Multipole & True Radial Ring Visualizer
Radial orientation is the thing we build. This tool draws the pole layout for any ring you configure, computes the surface field at a real measuring distance, and reports the pole pitch, decay length and encoder resolution that follow from it — across all five magnetization patterns a ring can carry.
interactive — pole layout & surface field
Multipole field decays exponentially away from the surface, with a length constant of only τ / 2π. Double the pole count on the same diameter and you halve the pole pitch, which halves the decay length — the field at a 1 mm sensor gap can fall by more than half even though the surface field barely moved. Resolution and reach pull in opposite directions, and this is the trade that decides most encoder ring designs.
the five patterns
| pattern | what it does | typical use |
|---|---|---|
| multipole radial | Alternating north and south segments around the circumference, each oriented along its own radius. Produces a repeating sinusoid as the ring turns. | BLDC and PMSM rotors, encoder rings, position sensing, magnetic couplings |
| true radial — single pole | One continuous pole over the entire outer surface and the opposite pole over the entire bore. Flux leaves the whole OD and returns through the surrounding circuit. | Sensor rotors, magnetic bearings, speaker and instrument assemblies, anywhere a uniform radial field is wanted |
| diametric | Two poles across the diameter — one half north, one half south. A single sinusoid per revolution. | Absolute rotary encoders, angle sensors over a single turn, small motor rotors |
| axial | Conventional through-thickness magnetization, one pole per flat face. | Holding, coupling through a plate, simple face-mounted sensing |
| multipole axial — face | Alternating poles arranged around one flat face rather than the circumference. | Axial-flux motors, face-reading encoders, pancake rotor assemblies |
This is the single most common specification error we see. A diametric ring has two poles across the diameter; the field direction is the same everywhere inside the ring. A radial ring has field pointing outward — or inward — along the radius at every point around the circumference. They look similar on a sketch and behave completely differently in an assembly. Sorting this out early is covered in magnetization directions explained.
sintered radial ring versus glued arc segments
Most rotors get built one of two ways: bond individual arc segments onto a hub in alternating polarity, or sinter a single ring already oriented radially and magnetize it into poles afterward. The second is what "true radial" means, and it is not just a cosmetic difference.
| criterion | sintered radial ring | glued arc segments |
|---|---|---|
| assembly labour | one part, one operation | one bond per pole, plus fixturing and polarity checks |
| pole-to-pole consistency | set by the magnetizing fixture — very repeatable | stacks segment tolerance on top of bond-line variation |
| weak spots at joints | none — continuous material | a gap at every joint, visible as cogging and harmonic content |
| retention risk | no individual pieces to release at speed | each bond is a failure path; usually needs a sleeve or band |
| tooling cost | press mould and magnetizing fixture required up front | little or none for standard arcs |
| economic crossover | favoured at production volume and for high pole counts | favoured for prototypes, one-offs and very large diameters |
| diameter ceiling | practical limits apply — talk to us about your size | essentially unlimited, segment by segment |
If you are prototyping, arcs are usually the right call — see arc & segment magnets. If you are heading into production with a repeating pole count, the sintered ring generally wins on total installed cost once assembly labour and scrap are counted. We build both: stocked true radial magnets, or custom tooling for your geometry.
how to specify one
A radial ring drawing needs more than three dimensions. The items below are what we ask for on every quote, and having them ready shortens the tooling conversation considerably:
| item | why it matters |
|---|---|
| OD, ID, axial length with tolerances | drives the press mould; tight bore tolerance may require post-grinding |
| pole count and pattern | determines the magnetizing fixture, which is separate tooling from the mould |
| pole orientation reference | where pole one sits relative to a keyway, flat or mark — otherwise assembly orientation is undefined |
| required surface field and where it is measured | a field spec without a measuring distance and probe type is not testable |
| operating temperature range | sets the coercivity class; check the geometry with the demagnetization calculator first |
| coating | NiCuNi, epoxy or parylene, and whether the bore is coated |
| skew, if any | skewing the poles axially reduces cogging at some cost in fundamental amplitude |
reference — the formulas used
| pole pitch at the surface | τ = π · D / N N = number of poles |
| multipole surface field | B̂(z) = k_a · Br(T) · (1 − e^(−2π·t/τ)) · e^(−2π·z/τ) · k_bi · k_fix k_a = (2/π) · sin(α_p · π/2)t = radial wall thickness, z = measuring distance, k_bi = back-iron factor, k_fix = magnetizing fixture efficiency |
| decay length | z(1/e) = τ / 2πthe distance over which surface field falls to 37 % — the number that kills high pole counts |
| true radial, into a return circuit | B_gap = Br(T) · t / (t + µ_rec · g)series magnetic circuit through the radial wall and the return gap |
| axial, on axis | Bz(z) = Bz,cyl(OD) − Bz,cyl(ID) Bz,cyl(z) = (Br/2)·[ (z+L)/√((z+L)²+R²) − z/√(z²+R²) ] |
| electrical frequency | f = (N/2) · n / 60 n = rpm |
| remanence vs. temperature | Br(T) = Br₂₀ · [1 + α · (T − 20) / 100] |
The multipole expression is the standard exponential result for a periodic magnetization pattern, and it is accurate where the wall is thick relative to pole pitch and the ring is long relative to its wall. It ignores axial end effects, magnetizing-fixture efficiency — real fixtures rarely saturate a ring perfectly at high pole counts — and any inter-pole transition width. Expect measured surface fields to land somewhat below the calculated value, and treat the numbers as design guidance rather than an acceptance limit. For a field spec you intend to put on a drawing, send us the geometry and we will quote against measured data from the actual fixture.
going deeper — the underlying magnetic-circuit theory is in magnets 201; ring geometry and tolerancing in ring & annular magnets; and if you are pairing one of these with a sensor, continue to the hall sensor air gap designer.