TECHNICAL FIELD The invention relates to a drivetrain for a horizontal axis wind turbine, comprising a main shaft provided with a rotor attachment flange adapted to be connected to a rotor of the wind turbine, and a main bearing housing carrying the main shaft via a first bearing and a second bearing. The invention also relates to a main bearing housing for a horizontal axis wind turbine, adapted to at least partially enclose a main shaft connected to a rotor of the wind turbine, the main bearing housing presenting a substantially circular component attachment flange for a connection of the main bearing housing to a further drivetrain component, where the further drivetrain component and the rotor are located at opposite ends of the main bearing housing. BACKGROUND

Horizontal axis wind turbines usually include a tower carrying on its top a nacelle, and a rotor mounted on a drivetrain which is housed in the nacelle. The drivetrain usually includes a main shaft, a gearbox and a generator. In many turbines, the main shaft is carried, via two axiaily offset bearings, by a main bearing housing. The connection between the main shaft and the gearbox input shaft can be of any suitable type, for example it could be a rigid connection, or it could include a low speed coupling as exemplified in WO2012052022A1 allowing some flexibility in the alignment of the shafts, in some cases, the main bearing housing, the gearbox and the generator each have their respective direct support to a carrying structure of the nacelle, in other cases the gearbox and the generator are cantilevered, as exemplified below, from the main bearing housing, and in such cases the joint between the main bearing housing and a fixed body of the gearbox will experience relatively large forces, in addition, certain types of low speed couplings have a relatively large diameter. This will require a large diameter of any fixed connection between the main bearing housing and the fixed body of the gearbox. For example, the main bearing housing and the gearbox might be connected via relatively large cover for a low speed coupling between the main shaft and the gearbox input shaft. However, it might be desired to provide a main shaft that has a relatively small diameter at its connection to the gearbox, and therefore a small bearing between the main shaft and the main bearing housing. SUMMARY

It is an object of the invention to provide in a wind turbine an effective and reliable main shaft bearing support, and an effective connection, in particular regarding loads, between a main bearing housing and a further drivetrain component, such as a gearbox.

This object is reached with a drivetrain for a horizontal axis wind turbine, drivetrain for a horizontal axis wind turbine, comprising

- a main shaft provided with a rotor attachment flange adapted to be connected to a rotor of the wind turbine, and

a main bearing housing having a main body at least partly enclosing the main shaft and extending from a first end to a second end, a distance from the second end to the rotor attachment flange being larger than a distance from the first end to the rotor attachment flange, the main body presenting at the second end a component attachment flange for a connection of the housing to a further drivetrain component,

the main bearing housing carrying the main shaft via a first bearing and a second bearing, a distance between the second bearing and the rotor attachment flange being larger than a distance between the first bearing and the rotor attachment flange,

the main bearing housing having a first seat for the first bearing, and a second seat for the second bearing,

the main bearing housing comprising a seat flange extending from the main body at least partly radially inwards, the second seat being located on a distal end of the seat flange.

Preferably, the main body of the main bearing housing extends along the rotational axis of the main shaft. In some embodiments, the further drivetrain component is a low speed coupling cover, in other embodiments, the component attachment flange of the main bearing housing is adapted for a connection of the housing directly to a fixed part of a gearbox or a generator.

The seat flange extending from the housing main body at least partly radially inwards, provides for the component attachment flange being located radially, in relation to a rotational axis of the main shaft, outside the second seat. I.e. the extension of the seat flange has a component in the radial direction; it could also have a component parallel to the rotational axis, but in any case the flange is oriented inwards from the main body, in a non-zero angle to the rotational axis.

The invention will allow for improving the force handling in drive trains where the further drivetrain component/components is/are cantiievered from the main bearing housing. The reason is that the connection between the further component(s) and the main bearing housing can be made relatively large to reduce the forces induced by the bending moment caused by the cantiievered arrangement, without compromising the design of the main shaft bearing arrangement, which can be kept relatively small as dictated by the size of the main shaft.

Thus, the reduced forces will allow for reducing the material. Also, since the connection between the main bearing housing and the further drivetrain component can be designed independently of the main shaft bearing arrangement, a relatively straight load path can be provided in the main bearing housing for stresses incurred by the cantiievered arrangement, with a large diameter at the connection to the further drivetrain component.

Also, the invention will be advantageous where a low speed coupling has a relatively large diameter requiring a large cover for the coupling. The component attachment flange can be made large for a connection to the cover, while the main shaft bearing can be kept considerably smaller to accommodate the main shaft.

Preferably, the seat flange is shaped as a truncated cone, where the distal end is the narrower end of the truncated cone, and where the distance from the distal end to the rotor attachment flange is larger than the distance from the wider end of the truncated cone to the rotor attachment flange. The truncated cone shape of the seat flange will provide an alignment of the seat flange with the force path associated with the second bearing, in particular where the second bearing is a tapered roller bearing, angled in the following manner (as exemplified in the drawings described below): The bearing rollers each have a first roller end and a second roller end, the distance between the rotor attachment flange and the first roller end being smaller than the distance between the rotor attachment flange and the second roller end, and the distance between the rotational axis of the main shaft and the first roller end being smaller than the distance between the rotational axis and the second roller end.

The invention also provides a main bearing housing for a horizontal axis wind turbine, adapted to at least partially enclose a main shaft connected to a rotor of the wind turbine, the main bearing housing presenting a substantially circular component attachment flange for a connection of the main bearing housing to a further drivetrain component, where the further drivetrain component and the rotor are located at opposite ends of the main bearing housing, the main bearing housing having a first seat for a first bearing between the main bearing housing and the main shaft, and a second seat for a second bearing between the main bearing housing and the main shaft, the distance from the component attachment flange to the first bearing seat being larger than the distance from the component attachment flange to the second bearing seat, the diameter of component attachment flange being larger than the diameter of the first bearing seat, and the diameter of the second bearing seat being equal to, or smaller than, the diameter of the first bearing seat.

Thereby, as has been mentioned above, the large component attachment flange provides for a good force handling capacity in view of the further drivetrain component being cantilevered from the main bearing housing, while a relatively small second bearing can be provided so as to be adapted for an optimised main shaft design.

Here the diameter of the component attachment flange is understood as the diameter of a region for transfer of loads between the component attachment flange and the further drivetrain component. For example, where there is a bolt connection between the component attachment flange and the further drivetrain component, with a single row of bolts and with the bolts oriented substantially in parallel to a rotational axis of the main shaft, the diameter of the component attachment flange is the diameter of an imaginary circle formed by the centre lines of the series of bolts distributed along the component attachment flange. The diameters of the bearing seats are understood as the diameters of respective surfaces of the seats facing radially inwards and supporting the respective bearings.

Preferably, the difference between the component attachment flange diameter and the second bearing seat diameter is at least twice, preferably three times, as large as the difference between the diameters of the first and second bearing seats. Preferably, the diameter of the component attachment flange is at least 30%, preferably at least 40%, more preferably at least 50%, larger than the second bearing seat diameter. DESCRIPTION OF THE DRAWINGS

Below embodiments of the invention will be described with reference to the drawings, in which

fig. 1 shows a partial side view of a horizontal axis wind turbine,

fig. 2 shows a cross-sectional view of a part of a drivetrain for a wind turbine, the section coinciding with the rotational axis of a rotor of the wind turbine,

fig. 3 shows a perspective view of the drivetrain part in fig. 2

fig. 4 shows a perspective view of the drivetrain partly shown in fig. 3,

fig. 5 shows a cross-sectional view of a part of a wind turbine drivetrain according to an alternative embodiment of the invention, the section coinciding with the rotational axis of a rotor of the wind turbine, and

fig. 8 shows a cross-sectional view of a part of a drivetrain for a wind turbine according to an alternative embodiment of the invention, the section coinciding with the rotational axis of a rotor of the wind turbine.

DETAILED DESCRIPTION The horizontal axis wind turbine 1 in fig. 1 includes a tower carrying on its top a nacelle 3 adapted to swing around a vertical axis in relation to the tower. The wind turbine also includes a rotor 4 with a hub 41 carrying three blades 42; alternatively there could be fewer or more than three blades on the hub 41. The hub is mounted on a drivetrain which is housed in the nacelle 3, The drivetrain includes a main shaft 5, gearbox 6 and a generator 7.

Fig. 2 shows parts of the drivetrain including the main shaft 5. The rotational axis of the rotor 4 and the main shaft 5 is indicated in Fig. 2 with a broken line A. The main shaft 5 is at a first end (to the left in fig. 2) provided with a rotor attachment flange 5C which is fixedly connected to the hub 41 (fig. 1 ). At a second end (to the right in Fig. 2), the main shaft 5 is connected to a low speed coupling 61 , in turn connected to an input shaft 62 of the gearbox 6. The main shaft 5 is carried via a main bearing arrangement 52, 53 by a main bearing housing 51 . As can be seen in fig. 3, the main bearing housing 51 is fixedly mounted on a bed frame 31 of the nacelle structure. As can be seen in fig. 2, the main bearing arrangement includes a first and a second bearing 52, 53 distributed along the rotational axis A of the main shaft 5. The first and second bearings are in this embodiment roller bearings, but any of them could alternatively be of any type of bearing, e.g. a ball bearing.

The first bearing 52 is located closer to the hub than the second bearing 53. The first bearing 52, or more specifically, an outer ring of the first bearing 52, is fitted in a first seat 51 1 presented by the main bearing housing 51. The second bearing 53 is located close to the low speed coupling 61 . An outer ring of the second bearing 53, is fitted in a second seat 512 also presented by the main bearing housing, inner rings of the first and second bearings 52, 53 are fitted in respective seats 5A, 5B presented by the main shaft 5.

The main bearing housing 51 comprises a main body 514 that encloses the main shaft 5 and extends substantially in parallel with the rotational axis A from a first end 5141 to a second end 5142. The first end is closer to the rotor that the second end. At the second end 5142 the main body 514 presents a component attachment flange 513 for a fixed connection of the main bearing housing 51 to a low speed coupling cover 61 1 , (see also fig. 3), arranged to cover the low speed coupling 61. The component attachment flange has a substantially circular cross-section. The component attachment flange 513 is located radially, in relation to the rotational axis A, outside the second seat 512. In other words, the component attachment flange 513 is located further away from the rotational axis A than the second seat 512.

As illustrated in fig. 4, the gearbox 6 and the generator 7 are not support directly on the nacelle structure. Instead they are cantiievered from the main bearing housing 51 via the low speed coupling cover 61 1. This means that the main bearing housing 51 and the low speed coupling cover 61 1 will be subjected to relatively large forces from the weights of the cantiievered components 6, 7. As can be seen in fig. 3, the main bearing housing main body 514 is at a portion 516 extending from the component attachment flange 513 towards the rotor attachment flange 5C shaped as a truncated cone. This will provide for a relatively straight load path for stresses in the main body caused by the cantiievered components.

To provide for the shorter radial distance of the second seat 512, the main bearing housing is provided with a seat flange 515. The seat flange 515 extends radially inwards from an inner surface of the main body 514. The seat flange 515 is provided with the second seat at its inner distal end. The seat flange 515 presents a non-zero angle to an imaginary plane oriented perpendicular to the rotational axis A, and has therefore the shape of a truncated cone with the end with the second seat 512 being the narrower end 5151 of the truncated cone and located further away from the rotor 4 compared to the wider end 5152 of the truncated cone where the seat flange 515 joins the main body 514. The junction between the seat flange 515 and the main body 514 is located at a distance from the second end 5142 and the component attachment flange 513.

The low speed coupling 61 can be of any type, e.g. one of the solutions described in WO2012052022A1 incorporated herein by reference. In this example, the low speed coupling has two flexible discs 612, which are at respective inner edges fixedly connected to the main shaft 5 and the gearbox input shaft 62, respectively. At respective outer edges the flexible discs 612 are fixedly joined via a cylinder 613. Such a low speed coupling will provide some flexibility to allow for misalignment and other un-desired phenomena that can occur in the drivetrain.

Due to the design of the low speed coupling 61 , it will have a relatively large diameter, and therefore so will also the low speed coupling cover 61 1 . As stated, the main bearing housing main body 514 is at a portion 516 extending from the component attachment flange 513 towards the rotor shaped as a truncated cone, (fig. 3), and this provides a relatively straight load path for stresses incurred by the cantilevered arrangement. In addition, the seat flange 515 will allow for the main bearing housing main body 514 to present the large diameter component attachment flange 513, while at the same time providing a rigid support for the second bearing 53. Actually, the diameter of the main body 514 is larger at the second end 5142 than at the first end 5141 . The large diameter of the component attachment flange 513 is beneficial in view of the loads, since the bending moment caused by the cantilevered arrangement will be counteracted by forces in the component attachment flange 513 that are lesser than they would have been with a smaller component attachment flange.

Furthermore, the truncated cone shape of the seat flange 515 provides an alignment of the flange with the force path associated with the second bearing 53, in particular where the latter is a tapered roller bearing, angled as indicated in the drawings. As can be seen in fig. 4, the bearing rollers each have a first roller end 531 and a second roller end 532, the distance between the rotor attachment flange 5C and the first roller end 531 being smaller than the distance between the rotor attachment flange 5C and the second roller end 532, and the distance between the rotational axis A of the main shaft 5 and the first roller end 531 being smaller than the distance between the rotational axis A and the second roller end 532. The diameter of component attachment flange 513 is larger than the diameter of the first bearing seat 51 1 , and the diameter of the second bearing seat 512 is smaller than the diameter of the first bearing seat 51 1. There is a bolt connection between the component attachment flange 513 and the low speed coupling cover 61 1 , with a single row of bolts 5131 oriented substantially in parallel to the rotational axis A. Here, the diameter of the component attachment flange 513 is understood as the diameter of an imaginary circle formed by the centre lines of the series of bolts 5131 distributed along the component attachment flange 513. The diameters of the bearing seats 51 1 , 512 are understood as the diameters of respective surfaces of the seats facing radially inwards and supporting the respective bearings 52, 53.

Fig. 5 shows an embodiment where the component attachment flange 513 is located adjacent to the outer periphery of the seat flange 515. I.e. the component attachment flange 513 is located adjacent to the connection between the seat flange 515 and the main bearing housing main body 514.

The invention is applicable also to turbines without any gearbox in the drivetrain, i.e. so called direct drive turbines. Thereby, the invention could be particularly useful where, as illustrated in fig. 6, the generator 7 is cantilevered from the main bearing housing 51 , and the main shaft 5 is connected to a rotor 71 of the generator 7, e.g. directly (as in fig. 6) or via a low speed coupling, e.g. as described in said WO2012052022A1.