Rotor assembly with solid low friction layer and press-fit sleeve

1. A rotor for an electric machine, comprising:

a rotor assembly having an outer surface and a plurality of poles with permanent magnets, the permanent magnets located in openings of the rotor assembly and positioned radially inwards from the outer surface;

a solid layer formed on the outer surface of the rotor assembly and radially outwards from the permanent magnets, the solid layer having an outer surface with a first coefficient of friction that is lower than a second coefficient of friction of the outer surface of the rotor assembly; and

an outer sleeve,

wherein the rotor assembly is press-fit into the outer sleeve.

2. The rotor assembly of claim 1 , wherein the solid layer has a thickness that is less than or equal to 0.2 mm.

3. The rotor assembly of claim 1 , wherein the outer sleeve comprises carbon fiber.

4. The rotor assembly of claim 1 , wherein the solid layer comprises a polymer material.

5. The rotor assembly of claim 4 , wherein the polymer material includes thermoplastic material.

6. The rotor assembly of claim 5 , wherein thermoplastic material is selected from a group consisting of polybenzimidazole, polytetrafluoroethylene, and polyoxymethylene.

7. The rotor assembly of claim 5 , wherein the solid layer has a compressive strength greater than or equal to 50 MPa and a thermal expansion coefficient less than or equal to 30 ppm/° C.

8. The rotor assembly of claim 4 , wherein a lubricating additive is added to the polymer material.

9. The rotor assembly of claim 8 , wherein the lubricating additive is selected from a group consisting of graphite, wax, and ceramic.

10. The rotor assembly of claim 1 , wherein the solid layer comprises a thermoset material.

11. The rotor assembly of claim 10 , wherein the thermoset material is selected from a group consisting of epoxy, phenolic and polyurethane.

12. The rotor assembly of claim 1 , wherein the solid layer comprises a ceramic material.

13. The rotor assembly of claim 12 , wherein the ceramic material is selected from a group consisting of tungsten disulfide, molybdenum disulfide, titanium nitride, and titanium carbide.

14. A method for manufacturing a rotor for an electric motor, comprising:

providing a rotor assembly including a plurality of rotor portions with permanent magnets and having an outer surface, the permanent magnets located in openings of the rotor assembly and positioned radially inwards from the outer surface;

forming a solid layer on the outer surface of the rotor assembly and radially outwards from the permanent magnets,

wherein the solid layer has a first coefficient of friction than is lower than a second coefficient of friction of the outer surface of the rotor assembly; and

press-fitting the rotor assembly into an outer sleeve.

15. The method of claim 14 , further comprising performing at least one of grinding, sanding and machining of an outer surface of the solid layer prior to the press-fitting.

16. The method of claim 14 , further comprising applying lubricant to an outer surface of the solid layer prior to the press-fitting.

17. The method of claim 14 , further comprising cooling the rotor assembly to a temperature less than or equal to −20° C. prior to the press-fitting.

18. The method of claim 14 , wherein:

the solid layer has a thickness that is less than or equal to 0.2 mm; and

the outer sleeve comprises carbon fiber.

19. The method of claim 14 , wherein the solid layer comprises a thermoplastic selected from a group consisting of polybenzimidazole, polytetrafluoroethylene, and polyoxymethylene.

20. The method of claim 14 , wherein the solid layer comprises one of:

a thermoset selected from a group consisting of epoxy, phenolic and polyurethane; or

ceramic selected from a group consisting of tungsten disulfide, molybdenum disulfide, titanium nitride, and titanium carbide.