Wheel hub motor for an electric bicycle and electric bicycle comprising said wheel hub motor

1. A wheel hub motor for an electric bicycle, comprising:

a fixed shaft which can be associated with a frame of an electric bicycle;

a hub shell, which is rotatable with respect to said fixed shaft, incorporatable in a wheel structure of said electric bicycle;

an electric motor being housed inside said hub shell, said electric motor having an internal stator rigidly connected to the fixed shaft and an external rotor rotatably movable with respect thereto, and a reduction unit which kinematically connects the rotation of said external rotor to that of said hub shell;

wherein said reduction unit comprises two reduction stages arranged alongside each other in the axial direction inside the hub shell:

a first reduction stage comprising at least one first pinion, rigidly connected to the external rotor and coaxial with the fixed shaft and meshing with at least one first gear rigidly connected to an eccentric shaft mounted on an intermediate member;

and a second reduction stage comprising at least one second pinion rigidly connected to said eccentric shaft and meshing with a second gear rigidly connected to said hub shell, said eccentric shaft traversing said intermediate member, a first portion of the eccentric shaft protruding from a first side of the intermediate member and a second portion of the eccentric shaft protruding from a second side of the intermediate member, said first gear and said second pinion being respectively supported by the first portion and by the second portion of the eccentric shaft.

2. The wheel hub motor according to claim 1 , wherein said intermediate member is rotationally constrained to the fixed shaft in at least one direction.

3. The wheel hub motor according to claim 2 , wherein said intermediate member is rotationally constrained to the fixed shaft by means of a freewheel.

4. The wheel hub motor according to claim 2 , wherein said intermediate member is rotationally constrained to the fixed shaft by means of a friction clutch.

5. The wheel hub motor according to claim 2 , wherein said intermediate member is rigidly rotationally constrained to the fixed shaft in both directions.

6. The wheel hub motor according to claim 2 , wherein said intermediate member is a rocker which supports the eccentric shaft at the end of a first arm.

7. The wheel hub motor according to claim 6 , wherein said rocker has a counterweight at the end of a second arm.

8. The wheel hub motor according to claim 1 , wherein at least one of the first pinion and the first gear of the first reduction stage is made, at least in the region of the meshing surface, of plastic material.

9. The wheel hub motor according to claim 8 , wherein said second pinion and said second gear are made, at least in the region of their meshing surface, of metallic material.

10. The wheel hub motor according to claim 9 , wherein said first gear has a peripheral portion made of plastic material co-moulded onto a sleeve made of metallic material.

11. The wheel hub motor according to claim 1 , wherein the first pinion and the first gear form a helical gearing, the second pinion and the second gear being of the straight-tooth type.

12. The wheel hub motor according to claim 11 , wherein said internal stator has a plurality of poles provided with shaped pole shoes made of sintered magnetic material.

13. The wheel hub motor according to claim 1 , wherein said electric motor is a synchronous electric motor, said external rotor comprising a permanent magnet moulded using plastoferrite.

14. The wheel hub motor according to claim 1 , wherein the transmission ratio provided by the reduction unit ranges between 1:14 and 1:20.

15. An electric bicycle comprising at least one wheel hub motor according to claim 1 .