China Halbach Cylinders
Get a first-pass center-field estimate for a two-pole Halbach cylinder, then use the sourcing and verification checklist to prepare a supplier RFQ.
Published September 25, 2026 · Evidence checked September 29, 2026
Dipole field estimate
Enter dimensions in millimeters. The estimate updates as you edit.
Default is illustrative. Replace it with the magnet supplier’s datasheet value at the operating temperature.
Confirm μr for the selected grade with the supplier.
Estimated center field
Analytical screening model for a two-pole (dipole) cylinder.
Estimate includes ideal dipole geometry, segment discretization, and a finite-length center-field correction.
Use as an initial screen
Gaps, sleeve materials, magnetization-angle error, temperature, demagnetization margin, and field uniformity are not modeled. Confirm the design with 3D FEA and a measured field map before release.
How to read the estimate
The tool models a two-pole cylinder and applies corrections for discrete segments and finite length. It does not decide whether a design is manufacturable or safe for a rotor.
- 1. InputsDimensions, remanence, and relative permeability.
- 2. Ideal dipole fieldBr / √μr × ln(Ro / Ri).
- 3. CorrectionsSegment discretization and finite length.
- 4. EstimatePreliminary center field in tesla.
Ideal field: Br / √μr × ln(Ro / Ri).
Segment factor: sin(2π/N) / (2π/N) for N equal segments in this dipole model.
Finite-length factor: uses length and mean radius to estimate the center-field end correction.
Combined model: B₀ = Br / √μr × ln(Ro / Ri) × sin(2π / N) / (2π / N) × L(L² + 6Rm²) / (L² + 4Rm²)^(3/2)
Ri = ID/2, Ro = OD/2, Rm = (Ro + Ri)/2; use the same length unit throughout. The μr adjustment is an approximation.
The equations follow the ideal dipole and correction factors referenced below. Air gaps, sleeve effects, segment-to-segment variation, thermal demagnetization, and bore uniformity are outside this estimate.
Sourcing from China: What to Expect
The IEA estimates China produced 94% of global sintered permanent magnets in 2024. That signals a concentrated supplier base, not proof that a specific cylinder will cost less.
Segment geometry, magnetization, fixtures, and retention design affect the schedule. Request a dated plan that separates material, tooling, assembly, inspection, and shipping.
Certifications and simulation tools vary by supplier. Ask for current site-specific certificates, the FEA assumptions, and measured field-map results for the quoted design.
Turn the design into a verifiable RFQ
There is no single tolerance, segment count, or retention method that fits every cylinder. Agree on the functional requirements and ask each supplier for evidence tied to the quoted geometry.
| RFQ item | Why it matters | Evidence to request |
|---|---|---|
| Field target and topology | The required bore field, uniformity, working volume, pole count, and direction set the design objective. | A field specification with measurement region and axes; simulation assumptions and a measured field map. |
| Dimensions and tolerances | ID, OD, length, concentricity, and runout affect assembly fit and the usable bore. | A drawing with datum references, agreed finished-part tolerances, and an inspection report after assembly. |
| Material and temperature | Remanence and coercivity change the achievable field and demagnetization margin. | The exact grade, lot certificate, Br/Hcj data, and relevant demagnetization curves at operating temperature. |
| Retention and speed | A rotor needs a retention design qualified for its loads, temperature, and maximum speed. | Sleeve or bandage material, stress calculation, safety factor, balance plan, and stated operating limits. |
| Export classification | Some listed rare-earth permanent-magnet materials are subject to Chinese export licensing. | Written confirmation from the exporter on item classification, license responsibility, and any information needed from the buyer. |
| Commercial terms and schedule | A unit price alone may omit engineering, tooling, inspection, shipping, or import costs. | Line-item quote, Incoterm, quote validity, delivery milestones, and who handles any export license. |
Have a field target and drawing ready? Ask for a design-specific review before tooling.
Request an engineering reviewRisks & Vendor Qualification
Assembly and retention
Brittle magnet segments can chip during handling, and a rotor can fail if its retention system is not rated for operating loads and speed.
Request the assembly sequence, fixture and bonding details, and a documented stress and balance review for rotating designs.
Export classification
China’s April 4, 2025 Announcement No. 18 also lists samarium-cobalt permanent-magnet materials, along with selected terbium- and dysprosium-containing NdFeB materials. It does not establish that every finished Halbach assembly is controlled.
Ask the exporter to classify the exact item and confirm license responsibility and required buyer information before placing the order.
Model limits
A first-pass analytical estimate does not capture every air gap, magnetization error, sleeve effect, temperature, or field uniformity requirement.
Compare supplier 3D FEA against the agreed target and require a measured field map on the finished assembly.
Example estimate: dipole cylinder
Illustrative inputs, not a customer case
Enter OD 50 mm, ID 20 mm, length 30 mm, 8 segments, Br 1.30 T, and μr 1.05 in the estimator. The analytical model returns about 0.877 T at the center. This is a reproducible screening result, not a measured production result or a rotor safety assessment.
Sources, dates, and model limits
- Magnet production share: The IEA 2024 chart reports China’s share of global sintered permanent-magnet production; it is not a price comparison or a cylinder-specific supplier-capability measure. The chart was updated on October 6, 2025.
- Export controls: MOFCOM Announcement No. 18 of 2025 was issued on April 4, 2025. It lists selected rare-earth items, including samarium-cobalt permanent-magnet materials and Tb- and Dy-containing NdFeB materials. Exporters must apply for a license for listed items. Classification is item-specific, and later rules or implementation updates may change requirements; verify the current treatment with the exporter before ordering.
- Later policy update: MOFCOM’s statement on the China–U.S. consultations says relevant export-control measures announced on October 9, 2025 would be suspended for one year. It is a separate policy update; verify its applicability to the buyer’s destination and the current rules with the exporter.
- Field estimate: The dipole calculation follows the analytical model and its segmentation and finite-length corrections described by Blümler and Soltner. It assumes ideal geometry and material input. Gaps, local tolerances, temperature-dependent demagnetization, sleeve effects, and field uniformity require design-specific analysis and measurement.
Evidence checked September 29, 2026. The cited production data is for 2024; Announcement No. 18 was issued on April 4, 2025. Check the official notice and current export-control guidance again when classifying a shipment.
Frequently Asked Questions
How long does it take to manufacture a custom Halbach cylinder in China?
There is no reliable universal lead time. Geometry, magnet availability, magnetization, retention design, inspection, and any export classification review can change the schedule. Ask for a dated schedule that separates material procurement, tooling, assembly, testing, and shipping.
Can you assemble Halbach cylinders with high-temperature SmCo?
SmCo can be considered when the operating temperature or demagnetization margin makes it a better fit. Compare the selected grade’s remanence and coercivity curves at the actual operating temperature, then confirm the assembly and retention method with the supplier.
How do you prevent the cylinder from demagnetizing during assembly?
Ask the supplier for its magnetization sequence, non-magnetic holding-fixture design, assembly work instruction, and demagnetization check. The right sequence depends on segment geometry, magnetization direction, and retention design.
What is the tightest dimensional tolerance for a completed array?
There is no universal tolerance for every assembled cylinder. State which surfaces are functional and request agreed limits for ID, OD, concentricity, runout, and post-assembly inspection before ordering.
Are there export restrictions on Halbach cylinders from China?
China’s Announcement No. 18 of 2025 lists selected rare-earth items, including samarium-cobalt permanent-magnet materials and terbium- or dysprosium-containing NdFeB materials. MOFCOM later said relevant measures announced on October 9, 2025 would be suspended for one year after China–U.S. consultations; that is a separate policy update. Whether a finished cylinder is controlled depends on its exact classification and destination. Ask the exporter to confirm the current rules and license responsibility in writing before shipment.
Can this estimator calculate a quadrupole or sextupole cylinder?
No. It is limited to a two-pole dipole Halbach cylinder. Multipole designs have different field quantities and should be evaluated with a topology-specific analytical model or 3D FEA.
Where should I get the Br and μr inputs?
Use values for the exact magnet grade and operating temperature from the supplier’s technical data or lot documentation. The defaults are illustrative and should not be treated as a material selection.
Does the center-field estimate predict bore uniformity?
No. It estimates a center value for an idealized geometry. Specify the working volume and allowable variation, then request a design-specific field map and a measured map from the finished assembly.
Can I use this estimate to approve a high-speed rotor?
No. The tool does not calculate centrifugal stress, sleeve loading, fatigue, adhesive performance, balance, or a safety factor. A qualified mechanical design review is required for a rotating assembly.
What should I ask the supplier to return with a quote?
Request the controlled drawing, exact material grade and temperature data, assembly and retention proposal, schedule assumptions, export-classification confirmation, simulation assumptions, and inspection and field-map plan.
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