Brushless DC electric motor

1. A method for forming a brushless DC electric motor comprising:

providing a hollow magnet having a multi-pole configuration;

inserting the hollow magnet into a collet;

inserting an output member into a cavity in a mold;

orienting the output member concentric with an outer surface of the hollow magnet;

closing the mold;

overmolding a plastic material onto an interior surface of the hollow magnet and thereby coupling the output member to the hollow magnet; and

inserting a rotor into a stator, the rotor comprising the hollow magnet, the output member and the plastic material that is overmolded onto the interior surface of the hollow magnet.

2. The method of claim 1 , wherein in a cross-section taken perpendicular to an axis about which the rotor is rotatable relative to the stator, an interior cavity defined by the interior surface of the hollow magnet has a non-circular cross-sectional shape.

3. The method of claim 2 , wherein the non-circular cross-sectional shape is a polygon.

4. The method of claim 3 , wherein the polygon has between five and ten sides.

5. The method of claim 1 , wherein the hollow magnet has a center of mass, and wherein the center of mass is offset from an axis about which the rotor rotates relative to the stator.

6. The method of claim 5 , further comprising machining an outside surface of the hollow magnet prior to overmolding the plastic material onto the interior surface of the hollow magnet.

7. The method of claim 6 , wherein the outside surface of the hollow magnet is ground in a centerless grinding operation.

8. The method of claim 6 , further comprising machining at least one of the hollow magnet and the overmolded plastic material after the plastic material has been overmolded onto the hollow magnet.

9. The method of claim 5 , wherein the plastic material that is overmolded onto the interior surface of the hollow magnet has a density that is within +/−10% of a density of the material that forms the hollow magnet when the rotor is at a temperature of 70 degrees Fahrenheit.

10. The method of claim 9 , wherein the density of the plastic material is within +/−5% of a density of the material that forms the hollow magnet.

11. The method of claim 1 , wherein the hollow magnet is formed of a powdered metal material.

12. The method of claim 1 , wherein the output member is a sleeve that is received in one end of the rotor such that the plastic material is disposed radially between the sleeve and the interior surface of the hollow magnet.

13. The method of claim 12 , wherein the sleeve defines a plurality of projections that are embedded into the plastic material.

14. The method of claim 13 , wherein the projections are castellations.

15. The method of claim 1 , wherein the output member is disposed in a first end of the rotor and a bearing is disposed in an opposite end of the rotor such that the plastic material is disposed radially between the bearing and the interior surface of the hollow magnet.

16. The method of claim 15 , wherein the stator includes a motor case, and wherein the motor case includes a hub onto which the bearing is received, the bearing supporting the rotor for rotation on the hub.

17. The method of claim 1 , wherein the collet is secured to a portion of the mold for movement therewith.

18. The method of claim 1 , wherein the output member comprises a shaft.

19. A method for forming a rotor, the method comprising:

providing an injection mold having a collet and first and second mold members, the collet being received between the first and second mold members, each of the first and second mold members defining a portion of a mold cavity;

installing a motor output member into one of the portions of the mold cavity;

installing a hollow cylindrical magnet into the collet;

positioning the first and second mold members in a closed position to locate an outer surface of the hollow cylindrical magnet relative to a rotational axis of the motor output member and to apply a compressive force onto the outer surface of the hollow cylindrical magnet;

injecting molten plastic under pressure between hollow cylindrical magnet and the motor output member; and

cooling the injected molten plastic to secure the motor output member to the hollow cylindrical magnet.

20. The method of claim 19 , wherein the hollow cylindrical magnet defines a non-round central aperture.

21. The method of claim 19 , wherein prior to installing the hollow cylindrical magnet into the collet, the method comprises grinding the outer surface of the hollow cylindrical magnet.

22. The method of claim 21 , wherein the outer surface of the hollow cylindrical magnet is ground in a centerless grinding operation.

23. The method of claim 19 , wherein positioning the first and second mold members in the closed position engages a pair of centering members to the motor output member, the centering members being configured to align the rotational axis of the motor output member coincident with a rotor axis that is defined by the first and second mold members.

24. The method of claim 23 , wherein one of the centering members is movable along the rotor axis.

25. The method of claim 23 , wherein each of the first and second mold members has a frusto-conically shaped collet-engaging surface that is configured to engage an associated one of a pair of frusto-conically shaped mold-engaging surfaces defined by the collet.

26. The method of claim 25 , wherein the collet is mounted to one of the first and second mold members for movement therewith.

27. The method of claim 26 , wherein the collet is mounted to the one of the first and second mold members in a manner that permits limited movement of the collet relative to the one of the first and second mold members.

28. The method of claim 19 , wherein cooling the injected molten plastic forms a solid overmold member and wherein at a temperature of 70 degrees F., the solid overmold member has a density that is within 10% of a density of the hollow cylindrical magnet.

29. The method of claim 28 , wherein the density of the solid overmold member is within 5% of the density of the hollow cylindrical magnet.

30. A method for forming a rotor, the method comprising:

providing an injection mold having first and second mold members, each of the first and second mold members defining a portion of a mold cavity;

installing a motor output member into one of the portions of the mold cavity;

coupling a hollow cylindrical magnet to one of the first and second mold members;

positioning the first and second mold members in a closed position to align a longitudinal axis of the motor output member coincidently with a rotational axis of a rotor that is defined by the first and second mold members, to locate the outer surface of the hollow cylindrical magnet concentrically about the rotor axis and to apply a compressive force to the outer surface of the hollow cylindrical magnet;

injecting molten plastic under pressure between hollow cylindrical magnet and the motor output member; and

cooling the injected molten plastic to secure the motor output member to the hollow cylindrical magnet

wherein cooling the injected molten plastic forms a solid overmold member and wherein at a temperature of 70 degrees F., the solid overmold member has a density that is within 10% of a density of the hollow cylindrical magnet.

31. The method of claim 30 , wherein a pressure of the molten plastic that is injected between the hollow cylindrical magnet and the motor output member is greater than or equal to 1000 p.s.i.g.

32. The method of claim 31 , wherein the pressure is greater than or equal to 2000 p.s.i.g.

33. The method of claim 32 , wherein the pressure is greater than or equal to 3000 p.s.i.g.

34. The method of claim 30 , wherein prior to coupling the hollow cylindrical magnet to the one of the first and second mold members, the method comprises grinding the outer surface of the hollow cylindrical magnet.

35. The method of claim 34 , wherein the outer surface of the hollow cylindrical magnet is ground in a centerless grinding operation.

36. The method of claim 30 , wherein the density of the solid overmold member is within 5% of the density of the hollow cylindrical magnet.

37. The method of claim 30 , wherein positioning the first and second mold members in the closed position engages a pair of centering members to the motor output member, the centering members being configured to align the rotational axis of the motor output member coincident with a rotor axis that is defined by the first and second mold members.

38. The method of claim 37 , wherein one of the centering members is movable along the rotor axis.