Ring & Annular Magnets: Wall Thickness, Magnetization & Encoders

Wall thickness and proportion

Three dimensions define a ring — outside diameter, inside diameter and height — but the number that governs whether it can be made economically is the wall, (OD − ID) / 2.

minimum wall: Around 0.5 mm is the practical floor for sintered NdFeB, but 1.5 mm or greater is where yields and handling become comfortable. Below 1 mm expect breakage in grinding, plating and assembly, and price accordingly.
wall as a fraction of OD: Aim for at least 10–15 % of OD. A large-diameter thin-wall ring is a hoop, and sintered magnet materials are poor hoops — they crack from residual stress before they ever reach an assembly.
height: For axially magnetised rings, height is the magnetisation length and therefore governs the permeance coefficient. Short wide rings sit low on the demagnetisation curve.
concentricity: ID to OD concentricity drives wall variation, which drives flux variation around the circumference. On sensing and encoder rings this shows up directly as angular error.
large diameters: Above roughly 100 mm OD, a segmented ring built from arcs is usually cheaper and more reliable than a single sintered ring. See arc and segment magnets.

Thin-wall rings crack from stresses you did not apply

Sintered rare-earth rings carry residual stress from sintering and grinding. In a thin wall that stress is a significant fraction of the material's strength, and rings can fracture during plating, during transport, or sitting on a shelf — with no external load and no handling error. If the design genuinely needs a thin large-diameter ring, discuss it before release: a segmented build, a bonded ring, or a supporting sleeve may all be better answers than a heroic single part.

Magnetization options

The ring is the only geometry that supports the full range of magnetisation patterns, and choosing among them is the main design decision.

Pattern	Poles located	Typical use	Cost / notes
Axial	One flat face north, the other south	Holding, speaker rings, simple sensor targets	Standard fixture; lowest cost
Diametric	One pole pair across the OD	Angular position sensing, BLDC rotor feedback	Sinusoidal output over a Hall sensor when rotated
Radial outward	All north on OD, all south on ID	Loudspeaker gap motors, coupling and sensing rings	Custom radial fixture; the true radial geometry
Multipole on the face	Alternating N/S around one flat face	Axial-read encoders, rotary position sensing	Multi-tooth fixture; pole count to order
Multipole on the OD	Alternating N/S around the circumference	Radial-read encoders, BLDC motor rings	Pole pitch must match the sensor array
Halbach ring	Rotating orientation around the ring	Field concentration inside or outside, self-shielding assemblies	Usually segmented; specialist fixture

True radial is not the same as multipole

The terms get used loosely and they describe different parts. A radially magnetised ring has a single continuous orientation — north over the entire outer surface, south over the entire inner surface. A multipole ring has alternating poles distributed around it. Both require radial-family fixtures, both are more expensive than axial, and specifying one when you meant the other produces a part that is useless in the application and technically to print. State the pole count explicitly, including “2-pole” where you mean diametric.

Radial and multipole magnetisation both require fixtures wound specifically for the geometry, which is a real non-recurring cost that should be surfaced at quotation rather than at first article.

Multipole rings for encoders

Multipole rings are the sensing element in most magnetic encoders, and they are specified against the sensor IC rather than in isolation. Get the sensor datasheet first; it dictates almost everything.

pole count: Sets resolution together with the sensor's interpolation factor. Higher pole counts give finer resolution but demand tighter pole pitch uniformity and a smaller air gap.
pole pitch: Must match the sensor array spacing. A differential Hall or magnetoresistive array has a defined pitch and reading a mismatched ring degrades signal amplitude badly.
pitch uniformity: Angular spacing error translates directly into position error. Specify ±2 % or better for precision work, and tighter for absolute encoders.
air gap: Typically 0.5–1.5 mm. Field falls steeply with gap and pole count — a fine-pitch ring has a short field reach, so high pole counts and generous gaps are mutually exclusive.
field amplitude: Sensor ICs specify a working peak-to-peak range, commonly 20–100 mT at the gap. Both too little and too much are failure conditions — saturating the sensor flattens the waveform.
transition sharpness: Pole transitions blur as pole count rises and as the gap opens. Signal quality is set by the combination, not by pole count alone.

Pole count and air gap are one decision

The commonest encoder ring failure is specifying a high pole count for resolution and then discovering that mechanical tolerances will not hold the sensor within the gap that pole count requires. The field from a fine-pitch multipole ring decays over a distance comparable to the pole pitch itself, so doubling pole count roughly halves the usable gap. Fix the mechanically achievable gap first, then choose the pole count that works at that gap.

Sensor selection, target design and the relationship between field and output are covered in magnets for sensors.

Mounting and retention

A ring on a shaft is a mechanical problem before it is a magnetic one, and sintered magnet materials are the worst possible candidate for the methods that would otherwise be obvious.

Never press a sintered magnet onto a shaft. The hoop stress from an interference fit is tensile, and sintered rare-earth material is weak in tension. It will crack, immediately or in service. This is the single most common ring magnet failure.
Bond, with a controlled gap. An adhesive bond with a designed glue line — typically 0.05–0.15 mm — both retains the ring and accommodates thermal expansion mismatch between magnet and shaft. Anaerobic retaining compounds and structural epoxies both work.
Use a carrier where load is real. Bond the ring into a non-magnetic sleeve or hub, then attach the hub to the shaft by conventional means. The carrier takes the mechanical load and the magnet only has to be a magnet.
Provide axial location. A shoulder, circlip or end cap keeps the ring positioned if the bond creeps at temperature.
Watch the shaft material. A steel shaft through an axially magnetised ring is part of the magnetic circuit and will change the field distribution. Where that is not wanted, use a non-magnetic shaft or a non-magnetic carrier.
Balance matters at speed. Wall variation and concentricity error make the ring an unbalanced mass as well as an unbalanced magnetic source.

Assemble unmagnetised where you can

A magnetised multipole ring will snap to any steel in reach, will not sit still on a fixture, and cannot be positioned angularly with any precision by hand. Bonding the ring unmagnetised and then magnetising the completed sub-assembly in a fixture removes all of that, and it lets the pole pattern be indexed to a mechanical datum — a keyway, a flat, a hub feature — to a precision that is simply not achievable by orienting a pre-magnetised ring. For encoder and commutation rings, where pole position relative to a mechanical reference is the whole point, this is usually the correct process. It has to be agreed at RFQ stage because it changes what the supplier ships.

Bond selection, surface preparation and temperature qualification are covered in bonding and mounting magnets and assembly and retention design.

Ring magnet RFQ checklist

OD, ID and height with tolerances, plus concentricity where it matters.
Material, grade and maximum operating temperature.
Magnetisation pattern named unambiguously — axial, diametric, radial outward or inward, or multipole with the pole count stated.
Pole indexing for multipole rings: which pole sits where relative to which mechanical feature, and with what angular tolerance.
Pole pitch uniformity for encoder rings.
Field acceptance — peak-to-peak amplitude at a stated gap, measured how and with what.
Coating, with dimensions stated as applying after coating.
Magnetised or unmagnetised on delivery, and if unmagnetised, who magnetises and with what fixture.
Edge break on ID and OD edges.
Sensor part number, if there is one. It is the fastest way for a supplier to catch a pole pitch or amplitude mismatch before tooling exists.

The general RFQ structure, including the commercial and quality sections, is in how to prepare a magnet RFQ. Standard ring sizes held in stock are listed under ring magnets, and true radial geometries under true radial magnets.

Ring magnets, standard and multipole

Ring and radially magnetised geometries are what we are known for. Encoder rings, sensor targets, motor rings and true radial magnetization in NdFeB, SmCo and ferrite — send us the pole count and the sensor you are designing against.

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Ring & Annular Magnets