Modular gear unit for a wind turbine

1. A planetary gear unit ( 21 ) of a gearbox of a wind turbine, the planetary gear unit ( 21 ) comprising only one central sun gear ( 27 ), at least one pair of planet gears ( 25 , 25 ′), a ring gear ( 24 ), a planet carrier ( 29 ), and an outer casing ( 31 ), the outer casing ( 31 ) being attached to a wind turbine nacelle, the ring gear ( 24 ) being mounted on the outer casing ( 31 ), the planet carrier ( 29 ) comprising a bogie plate ( 35 ), the bogie plate ( 35 ) supporting planet shafts ( 41 ), wherein the at least one pair of planet gears ( 25 , 25 ′) is rotatably mounted by means of a first set of bearings ( 37 , 37 ′) on one of the planet shafts ( 41 ), one planet gear ( 25 ) of the at least one pair of planet gears ( 25 , 25 ′) being placed at one side of said bogie plate ( 35 ), and another planet gear ( 25 ′) of the at least one pair of planet gears ( 25 , 25 ′) being placed at an other side of said bogie plate ( 35 ),

wherein the planet carrier ( 29 ) comprises a main body ( 51 ) and a backplate ( 53 ) which can be joined/disjoined to/from the main body ( 51 ) for facilitating assembly/disassembly of the planetary gear unit ( 21 ), and

wherein the planet carrier ( 29 ) is mounted on said outer casing ( 31 ) by means of a first bearing ( 72 ) and a second bearing ( 73 ), wherein the first bearing ( 72 ) and the second bearing ( 73 ) form a second set of bearings, the first bearing ( 72 ) being in contact with said main body ( 51 ) at a rotor side of the planetary gear unit ( 21 ) so that the planet carrier ( 29 ) is able to rotate, and the second bearing ( 73 ) being in contact with said backplate ( 53 ) at a generator side of the planetary gear unit ( 21 ).

2. The planetary gear unit ( 21 ) according to claim 1 , wherein said main body ( 51 ) of the planet carrier ( 29 ) extends from the rotor side of the planetary gear unit ( 21 ) to at least a hypothetical vertical plane (A-A) that is located by an end of the sun gear ( 27 ) at the generator side of the planetary gear unit ( 21 ).

3. The planetary gear unit ( 21 ) according to claim 1 , wherein the main body ( 51 ) of the planet carrier ( 29 ) comprises a frontal carrier ( 61 ) and a plurality of struts ( 63 ) to support the bogie plate ( 35 ), the struts ( 63 ) include, at the generator side of the planetary gear unit, contact flanges ( 65 ) with said backplate ( 53 ) for joining/disjoining the backplate ( 53 ) to/from the main body ( 51 ).

4. The planetary gear unit ( 21 ) according to claim 3 , wherein said backplate ( 53 ) comprises an annular-shaped part ( 71 ) and a tubular, part ( 75 ), the annular-shaped part ( 71 ) comprising cooperating areas ( 73 ) with said contact flanges ( 65 ) for joining/disjoining the backplate ( 53 ) to/from the main body ( 51 ), and the tubular part ( 75 ) protruding from an inner border of the annular-shaped part ( 71 ) and being in contact with said second bearing ( 73 ).

5. The planetary gear unit ( 21 ) according to claim 4 , wherein the backplate ( 53 ) is joined to said contact flanges ( 65 ) by means of screws.

6. The planetary gear unit ( 21 ) according to claim 4 , wherein the backplate ( 53 ) is joined to said contact flanges ( 65 ) by a combination of screws and pins.

7. The planetary gear unit ( 21 ) according to claim 1 , wherein said first bearing ( 72 ) is a cylindrical roller bearing or a taper rolling bearing or a ball bearing.

8. The planetary gear unit ( 21 ) according to claim 1 , wherein said second bearing ( 73 ) is a cylindrical roller bearing or a taper rolling bearing or a ball bearing.

9. The gearbox of the wind turbine comprising the planetary gear unit ( 21 ) according to claim 1 .

10. The wind turbine comprising the gearbox according to claim 9 .