The present invention relates to the field of bearing arrangements for supporting radial loads and axial loads in both directions comprising bearings with a spherical outer raceway. More particularly, the invention is directed to a bearing arrangement of this kind which is suitable for supporting a wind turbine blade relative to a wind turbine hub, to enable the blade to rotate about a blade pitch axis. BACKGROUND TO THE INVENTION

Typically, wind turbine blades are connected to the hub using a slewing bearing with a diameter that is approximately equal to the diameter of the blade root. Wind turbines are becoming larger and larger, and the blade root can have a diameter of more than 3 metres. A slewing bearing with an equivalent diameter generates a substantial amount of friction. Also, the slewing bearings experience a high level of wear, due to the relatively small back and forth rotations that the bearing undergoes during operation, especially when individual blade pitch control is employed, and the associated difficulty of maintaining a good lubrication film. Consequently, the use of slewing bearings as pitch bearings in wind turbines has disadvantages.

An alternative design for a pitch bearing is disclosed in WO2012/146745. In one embodiment, the pitch bearing comprises a spherical plain bearing for supporting radial loads. The spherical plain bearing is considerably smaller in diameter than the blade root diameter, meaning that relatively low friction is generated. The use of a spherical plain bearing is further advantageous due to the self-aligning properties and excellent wear behaviour. The disclosed pitch bearing further comprises an arrangement of rod connections with swivelling rod ends, for taking up the axial loads and bending moment of the blade. The contact area of the swivelling rod ends is also relatively low, and the pitch bearing as a whole generates considerably less friction in comparison with a slewing bearing.

A further alternative design for a pitch bearing assembly is disclosed in DE 2855992. The assembly comprises two axially spaced bearings which are connected to the blade and hub respectively via two mutually overlapping conical structures. The two bearings have a diameter that is considerably less than the blade root diameter, which is advantageous in terms of reducing friction. Needless to say, when bearings of relatively small diameter are used to support a wind turbine blade, the bearing construction must be capable of transmitting the high axial and radial forces concerned. A radial spherical bearing is able to accommodate radial and axial loading, although the radial load-carrying capacity is significantly greater. Therefore, depending on the magnitude of the axial loads in either direction, it may be necessary to use one or two thrust bearings for accommodating axial loading in one or two directions. A conventional bearing solution for supporting radial load and axial loads in both directions is to mount a radial bearing between two thrust bearings. However, especially when the radial bearing has a larger inside diameter than the thrust bearings, mounting and dismounting the bearings has to be performed from either side of the shaft. This is not always possible or desirable.

Consequently, there room for improvement. SUMMARY OF THE INVENTION

The present invention resides in a bearing arrangement that comprises a first bearing unit adapted to accommodate axial loads acting in a first direction from a first axial side of the bearing arrangement towards a second axial side, and comprises a second bearing unit adapted to accommodate radial loads and axial loads acting in an opposite, second axial direction. The first bearing unit comprises a first thrust bearing with a spherical geometry, and the second bearing unit comprises a radial bearing with a spherical geometry. An outer ring of the radial bearing is mounted in fixed connection with a housing component and an inner ring of the radial bearing is mounted in fixed connection with a shaft component. According to the invention, the first thrust bearing is arranged at the second axial side of the radial bearing. Furthermore, an inner ring of the first thrust bearing is mounted in fixed connection with the housing component and an outer ring of the first thrust bearing is mounted in fixed connection with the shaft component. Thus, when e.g. the inner ring of the second bearing unit is rotational in use, it is the outer ring of the first bearing unit that is rotational in use.

In conventional bearing arrangements comprising a radial bearing and a thrust bearing of the above kind, the thrust bearing would be arranged at the first axial side of the radial bearing and each of the inner and outer rings would be mounted to the same component. If the radial bearing has a larger bore diameter than the thrust bearing, and the shaft component is non-separable, then the first thrust bearing has to be mounted from the first axial side.

In a bearing arrangement according to the invention, the radial bearing and the thrust bearing can be mounted and dismounted from the second axial side.

Depending on the magnitude of the axial load acting in the second axial direction, the radial bearing may not be able to accommodate this load and retain a sufficient bearing life. Thus, in an embodiment of the invention, the second bearing unit further comprises a second thrust bearing with a spherical geometry for accommodating loads in the second axial direction. The second thrust bearing is arranged between the radial bearing and the first thrust bearing. In the conventional manner, an inner ring of the second thrust bearing is mounted to the shaft component, while its outer ring is mounted to the housing component.

Suitably, each of the spherical bearings in the bearing arrangement according to the invention is concentrically arranged so as to have a common centre point. This ensures that the individual spherical bearings retain their self-aligning properties.

In one example, the spherical bearings are roller bearings. The first bearing unit comprises a spherical roller thrust bearing, and the second bearing unit comprises a spherical roller bearing and, optionally, a spherical roller thrust bearing. The advantage of using roller bearings is that extremely low friction is generated.

In a further example, the spherical bearings are plain bearings. Spherical plain bearings have a convex inner race and a concave outer race. The first bearing unit may comprise an angular contact spherical plain bearing or a thrust spherical plain bearing. The second bearing unit comprises a radial spherical plain bearing and, optionally, an angular contact or thrust spherical plain bearing. The advantage of using plain bearings is improved wear resistance. Angular contact and thrust spherical plain bearings can both accommodate some radial loading. A thrust spherical plain bearing can take up only a small ratio of radial loading, while an angular contact spherical plain bearing can take up a larger ratio of radial loading. Radial loading on a thrust spherical plain bearing reduces the life of the bearing, and it thus advantageous to prevent radial loads being transmitted through a thrust bearing.

In an embodiment of the invention, where the first thrust bearing is a thrust spherical plain bearing, the inner ring of the first thrust bearing is resiliently retained in a radial direction against the housing component, such that radial deformations of the housing component due to radial loads are not transmitted to the shaft component through the first thrust bearing. For example, a small spring may be provided between the bore of the housing component and an outer diameter of the inner ring. Radial location of bearing inner ring can be beneficial if wear occurs between the concave race of the outer ring and the convex race of the inner ring. The inner ring is then forced to maintain its position, but any radial deformations of the housing due to radial load are taken up by the spring and are not passed on to the thrust spherical plain bearing.

Similarly, when the second bearing unit comprises a thrust spherical plain bearing, the outer ring of this bearing may be resiliently retained against the housing in radial direction.

A bearing arrangement according to the invention is particularly suitable for use as part of a bearing construction for rotationally supporting a wind turbine blade relative to a wind turbine hub. The construction enables the bearings to be replaced without detaching the blade and the bearing arrangement of the invention enables the bearings to be dismounted from an accessible side. Other advantages of the invention will become apparent from the detailed description and accompanying figures.

DRAWINGS

The invention will now be described further, with reference to the following Figures, in which:

Fig. 1 shows a cross-sectional view of a bearing construction comprising an example of a bearing arrangement according to the invention; Fig. 2a, 2b show an exploded cross-sectional-view of the bearing arrangement from Fig. 1 ;

Fig. 3 shows a cross-sectional-view of a further example of a bearing arrangement according to the invention. DETAILED DESCRIPTION

Fig. 1 depicts a bearing construction 10 for rotationally supporting a wind turbine blade 20 relative to a wind turbine hub 30. The hub comprises a static frame 40 with a first shaft component 1 10 and a second shaft component 21 0. The construction further comprises a dynamic frame 50 which has a first housing component 120 and a second housing component 220, whereby the blade 20 is attached to the dynamic frame 50 or whereby the dynamic frame forms an integral part of the blade. The dynamic frame is rotationally supported relative to the static frame 40 by a first bearing arrangement 100 and a second bearing arrangement 200. The first bearing arrangement connects the first shaft component 1 1 0 and the first housing component 120; the second bearing arrangement connects the second shaft component 21 0 and the second housing component 220. Further, both the static frame 40 and the dynamic frame 50 comprise legs with openings in between. Each frame has three legs in the depicted example. The legs 55 of the dynamic frame pass though the openings between the legs 45 of the static frame, to enable a limited amount of relative rotation between the frames about a blade pitch axis 60.

A bearing construction of the type depicted in Fig. 1 enables the use of bearings with a diameter that is considerably smaller than a diameter of the blade root. A smaller contact radius leads to reduced friction, which improves the energy efficiency of the wind turbine. In the depicted example, the second bearing arrangement consists of a radial spherical plain bearing, which is configured for transmitting radial loads on the blade to the second shaft section 210, which are then transmitted to the hub 30 via the static frame legs 55. The first bearing arrangement 100 is configured for transmitting radial loads and the axial loads on the blade to hub 30, via the first shaft component 1 10. The first shaft component is bolted via a flange part to a central shaft 35 of the hub, which shaft is coupled to a main shaft of the turbine generator. The central shaft 35 interconnects a further two bearing constructions (not shown) which support a further two turbine blades (not shown).

The first bearing arrangement 1 00 is depicted in greater detail in Fig. 2a, and comprises a radial spherical plain bearing 1 30 and a thrust spherical plain bearing 140. Spherical plain bearings are used, because of their self-aligning properties and good wear resistance to the frequent, back-and forth angular adjustments about the blade pitch axis 60, which are typically experienced in pitch applications. An inner ring 1 31 of the radial bearing 130 is mounted to the first shaft component 1 10. An outer ring 132 of the radial bearing is mounted to the housing component 120 via a housing sleeve 125, in the depicted example. The housing sleeve 125 has a conical outer surface and the housing component 120 has a tapered bore. The conical housing sleeve facilitates replacement of the radial bearing 130 and eliminates misalignment between the shaft component and housing component that occurs when the radial bearing is dismounted.

Radial spherical plain bearings can withstand some axial loading, and in the depicted example, the radial bearing is used to transmit axial forces acting from a blade side 180 to a hub side 190 of the bearing arrangement. The axial forces in this direction are predominantly due to the weight of an upwardly oriented blade, and are transmitted from the dynamic frame 50 to the first shaft component 1 1 0 of the hub 30 through the radial bearing 1 30 as indicated by the arrows in Fig. 2a.

In the opposite axial direction, from the hub side 190 to the blade side 180 of the bearing arrangement, the axial forces are due to centrifugal forces on the blade during operation and the weight of the blade. These axial forces are too high to be carried by the radial bearing and a thrust spherical plain bearing 140 is provided. Conventionally, such a bearing would be arranged at the hub side 190 of the radial bearing 130 in order to transmit loads in the required direction. In the bearing arrangement according to the invention, however, the thrust bearing 140 is arranged at the blade side 180 of the radial bearing 130.

Due to the geometry of spherical plain bearings, a thrust spherical plain bearing with a certain bore diameter has a larger contact radius than a radial spherical plain bearing with the same bore diameter. In order to minimize friction, the contact radius of the plain bearings should also be minimized. For the depicted bearing construction, the application loads require the radial spherical thrust bearing 130 to have a certain size. The thrust spherical plain bearing 140 may have a smaller diameter. As a result, the first shaft component 1 10 has a stepped portion 1 1 2 with a smaller diameter than the portion on which the radial bearing 130 is mounted. Due to the high loads on the first shaft component 1 10, it is preferably executed a single sold piece. Also, at the hub side 190 of the first bearing arrangement, the shaft diameter is preferably as large as possible to provide sufficient robustness. Consequently, the thrust bearing 140 cannot be mounted at the hub side 1 90 of the radial bearing 1 30, and is provided at the blade side 190 of the radial bearing.

According to the invention, the thrust bearing 140 is able to transmit the axial loads in the required direction in that an inner ring 141 of the thrust bearing is mounted to the housing component 120 and an outer ring 142 is mounted to the shaft component 1 10. In the depicted example, as shown in Fig. 2b, the conical housing sleeve 1 25 has one axial side face that retains the outer ring 132 of the radial bearing, and has an oppositely oriented side face against which the inner ring 141 of the thrust bearing is mounted. In spherical plain bearings, the inner ring has a convex inner race and the outer ring has a concave outer race. The outer ring 142 is mounted on the stepped portion 1 12 of the shaft component and is axially retained by e.g. a locking plate 1 60 that is bolted to the shaft component. The inner ring 141 is mounted around the stepped portion 1 12 with a clearance. The axial forces acting from the hub side 190 to the blade side 1 80 are transmitted from the housing component 120 to the shaft component 1 10 through the conical housing sleeve 125, through the thrust bearing and through the locking plate 160 as shown by the arrows in Fig. 2b. In a bearing construction of the type shown in Fig. 1 , the first shaft component 1 10 is accessible from the blade side 190. Thus, the bearing arrangement of the invention facilitates dismounting of the bearings while ensuring that the construction is strong enough to transmit the required loads. A further example of a bearing arrangement according to the invention is depicted in Fig. 3. Again the bearing arrangement 201 forms part of a bearing construction for rotationally supporting a turbine blade, via a dynamic frame, relative to a wind turbine hub 205. The bearing arrangement comprises a radial spherical plain bearing 230, and first and second thrust spherical plain bearings 240, 250. The second thrust bearing 250 is configured to take up axial loads acting from the blade side 190 to the hub side 180 of the bearing arrangement. The radial bearing 230 and the second thrust bearing 250 form a bearing unit, whereby an inner ring 231 , 251 of each bearing 230, 250 is mounted to the shaft component 21 0 and an outer ring 232, 252 of each bearing is mounted to the housing component 220. In accordance with the invention, the first thrust bearing 240, which takes up axial loads acting from the blade side 190 to the hub side 180, is arranged at the blade side of the bearing unit 230, 250. Also, the inner ring 241 of the first thrust bearing is in fixed connection with the housing component 222 and is not in contact with the shaft component 210. The outer ring 242 of the first thrust bearing is mounted to the shaft component.

The second thrust bearing 250 has a slightly smaller bore diameter than the radial bearing 230 and the first thrust bearing 240 has a slightly smaller bore diameter than the second thrust bearing. Each of the bearings can be mounted and dismounted from the blade side 1 80, to facilitate replacement of the bearings when necessary.

The outer race of a spherical bearing forms part of a sphere with a certain spherical centrepoint. This is depicted with reference numeral 290 in Fig. 3. Suitably, each spherical bearing in an arrangement according to the invention is configured to have the same centre point. This ensures that each bearing retains its self-aligning properties. The invention has been described with reference to a pitch bearing application in which the housing component is rotational about the blade pitch axis. It is also possible for the housing component to form part of the hub structure and for the shaft component to form part of the structure that supports the blade and that is rotational about the blade pitch axis. Furthermore, although the invention is beneficial in pitch bearing applications, the inventive bearing arrangement may be applied in any application where the thrust bearing is preferably mounted and dismounted at the side of the bearing arrangement towards which the axial force is acting. A number of aspects/embodiments of the invention have been described. It is to be understood that each aspect/embodiment may be combined with any other aspect/embodiment. Moreover the invention is not restricted to the described embodiments, but may be varied within the scope of the accompanying patent claims.