Radial Multi-pole Ring Magnets for Robot Joint Motors: What to Put on the Drawing

Electrical frequency is pole pairs times mechanical revolutions per second. At 3,000 rpm, a 14-pole ring gives 350 Hz and an 8-pole ring gives 200 Hz. More poles shorten the pole pitch and change the yoke and winding design. The magnetizing fixture must suit the requested pole count and diameter. Ask for confirmation of that pair, including any skew, before freezing the electrical design.

The 29 November 2025 external-OD scans compare nominal Ø36.6 × Ø29.8 × 17 mm, 10-pole specimens: radial N45SH, segmented N50SH and integrated Halbach N45SH. Recorded N/S peaks were 415.74/−423.29 mT, 521.39/−519.46 mT and 629.00/−627.71 mT respectively. Probe standoff and measurement uncertainty were not recorded, and the reports give no pass judgement.

These are historical surface-field traces, not air-gap, back-EMF, torque or bore-field measurements. The N50SH specimen is retained as historical evidence, not a current Mainrich offer. Using the N peaks, 629.00 mT is about 51% above 415.74 mT and about 21% above 521.39 mT. Using mean absolute N/S peaks instead gives about 50% and 21%.

Neither calculation establishes the effect in another magnetic circuit. See the full comparison.

The radial-ring TDS, Rev A of 31 July 2026, states OD 20 to 75 mm, ID 15 to 68 mm, wall 2 to 7 mm and height 5 to 50 mm, with ±0.05 mm standard dimensional tolerance within the qualified route. These limits are not freely combinable. Confirm the drawing, grade, pole pattern and individual features before quotation.

The separate integrated-Halbach route states ±0.02 mm on critical features, subject to feature and drawing confirmation. Give wall variation, coaxiality, cylindricity and assembly datums their own drawing requirements. ISO 21940-11 covers rotors with rigid behaviour.

At G2.5 and 30,000 rpm, e_per = G/ω gives about 0.796 µm for the complete rotor. With rotor mass in kg and eccentricity in µm, their product is residual unbalance in g·mm. Allocate that allowance among the parts and assembly process.

Select the grade using the actual hot demagnetization curves, the magnetic circuit working point and the worst opposing field. A suffix is a nominal material class, not a guaranteed motor temperature limit or a chemical recipe. Mainrich’s sintered NdFeB range is N30 to N56, with no AH grades. The top grade depends on the temperature series: N56, N54M, N52H, N52SH, N50UH and N45EH.

Grades up to N52SH can be made without a Chinese export licence, so we offer those first; UH and EH grades need a licence, applied for per order. Confirm the grade, suffix, geometry and production route for each drawing. This summary does not offer every combination; grades outside it in educational examples are not Mainrich supply offers.

Request hot curves and the allowed irreversible loss after the agreed thermal and reverse-field duty. The educational grade pages are a reference for reading those properties.

For a ring used by a position sensor, define field-direction error separately from the pole-position and peak-field tolerances. Specify the scan radius, sensor orientation, temperature, angular reference and data format. The appropriate sampling plan depends on the failure risk and demonstrated process capability. A published sample trace does not establish a production-lot variance or a 100% inspection commitment.

Compare alloy and diffusion route, tooling, machining, assembly and inspection on one drawing. The required coercivity can be reached by different compositions, so the suffix alone cannot price the rare earth input. Agree tooling, sample quantity, inspection and production milestones in the quotation.

Export release depends on any required authorization. No standard prototype quantity or fixed delivery interval is promised here. If buying a rotor assembly, include the shaft datum, bond line, retention and final field measurement in the acceptance plan.