Abstract:
A bearing with at least one row of rolling elements, where at least one of the at least one row of rolling elements includes a plurality of first rolling elements adapted for substantially transferring axial forces, and a plurality of second rolling elements adapted for substantially transferring radial forces. To minimize friction, at least one of the at least one row of rolling elements includes a mechanism for separating rolling elements, where the mechanism for separating rolling elements includes one or more cages for retaining the rolling elements, where the one or more cages include a pocket adapted for allowing the rolling element to be displaced in the longitudinal direction of the row of elements. A method for transferring forces through a bearing of a wind turbine is also contemplated.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application is a continuation of pending International patent application PCT/DK2007/000545 filed on Dec. 14, 2007 which designates the United States and claims priority from Danish patent application PA 2006 01659 filed on Dec. 18, 2006, the content of which is incorporated herein by reference. 
       FIELD OF THE INVENTION 
       [0002]    The invention relates to a bearing with at least one row of rolling elements, where at least one of the at least one row of rolling elements comprise a plurality of first rolling elements adapted for substantially transferring axial forces, and a plurality of second rolling elements adapted for substantially transferring radial forces and a method for transferring forces through a bearing of a wind turbine. 
       BACKGROUND OF THE INVENTION 
       [0003]    In general, larger wind turbines use special configuration, four-point contact ball bearings or crossed roller bearings for applications throughout the wind turbine. This is the case for pitch and yaw bearings especially. 
         [0004]    The balls and/or rollers of the bearings carries loads applied to their supporting structures, comprising bearing rings, and provides low friction movement around the rotating axial axis of the bearing. 
         [0005]    The mentioned types of bearings have distinct load paths for the rolling elements and are capable of carrying simultaneous moments and radial and axial loads. Hardened surfaces on inner and outer rings provide points of contact between stationary and moving elements of the bearing. 
         [0006]    German patent application DE 195 10 182 A1 discloses, for use in a wind turbine, a yaw roller bearing where rollers ( FIG. 3 , item 7) are positioned in an 45 degree angle relative to the resulting axial forces. 
         [0007]    A disadvantage of the mentioned type of roller bearing is that, as pitch bearings primarily are influenced by axial loads, rollers positioned in a 45 degree angle are not loaded in their primarily direction and the load on each separate roller and on the bearing construction itself is therefore is increased. 
         [0008]    German patent application DE 102 57 195 A1 discloses a general purpose cross roller bearing, where one or more ball elements are inserted in the row of roller elements to adjust the gap between roller elements. 
         [0009]    A disadvantage of the described bearings is that for large scale wind turbine bearings carrying great loads, inertia forces between adjacent roller elements results in increased deterioration of the bearing and a reduction of its lifetime. Further roller contact angles, which is not in the direction of the loading compromises the life expectancy. As an example the maximum moment on a modern 3 MW wind turbine is approximately 8,000 kNm. 
         [0010]    It is an object of the invention to provide technique without the above mentioned disadvantages and especially it is an object to simplify the construction of the bearings and hereby increase their operational reliability. 
       SUMMARY OF THE INVENTION 
       [0011]    The invention relates to a bearing with at least one row of rolling elements, where at least one of said at least one row of rolling elements comprise a plurality of first rolling elements adapted for substantially transferring axial forces, and a plurality of second rolling elements adapted for substantially transferring radial forces, wherein at least one of said at least one row of rolling elements comprises means for separating rolling elements, wherein said means for separating rolling elements comprises one or more cages for retaining said rolling elements, where said one or more cages comprise a pocket adapted for allowing the rolling element to be displaced in the longitudinal direction of said row of elements. 
         [0012]    Hereby it is achieved that the rolling elements are separated and retained in a desired position in relation to adjacent rolling elements and in relation to the total construction of the bearing, and the pocket that allows the rolling element to be displaced in the longitudinal direction of said row of elements, thereby reducing friction of the bearing, in particular by relatively small angular movements of the bearing. By using separate rolling elements for transferring axial and radial forces respectively in a bearing it is ensured that the bearing can be dimensioned properly in order to transfer forces acting on the bearing. 
         [0013]    It is also ensured that said separate rolling elements of a bearing are substantially only transferring forces that it is designed to whereby wear and tear on the bearing is minimized. 
         [0014]    Furthermore it is ensured that the number of rows of rolling elements can be minimized whereby the construction of the bearing is simplified and even further the production and installation costs can be reduced. 
         [0015]    In another aspect of the invention said first rolling elements have a rotational axis directed substantially perpendicular to the rotational axis of said bearing. Hereby it is ensured that said first rolling elements are transferring moments and primarily axial forces acting on structures attached to the bearing and that the bearing can be designed to handle said moments and forces. 
         [0016]    In another aspect of the invention said second rolling elements have a rotational axis directed substantially in parallel to the rotational axis of said bearing. Hereby it is ensured that said second rolling elements are transferring primarily radial forces acting on structures attached to the bearing and that the bearing can be designed to handle said forces. 
         [0017]    In another aspect of the invention said first rolling elements are different from said second rolling elements. Hereby it is ensured that axial and radial forces acting on the bearing are transferred to the bearing supporting structures substantially by separate elements whereby the design of the bearing and the bearing supporting structures can be simplified. 
         [0018]    In another aspect of the invention said bearing comprises at least two rows of rolling elements. Hereby it is ensured that the bearing structure can be better stabilized and optimized regarding distribution of moments and forces acting on the bearing, slip in the bearing etc. 
         [0019]    The rolling elements may have any arbitrary profile, however according to one aspect of the invention the rolling elements are substantially cylindrical rollers such as crowned rollers or ball rollers. Hereby it is ensured that the bearing can be designed to transfer maximal load in desired directions and hereby utilizing maximal capacity of the bearing to specific purposes. 
         [0020]    In another aspect of the invention said rolling elements make contact with supporting structures such as bearing rings on raceways. It is herby ensured that the contact between rolling elements and supporting structures is on hardened structures designed to be able to handle the resulting moments and forces and designed to minimize wear and tear on said supporting structures. 
         [0021]    In another aspect of the invention the ratio between the number of said first rolling elements and the number of said second rolling elements is in the range from 1:2 to 1:50, more preferred in the range from 1:5 to 1:20 such as 1:8. Hereby it is ensured that the bearing can be designed to transfer the actual radial and axial forces acting on the bearing also taking into account the relationship in size between said forces. 
         [0022]    Another aspect of the invention relates to a wind turbine comprising a bearing as outlined above. The bearing is very suitable for heavy duty bearings for wind turbines, especially bearings which are often subjected to small angular movements. 
         [0023]    In another aspect of the invention said bearing is a pitch bearing for pitching one or more blades of said wind turbine in relation to a hub of said wind turbine. Hereby it is ensured that the said pitch bearing can be designed to handle the forces acting on a wind turbine rotor blade to hub connection ensuring that the control of the pitch of the rotor blade can be done easily and accurate. Furthermore it is ensured that the bearing can be designed and optimized to specific operational parameters such as working range. By working range is meant the angle range at which said bearing is used e.g. a non-full rotational range such as 90 degrees. As an example pitch motions are often in the range of 20 degrees, and small corrections of the pitch angle, i.e. pitch motions of up to 5 degrees, are common. 
         [0024]    In another aspect of the invention said bearing is a yaw bearing for yawing a nacelle of said wind turbine in relation to a tower of said wind turbine. Hereby it is ensured that the said yaw bearing can be designed to handle the forces acting on a wind turbine nacelle to tower connection ensuring that the control of the yaw of the nacelle can be done easily and accurate. 
         [0025]    By longitudinal direction of said row of elements is meant the direction of rolling for the rolling elements i.e. the direction of its raceway. For a circular bearing that is a direction that is tangential to the periphery (circular arch) of the bearing. 
         [0026]    The wider diameter of the pocket d i  compared to the diameter of the rolling element d r  allows a higher range of rolling motion for the rolling element before reaching the walls of said cage, instead of sliding due to different velocity of raceways for axial and radial rollers respectively. 
         [0027]    Especially it is ensured for bearings with motion back and forth around some average point, such as a pitch bearing where motion is not a repeated 360 degree turn, that the radial rolling element retained in said cage comprising a pocket can roll freely within the limits of the pocket. 
         [0028]    Furthermore the invention also relates to a method for transferring forces through a comprising the steps of providing the bearing with at least one row of rolling elements, where at least one of said at least one row of rolling elements comprises a plurality of first rolling elements adapted for substantially transferring axial forces, and a plurality of second rolling elements adapted for substantially transferring radial forces, further comprising the additional step of providing at least one of said at least one row of rolling elements with means for separating rolling elements, wherein said means for separating rolling elements comprises one or more cages for retaining said rolling elements, where said one or more cages comprise a pocket adapted for allowing the rolling element to be displaced in the longitudinal direction of said row of elements. 
         [0029]    Hereby an advantageous method is obtained in that relatively large loads may be transferred with relatively low friction, especially for small angular movements of the bearing. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0030]    The invention will be described in the following with reference to the figures in which 
           [0031]      FIG. 1  illustrates a large modern wind turbine including three wind turbine blades in the wind turbine rotor, 
           [0032]      FIG. 2  illustrates schematically forces and moment acting on a wind turbine rotor blade pitch bearing 
           [0033]      FIG. 3  illustrates a part of a cross section of a pitch bearing comprising ball rollers as known in the art, 
           [0034]      FIG. 4   a  illustrates schematically the principle of a conventional single row bearing comprising spacers, 
           [0035]      FIG. 4   b  illustrates schematically the principle of a conventional single row bearing comprising a cage, 
           [0036]      FIG. 5  illustrates a part of a cross section of a wind turbine pitch bearing comprising rollers as known in the art, 
           [0037]      FIG. 6  illustrates schematically the principle of one embodiment of the invention as a crossed roller bearing comprising a combination of spacers and cages, 
           [0038]      FIG. 7   a  illustrates schematically the principle of a bearing according to one embodiment of the invention with a cage comprising a pocket, 
           [0039]      FIG. 7   b  illustrates schematically the principle of another embodiment of the invention as a crossed roller bearing comprising a combination of spacers and cages comprising a pocket, 
           [0040]      FIG. 8  illustrates schematically the principle of a bearing according to another embodiment of the invention comprising two rows of crossed rollers comprising spacers and cages comprising a pocket, and 
           [0041]      FIG. 9  illustrates for one embodiment of the invention a bearing according to the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0042]      FIG. 1  illustrates a modern wind turbine  1  with a tower  2  and a wind turbine nacelle  3  positioned on top of the tower. 
         [0043]    The wind turbine rotor, comprising at least one blade such as three wind turbine blades  5  as illustrated, is connected to the hub  4  through pitch mechanisms  6 . Each pitch mechanism includes a blade bearing and individual pitch actuating means which allows the blade to pitch. The pitch process is controlled by a pitch controller. 
         [0044]    As indicated on the figure, wind over a certain level will activate the rotor and allow it to rotate in a substantially perpendicular direction to the wind. The rotation movement is converted to electric power which usually is supplied to the utility grid as will be known by skilled persons within the area. 
         [0045]      FIG. 2  illustrates a front view of a wind turbine hub  4 . Wind turbine rotor blades  5  are connected to the hub  4  through a pitch mechanism  6  comprising pitch bearings that supports the rotor blades  5  to forces acting in both axial (longitudinal) and radial direction as well as to moments M as indicated on the figure. 
         [0046]      FIG. 3  schematically illustrates parts of a cross section of one embodiment of a double row ball bearing as used in wind turbines  1  known in the art. 
         [0047]    The bearing comprise a nose bearing ring  8 , a split C-bearing ring  9  and two rows of ball rolling elements  10  managing bending moments and axial and radial forces. The directions of the applied moment and forces are indicated by arrows on the figure. 
         [0048]    Furthermore the bearing comprise through holes  11  for attachment to its supporting structures by attachment means such as screws, studs, bolts or rivets. 
         [0049]      FIG. 4   a  illustrates schematically the principle of spacers  12  used in one embodiment of a conventional roller bearing. Spacers  12  are located in between rolling elements  13  and are formed as to substantially fit the shape of its adjacent rolling elements. 
         [0050]    In one embodiment of the invention rolling elements  13  are formed as crowned rollers i.e. rollers whose diameter is intentionally larger in the middle than at the ends. Crowning of the rolling elements is expected to reduce roller edge loading. 
         [0051]    This type of roller bearings can support high loads. In many embodiments this type of bearings can be taken apart and can therefore be fitted and dismantled more easily. 
         [0052]    Spacers  12  can be made of various wear well materials such as aluminum, brass, stainless steel, chrome steel, bronze, synthetic materials etc. and is used to separate and position the rolling elements  13  at substantially equal intervals around the bearing raceways  14  in order to maximize efficiency and minimize wear. Furthermore spacers  12  reduce vibratory impulses which might be imposed by the surfaces of the races. 
         [0053]      FIG. 4   b  illustrates schematically the principle of a cage  15  comprising rolling elements  13  used in one embodiment of a conventional roller bearing. 
         [0054]    Cages  15  have the same purpose of separating and positioning as said spacers  12  and can be made of the same said materials. 
         [0055]      FIG. 5  illustrates schematically parts of cross sections of one embodiment of a 3-row roller pitch bearing  16  as used in wind turbines  1  known in the art. 
         [0056]    The bearing comprise a nose bearing ring  8 , a split C-bearing ring  9 , rows of rolling elements managing axial forces  17  and rows of rolling elements managing radial forces  18  and where axial and radial rolling elements are positioned substantially perpendicular to each other. Rolling elements  17 ,  18  are comprised in cages  15 . 
         [0057]    Bearing raceways  14  on nose bearing ring  8  and C-bearing rings  9  ensure that wear and tear from the contact between rolling elements  17 ,  18  and bearing rings  8 ,  9  is minimized resulting in a prolonged lifetime for the bearing. Of course the opposite lay-out is equally possible, i.e. the nose bearing ring/C-bearing rings can be used both as outer ring and inner ring. 
         [0058]    Furthermore the bearing comprise through holes  11  for attachment of its supporting structures by attachment means such as screws, studs, bolts or rivets. 
         [0059]    The directions of the applied axial and radial forces are indicated by arrows on the figure. 
         [0060]      FIG. 6  illustrates for one embodiment of the invention the principle of a cross roller bearing  16  where a single row of rolling elements comprise rolling elements that manage axial forces  17  and rolling elements that manage radial forces  18  in combination. 
         [0061]    Compared to a conventional bearing comprising only rolling elements that manage axial forces  17 , a number of rolling elements are exchanged by substantially perpendicular positioned radial rolling elements  18 . Spacers  12  between two axial rolling elements  17  is not modified and has a similar form compared to a conventional bearing A special formed cage  15  supporting one or more rolling elements  18  is formed as to fit the shapes of its adjacent rolling elements  17 . 
         [0062]    For another embodiment of the invention, spacers  12  and/or cages  15  are used to separate and position some or all rolling elements  17 ,  18 . 
         [0063]    For one embodiment of the invention every second rolling element is an axial rolling element  17  and every other second rolling element is a radial rolling element  18  i.e. the ratio between the number of rolling elements managing axial forces  17  and the number of rolling elements managing radial forces  18  is 1:1. 
         [0064]    For other preferred embodiments of the invention the ratios between the number of rolling elements managing axial forces and the number of rolling elements managing radial forces are more than 1:1, preferred in the range from 1:2 to 1:50, more preferred in the range from 1:5 to 1:20 such as 1:8 i.e. the number of axial rolling elements  17  is greater than the number of radial rolling elements  18 . 
         [0065]    For a bearing according to the present invention, said ratio is determined depending on demands to loadings, moments and deflections in axial and radial direction respectively that the bearing must be able to manage. 
         [0066]    In a rotating bearing of the invented type, the velocity of the center of the radial rolling elements  18  may be different than the velocity of the center of the axial rolling elements  16  e.g. due to different length in raceways  14 . As the radial rolling elements are held in position by cages this may result in a partly sliding instead of rolling motion for said radial rolling elements. 
         [0067]      FIG. 7   a  and  FIG. 7   b  illustrates for another embodiment of the invention the principal construction of one row of a bearing comprising axial rolling elements  17  separated by spacers  12  and cages  15  comprising radial rolling elements  18 . 
         [0068]      FIG. 7   a  illustrates a frontal view of one cage  15  comprising one radial rolling element  18  and where the cage  15  is constructed in a way that it includes a pocket  19  i.e. room for free play for the radial roller  18  in the direction of movement as indicated by arrows. The diameter d i  of the pocket  19  in the cage  15  is wider than the diameter d r  of the radial rolling element  18 . The wider diameter of d i  compared to d r  enables a higher degree of rolling motion for the radial rolling element  18  instead of sliding due to the velocity of the center of the radial rolling elements  18  may be different than the velocity of the center of the axial rolling elements  16  e.g. due to different length in raceways  14 . Hereby the radial rolling element  18  can move freely over a distance range before reaching the walls of its surrounding cage  15 . 
         [0069]    It is currently expected that the maximum play will be the diameter of the radial rolling element (i.e. d i =2*d r ), but in many cases the play will be in the interval of 0.1−0.5 (i.e. d i =1.1*d r  to 1.5*d r ), and often the play will be approximately 0,25 (i.e. d i =1.25*d r ). As an example the bearing may be a pitch bearing having a diameter of 1 m to 5 m. With a pitch bearing diameter of 2 m, a pitch motion of e.g. 5 degrees equals a distance of less than 9 cm, which the rolling elements must travel. With the mentioned play, the radial rolling elements may travel at least a part of this distance freely. 
         [0070]      FIG. 7   b  illustrates part of one row of a bearing comprising axial rolling elements  17 , spacers  12  separating said axial rolling elements  17 , and radial rolling elements  18  comprised in cages  15  comprising pockets  19  enabling free move for the radial rolling element  18 . 
         [0071]      FIG. 8  illustrates parts of a cross section of a pitch bearing according to one embodiment of the invention. 
         [0072]    The bearing comprises a nose bearing ring  8 , a split C-bearing ring  9  and rows of rolling elements  17 ,  18  managing both axial and radial forces together with spacers  12  and cages  15 , where said axial and radial rolling elements  17 ,  18  are positioned substantially perpendicular to each other. The ratio between the number of axial and radial rolling elements  17 ,  18  can be e.g. in the range between 1:1 to 1:100, preferred in the range from 1:2 to 1:50, more preferred in the range from 1:5 to 1:20 such as 1:8 i.e. the number of axial rolling elements  17  is greater than the number of radial rolling elements  18 . 
         [0073]    The directions of the applied axial and radial forces are indicated by arrows on the figure. 
         [0074]    Furthermore the bearing comprise through holes  11  for attachment on its supporting structures by attachment means such as screws, studs, bolts or rivets. 
         [0075]    As can be seen on the figure hardened bearing raceways  14  on nose and C-bearing rings  8 ,  9  ensure that wear and tear from the contact between rolling elements  17 ,  18  and bearing rings  8 ,  9  is minimized. 
         [0076]      FIG. 9  illustrates schematically one embodiment of the invention where a row of rolling elements  13  comprises both axial and radial rolling elements  17 ,  18  and is forming a full 360 degree circle. For this embodiment the ratio between the number of axial and radial rolling elements  17 ,  18  is 1:4. 
         [0077]    The skilled person will find no difficulty in assessing a suitable number of rolling elements for a given application. For a pitch bearing having a diameter in the range of 1 to 5 m, the number of rolling elements in each row would typically be from 50 to 500 rolling elements per row. Similarly it will be straight forward for the skilled person to establish suitable dimensions of the rolling elements for a given application. For pitch bearings of the mentioned size, suitable dimensions are from 30 mm to 150 mm in length and diameter.