Patent Publication Number: US-2018051678-A1

Title: Wind turbine main shaft bearing and method of upgrading a main shaft bearing

Description:
BACKGROUND 
     The present invention relates to a bearing system for use in supporting the main shaft of a wind turbine assembly. 
     In a modular gear drive wind turbine with a 3-point mount main rotor bearing arrangement, the main bearing must support the weight of the hub as radial load, along with thrust from the wind as an axial load, while accommodating overturning moments that result in dynamic misalignment. A spherical roller bearing is typically used in order to accommodate this combination of loads. Any thrust load not absorbed by the main bearing is transmitted through the gearbox carrier bearings to the torque arms, and then to the nacelle bedplate. Spherical roller main bearings in 3-point wind turbine models normally show significant wear and damage before their designed 20 year life. This damage often causes the main bearing to fail prematurely, but also results in the bearing losing its designed roller to raceway conformity, thus transmitting more of the system thrust into the gearbox. The gearbox planetary section often experiences increasing thrust damage over time as the main bearing wears. 
     SUMMARY 
     The mechanism responsible for the life-limiting wear mode of micro-pitting that afflicts main shaft spherical roller bearings is roller/raceway sliding in low lambda conditions. These conditions are unavoidable for spherical roller bearings operating in main shaft pillow blocks of wind turbines. The present invention recognizes this shortcoming of spherical roller bearings in the wind turbine main shaft bearing application, and provides an improved solution, utilizing tapered roller bearings. The use of preloaded tapered roller bearings to replace spherical roller bearings results in improved system stiffness while still operating effectively in high misalignment conditions. The preloaded, double-row tapered roller bearing design facilitates proper load share between the two rows of rollers to help reduce or eliminate the roller/raceway sliding and skidding/smearing associated with conventional spherical roller bearing damage. The transmission of thrust loads to the gearbox is also minimized, resulting in longer main bearing life and less gearbox planetary section damage. 
     In one embodiment, the invention provides a wind turbine bearing system for supporting a rotating main shaft of a wind turbine. The system includes a double-row tapered roller bearing assembly having a one-piece inner race element defining an axial bore through which, in use, a rotating main shaft of a wind turbine passes. The one-piece inner race element defines first and second inner raceways on an outer diameter of the inner race element. The roller bearing assembly further includes first and second sets of tapered rollers, the first set of tapered rollers seated within the first inner raceway and the second set of tapered rollers seated within the second inner raceway. The bearing assembly also includes a two-piece outer race element, each piece of the two-piece outer race element defining a respective outer raceway on an inner diameter on which a corresponding set of the tapered rollers is seated. Each piece of the two-piece outer race element has a cylindrical outer diameter defining an outer contact surface. The system further includes a pillow block housing assembly configured for attachment to a stationary support structure. The pillow block housing assembly defines a cylindrical inner diameter substantially matched to the outer contact surfaces of the pieces of the two-piece outer race element to define an interface between the pillow block housing assembly and the bearing assembly. 
     In another embodiment, the invention provides a method of upgrading a wind turbine main shaft bearing system having a pillow block housing assembly configured for attachment to a stationary support structure, the pillow block housing assembly defining a bearing envelope around a bearing assembly installed in the pillow block housing. The method includes removing a double-row spherical roller bearing assembly from within the pillow block housing assembly and installing a double-row tapered roller bearing assembly into the pillow block housing assembly, such that once installed, the bearing envelope defined by the pillow block housing is unchanged or changed only for obtaining a desired preload on the double-row tapered roller bearing assembly. 
     Other aspects of the invention will become apparent by consideration of the detailed description and accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a partial sectional view of a prior art pillow block bearing assembly for the main shaft of a wind turbine, the pillow block bearing assembly including a spherical roller bearing assembly. 
         FIG. 2  a partial sectional view of a pillow block bearing assembly embodying the present invention, and including a tapered roller bearing assembly. 
     
    
    
     DETAILED DESCRIPTION 
     Before any embodiments of the invention are explained in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the following drawings. The invention is capable of other embodiments and of being practiced or of being carried out in various ways. 
       FIG. 1  illustrates an example of a prior art wind turbine bearing system  10  for supporting a rotating main shaft of a wind turbine. The turbine main shaft  14  is supported by a double-row spherical roller bearing assembly  18 , which is housed within a pillow block housing assembly  22 . The pillow block housing assembly  22  is configured to be attached to a stationary support structure of the wind turbine and can include multiple structural elements, including in the illustrated embodiment, a main housing portion  26  and elements attached to the main housing portion  26 , such as end plates  30 , seals  34 , seal carriers  38 , and a clamp ring  40 . Together, the elements of the pillow block housing assembly  22  define an envelope  42  into which the spherical roller bearing assembly  18  fits. It is to be understood that while the section view of  FIG. 1  illustrates only the portion of the bearing system  10  above the shaft  14 , a mirror image exists below the shaft  14  defining the other half of the sectioned envelope  42 . The illustrated portion of the envelope  42  is defined at its upper end by a cylindrical inner diameter surface  46  of the pillow block housing assembly  22 , on its axial sides by axial inner surfaces  50  of the pillow block housing assembly  22 , and at its lower end by the outer diameter  54  of the turbine main shaft  14 . As seen in  FIG. 1 , the illustrated portion of the envelope  42  is generally rectangular in sectional shape. 
     As discussed above, the prior art wind turbine bearing system  10  suffers from problems associated with the use of the spherical roller bearing assembly  18 .  FIG. 2  illustrates the wind turbine bearing system  60  of the present invention, in which the prior art spherical roller bearing assembly  18  is replaced by a double-row, tapered roller bearing assembly  64 . The pillow block housing assembly  22  and the turbine main shaft  14  are unchanged, with like parts given like reference numerals. Therefore, according to the present invention, the tapered roller bearing assembly  64  is configured to fit within and occupy the existing envelope  42  provided by the prior art wind turbine bearing system  10  with no, or only slight, modification to the envelope  42 , as will be explained in detail below. Likewise, it is understood that existing wind turbine bearing systems can have varying envelopes, yet the present invention contemplates designing a tapered roller bearing assembly to fit within and occupy substantially any existing envelope in a wind turbine bearing system that had previously utilized a double-row spherical roller bearing assembly. 
     The tapered roller bearing assembly  64  includes a one-piece inner race element  68  having an axial bore  70  through which, in use, the rotating main shaft  14  passes. The one-piece inner race element  68  defines first and second inner raceways  72 ,  76 , respectively, on an outer diameter of the inner race element  68 . The one-piece inner race element  68  includes a central rib  80  between the first and second inner raceways  72 ,  76 , the central rib  80  defining a first shoulder  84  and a second shoulder  88 . First and second outer ribs  92 ,  96 , defining respective first and second outer rib shoulders  100 ,  104 , are also formed on the inner race element  68 . This one-piece inner race element  68  means that the tapered roller bearing assembly  64  is of the type commonly referred to as a tapered double inner (TDI) roller bearing assembly. The TDI indicator means that the inner race element  68  has two tapered raceways  72 ,  76  formed on a single or one-piece inner race element  68 . 
     The tapered roller bearing assembly  64  further includes a first set or row of tapered rollers  108  and a second set or row of tapered rollers  112 . The first set of tapered rollers  108  is seated on and within the first inner raceway  72  and the second set of tapered rollers  112  is seated on and within the second inner raceway  76 . The rollers  108 ,  112  and raceways  72 ,  76 ,  132 ,  140  can be selectively radiused/crowned in order to account for the misalignment otherwise expected in other main shaft bearing systems. The first and second shoulders  84  and  88  are each in facing relation to axial ends of the respective first and second sets of tapered rollers  108 ,  112 , with the central rib  80  sized and configured to selectively maintain the first and second sets of tapered rollers  108 ,  112  in position within the respective first and second inner raceways  72 ,  76 . In the illustrated embodiment, a first retainer  116  positions the first set of tapered rollers  108  within the bearing assembly  64 , and a second retainer  120  positions the second set of tapered rollers  112  within the bearing assembly  64 . 
     The tapered roller bearing assembly  64  further includes a two-piece outer race element  124  having a first piece or outer ring  128  defining a first outer raceway  132  on its inner diameter, and a second piece or outer ring  136  defining a second outer raceway  140  on its inner diameter. The first set of rollers  108  ride on the first outer raceway  132 , and the second set of rollers  112  ride on the second outer raceway  140  Each outer ring  128 ,  136  has a cylindrical outer diameter defining a cylindrical outer contact surface  144  and  148 , respectively. The outer contact surfaces  144 ,  148  are substantially matched in diameter to the cylindrical inner diameter surface  46  of the pillow block housing assembly  22  so as to fit in the existing envelope  42 . The interface between the cylindrical inner diameter  46  of the pillow block housing assembly  22  and the outer contact surfaces  144 ,  148  of the two-piece outer race element  124  can be a loose fit, a transition fit, or a tight fit as desired. Those fits, and the selected usage of those fits, are well-understood by those skilled in the bearing art. 
     The tapered roller bearing assembly  64  also includes a spacer  152  positioned between the two rings  128 ,  136  of the two-piece outer race element  124 . The illustrated spacer  152  includes an outer diameter  156  that at least partially corresponds in size to the outer diameter defining the contact surfaces  144 ,  148  of each outer ring  128 ,  136 . The outer diameter  156  of the spacer  152  further includes a recessed portion or annular groove  160  having an outer diameter  164  smaller than the outer diameter defining the contact surfaces  144 ,  148  of each outer race ring  128 ,  136 . The annular groove  160  provides a channel for lubricant flow. Radial holes  166  (only one is shown) provide communication between the groove  160  and the rollers  108 ,  112  for greasing the bearing assembly  64 . 
     In a prior design of a tapered roller bearing assembly to be used for a main shaft bearing application (described in U.S. Pat. No. 8,075,196), a generally spherical or ball-and-socket interface was provided between the outer bearing race element and the inner surface of the pillow block housing. This spherical interface was intended to accommodate misalignment, but resulted in the need for more complicated seals and perhaps an anti-friction liner at the interface. Unlike that prior assembly, which was a tapered double outer (TDO) roller bearing, the cylindrical interface between the cylindrical inner diameter  46  of the pillow block housing assembly  22  and the outer contact surfaces  144 ,  148  of the two-piece outer race element  124  does not require any modification to the envelope  42  (including the seals) used in existing prior art wind turbine bearing systems having spherical roller bearing assemblies. Thus, the present invention contemplates a drop-in replacement solution using a TDI tapered roller bearing assembly  64  configured to fit in existing envelopes  42 . 
     Therefore, the invention also contemplates a method of upgrading a wind turbine main shaft bearing system by removing a double-row spherical roller bearing assembly  18  from within the pillow block housing assembly  22  and installing a double-row tapered roller bearing assembly  64  into the pillow block housing assembly  22  either without making any modifications to the pillow block housing assembly  22 , or with minor modifications only for obtaining the desired preload on the bearing assembly. In the first situation, in which absolutely no modification is required, once the double-row tapered roller bearing assembly  64  is installed, the bearing envelope  42  defined by the pillow block housing  22  is unchanged from the configuration and size it defined when occupied by the double-row spherical roller bearing assembly  18 . 
     In the second situation, in which a minor modification is made to the envelope  42  only for obtaining the desired preload on the bearing assembly  64 , the clamp ring  40  may be adjusted to ever-so-slightly change the axial length of the envelope  42  adjacent the clamp ring  40 . This modification is a minor surface machining operation to a surface of the clamp ring  40 , in order to enable modification of the envelope  42  so slightly. Specifically, as shown in  FIG. 2 , the clamp ring  40  includes a finger portion  170  that defines an axial inner surface  50  of the clamp ring finger portion  170 . The engagement between that axial inner surface  50  of the clamp ring finger portion  170  and the axial end of the first outer ring  128  dictates the positive clamp needed to establish the desired preloading of the bearing assembly  64 . The engagement, and therefore the preloading, is facilitated by a gap  174  defined between a face surface  178  of the clamp ring  40  and a face surface  182  of the adjacent end plate  30  (or other portion of the pillow block housing assembly  22 ). This gap  174  may be about 0.2 mm so as to prevent the face surfaces  178  and  182  from engaging and “bottoming out” prior to the clamp ring  40  being sufficiently tightened down (via fasteners) for preloading the bearing assembly  64 . In the event the desired preload cannot otherwise be achieved before the face surfaces  178  and  182  engage, the face surface  178  of the clamp plate  40  (or the face surface  182  of the end plate  30 ) may be machined to provide for the needed gap  174 , and therefore the desired preload. 
     The amount of adjustment applied to the clamp ring  40 , along with any machining of either of the face surfaces  178 ,  182  to allow for further tightening, can result in a slight alteration (i.e., increase or reduction) of the axial length of the envelope  42  adjacent the first outer ring  128 , as compared to the original envelope  42  occupied by the spherical roller bearing assembly  18 . This slight alternation ensures that the axial inner surface  50  defined by the finger portion  170  of the clamp ring  40  is in contact with the first outer ring  128  prior to face surfaces  178  and  182  engaging. As used herein and in the appended claims, reference to changes or modification(s) made to the envelope only for obtaining the desired preload on the bearing assembly means changes or modification(s) to the envelope of the above-described nature, for the sole purpose of achieving the desired preloading on the bearing assembly  64 . 
     The use of preloaded tapered roller bearings to replace spherical roller bearings results in improved system stiffness while still operating effectively in high misalignment conditions. The preloaded, double-row tapered roller bearing design facilitates proper load share between the two rows of rollers to help reduce or eliminate the roller/raceway sliding and skidding/smearing associated with conventional spherical roller bearing damage. The preloaded, double-row tapered roller bearing design also minimizes the transmission of thrust loads to the gearbox, resulting in longer main bearing life and less gearbox planetary section damage. 
     Installing the double-row tapered roller bearing assembly  64  includes installing the one-piece inner race element  68  (with the first and second sets of tapered rollers  108 ,  112  and retainers  116 ,  120 ), and installing the two-piece outer race element  124  (with spacer  152 ) such that the cylindrical outer diameter defining the outer contact surfaces  144 ,  148  fits within the bearing envelope  42 . In some situations, no modifications to the envelope  42 , including those requiring machining or otherwise re-working the components of the pillow block housing assembly  22 , are required. Alternatively, modifications may be made to the envelope  42  only for obtaining the desired preload on the bearing assembly  64 . 
     It should be understood that while the illustrated embodiments show one example pillow block housing assembly  22  for a wind turbine that can be upgraded with a tapered roller bearing assembly  64 , the present invention contemplates upgrading\replacing spherical roller bearing assemblies of different sizes used in virtually any prior art wind turbine pillow block housing assemblies. The replacement tapered roller bearing assembly can be sized to fit virtually any existing envelope, making upgrade/replacement easy. 
     Various features and advantages of the invention are set forth in the following claims.