High-Speed Rotor Magnet Retention: Sleeves, Bonding, and Burst Margin

Tip speed is v = ωr. A 40 mm diameter rotor at 30,000 rpm has a tip speed of about 62.8 m/s. This calculation does not certify that rotor as safe. For a fixed geometry, centrifugal loads scale with speed squared; doubling speed gives four times the load. Supply both diameter and speed, then agree the overspeed condition and test plan using the applicable machine requirements. Published speeds for another rotor do not establish an allowable speed for a new sleeve.

Adhesive bonds position magnets and may carry structural loads if qualified for that duty. Assess shear, peel and tensile loading on the actual coated substrate at operating temperature. Surface preparation, cured thickness, cure schedule and ageing affect strength. A room-temperature lap-shear number cannot be used as the rotor allowable. Determine whether a separate mechanical retention path is needed through analysis and testing.

Metal sleeves can carry hoop load and apply radial pressure to the magnets. Select the alloy, thickness, fit and heat treatment using temperature-dependent strength and fatigue data. Electrical conductivity means harmonic fields can induce sleeve currents. Model both sleeve loss and any change to magnet loss, including the heat-removal path. The balance between loss, mass and strength is specific to the rotor.

Carbon-fibre composite sleeves can provide high strength at low mass. Their electrical and thermal properties depend on fibre directions, fibre contacts, resin and manufacturing. Carbon fibre is electrically conductive; a CFRP sleeve cannot be assumed to have zero eddy-current loss. Cooper et al., 2005 measured electrical behaviour in carbon-fibre structures.

That work establishes conductivity, not the loss in a particular motor sleeve. Use the actual laminate properties in electromagnetic, thermal and structural models, and validate the manufactured sleeve.

Diametral interference is the magnet/core outside diameter minus sleeve inside diameter before assembly. If the core grows more than the sleeve on heating, the thermal contribution to interference increases. If the sleeve grows more, it decreases. Composite expansion depends on layup and direction; a generic near-zero expansion value is insufficient.

Centrifugal growth, elastic deformation, adhesive behaviour and temperature gradients also change contact pressure. Calculate these effects together and check both contact and material stress limits at the relevant hot and overspeed conditions. A cold bench test alone does not establish that margin.

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.

Include sleeve and magnet loss in the thermal model. The reported N45H magnetic comparison and the separate historical 50H Dy-reduction example have different scopes. See the GBD evidence and limitations. Finished-part savings require a quotation and qualification of the proposed route.

Send the complete mechanical and thermal duty with the drawing so that retention and magnetic performance can be assessed together.