plus eddy-current heating. Match duty to IEC 60034-1 (S1 vs S3).
Why Choose GreenSky?
GreenSky supplies the drive behind the coupling
Magnetic gear pumps are only as good as the motor and variable-frequency drive that spin the outer magnet. GreenSky Power designs and manufactures the BLDC and PMSM motors and FOC drives that turn a sealless gear pump into a precise, efficient metering system:
- IE4 / IE5 permanent-magnet motors that meet the 2027 DOE 10 CFR 431 and EU Ecodesign efficiency mandates while recovering the magnetic-coupling loss at system level.
- Closed-loop FOC with encoder / Hall feedback for a clean, linear RPM-to-flow relationship — the foundation of accurate metering.
- Custom shaft and mounting adaptation to mate directly with the outer magnet carrier of your magnetic coupling, including co-engineered drive packages for OEM pump builders.
- Torque-matched sizing so the motor’s breakdown torque and the coupling’s pull-out torque are verified together against your worst-case Δp and viscosity.
- Duty-cycle and thermal design to IEC 60034-1 (S1/S3) and insulation classes B/F/H, with documentation for OEM integration and quality control.
Whether you build micro-dosing pumps or industrial sealless gear pumps, our engineering team can specify the motor–drive–coupling combination as one validated system. See also our magnetic drive pump guide for the centrifugal counterpart.
References
- IEC 60034-1: Rotating electrical machines — Rating and performance (duty cycles S1–S10, insulation classes, thermal limits). https://www.iec.ch/standard/60034-1
- IEC 60034-30-1: Rotating electrical machines — Efficiency classes for line-operated AC motors (IE1–IE5). https://www.iec.ch/standard/60034-30-1
- IEC 60034-30-2: Rotating electrical machines — Efficiency classes for VSD-fed motors (IE-code extension to variable-speed duty). https://www.iec.ch/standard/60034-30-2
- IEEE Xplore — Montague, R.G., Bingham, C., Atallah, K., “Servo Control of Magnetic Gears,” IEEE/ASME Trans. Mechatronics, 2012 (magnetic coupling torque, pole-slipping and overload behaviour). https://doi.org/10.1109/TMECH.2011.2104960
- IEEE Xplore — Cheng, X., Liu, W., Zhang, Y., et al., “A Concise Transmitted Torque Calculation Method for Pre-Design of Axial Permanent Magnetic Coupler,” IEEE Trans. Energy Conversion, 2020 (T ∝ 1/g² design method). https://doi.org/10.1109/TEC.2019.2959960
- NEMA MG 1-2021: Motors and Generators (North American efficiency tables, Design letters, basis of DOE compliance). https://www.nema.org/standards/view/mg-1-motors-and-generators
- U.S. DOE 10 CFR Part 431: Energy Conservation Program for Electric Motors (IE3 baseline, IE4 for mid-range from June 2027). https://www.ecfr.gov/current/title-10/subtitle-B/chapter-II/subchapter-D/part-431
- IEA — Energy Efficiency of Motor-Driven Systems (motor energy consumption and savings potential). https://www.iea.org/reports/energy-efficiency-of-motor-driven-systems
- SKF — Bearing solutions for pumps (product-lubricated andprocess pump bearing selection). https://www.skf.com/group/industries/pumps
- Siemens — SIMOTICS motors & SINAMICS drives (IE4/IE5 PM motor and VFD platforms). https://www.siemens.com/global/en/products/drives.html
Standards referenced in-text: API 676 (4th ed., 2022 — rotary positive-displacement pumps), API 685 (3rd ed., 2022 — sealless pumps), ISO 15783 / ISO 5199 (pump design), and ATEX for explosive atmospheres.
