FIELD OF THE INVENTION

The present invention relates to a bearing, a wind energy converter and a method of manufacturing a bearing.

BACKGROUND

Wind energy converters (hereinafter "WECs") convert kinetic energy from wind into electrical energy. WECs generally comprise a rotor having a number of blades connected to a hub. The hub drives a shaft which in turn drives a generator to produce electrical energy.

The load on the rotor is controlled by pitching the blades. To this end, each blade connects to the hub via a pitch bearing. This allows each blade to be turned around its longitudinal axis in and out of the wind. In fact, on some modern WECs the blades are turned 98% of the time during operation.

As the size of WECs increases, pitch bearings have to take up increasingly high loads. These high loads, most of all bending moments, deform the bearings which results in a number of negative effects.

In the past, different approaches have been suggested to this problem.

One approach is to increase the size and stiffness of such bearings, which is however limited by weight and cost considerations .

Another approach is disclosed in document US 2008/0213095 Al . Said document describes a pitch bearing provided with flexi- bility enhancing means. Such means ensure the functionality of the bearing even though the rings are distorted by the load.

SUMMARY

The present invention provides a bearing, in particular a pitch bearing for a wind energy converter, comprising an outer ring, an inner ring, a plurality of rollers riding between the outer and inner ring and means to reduce fluctuations in turning resistance.

Further, the present invention provides a wind energy converter, comprising a blade, a hub and a bearing according to the invention connecting the blade to the hub in a rotatable manner .

Even further, the present invention provides a method for manufacturing a bearing according to invention, wherein the first and second cross-section of the outer and/or inner ring is produced by rolling.

Even further, the present invention provides a bearing, in particular a pitch bearing for a wind energy converter, comprising an outer ring, an inner ring, a plurality of rollers riding between the outer and inner ring, wherein the outer and/or the inner ring has a cross-section that varies in a circumferential direction of the outer or inner ring.

Even further, the present invention provides a bearing, in particular a pitch bearing for a wind energy converter, comprising an outer ring, an inner ring, a plurality of rollers riding between the outer and inner ring, wherein the rollers are formed as balls running in respective grooves of the inner and outer ring, the respective grooves defining a raceway. Even further, the present invention provides a method for manufacturing a bearing according to the invention, wherein the rings are elastically deformed during finish cutting manufacturing of the raceway in order to gain a smooth 3- dimensional surface of the raceway. In some embodiments, a varying pretension is produced by grinding or hard turning in the finishing process.

Even further, the present invention provides a bearing, in particular a pitch bearing for a wind energy converter, comprising an outer ring, an inner ring, a plurality of rollers riding between the outer and inner ring, wherein the rollers are held in at least two carriage segments, with at least one roller spacing the two carriage segments apart from each other .

Even further, the present invention provides a bearing, in particular a pitch bearing for a wind energy converter, comprising an outer ring, an inner ring and a plurality of rollers riding between the outer and inner ring, wherein the rollers are held in a cage, the cage comprising webs between adjacent rollers, wherein the webs have a progressive spring characteristic .

Even further, the present invention provides a bearing, in particular a pitch bearing for a wind energy converter, comprising an outer ring, an inner ring, a plurality of rollers riding between the outer and inner ring, wherein the outer and/or the inner ring has a raceway geometry that varies in a circumferential direction of the outer or inner ring.

One idea of the present invention is to provide the bearing with means to reduce fluctuations in turning resistance. It has been recognized that the loads acting on a bearing, in particular a pitch bearing, change the turning resistance. The reason for this is, in particular, that elastic deformation of the rings and rollers of the bearing changes the contact angle of at least some of the rollers, thus changing the speed of the individual rollers as they move along the rings. The different speeds of the rollers cause reaction forces that ultimately increase the turning resistance. By reducing the fluctuations in turning resistance wear of the bearing can be reduced and the efficiency of the WEC as a whole increased.

The reduction of fluctuations in turning resistance may be achieved in numerous ways as presented hereinafter. For example, the reduction may be achieved by varying the cross- section of the outer and inner ring appropriately, by providing a suitable design of the groove of a respective ring, in which the rollers run, or by providing an appropriate cage design. Of course, the most effective way of reducing fluctuations in turning resistance may be to combine the different solutions described herein.

The dependent claims describe advantageous embodiments of the present invention.

According to one embodiment of the bearing of the present in- vention, the outer and/or the inner ring has a cross-section that varies in a circumferentia1 direction of the outer or inner ring. Deformation of the outer and inner ring may thus be controlled so as to be substantially constant along the cir- cumference of the inner and outer ring. As a result, fluctua- tions in turning resistance are reduced.

According to another embodiment of the bearing of the present invention, the variation in cross-section comprises at least a first and a second cross-section, wherein a radial thickness and/or an axial width of the first cross-section is larger than a radial thickness and/or an axial width of the second cross-section. Thus, the cross-sections may be designed so as to obtain the desired deformation under load.

According to another embodiment of the bearing of the present invention, the outer ring has an outer surface with an oval contour and/or the inner ring has an inner surface with an oval contour. In this manner, suitable first and second cross- sections may be easily obtained .

According to another embodiment of the bearing of the present invention, the outer ring has an outer surface with an oval contour and/or the inner ring has an inner surface with a substantially triangular contour, or the outer ring has an outer surface with a substantially triangular contour and/or the inner ring has an inner surface with an oval contour. This is another way of obtaining suitable first and second cross- sections .

According to another embodiment of the bearing of the present invention, the rollers are formed as balls running in respective grooves of the inner and outer ring. Typically, the bearing comprises one, two or three rows of balls.

According to another embodiment of the bearing of the present invention, the radius of a curvature of at least one of the grooves varies as a function of an angle measured in a plane transversal to the groove. The result of the groove having a radius that varies as a function of an angle is that the relationship between said radius and said angle may be chosen so as to reduce fluctuations in turning resistance. According to another embodiment of the bearing of the present invention, the raceway geometry of at least one of the grooves varies along the circumference of a respective ring. Due to the varying geometry, the contact angle of the balls is adjusted around the circumference of the outer and inner ring so as to ensure the same speed of each ball. Thus, fluctuations in turning resistance can be reduced.

According to another embodiment of the bearing of the present invention, the rollers are held in at least two cage segments, wherein at least one roller is placed between the two cage segments .

According to another embodiment of the bearing of the present invention, the cage segments comprise opposing ends, each end having an open pocket, wherein the at least one roller is held between the open pockets. The open pockets are designed in a manner that loads are transferred over the segment ends and not over the roller.

According to another embodiment of the bearing of the present invention, the cage segments comprise opposing straight ends, wherein the at least one roller is held between the straight ends. This results in a simple design.

According to another embodiment of the bearing of the present invention, two rows of rollers are held by each cage segment, wherein the rows are arranged in a staggered relationship to each other. This cage design also reduces fluctuations in turning resistance.

According to another embodiment of the bearing of the present invention, the rollers are held in a cage, the cage comprising webs between adjacent rollers, wherein the webs have a pro- gressive spring characteristic. In this way, when the rollers collide with the webs as the outer and inner ring turn with respect to each other, a reaction force can be built up slowly, thus reducing fluctuations in turning resistance.

According to another embodiment of the bearing of the present invention, the outer and/or the inner ring has a pretension that varies in a circumferential direction of the outer or inner ring. This variation is then provided in a manner so as to reduce fluctuations in turning resistance.

According to one embodiment of the WEC of the present invention, the first cross-section is orientated in the circumferential direction so as to absorb a maximum tension or compression load. Since the first cross-section is larger than the second cross-section, it will be deformed less. Thus, the deformation of the outer and inner ring will be substantially constant along their circumference. As a result, fluctuations in turning resistance are reduced.

According to another embodiment of the WEC of the present invention, the outer and inner ring each comprise a first portion which is loaded in tension when the bearing is subjected to a bending moment tending to tilt the inner ring with respect to the outer ring and a second portion which is loaded in compression when the bearing is subjected to said bending moment, wherein the first portion has the first cross-section and the second portion has the second cross section. As a result, fluctuations in turning resistance are reduced.

According to another embodiment of the WEC of the present invention, the outer or inner ring has a toothed section to cooperate with a gear for turning the blade around its longitudinal axis, wherein the toothed section has the second cross- section. As a result, fluctuations in turning resistance are reduced.

According to another embodiment of the WEC of the present invention, the blade has a cross-section that varies in a circumferential direction of the blade.

According to another embodiment of the WEC of the present invention, the variation in cross-section of the blade comprises at least a first and a second cross-section, wherein a radial thickness the first cross-section of the blade is larger than a radial thickness of the second cross-section of the blade, wherein the second cross-section of the blade is connected to the toothed section of the outer or inner ring. Thus, the blade is designed so as to ensure a constant deformation of the bearing. As a result, fluctuations in turning resistance are reduced.

According to another embodiment of the WEC of the present invention, a plurality of bolts fasten the inner ring to the blade and the outer ring to the hub, or the inner ring to the hub and the outer ring to the blade .

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is further described by way of example with reference to the accompanying figures, in which:

Fig. 1 shows a cross-section of a bearing; Fig. 2 shows an detail view I from Fig. 1; Fig. 3 shows a partial section of the bearing of Fig. 2;

Fig. 4 shows a cage and rollers at different speeds; shows the cage and rollers of Fig. 4 at the same speed; shows a diagram of a tilting moment as a function of time ; shows a perspective view of a WEC; shows a vector of the tilting moment; shows a diagram of the ideal load distribution in the raceway system caused by a tilting moment of the bearing of Fig. 1; shows the tension side of the bearing of Fig. 1 when subjected to the tilting moment; shows the compression side of the bearing of Fig. 1 when subjected to the tilting moment; shows a bearing according to one embodiment having an outer and inner ring, each having a varying cross- section; shows a bearing according to another embodiment hav- ing an outer and inner ring, each having a varying cross- section; shows a bearing according to another embodiment hav- ing an outer and inner ring, each having a varying cross- section; shows a bearing according to another embodiment hav- ing an outer and inner ring, the inner ring having a toothed portion; Fig. 16 shows a section of a bearing according to another embodiment having a ring with varying angle of curvature of a groove;

Fig. 17 shows the bearing of Fig. 16 in a side view;

Fig. 18 shows a diagram of the manufactured contact angle as a function of an angular position;

Figs. 19a and 19b show, in a top view, a cage according to one embodiment;

Fig. 20 shows, in a top view, a cage according to another embodiment;

Fig. 21 shows, in a top view, a cage according to yet another embodiment; and

Fig. 22 shows, in a top view, a cage according to yet another embodiment .

In the figures, the same reference numbers refer to the same or functionally equivalent components unless stated otherwise.

DETAILED DESCRIPTION

Fig. 1 shows a double row slewing bearing 100 capable of taking up axial loads, radial loads and tilting moments. The bearing 100 comprises an outer ring 102 and an inner ring 104. Two rows 106 of balls 108 run between an inner surface 110 of the outer ring 102 and an outer surface 112 of the inner ring 104. The outer and inner ring 102, 104 comprise bolt holes 114, 116 for attaching the inner ring 104 to a blade of a WEC (not shown in Fig. 1) and the outer ring 102 to the hub of the WEC. Corresponding bolts 119 are indicated by a dashed line. Thus, the blade turns with respect to the hub around the bearing axis 118. Alternatively, the outer ring 102 may also be attached to the blade and the inner ring 104 to the hub.

Fig. 2 shows a detail view I from Fig. 1. The balls 108 of a respective set 106 run in between two grooves 200 formed in the inner and outer surface 110, 112 of the outer and inner ring 102, 104. The two grooves 200 together define a raceway that contain the balls 108 of a respective set 106. The grooves 200 may each comprise a lubrication groove 202.

Fig. 2 also illustrates that the ball 108 has four contact points 204 with the outer and inner ring 102, 104.

Fig. 3 shows an angle a, the so called "contact angle", measured, in a plane 301 (corresponds to the plane of the paper) transversal to the groove 200, between a symmetry line 300 of the groove 200 extending radially with respect to the bearing axis 118 and a line 302 through one of the four contact points 204 and the center of a ball 108. The contact angle a is changed by the load acting on the bearing 100 and the resulting deformation. Thus, when the contact angle a changes as the ball 108 moves between the outer and inner ring 102, 104, the speed of the ball 108 also changes.

Fig. 4 and 5 show two balls 108, 108' of a single row 106 of balls 108 and a cage 400 of the bearing 100 separating the balls 108, 108' from each other. Under load, the ball 108' may travel faster than the ball 108 due to a variation of the contact angle a. As a result, the ball 108' also comes into contact with the cage 400 as shown in Fig. 5. Hereafter, the balls 108, 108' travel at the same speed. However, in this process a significant amount of rotational energy has been converted into frictional energy, thus increasing the turning resistance of the bearing 100 as whole. Fig. 6 shows the absolute value of a tilting moment vector M _xy (depicted as a solid line) as a function of time as well as the angle φ (depicted as a dashed line) of said vector as a function of time as is typically encountered in a WEC. Evidently, the tilting moment changes in value and direction with time and repeats itself more or less periodically for every full rotation of the rotor of the WEC.

Fig. 7 shows a WEC 700 comprising a rotor 702 . The rotor 702 comprises three blades 704 each connected to a hub 706 via a bearing 100 . The hub 706 drives a shaft (not visible) which in turn drives a generator inside a nacelle 708 atop a tower 710 .

The tilting moment vector M _xy is a moment vector in the XY- plane and expresses the moment that tends to tilt a respective blade 704 about its base 712 , where the blade 704 attaches via the bearing 100 to the hub 70 6 . To improve understanding, reference will be made only to a single blade 704 hereinafter, but the following applies to all blades 704 .

The coordinate system X, Y, Z has its origin at the base 712 of the blade 704 . The acting forces and moments are designated as Fx, Fy, Fz, Mx, My and Mz . The tilting moment vector M _xy is most of all a result of wind loads and gravity acting on the blade 704 . The Z-axis corresponds to the longitudinal axis of the blade 704 . The angle cp (see Fig. 6 ) changes as the blade 704 is turned inside its bearing 100 to respond to, for example, changing wind conditions, and as the blade 704 revolves about the shaft axis 714 to drive the generator. The tilting moment M _xy is absorbed inside the bearing 100 .

Fig. 8 illustrates the tilting moment vector M _xy in the XY plane and the definition of the angle cpMxy. Fig. 9 shows an ideal load distribution in the bearing, caused by tilting moment. The load distribution in the real bearing differs strongly from Fig. 9, because of the following effects .

One side is termed the compression side 900 and the other side the tension side 902. The tension side is illustrated in Fig. 10 and the compression side is illustrated in Fig. 11. The large arrows in Fig. 10 and 11 indicate the forces exerted by the bolts 119 in the respective holes 114 and 116 on the outer and inner ring 102, 104. The smaller arrows indicate the compressive forces acting on each ball 108. On the tension side (Fig. 10) bolts 119 connecting the blade 704 to the inner ring 104 are loaded in tension, whereas on the compression side the load in the corresponding bolts 119 is reduced. Thus, the inner ring 104 is deformed more on the tension side than on the compression side. Due to the larger deformations on the tension side 902 than on the compression side 900, the contact angle a (see Fig. 3) also changes more on the tension side 902. This again changes the speeds of the individual balls 108 as illustrated in Fig. 4 and 5, which eventually causes increased turning resistance.

It has thus been discovered that fluctuations in turning resistance need to be reduced to improve bearing performance.

Fig. 12 shows a bearing 100 according to one embodiment of the present invention.

The outer and inner ring 102, 104 each have a cross-section that varies in a circumferential direction (indicated by a double arrow) of the outer or inner ring 102, 104. Specifically, the variation in cross-section comprises at least a first and a second cross-section 1200, 1202 of the outer ring 102 and a first and a second cross-section 1200', 1202' of the inner ring 104. In this embodiment, the radial thickness 1204, i.e. the thickness measured in a direction radial to the cen- terline 118 of the bearing 100, at the first cross-section 1200 of the outer ring 102 is larger than the radial thickness at the second cross-section 1202 of the outer ring 102. Further, the radial thickness 1204' of the inner ring 104 at the first cross-section 1200 ' is larger than the radial thickness 1204' at the second cross-section 1202'. The axial width 120 (see Fig. 1), i.e. the width measured in a direction normal to the plane of the paper, may be constant at all cross-sections 1200, 1202, 1200', 1202', or vary as well.

First portions 1206, 1206' of the outer and inner ring 102, 104 comprising the first cross-sections 1200, 1200' correspond to the tension side 902 of the bearing 100, see Fig. 9, when the bearing 100 is installed inside the WEC 700 to support the blade 704. Second portions 1208, 1208' of the outer and inner ring 102, 104 comprising the second cross-sections 1202, 1202' correspond to the compression side 900 of the bearing 100, see Fig. 9.

The first portions 1206, 1206' of the outer and inner ring 102, 104 having the increased radial thickness 1204 see the higher loads on the tension side 902. The first portions 1206, 1206' are thus deformed to a lesser and therefore similar extent as the second portions 1208, 1208', which see the lower loads on the compression side 900 and are thus deformed to a lesser extent per se. As a result, the contact angle a of the balls 108 varies to a lesser extent over the entire circumference of the inner and outer ring 102, 104, providing for reduced fluctuations in turning resistance. The fact that loads of different magnitudes are applied to the bearing 100 around its circumference (the compression and tension scenario above being only one example) may also be a result of the supporting structure supporting the bearing 100 as the bearing 100 takes up the tilting moment M _xy from the blade 704. For example, if the supporting structure, in particular the hub 706, comprises a man hole (not shown) this will reduce the stiffness of the bearing 100 in a corresponding portion. If now the first portions 1206, 1206' of the bearing 100 are located adjacent to said man hole, deformations in the first and second portion 1206, 1206', 1208, 1208' will be similar, thus providing for a reduced turning resistance.

The variation in radial thickness 1204, 1204' is, according to the present embodiment, achieved by providing the outer ring 102 with an outer surface 1209 having an oval contour and the inner ring 104 with an inner surface 1210 also having an oval contour. The inner surface 110 of the outer ring 102 and the outer surface 112 of the inner ring 104 each have a circular contour .

Instead or in addition to the variation in cross-section, the bolt holes 114, 116, see Fig. 1, with their corresponding bolts 119 may be arranged asymmetrically around the axis of rotation Z of the blade 704. The bolt holes 114, 116 are then arranged so as to reduce fluctuations in turning resistance.

Instead or in addition to the variation in cross-section and/or the asymmetric arrangement of the bolt holes, the blade 704 may, in a portion where it connects to the bearing 100, have a cross-section that varies in a circumferential direction of the blade 704. This variation is then provided in a manner so as to reduce fluctuations in turning resistance. The first and second portions 1206, 1206', 1208, 1208' having the first and second cross-sections 1200, 1200', 1202, 1202' may be produced by rolling.

Fig. 13 shows a bearing 100 according to further embodiment of the present invention. Only the differences between the embodiments of Fig. 12 and Fig. 13 will be explained below.

The variation in radial thickness is, according to the present embodiment, achieved by providing the outer ring 102 with an outer surface 1209 having an oval contour. The oval contour has the shape of a circle that has been flattened on opposite sides 1300, 1302. The oval contour may, in particular, have an elliptical shape. As a result, the outer ring 102 comprises two first portions 1206 opposing each other and two second portions 1208 also opposing each other. The first portions 1206 each comprise a cross-section 1200 with a radial thickness 1204 that is larger than a radial thickness 1204 of a cross-section 1202 of either of the second portions 1208.

Preferably, the outer ring 102 is symmetric about an axis 1304 radially through the bearing axis 118 as well as about an axis 1306 at right angles to the axis 1304 and also passing radially through the bearing axis 118.

When the bearing 100 is installed on the WEC 700, the portions 1206 of the bearing may be arranged in the plane in which the maximum tilting moment M _xy acts.

Or, when the bearing 100 is installed on the WEC 700, the portions 1206 of the bearing may each be arranged adjacent to a portion of the supporting structure having a reduced stiffness, which may, for example, result from a manhole. The inner ring 104 may comprise an inner surface 1210 also having an oval contour corresponding to a circle that has been flattened on three sides 1308, 1310, 1312. The shape of the inner surface 1210 thus resembles a triangle, and is therefore referred to as triangular herein. As a result, the inner ring 104 has three first portions 1206' and three second portions 1208' arranged in alternation in a circumferential direction (indicated by a double arrow) around the bearing axis 118. The first portions 1206' each comprise a cross-section 1200' with a radial thickness 1204' that is larger than a radial thickness 1204' of a cross-section 1202' of any of the second portions 1208'. Preferably, the inner ring 104 is symmetric about an axis 1314 radially through the bearing axis 118, as well as axes (not shown) arranged at an angle of 120 degrees to the axis 1304 in the clockwise and counterclockwise direction.

The inner surface 1210 may also have an oval contour corresponding to a circle that has been flattened on more than three sides, for example, four, five or six sides. The same applies to the outer surface 1209.

Fig. 14 shows a bearing 100 according to further embodiment of the present invention.

In this embodiment the outer ring 102 comprises an outer surface 1209 having the triangular shape explained in connection with the inner surface 1210 of the inner ring 104 of Fig. 13. The inner ring 102 comprises an inner surface 1210 having the oval shape explained in connection with the outer surface 1209 of the outer ring 102 of Fig. 13. The features explained in connection with Fig. 13 thus apply analogously to Fig. 14.

Fig. 15 shows a bearing 100, a gear 1500 and the blade 704 according to a further embodiment of the present invention. The blade 704 is shown in cross-section. The blade 704 partially covers the inner ring 104.

The bearing 100 comprises an outer ring 102 having a cross- section 1200 with a constant radial thickness 1204 over the entire circumference of the outer ring 102.

The inner ring has a first portion 1206' and a second portion 1208'. The first portion 1206' comprises a cross-section 1200' having a constant radial thickness 1204'. The second portion 1208' comprises multiple cross-sections 1202' having a smaller radial thickness 1204' than the radial thickness 1204' of the first portion 1206' . This is a result of the second portion 1208' comprising teeth 1502 formed along the inner surface 1210 of the inner ring 102. The teeth 1502 cooperate with the gear 1500 for turning the blade 704, which the bearing 100 supports inside the WEC 700, around its longitudinal axis Z (identical with the bearing axis 118) .

When the first portion 1206' is arranged so as to correspond to the tension side 902, see Fig. 11, and the second portion 1208' is arranged so as to correspond to the compression side 900, see Fig. 11, then deformations in the first and second portion 1206', 1208' (and in the respective portions of the outer ring 102) will be similar, thus providing for a reduced turning resistance.

The blade 704 may be fixedly connected at its base 712 to the inner ring 102. Advantageously, the blade 704 has a first and a second portion 1506, 1508. The first portion 1506 comprises a first cross-section 1504 and is connected to the first portion 1206' of the inner ring 102. The second portion 1508 comprises a second cross-section 1510 and is connected to the second portion 1208' of the inner ring 102. The first cross- section 1504 has a larger radial thickness 1512 than the second cross-section 1510. To this end, the blade 704 is formed with a circular outer surface 1514 and an oval inner surface 1516. The inner surface 1516 may also be formed triangular or in any of the other shapes discussed in connection with Fig. 13 and 14.

Again, in this manner deformations in the first and second portion 1206', 1208' of the inner ring 104 (and in the respective portions of the outer ring 102) of the bearing 100 will be similar, thus providing for a reduced turning resistance.

According to another embodiment, the blade 704 is connected to the outer ring 102. In this case, the inner ring 104 is connected to the hub 706. The foregoing ideas equally apply to this setup.

Fig. 16 illustrates a portion of a bearing 100 according to another embodiment of the present invention.

The groove 200 has radius R _g that varies as a function of an angle β . The angle β is measured, in the plane 301 transversal to the groove 200, between the symmetry line 300 of the groove 200 extending radially with respect to the bearing axis 118 and a respective radius R _g to a point on the surface of the groove 200. Thus, the groove 200 has at an angle β ι a radius R _g i and at an angle β ₂ a radius R _g2 , where βι ≠ βι and R _g i ≠ R _g2 . Virtually any suitable function may be chosen that relates the radius R _g and the angle β .

The result of the groove 200 having a radius R _g that varies as a function of an angle β is that the relationship between the radius R _g and β may be chosen so as to reduce fluctuations in turning resistance. To this end, the relationship between the radius R _g and the angle β is chosen so as to match the deformations of the groove 200 and balls 108, when the bearing 100 is under load.

Fig. 17 shows a bearing 100 according to another embodiment of the present invention.

The embodiment of Fig. 16 may be even improved when the raceway geometry according to Fig. 3 also varies as a function of an angle γ. The angle γ is measured in the circumferential direction of the bearing 100, i.e. the circumferential direction of the outer and inner ring 102, 104.

For example, the manufactured contact angle a may vary as a function of the angle γ as illustrated in Fig. 18. According to this embodiment, loads 1700 (as shown in Fig. 17) are introduced at specific locations into the outer ring 102 from the supporting structure 706, when the bearing 100 is installed inside the WEC 700. The groove 200 of the outer ring 102 has a smaller manufactured contact angle a in the locations corresponding to the loads 1700 than in other locations as can be seen in Fig. 18. The contact angle a of the balls 108 is adjusted around the circumference of the outer and inner ring 102, 104 so as to ensure the same speed of each ball 108.

Thus, fluctuations in turning resistance can be reduced.

Figs. 19a and 19b illustrate a portion of a bearing 100 according to another embodiment of the present invention.

The bearing 100 has a cage 1900 comprising segments 1902 and 1904. Each segment 1902, 1904 has a number of pockets 1906 each holding a ball 108. At their respective ends 1908, 1910, the segments 1902, 1904 each comprise pockets 1912, 1914 that are open in the circumferential direction (indicated by a dou- ble arrow) of the bearing 100. The open pockets 1912, 1914 are designed in a manner that loads are transferred over the segment ends 1908, 1910 and not over the roller 108. In particular, the open pockets 1912, 1914 are dimensioned to form a combined pocket 1906' that has a greater dimension in the circumferential direction than the diameter of the ball 108. As a result, when a roller 108 is disposed within the combined pocket 1906', the respective ends 1908, 1910 of the segments 1902, 1904 abut, permitting load transfer over the ends 1908, 1910. In the illustrated embodiment, the combined pocket 1906' has the same circumferential dimension as the closed pockets 1906. The distances 1918 between the balls 108 in the circumferential direction are kept the same.

This cage design also reduces fluctuations in turning resistance .

Fig. 20 illustrates a portion of a bearing 100 according to another embodiment of the present invention.

Instead of the pockets 1912, 1914 as shown in Fig. 19, the segments 1902 and 1904 of Fig. 20 have straight ends 1908, 1910. Otherwise, the embodiment of Fig. 20 corresponds to the embodiment of Fig. 19.

Fig. 21 illustrates a portion of a bearing 100 according to another embodiment of the present invention.

The cage 1900 comprises two rows 106, 106' of pockets 1906 holding two rows of balls 108. The pockets 1906, 1906' are arranged in a staggered relationship to one another, thus two pockets 1906, 1906' of different rows 106, 106' being offset to each other in the circumferential direction. The offset is indicated at 2100. This offset 2100 also applies to the open pockets 1912, 1914. Each pair of open pockets 1912, 1914 has an offset 2100 with respect to another pair of open pockets 1912', 1914'. Otherwise, the embodiment of Fig. 21 corresponds to the embodiment of Fig. 19.

Fig. 22 illustrates a portion of a bearing 100 according to another embodiment of the present invention.

The bearing 100 has a cage 1900 comprising pockets 1906 each holding a roller 108, for example a ball or a cylindrical roller. Adjacent pockets 1906 are defined by a web 2200 between them. Each web 2200 is designed with a progressive spring characteristic, i.e. the derivative of the force required to compress each web 2200 in the circumferential direction (indicated by a double arrow) increases with compression.

In this way, when the rollers 108 collide with the webs 2200 as the outer and inner ring 102, 104 turn with respect to each other, a reaction force can be built up slowly, thus reducing fluctuations in turning resistance.

Although the present invention has been described in accordance with preferred embodiments, it is obvious for a person skilled in the art that modifications are possible in all embodiments without departing from the scope of the claims.

Also, the embodiments described herein may be combined. For example, the embodiments of Fig. 12, 16 and 19 may be combined to achieve a minimum turning resistance. LIST OF REFERENCE SIGNS

100 bearing

102 outer ring

104 inner ring

106 row

108 ball

108 ' ball

110 inner surface

112 outer surface

114 hole

116 hole

118 bearing axis

119 bolt

120 axial width

200 groove

202 lubrication groove

204 contact point

300 axis of symmetry

301 plane

302 line

400 cage

700 WEC

702 rotor

704 blade

706 hub

708 nacelle

710 tower

712 base

900 compression side

902 tension side

1200 cross-section

1202 cross-section

1204 radial thickness 1204 ' radial thickness

1206 first portion

1206' first portion

1208 second portion

1208 ' second portion

1209 outer surface

1210 inner surface

1300 side

1302 side

1304 axis of symmetry

1306 axis of symmetry

1308 side

1310 side

1312 side

1314 axis

1500 gear

1502 tooth

1504 cross-section

1506 first portion

1508 second portion

1510 cross-section

1512 radial thickness

1514 outer surface

1516 inner surface

1700 load

1900 cage

1902 segment

1904 segment

1906 pocket

1906' pocket

1908 end

1910 end

1912 open pocket

1912 ' open pocket 1914 open pocket

1914' open pocket

1918 distance

2200 web

C compression side

F Force

M _xy tilting moment

R _g groove radius

R _b ball radius

T tension side

X direction

Y direction

Z direction a angle

β angle

Y angle

cp angle