Magnetic Drives and Magnetic Couplings

A magnetic coupling transmits torque through a wall. There is no shaft passing through the housing, so there is no packing, no mechanical seal and no leak path. The pump, the agitator or the vacuum chamber stays sealed while the motor outside drives it.

PinyiGear builds magnetic drives and assemblies to the drawing: the magnetic circuit, the hub and carrier geometry, the containment sleeve, and the assembled unit. This page sets out how the parts work, what a magnetic circuit design has to settle, and what an inquiry needs to carry before a quotation can be made.

How the torque crosses the wall

Two rotors face each other across a stationary barrier.

• The inner rotor sits on the process side, on the pump or agitator shaft. It carries the driven magnets.
• The outer rotor sits outside, driven by the motor. It carries the driving magnets.
• The containment sleeve separates them. It is a non-magnetic, pressure-tight wall — thin enough to keep the air gap small, thick enough to hold the pressure and stay stiff.

When the outer rotor turns, the magnetic field pulls the inner rotor with it. Slip is small under normal load and grows as load rises. Past the maximum torque the coupling breaks away: the outer rotor spins, the inner rotor stops. That behaviour is why magnetic drives appear in chemical and pharmaceutical duty — an overload stops the process instead of destroying the seal, because there is no seal to destroy.

What the coupling cannot do is isolate a jammed rotor while the motor keeps pushing. Breakaway torque is a design figure, and it has to sit above the working torque with margin and below whatever the driven shaft can survive.

Magnetic gears

A magnetic gear does the same job but changes speed. Instead of two rotors coupled one-to-one, pole pieces modulate the field between a high-speed rotor with few pole pairs and a low-speed rotor with many. The result is a ratio without tooth contact: no lubricant, no tooth wear, no backlash in the usual sense, and inherent overload protection.

The trade is torque density. For the same torque a magnetic gear is physically larger than a mechanical pair, which is why it appears in sealed or clean applications rather than as a general replacement for a gearbox. Where the process side cannot be lubricated or opened, that trade is often worth making.

What the magnetic circuit design has to settle

The magnets are the least interesting part of the design. What decides whether the coupling works is the circuit around them.

Air gap. Torque falls steeply as the gap grows. The gap is the sleeve thickness plus running clearance, so sleeve material and wall thickness are a torque decision, not only a pressure decision.

Magnet grade and volume. NdFeB covers the higher energy grades — N35 to N52 is the usual span — and gives the smallest package for a given torque. SmCo holds its output at higher temperature and resists corrosion better without coating. Ferrite is the low-cost route for low torque and benign conditions. Grade selection follows temperature, duty cycle and what the surrounding structure tolerates.

Temperature at the magnets. The figure that matters is not ambient but the temperature at the magnet face, which includes eddy-current heating from slip. NdFeB loses output as it approaches its maximum working temperature and demagnetises permanently past it. Working range we design to runs from −40 °C to +150 °C, with the actual limit set by the grade chosen.

Eddy currents in the sleeve. The rotating field induces currents in the containment sleeve, which heats it and wastes torque. Metallic sleeves need to be thin or made from a low-conductivity material; composite sleeves avoid the loss but are more delicate.

Torque margin. Breakaway torque has to clear working torque by a margin that accounts for temperature derating, gap tolerance and magnet ageing. A coupling specified exactly at working torque will slip in service.

Stray field. Outside the coupling the field has to be low enough not to disturb instruments, sensors or nearby equipment. That is a shielding and geometry question, and it belongs in the design review.

Materials and build

Assembly and test

A magnetic assembly is not parts bolted together. The sequence runs:

1. Circuit design review against the duty — torque, speed, gap, temperature, pressure.
2. Magnet verification — grade and coating checked against the specification, because a substituted grade silently changes the breakaway torque.
3. Rotor build — magnets bonded and mechanically retained, balance checked on the rotating assembly.
4. Assembly of the coupling with the gap set and measured.
5. Breakaway torque test on the completed unit.
6. Run-in and temperature check where the duty calls for it.
7. Documentation — magnet grade, measured breakaway torque and gap setting travel with the unit.

The breakaway test is the one that matters most. It is the only measurement that confirms the whole assembly — magnets, geometry, gap — behaves as designed. Every unit we ship is tested and the measured figure recorded against the serial.

Where magnetic drives are used

• Chemical pumps and agitators — no seal, no leak, no flushing plan
• Vacuum equipment — torque into a chamber without a rotary feedthrough
• Food and pharmaceutical production — cleanable, no lubricant in the process zone
• Precision instruments — smooth transmission, no backlash, no wear debris
• High-pressure vessels and subsea equipment — where a dynamic seal is the weak point

What an inquiry needs

For a coupling, the useful information is duty rather than geometry:

• Working torque and peak torque, or motor power and speed
• Shaft diameters on both sides and the mounting arrangement
• Process-side pressure and temperature
• Fluid or environment, so wetted materials can be chosen
• Required breakaway torque, if specified
• Speed and duty cycle, so eddy-current heating can be checked
• Any restriction on stray field

Send those and circuit, magnets and sleeve can be designed and quoted together. Send only “a coupling for a 2.2 kW pump” and the first reply will be a set of questions.

Send a magnetic drive inquiry

FAQ

Does a magnetic coupling slip under normal load? It runs with a small slip angle proportional to load. The inner rotor lags the outer by an angle that grows with torque. Above breakaway torque it decouples entirely.

What happens when the driven side jams? The coupling breaks away and stops transmitting. That protects the seal and the process, but the motor keeps running — so the control system needs a way to detect the stop, from process feedback or from the outer rotor load.

Can you replace an existing coupling without changing the pump? Usually. Gap, shaft fits and sleeve diameter are what have to match. Send the existing part drawing or the pump model; we work from the interface rather than from a catalogue.

Which magnet grade should be used? That follows the temperature at the magnet face and the torque density needed. NdFeB gives the most torque per volume, SmCo holds up better at temperature, ferrite is the low-cost route for light duty. The choice is made in the circuit review.

Is the assembly balanced? Rotating assemblies are balanced to the speed class agreed with the customer. High-speed couplings are balanced as an assembly, not as separate parts, because inner and outer rotors interact.

Related pages

• CNC machined parts
• Precision shafts and splines
• Custom gear manufacturing