Root bushing, wind turbine rotor blade and method

1 . A root bushing for a wind turbine rotor blade, comprising:

an insert having a first end, a second end, and a bore extending from the first end to the second end, wherein the insert further includes a thread section configured to engage a bolt insertable within the bore through the first end, and a receiving section having an inner diameter that gradually increases toward the second end;

a first plurality of fiber material layers wound around an outer surface of the insert to embed the insert in the first plurality of fiber material layers, wherein the outer surface is dimensioned to form a first scarf and a second scarf such that at least a part of the first plurality of fiber material layers forms a double scarf joint for transferring loads from the insert to the first plurality of fiber material layers, and vice versa; and

a rod arrangement partly received within the receiving section through the second end of the insert, the rod arrangement comprising a pultrusion rod covered by a second plurality of fiber material layers, wherein the pultrusion rod has a frustoconical or cone-shaped end inserted into receiving section;

wherein at least a part of the first plurality of fiber material layers contacts the outer surface of the insert at the receiving section and at least a part of the second plurality of fiber material layers contacts an inner surface of the insert at the receiving section;

wherein the outer surface comprises a first double cone section, a second double cone section, and a cylindrical section sandwiched between the first double cone section and the second double cone section, each of the first double cone section and the second double cone section comprising a first cone area and a second cone area, the first cone area and the second cone area being arranged inversely, further wherein a cylindrical area is sandwiched between the first cone area and the second cone area, the first cone area and the second cone area both open out starting out from the cylindrical area;

wherein the cylindrical area has a smaller outer diameter than the cylindrical section;

wherein each of the first and second double cone sections is filled with the first plurality of fiber material layers arranged in a stepped or staggered way to form the double scarf joint.

2 . The root bushing according to claim 1 , wherein the double scarf joint is formed at the first double cone section and/or the second double cone section.

3 . The root bushing according to claim 1 , wherein the receiving section is cone shaped, and the second plurality of fiber material layers are arranged in a staggered way to form the cone shaped end that is received within the receiving section.

4 . The root bushing according to claim 1 , further comprising two C-shaped pultrusion wedges, wherein the insert and the pultrusion rod are sandwiched between the two C-shaped pultrusion wedges.

5 . A wind turbine rotor blade for a wind turbine, comprising a plurality of root bushings according to claim 1 .

6 . The root bushing according to claim 1 , wherein a section of the rod arrangement that extends from the second end of the insert is cut, leaving an oblique face at a cut end of the rod arrangement.

7 . The root bushing according to claim 1 , further comprising a closing fiber material layer covering the insert and the second plurality of fiber material layers.

8 . The root bushing according to claim 1 , wherein the first plurality of fiber material layers are wound on the insert in a dry state and subsequently impregnated with a resin in a resin transfer molding process.

9 . A method for manufacturing a root bushing for a wind turbine rotor blade, the method comprising:

a) providing two inserts, each insert having a first end, a second end, and a bore extending from the first end to the second end, wherein the insert further includes a thread section configured to engage a bolt insertable within the bore through the first end, and a receiving section having an inner diameter that gradually increases toward the second end;

b) embedding the two inserts in a first plurality of fiber material layers;

c) impregnating the first plurality of fiber material layers with a resin;

d) forming a double scarf joint at each insert by at least a part of the first plurality of fiber material layers for transferring loads from the two inserts to the first plurality of fiber material layers, and vice versa;

e) providing a pultrusion rod covered by a second plurality of fiber material layers; and

f) engaging the pultrusion rod with the receiving section of the two inserts to sandwich the pultrusion rod between the two inserts;

wherein at least a part of the first plurality of fiber material layers contacts the outer surface of each insert at the receiving section and at least a part of the second plurality of fiber material layers contacts an inner surface of each insert at the receiving section;

wherein the outer surface comprises a first double cone section, a second double cone section, and a cylindrical section sandwiched between the first double cone section and the second double cone section, each of the first double cone section and the second double cone section comprising a first cone area and a second cone area, the first cone area and the second cone area being arranged inversely, further wherein a cylindrical area is sandwiched between the first cone area and the second cone area, the first cone area and the second cone area both open out starting out from the cylindrical area;

wherein the cylindrical area has a smaller outer diameter than the cylindrical section;

wherein each of the first and second double cone sections is filled with the first plurality of fiber material layers arranged in a stepped or staggered way to form the double scarf joint.

10 . The method according to claim 9 , wherein the second plurality of fiber material layers is wound on the pultrusion rod in a staggered way to form two cone shaped ends on the pultrusion rod, and wherein each cone shaped end is received in one of the receiving sections of the two inserts.

11 . The method according to claim 9 , wherein the pultrusion rod and the two inserts are sandwiched between two C-shaped pultrusion wedges to form a blank.

12 . The method according to claim 11 , wherein the blank is cut into two root bushings.

13 . The method according to claim 12 , wherein the cut is made at an oblique angle.