Patent Publication Number: US-2011057451-A1

Title: Yaw bearing assembly for use with a wind turbine and a method for braking using the same

Description:
BACKGROUND OF THE INVENTION 
     The embodiments described herein relate generally to yaw bearing assemblies and, more particularly, to yaw bearing assemblies for use with wind turbines. 
     Known wind turbines include a yaw bearing assembly to rotate a nacelle assembly with respect to a tower. At least some known yaw bearing assemblies include ball bearings that enable the nacelle assembly to rotate on the tower. At least some known ball bearing assemblies require that yaw motors and the braking system are constantly activated to maintain a yaw direction of the nacelle assembly. Further, ball bearings are relatively costly when compared to other bearings. 
     At least some other known wind turbines include yaw bearing assemblies that include sliding bearings. At least one known wind turbine having a sliding bearing assembly includes active braking modules and passive braking modules coupled about a ring gear. A sliding track is positioned between a frame of a nacelle assembly and a ring gear. The active brake assembly includes a friction pad that contacts a friction pad on the ring gear. While such a brake assembly facilitates maintaining a yaw direction of the nacelle assembly, the yaw motors use energy to overcome the combined effect of aerodynamic loads and friction forces between the friction pads. At least some known brake assemblies include multiple sliding tracks, which may make replacing and/or servicing friction pads within the brake assembly difficult. Further, at least some known wind turbines include a yaw bearing assembly and a separate yaw brake system, which add to the complexity of wind turbine. 
     As such, it would be desirable for a yaw bearing assembly to include a sliding bearing having components that do not require the yaw motor to use energy overcome forces between two friction surfaces and/or that apply additional friction forces to resist wind turbine yawing loads during normal operation. Further, it is desirable to have an integrated yaw bearing and brake system. 
     BRIEF DESCRIPTION OF THE INVENTION 
     In one aspect, a yaw bearing assembly is provided. The yaw bearing assembly includes a first component having a first sliding surface and a second sliding surface, and a brake assembly coupled with respect to the first component and adjacent to at least one of the first sliding surface and the second sliding surface. The brake assembly includes a piston, a sliding pad coupled to the piston, and an activation device coupled to the piston for moving the sliding pad with respect to at least one of the first sliding surface and the second sliding surface. 
     In another aspect, a wind turbine is provided. The wind turbine includes a tower, a nacelle assembly having a base, and a yaw bearing assembly operationally coupling the nacelle assembly to the tower. The yaw bearing assembly is configured to rotate the nacelle assembly with respect to the tower. The yaw bearing assembly includes a first component having a first sliding surface and a second sliding surface, and a brake assembly coupled with respect to the first component and adjacent to at least one of the first sliding surface and the second sliding surface. The brake assembly includes a piston, a sliding pad coupled to the piston, and an activation device coupled to the piston for moving the sliding pad with respect to at least one of the first sliding surface and the second sliding surface. 
     In yet another aspect, a method for braking a wind turbine is provided. The wind turbine includes a yaw bearing assembly coupled to a nacelle and a tower of the wind turbine. The yaw bearing assembly includes a first component having a first sliding surface and a second sliding surface and a brake assembly coupled with respect to the first component and adjacent to at least one of the first sliding surface and the second sliding surface. The brake assembly includes a piston, a sliding pad coupled to the piston, and an activation device coupled to the piston. The method includes rotating the nacelle with respect to the tower using the yaw bearing assembly, and moving the sliding pad toward at least one of the first sliding surface and the second sliding surface to apply a braking force to the one of the first sliding surface and the second sliding surface. 
     The embodiments described herein include a brake assembly having a sliding pad that acts against a sliding surface to facilitate maintaining a yaw position of a nacelle assembly. As such, the embodiments described herein require less force to change the yaw direction of the nacelle assembly, as compared to brake assemblies that include friction pads and/or surfaces. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1-18  show exemplary embodiments of the assemblies and methods described herein. 
         FIG. 1  is a side elevation view of an exemplary wind turbine. 
         FIG. 2  is a schematic cross-sectional view of an exemplary nacelle assembly that may be used with the wind turbine shown in  FIG. 1 . 
         FIG. 3  is perspective view of a bearing ring that may be used with the nacelle assembly shown in  FIG. 2 . 
         FIG. 4  is a schematic cross-sectional view of an exemplary yaw bearing assembly that may be used with the nacelle assembly shown in  FIG. 2 . 
         FIG. 5  is a schematic bottom view of the yaw bearing assembly shown in  FIG. 4 . 
         FIG. 6  is a schematic cross-sectional view of a first alternative exemplary yaw bearing assembly that may be used with the nacelle assembly shown in  FIG. 2 . 
         FIG. 7  is a schematic bottom view of the yaw bearing assembly shown in  FIG. 6 . 
         FIG. 8  is a schematic cross-sectional view of a second alternative exemplary yaw bearing assembly that may be used with the nacelle assembly shown in  FIG. 2 . 
         FIG. 9  is a schematic side view of the yaw bearing assembly shown in  FIG. 8 . 
         FIG. 10  is a schematic cross-sectional view of a third alternative exemplary yaw bearing assembly that may be used with the nacelle assembly shown in  FIG. 2 . 
         FIG. 11  is a schematic cross-sectional view of a fourth alternative exemplary yaw bearing assembly that may be used with the nacelle assembly shown in  FIG. 2 . 
         FIG. 12  is a schematic cross-sectional view of a fifth alternative exemplary yaw bearing assembly that may be used with the nacelle assembly shown in  FIG. 2 . 
         FIG. 13  is a schematic cross-sectional view of a sixth alternative exemplary yaw bearing assembly that may be used with the nacelle assembly shown in  FIG. 2 . 
         FIG. 14  is a schematic cross-sectional view of a seventh alternative exemplary yaw bearing assembly that may be used with the nacelle assembly shown in  FIG. 2 . 
         FIG. 15  is a schematic cross-sectional view of the yaw bearing assembly shown in  FIG. 14 . 
         FIG. 16  is a schematic cross-sectional view of an eighth alternative exemplary yaw bearing assembly that may be used with the nacelle assembly shown in  FIG. 2 . 
         FIG. 17  is a schematic cross-sectional view of a ninth alternative exemplary yaw bearing assembly that may be used with the nacelle assembly shown in  FIG. 2 . 
         FIG. 18  is a flowchart of an exemplary method that may be used with any of the embodiments shown in  FIGS. 3-17 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The embodiments described herein include yaw bearing assemblies having a brake assembly that facilitate maintaining a yaw position of a nacelle with respect to a tower. The brake assembly includes an activation device that applies a pre-loaded force to a ring gear to facilitate maintaining the yaw position of the nacelle. More specifically, the pre-loaded force can be adjusted by adjusting a position of a sliding pad of the braking assembly with respect to a sliding surface of the ring gear. Further, the embodiments described herein use a sliding bearing rather than ball bearings to facilitate rotating the nacelle with respect to the tower. Moreover, a segmented ring is described herein that can used with any bearing ring and/or supporting ring described herein. 
     The yaw bearing assemblies described herein include brake assemblies that are configured to have one or more of the following load paths: (1) a motor (electrical signals to rotational movement) to a spindle (rotational movement to linear movement), to a lever (linear movement to rotational movement), to a spindle (rotational movement to linear movement), to a piston that linearly moves a sliding pad; (2) a cylinder (pressure force to linear movement) to a lever (linear movement to rotational movement), to a spindle (rotational movement to linear movement), to a piston that linearly moves a sliding pad; (3) a cylinder (pressure force to linear movement) to a lever (linear movement to rotational movement), to a piston that linearly moves a sliding pad; (4) a cylinder (pressure force to linear movement) to a piston that linearly moves a sliding pad; and/or (5) a mechanism, such as springs (pressure force to linear movement) to a piston that linearly moves a sliding pad. 
     Further, the embodiments described herein may be used with any suitable wind turbine and/or nacelle assembly rather than being limited to use with the wind turbine and nacelle assembly described herein. 
       FIG. 1  is a perspective view of an exemplary wind turbine  10 . In the exemplary embodiment, wind turbine  10  is a nearly horizontal-axis wind turbine, however, wind turbine  10  may have any suitable tilt angle. Alternatively, wind turbine  10  may be a vertical axis wind turbine. In the exemplary embodiment, wind turbine  10  includes a tower  12  that extends from a supporting surface  14 , a nacelle assembly  16  mounted on tower  12 , and a rotor  18  that is coupled to nacelle assembly  16 . Rotor  18  includes a rotatable hub  20  and at least one blade  22  coupled to and extending outward from hub  20 . In the exemplary embodiment, rotor  18  has three blades  22 . In an alternative embodiment, rotor  18  includes any suitable number of blades  22 . In the exemplary embodiment, tower  12  is fabricated from tubular steel such that a cavity (not shown in  FIG. 1 ) is defined between supporting surface  14  and nacelle assembly  16 . In an alternative embodiment, tower  12  is any suitable type of tower. A height of tower  12  may be selected based upon factors and conditions known in the art. 
     Blades  22  are spaced about hub  20  to facilitate rotating rotor  18  to enable kinetic energy to be transferred from the wind into usable mechanical energy, and subsequently, electrical energy. Blades  22  are mated to hub  20  by coupling a blade root portion  24  to hub  20  at a plurality of load transfer regions  26 . Load transfer regions  26  have a hub load transfer region and a blade load transfer region (both not shown in  FIG. 1 ). Loads induced to blades  22  are transferred to hub  20  via load transfer regions  26 . 
     In the exemplary embodiment, blades  22  have a length ranging from about 15 meters (m) to about 91 m. Alternatively, blades  22  may have any length that enables wind turbine  10  to function as described herein. For example, other non-limiting examples of blade lengths include 10 m or less, 20 m, and 37 m. As wind strikes blades  22  from a direction  28 , rotor  18  is rotated about an axis of rotation  30 . As blades  22  are rotated and subjected to centrifugal forces, blades  22  are also subjected to various forces and moments. As such, blades  22  may deflect and/or rotate from a neutral, or non-deflected, position to a deflected position. Moreover, a pitch angle of blades  22 , i.e., an angle that determines a perspective of blades  22  with respect to direction  28  of the wind, may be changed by a pitch adjustment system  32  to control power generated by wind turbine  10  by adjusting an angular position of a profile of at least one blade  22  relative to wind vector. Pitch axes  34  for blades  22  are shown in  FIG. 1 . In the exemplary embodiment, a pitch of each blade  22  is controlled individually by a control system  36 . Alternatively, the blade pitch for all blades may be controlled simultaneously by control system  36 . Further, in the exemplary embodiment, as direction  28  changes, a yaw direction of nacelle assembly  16  may be controlled about a yaw axis  38  to position blades  22  with respect to direction  28 . 
     In the exemplary embodiment, control system  36  is shown as being centralized within nacelle assembly  16 , however control system  36  may be a distributed system throughout wind turbine  10 , on supporting surface  14 , within a wind farm, and/or at a remote control center. Control system  36  includes a processor  40  configured to perform the methods and/or steps described herein. Further, many of the other components described herein include a processor. As used herein, the term “processor” is not limited to integrated circuits referred to in the art as a computer, but broadly refers to a controller, a microcontroller, a microcomputer, a programmable logic controller (PLC), an application specific integrated circuit, and other programmable circuits, and these terms are used interchangeably herein. It should be understood that processor  40  and/or control system  36  can also include memory, input channels, and/or output channels. 
     In the embodiments described herein, memory may include, without limitation, a computer-readable medium, such as a random access memory (RAM), and a computer-readable non-volatile medium, such as flash memory. Alternatively, a floppy disk, a compact disc-read only memory (CD-ROM), a magneto-optical disk (MOD), and/or a digital versatile disc (DVD) may also be used. Also, in the embodiments described herein, input channels may include, without limitation, sensors and/or computer peripherals associated with an operator interface, such as a mouse and a keyboard. Further, in the exemplary embodiment, output channels may include, without limitation, a control device, an operator interface monitor and/or a display. 
     Processors described herein process information transmitted from a plurality of electrical and electronic devices that may include, without limitation, sensors, actuators, compressors, control systems, and/or monitoring devices. Such processors may be physically located in, for example, a control system, a sensor, a monitoring device, a desktop computer, a laptop computer, a PLC cabinet, and/or a distributed control system (DCS) cabinet. RAM and storage devices store and transfer information and instructions to be executed by the processor(s). RAM and storage devices can also be used to store and provide temporary variables, static (i.e., non-changing) information and instructions, or other intermediate information to the processors during execution of instructions by the processor(s). Instructions that are executed may include, without limitation, wind turbine control commands. The execution of sequences of instructions is not limited to any specific combination of hardware circuitry and software instructions. 
       FIG. 2  is a schematic cross-sectional view of an exemplary nacelle assembly  16  that may be used with wind turbine  10  (shown in  FIG. 1 ). Additional or alternative components are indicated by dashed lines in  FIG. 2 . 
     Nacelle assembly  16  includes a nacelle housing  52  coupled about a generator  54 , a gearbox  56 , and a base  58 . Generator  54  and gearbox  56  are supported on base  58 . A low speed shaft  60  extends between gearbox  56  and hub  20  and is supported on base  58  by a main bearing  62 . A high speed shaft  64  extends between gearbox  56  and generator  54 . During operation of wind turbine  10 , wind rotates blades  22 , which rotates hub  20  and low speed shaft  60 . Through gearbox  56 , low speed shaft  60  rotates high speed shaft  64  to drive generator  54  to generate power. In the exemplary embodiment, high speed shaft  64  rotates at a higher rotation per minute (rpm) than an rpm at which low speed shaft  60  rotates. Alternatively, wind turbine  10  does not include gearbox  56 . 
     At least one yaw motor  66  is coupled to base  58  for controlling a yaw direction of nacelle assembly  16 . In the exemplary embodiment, control system  36  is coupled in communication with, such as coupled in operational control communication with, yaw motor  66  to facilitate aligning nacelle assembly  16 , hub  20 , and/or blades  22  with a yaw direction as determined by control system  36  and/or an operator of wind turbine  10 . As used herein, the term “operational control communication” refers to a link, such as a conductor, a wire, and/or a data link, between two or more components of wind turbine  10  that enables signals, electric currents, and/or commands to be communicated between the two or more components. The link is configured to enable one component to control an operation of another component of wind turbine  10  using the communicated signals, electric currents, and/or commands. Each yaw motor  66  includes a pinion gear  68  coupled to a shaft  70  such that yaw motor  66  drives shaft  70  to rotate pinion gear  68 . 
     In the exemplary embodiment, nacelle assembly  16  also includes a yaw bearing assembly  72  operationally coupling nacelle assembly  16 , at base  58 , to tower  12 . More specifically, yaw bearing assembly  72  includes a ring gear  74  and a support ring  76 . In the exemplary embodiment, ring gear  74  is coupled to tower  12 , and support ring  76  is coupled to base  58 . Alternatively, ring gear  74  is coupled to base  58  and support ring  76  is coupled to tower  12 . In the exemplary embodiment, ring gear  74  includes teeth  78  that engage pinion gear  68  to rotate base  58  with respect to tower  12 . 
     Supporting ring  76  includes at least one brake assembly  80  in contact with ring gear  74 . More specifically, brake assembly  80  is in contact with a sliding surface  82  of ring gear  74 . In the exemplary embodiment, each brake assembly  80  includes a sliding pad  84  coupled thereto and an activation device  86  configured to move sliding pad  84  of brake assembly  80  with respect to, such as toward and away from, ring gear  74 . In one embodiment, activation device  86  is configured to apply a pre-loaded force to ring gear  74 . Control system  36  is coupled in communication with, such as coupled in operational control communication with, brake assembly  80  and/or activation device  86  to control operation of brake assembly  80  and/or activation device  86 . When more than one brake assembly  80  is included in yaw bearing assembly  72 , brake assemblies  80  are spaced circumferentially about supporting ring  76  as shown in  FIG. 2 . 
     To control a yaw of nacelle assembly  16 , yaw motors  66  are activated to rotate nacelle assembly  16  about yaw axis  38  to position rotor  18  with respect to direction  28 . Once nacelle assembly  16  is at a desired yaw position, brake assembly  80  is activated to maintain the yaw position of nacelle assembly  16 . 
       FIG. 3  is perspective view of a ring  90  that may be used with nacelle assembly  16  (shown in  FIG. 2 ) as ring gear  74  and/or supporting ring  76 . In the exemplary embodiment, ring  90  includes a plurality of segments  92  that are coupled circumferentially about tower  12  (shown in  FIG. 2 ) and/or base  58  (shown in  FIG. 2 ). More specifically, a pin  94  is positioned between each segment  92  to coupled segments  92  together to form ring  90 . Each segment includes a pin channel  96  defined in an end surface  98  thereof for retaining pin  94  between segments  92 . When two segments  92  are abutted at end surfaces  98 , adjacent pin channels  96  define a pin opening into which a pin  94  can be inserted. Segments  92  and/or pins  94  are configured to facilitate maintaining and/or replacing a pad, such as a sliding pad coupled to a sliding surface, and/or a gear, such as teeth  78  (shown in  FIG. 2 ) on ring gear  74  (shown in  FIG. 2 ). Although ring  90  is described as having teeth  78  defined about an outer surface of ring  90 , ring  90  can alternatively or addition include teeth  78  defined along an inner surface of ring  90 . 
       FIG. 4  is a schematic cross-sectional view of an exemplary yaw bearing assembly  100  that may be used with nacelle assembly  16  (shown in  FIG. 2 ) as yaw bearing assembly  72  (shown in  FIG. 2 ).  FIG. 5  is a schematic bottom view of yaw bearing assembly  100 . Components shown in  FIGS. 1 ,  2 ,  4 , and  5  are numbered similarly. 
     In the exemplary embodiment, yaw bearing assembly  100  includes a ring gear  102  coupled to tower  12  and a supporting ring  104  coupled to base  58 . Ring gear  102  includes a substantially vertically-aligned tooth face  106 , a substantially horizontally-aligned lower face  108 , and an upper face  110  having a substantially horizontal portion  112  and a tapered portion  114 . A substantially vertically-aligned inner face  116  extends between upper face  110  and lower face  108 . Tooth face  106  includes a plurality of teeth  78  configured to engage pinion gear  68  (shown in  FIG. 2 ). Lower face  108  includes a first or lower sliding surface  118 , and tapered portion  114  of upper face  110  includes a second or upper sliding surface  120 . 
     Supporting ring  104  includes an upper portion  122  coupled to base  58  and a lower portion  124  coupled to upper portion  122 . Upper portion  122  and lower portion  124  define a groove  126  that is shaped to correspond to a shape of ring gear  102 . More specifically, in the exemplary embodiment, upper portion  122  includes an angled lower face  128  that corresponds to tapered portion  114  of upper face  110 , and lower portion  124  includes a substantially horizontally-aligned upper face  130  that corresponds to lower face  108  of ring gear  102 . Lower face  128  is positioned adjacent upper sliding surface  120 , and upper face  130  is positioned adjacent lower sliding surface  118 . An upper sliding pad  132  is coupled to lower face  128  of supporting ring  104  adjacent to upper sliding surface  120  of ring gear  102 . In the exemplary embodiment, upper sliding pad  132  extends circumferentially about lower face  128  and is in contact with upper sliding surface  120 . Upper sliding pad  132  can be segmented to facilitate maintenance and/or repair. Alternatively, upper sliding pad  132  includes a plurality of pads that are spaced circumferentially about lower face  128 . 
     In the exemplary embodiment, a brake assembly  134  is coupled to lower portion  124  of supporting ring  104  and at least partially extends through upper face  130 . Brake assembly  134  includes at least one lever arm  136 , a caliper  138  coupled to lever arm  136 , a piston  140  coupled to caliper  138 , a shoe  142  coupled to piston  140 , and a lower sliding pad  144  coupled to shoe  142 . Piston  140  and/or shoe  142  is threaded to convert rotational movement into linear movement of lower sliding pad  144 . Alternatively, brake assembly  134  does not include shoe  142  and sliding pad  144  is coupled to piston  140 . Further, it should be understood that caliper  138  can include any suitable number of pistons  140  coupled thereto. In the exemplary embodiment, lever arms  136  are each configured to rotate with respect to caliper  138 . At least a portion of piston  140  and/or shoe  142  is positioned within an opening  146  defined through lower portion  124  of supporting ring  104 . Lower sliding pad  144  is positioned to extend beyond upper face  130  to contact lower sliding surface  118 . In the exemplary embodiment, upper sliding pad  132  and lower sliding pad  144  are formed from any suitable material that enables supporting ring  104  to rotate with respect to ring gear  102 . More specifically, materials for upper sliding pad  132  and lower sliding pad  144  and/or material for upper sliding surface  120  and/or lower sliding surface  118  are selected to enable yaw bearing assembly  100  to function as described herein. In a particular embodiment, upper sliding surface  120  and lower sliding surface  118  are steel and upper sliding pad  132  and lower sliding pad  144  are a conventional sliding material. 
     An activation device  148  is operationally coupled to lever arm  136  for controlling movement of piston  140  with respect to, such as toward and away from, lower sliding surface  118 . In the exemplary embodiment, lower sliding pad  144  is in contact with lower sliding surface  118 . Further, in the exemplary embodiment, brake assembly  134  includes two lever arms  136  coupled to caliper  138  as shown in  FIG. 5 , however, it should be understood that brake assembly  134  may include any suitable number of lever arms  136 . 
     As shown in  FIG. 5 , activation device  148  includes a double-threaded spindle  150  in the exemplary embodiment. More specifically, spindle  150  includes a first end  152  having threads  154  in a first direction and a second end  156  having threads  158  in a second direction opposite to the first direction. Each lever arm  136  includes an opening  160  having threading corresponding to threads  154  or threads  158  of spindle  150 . Yaw bearing assembly  100  includes a motor, such as a hydraulic unit and/or any other suitable type of motor. Motor  162  is operatively coupled to at least one spindle  150 . Motor  162  rotates spindle  150  to move lower sliding pad  144  toward or away from lower sliding surface  118 . More specifically, when spindle  150  is rotated in a first direction, spindle  150  moves toward caliper  138  and shoe  142  and/or piston  140  is rotated by lever arm  136  to apply a braking force to ring gear  102  by moving lower sliding pad  144  toward lower sliding surface  118 . When motor  162  rotates spindle  150  in a second direction, spindle  150  moves away from caliper  138  and shoe  142  and/or piston  140  is rotated by lever arm  136  to lessen the brake force and enable supporting ring  104  to move rotationally with respect to ring gear  102 . In the exemplary embodiment, control system  36  (shown in  FIG. 2 ) is coupled in communication with, such as in operational control communication with, motor  162  for controlling a rotation of spindle  150 . 
     During operation of wind turbine  10  (shown in  FIGS. 1 and 2 ), a yaw direction of nacelle assembly  16  is controlled using yaw motors  66  (shown in  FIG. 2 ) and yaw bearing assembly  100 . More specifically, to change the yaw direction of nacelle assembly  16 , spindle  150  is rotated to move caliper  138  away from lower sliding surface  118 . As such, piston  140 , shoe  142 , and lower sliding pad  144  are moved away from lower sliding surface  118  with lower sliding pad  144  maintaining contact with lower sliding surface  118 . By moving lower sliding pad  144  away from lower sliding surface  118 , a braking force against ring gear  102  is reduced to allow movement between ring gear  102  and supporting ring  104 . Yaw motors  66  are activated to rotate base  58  with respect to tower  12  to position nacelle assembly  16  at a desired yaw direction and then are deactivated. 
     To maintain the yaw direction of nacelle assembly  16 , motor  162  is activated to rotate spindle  150  to move caliper  138  toward lower sliding surface  118 . As such, piston  140 , shoe  142 , and lower sliding pad  144  are moved toward lower sliding surface  118  to increase the braking force applied to lower sliding surface  118 . Accordingly, the ability of supporting ring  104  to move with respect to ring gear  102  is reduced such that the yaw direction of nacelle assembly  16  is maintained. 
       FIG. 6  is a schematic cross-sectional view of a first alternative exemplary yaw bearing assembly  200  that may be used with nacelle assembly  16  (shown in  FIG. 2 ) as yaw bearing assembly  72  (shown in  FIG. 2 ).  FIG. 7  is a schematic bottom view of yaw bearing assembly  200 . Unless otherwise described, yaw bearing assembly  200  includes components that are similar to the components described above with reference to yaw bearing assembly  100 . Yaw bearing assembly  200  includes activation device  202  rather than activation device  148  (shown in  FIGS. 4 and 5 ). 
     In the exemplary embodiment, activation device  202  includes a cylinder  204  having a first rod  206  and a second rod  208  coupled thereto. Each rod  206  and  208  is coupled to a respective lever arm  136  at a pivot device  210 . In a particular embodiment, cylinder  204  is a hydraulic cylinder. Alternatively, cylinder  204  is a pneumatic cylinder. In the exemplary embodiment, cylinder  204  is configured to move rods  206  and  208  toward and away from each other to move lower sliding pad  144  with respect to lower sliding surface  118 . 
     Motor  162  is coupled to at least one cylinder  204  to control movement of lower sliding pad  144  with respect to lower sliding surface  118 . More specifically, when cylinder  204  forces rods  206  and  208  in a first direction, such as away from each other, cylinder  204  is moved toward caliper  138  and show  142  and/or piston  140  is rotated by lever arm  136  to apply a braking force to ring gear  102  by moving lower sliding pad  144  toward lower sliding surface  118 . When cylinder  204  forces rods  206  and  208  in a second direction, such as toward each other, cylinder  204  is moved away from caliper  138  and shoe  142  and/or piston  140  is rotated by lever arm  136  to lessen the brake force and to enable supporting ring  104  to move rotationally with respect to ring gear  102 . In the exemplary embodiment, control system  36  (shown in  FIG. 2 ) is coupled in communication with, such as in operational control communication with, motor  162  for controlling a movement of cylinder  204 . 
     During operation of wind turbine  10  (shown in  FIGS. 1 and 2 ), a yaw direction of nacelle assembly  16  is controlled using yaw motors  66  (shown in  FIG. 2 ) and yaw bearing assembly  200 . More specifically, to change the yaw direction of nacelle assembly  16 , cylinder  204  is activated to move caliper  138  away from lower sliding surface  118 . As such, piston  140 , shoe  142 , and lower sliding pad  144  are moved away from lower sliding surface  118  with lower sliding pad  144  maintaining contact with lower sliding surface  118 . By moving lower sliding pad  144  away from lower sliding surface  118 , a braking force against ring gear  102  is reduced to allow movement between ring gear  102  and supporting ring  104 . Yaw motors  66  are activated to rotate base  58  with respect to tower  12  to position nacelle assembly  16  at a desired yaw direction and then are deactivated. 
     To maintain the yaw direction of nacelle assembly  16 , motor  162  activates cylinder  204  to move lever arms  136 . As such, piston  140 , shoe  142 , and lower sliding pad  144  are moved toward lower sliding surface  118  to increase the braking force applied to lower sliding surface  118 . Accordingly, the ability of supporting ring  104  to move with respect to ring gear  102  is reduced such that the yaw direction of nacelle assembly  16  is maintained. 
       FIG. 8  is a schematic cross-sectional view of a second alternative exemplary yaw bearing assembly  300  that may be used with nacelle assembly  16  (shown in  FIG. 2 ) as yaw bearing assembly  72  (shown in  FIG. 2 ).  FIG. 9  is a schematic side view of yaw bearing assembly  300 . Unless otherwise described, yaw bearing assembly  300  includes components that are similar to the components described above with reference to yaw bearing assembly  100 . 
     More specifically, yaw bearing assembly  300  includes activation device  302  rather than activation device  148  (shown in  FIGS. 4 and 5 ). Further, yaw bearing assembly  300  includes lever brackets  304  rather than lever arms  136 . Each lever bracket  304  is substantially L-shaped having a first portion  306  and a second portion  308  that is substantially perpendicular to first portion  306  and coupled to first portion  306  at a corner  310 . In the exemplary embodiment, first portion  306 , corner  310 , and second portion  308  are formed integrally as one-piece, however, lever bracket  304  can be formed from any suitable number of separate components. Corner  310  is coupled to caliper  138  at a pivot device  312 . Second portion  308  is coupled to piston  140  at a pivot point  313  for moving piston  140  with respect to lower sliding surface  118 . 
     Further, in the exemplary embodiment, activation device  302  includes a cylinder  314  having a first rod  316  and a second rod  318  coupled thereto. Alternatively, activation device  302  includes spindle  150  rather than cylinder  314 , first rod  316 , and second rod  318 . In the exemplary embodiment, each rod  316  and  318  is coupled to a respective lever bracket  304  at a pivot point  320 . Cylinder  314  may include a hydraulic cylinder and/or a pneumatic cylinder. In the exemplary embodiment, cylinder  314  is configured to move rods  316  and  318  with respect to each other, such as toward and away from each other, to move second portion  308  of lever brackets  304  with respect to, for example, toward and away from, lower sliding surface  118 . In an alternative embodiment, rather than including cylinder  314  and rods  316  and  318 , activation device  302  includes spindle  150  (shown in  FIGS. 4 and 5 ). Further, although lever brackets  304  are shown applying a vertical or axial force to piston  140 , lever brackets  304  can apply a radial force to piston  140  in an alternative embodiment. 
     In the exemplary embodiment, motor  162  is coupled to at least one cylinder  314  to control movement of lever brackets  304  with respect to, such as toward or away from, lower sliding surface  118 . More specifically, when cylinder  314  forces rods  316  and  318  in a first direction, such as toward each other, piston  140  is moved toward lower sliding surface  118  to apply a braking force to ring gear  102 . When cylinder  314  forces rods  316  and  318  in a second direction, such as away from each other, piston  140  is moved away from lower sliding surface  118  and the brake force is lessened to enable supporting ring  104  to move rotationally with respect to ring gear  102 . In the exemplary embodiment, control system  36  (shown in  FIG. 2 ) is coupled in communication with, such as in operational control communication with, motor  162  for controlling a movement of cylinder  314 . In an alternative embodiment, rather than including cylinder  314  and rods  316  and  318 , activation device  302  includes spindle  150  (shown in  FIGS. 4 and 5 ). 
     During operation of wind turbine  10  (shown in  FIGS. 1 and 2 ), a yaw direction of nacelle assembly  16  is controlled using yaw motors  66  (shown in  FIG. 2 ) and yaw bearing assembly  300 . More specifically, to change the yaw direction of nacelle assembly  16 , cylinder  314  is activated to move piston  140  away from lower sliding surface  118 . As such, shoe  142  and lower sliding pad  144  are moved away from lower sliding surface  118  with lower sliding pad  144  maintaining contact with lower sliding surface  118 . By moving lower sliding pad  144  away from lower sliding surface  118 , a braking force against ring gear  102  is reduced to allow movement between ring gear  102  and supporting ring  104 . Yaw motors  66  are activated to rotate base  58  with respect to tower  12  to position nacelle assembly  16  at a desired yaw direction and then are deactivated. 
     To maintain the yaw direction of nacelle assembly  16 , motor  162  activates cylinder  314  to move piston  140  toward lower sliding surface  118 . As such, shoe  142  and lower sliding pad  144  are moved toward lower sliding surface  118  to increase the braking force applied to lower sliding surface  118 . Accordingly, the ability of supporting ring  104  to move with respect to ring gear  102  is reduced such that the yaw direction of nacelle assembly  16  is maintained. 
       FIG. 10  is a schematic cross-sectional view of a third alternative exemplary yaw bearing assembly  400  that may be used with nacelle assembly  16  (shown in  FIG. 2 ) as yaw bearing assembly  72  (shown in  FIG. 2 ). Unless otherwise described, yaw bearing assembly  400  includes components that are similar to the components described above with reference to yaw bearing assembly  100 . Yaw bearing assembly  400  includes activation device  402  rather than activation device  148  (shown in  FIGS. 4 and 5 ). 
     In the exemplary embodiment, activation device  402  includes cylinder  204  and/or cylinder  314  (shown in  FIGS. 6-9 ), springs  404 , and/or spindle  150  (shown in  FIGS. 4 and 5 ) coupled between piston  140  and opening  146 . In the exemplary embodiment, cylinder  204  and/or cylinder  314 , springs  404 , and/or spindle  150  is configured to directly move piston  140  toward and away from lower sliding surface  118 . Further, as an alternative or an addition to cylinder  204  and/or cylinder  314 , springs  404 , and/or spindle  150 , activation device  402  includes mechanical levers positioned within opening  146  to move piston  140 . 
     In the exemplary embodiment, motor  162  is coupled to activation device  402  for control of the components thereof, such as cylinder  204  and/or  314 , springs  404 , and/or spindle  150 , for moving piston  140  with respect to, such as toward or away from, lower sliding surface  118 . More specifically, activation device  402  forces piston  140  in a first direction toward lower sliding surface  118  to apply a braking force to ring gear  102 . Activation device  402  forces piston  140  in a second direction away from lower sliding surface  118  to lessen the brake force to enable supporting ring  104  to move rotationally with respect to ring gear  102 . In the exemplary embodiment, control system  36  (shown in  FIG. 2 ) is coupled in communication with, such as in operational control communication with, motor  162  for controlling a movement of activation device  402 . 
     During operation of wind turbine  10  (shown in  FIGS. 1 and 2 ), a yaw direction of nacelle assembly  16  is controlled using yaw motors  66  (shown in  FIG. 2 ) and yaw bearing assembly  400 . More specifically, to change the yaw direction of nacelle assembly  16 , activation device  402  is activated to move piston  140  away from lower sliding surface  118 . As such, shoe  142  and lower sliding pad  144  are moved away from lower sliding surface  118  with lower sliding pad  144  maintaining contact with lower sliding surface  118 . By moving lower sliding pad  144  away from lower sliding surface  118 , a braking force against ring gear  102  is reduced to allow movement between ring gear  102  and supporting ring  104 . Yaw motors  66  are activated to rotate base  58  with respect to tower  12  to position nacelle assembly  16  at a desired yaw direction and then are deactivated. 
     To maintain the yaw direction of nacelle assembly  16 , motor  162  activates activation device  402  to move piston  140  toward lower sliding surface  118 . As such, shoe  142  and lower sliding pad  144  are moved toward lower sliding surface  118  to increase the braking force applied to lower sliding surface  118 . Accordingly, the ability of supporting ring  104  to move with respect to ring gear  102  is reduced such that the yaw direction of nacelle assembly  16  is maintained. 
       FIG. 11  is a schematic cross-sectional view of a fourth alternative exemplary yaw bearing assembly  500  that may be used with nacelle assembly  16  (shown in  FIG. 2 ) as yaw bearing assembly  72  (shown in  FIG. 2 ). Similar components shown in  FIGS. 1 ,  2 , and  11  are numbered similarly. 
     In the exemplary embodiment, yaw bearing assembly  500  includes a ring gear  502  coupled to tower  12  and a supporting ring  504  coupled to base  58 . Ring gear  502  includes a substantially vertically-aligned tooth face  506 , a substantially horizontally-aligned lower face  508 , and an upper face  510  having a substantially horizontal portion  512  and a tapered portion  514 . A substantially vertically-aligned inner face  516  extends between upper face  510  and lower face  508 . Tooth face  506  includes a plurality of teeth  78  configured to engage pinion gear  68  (shown in  FIG. 2 ). Lower face  508  includes a lower sliding surface  518 , and tapered portion  514  of upper face  510  includes an upper sliding surface  520 . 
     Supporting ring  504  is coupled to base  58 . Alternatively, supporting ring  504  includes an upper portion coupled to base  58  and a lower portion coupled to the upper portion, as shown and described with respect to  FIGS. 4 and 5 . In the exemplary embodiment, base  58  and supporting ring  504  define a groove  522  that is shaped to correspond to a shape of ring gear  502 . More specifically, in the exemplary embodiment, base  58  includes an angled lower face  524  that corresponds to tapered portion  514  of upper face  510 , and supporting ring  504  includes a substantially horizontally-aligned upper face  526  that corresponds to lower sliding surface  518  of ring gear  502 . Lower face  524  is positioned adjacent upper sliding surface  520 , and upper face  526  is positioned adjacent to lower sliding surface  518 . A lower sliding pad  528  is coupled to upper face  526  of supporting ring  504  adjacent to lower sliding surface  518  of ring gear  502 . In the exemplary embodiment, lower sliding pad  528  extends circumferentially about upper face  526  and is in contact with lower sliding surface  518 . Lower sliding pad  528  can be segmented to facilitate maintenance and/or repair. Alternatively, lower sliding pad  528  includes a plurality of pads that are spaced circumferentially about upper face  526 . 
     In the exemplary embodiment, a brake assembly  530  is coupled to base  58  and at least partially extends through lower face  524 . Brake assembly  530  includes an activation device  532 , a piston  534  coupled to activation device  532 , a shoe  536  coupled to piston  534 , and an upper sliding pad  538  coupled to shoe  536 . It should be understood that brake assembly  530  can include any suitable number of pistons  534  coupled to activation device  532 . In an alternative embodiment, brake assembly  530  does not include shoe  536  and upper sliding pad  538  is coupled to piston  534 . In the exemplary embodiment, at least a portion of piston  534  and/or shoe  536  is positioned within an opening  540  defined within lower face  524  of base  58 . Upper sliding pad  538  is positioned to extend outwardly from lower face  524  to contact upper sliding surface  520 . In the exemplary embodiment, upper sliding pad  538  and lower sliding pad  528  are formed from any suitable material that enables supporting ring  504  to rotate with respect to ring gear  502 . More specifically, materials for upper sliding pad  538  and lower sliding pad  528  and/or material for upper sliding surface  520  and/or lower sliding surface  518  are selected to enable yaw bearing assembly  500  to function as described herein. In a particular embodiment, upper sliding surface  520  and lower sliding surface  518  are steel and upper sliding pad  538  and lower sliding pad  528  are a conventional sliding material. 
     Activation device  532  is directly or indirectly coupled to piston  534  for controlling movement of piston  534  toward and away from upper sliding surface  520 . Activation device  532  is any of activation device  148  (shown in  FIGS. 4 and 5 ), activation device  202  (shown in  FIGS. 6 and 7 ), activation device  302  (shown in  FIGS. 8 and 9 ), and/or activation device  402  (shown in  FIG. 10 ). A motor  542  is coupled to activation device  532  for control of the components thereof, such as cylinder  204  and/or cylinder  314  (shown in  FIGS. 6-9 ), springs  404  (shown in  FIG. 10 ), and/or spindle  150  (shown in  FIGS. 4 and 5 ), for moving piston  534  toward or away from upper sliding surface  520 . More specifically, activation device  532  forces piston  534  in a first direction toward upper sliding surface  520  to apply a braking force to ring gear  502 . Activation device  532  forces piston  534  in a second direction away from upper sliding surface  520  to lessen the brake force to enable supporting ring  504  to move rotationally with respect to ring gear  502 . In the exemplary embodiment, control system  36  (shown in  FIG. 2 ) is coupled in communication with, such as in operational control communication with, motor  542  for controlling a movement of activation device  532 . 
     During operation of wind turbine  10  (shown in  FIGS. 1 and 2 ), a yaw direction of nacelle assembly  16  is controlled using yaw motors  66  (shown in  FIG. 2 ) and yaw bearing assembly  500 . More specifically, to change the yaw direction of nacelle assembly  16 , activation device  532  is activated to move piston  534  away from upper sliding surface  520 . As such, shoe  536  and upper sliding pad  538  are moved away from upper sliding surface  520  with upper sliding pad  538  maintaining contact with upper sliding surface  520 . By moving upper sliding pad  538  away from upper sliding surface  520  a braking force against ring gear  502  is reduced to allow movement among ring gear  502 , base  58 , and/or supporting ring  504 . Yaw motors  66  are activated to rotate base  58  with respect to tower  12  to position nacelle assembly  16  at a desired yaw direction and then are deactivated. 
     To maintain the yaw direction of nacelle assembly  16 , motor  542  activates activation device  532  to move piston  534  toward upper sliding surface  520 . As such, shoe  536  and upper sliding pad  538  are moved toward upper sliding surface  520  to increase the braking force applied to upper sliding surface  520 . Accordingly, the ability of supporting ring  504  and/or base  58  to move with respect to ring gear  502  is reduced such that the yaw direction of nacelle assembly  16  is maintained. 
       FIG. 12  is a schematic cross-sectional view of a fifth alternative exemplary yaw bearing assembly  600  that may be used with nacelle assembly  16  (shown in  FIG. 2 ) as yaw bearing assembly  72  (shown in  FIG. 2 ). Similar components shown in  FIGS. 1 ,  2 , and  12  are numbered similarly. 
     In the exemplary embodiment, yaw bearing assembly  600  includes a ring gear  602  coupled to tower  12  and a supporting ring  604  coupled to base  58 . Ring gear  602  includes a substantially vertically-aligned tooth face  606 , a substantially horizontally-aligned upper face  608 , and a lower face  610  having a substantially horizontal portion  612  and a tapered portion  614 . A substantially vertically-aligned inner face  616  extends between upper face  608  and lower face  610 . Tooth face  606  includes a plurality of teeth  78  configured to engage pinion gear  68  (shown in  FIG. 2 ). Upper face  608  includes an upper sliding surface  618 , and tapered portion  614  of lower face  610  includes a lower sliding surface  620 . Horizontal portion  612  of lower face  610  is coupled to tower  12 . 
     Supporting ring  604  includes an upper portion  622  coupled to base  58  and a lower portion  624  coupled to upper portion  622 . Upper portion  622  and lower portion  624  define a groove  626  that is shaped to correspond to a shape of ring gear  602 . More specifically, in the exemplary embodiment, upper portion  622  includes a substantially horizontally-aligned lower face  628  that corresponds to upper face  608 , and lower portion  624  includes an angled upper face  630  that corresponds to tapered portion  614  of lower face  610  of ring gear  602 . Lower face  628  is positioned adjacent upper sliding surface  618 , and upper face  630  is positioned adjacent to lower sliding surface  620 . An upper sliding pad  632  is coupled to lower face  628  of supporting ring  604  adjacent to upper sliding surface  618  of ring gear  602 . In the exemplary embodiment, upper sliding pad  632  extends circumferentially about lower face  628  and is in contact with upper sliding surface  618 . Upper sliding pad  632  can be segmented to facilitate maintenance and/or repair. Alternatively, upper sliding pad  632  includes a plurality of pads that are spaced circumferentially about lower face  628 . 
     In the exemplary embodiment, a brake assembly  634  is coupled to lower portion  624  of supporting ring  604  and at least partially extends through upper face  630 . Brake assembly  634  includes an activation device  636 , a piston  638  coupled to activation device  636 , a shoe  640  coupled to piston  638 , and a lower sliding pad  642  coupled to shoe  640 . It should be understood that brake assembly  634  includes any suitable number of pistons  638  coupled to activation device  636 . In an alternative embodiment, brake assembly  634  does not include shoe  640  and lower sliding pad  642  is coupled to piston  638 . In the exemplary embodiment, at least a portion of piston  638  and/or shoe  640  is positioned within an opening  644  defined within upper face  630  of lower portion  624 . Lower sliding pad  642  is positioned to extend outwardly from upper face  630  to contact lower sliding surface  620 . In the exemplary embodiment, upper sliding pad  632  and lower sliding pad  642  are formed from any suitable material that enables supporting ring  604  to rotate with respect to ring gear  602 . More specifically, materials for upper sliding pad  632  and lower sliding pad  642  and/or material for upper sliding surface  618  and/or lower sliding surface  620  are selected to enable yaw bearing assembly  600  to function as described herein. In a particular embodiment, upper sliding surface  618  and lower sliding surface  620  are steel and upper sliding pad  632  and lower sliding pad  642  are a conventional sliding material. 
     Activation device  636  is directly or indirectly coupled to piston  638  for controlling movement of piston  638  with respect to, such as toward and away from, lower sliding surface  620 . Activation device  636  is any of activation device  148  (shown in  FIGS. 4 and 5 ), activation device  202  (shown in  FIGS. 6 and 7 ), activation device  302  (shown in  FIGS. 8 and 9 ), and/or activation device  402  (shown in  FIG. 10 ). A motor  646  is coupled to activation device  636  for control of the components thereof, such as cylinder  204  and/or cylinder  314  (shown in  FIGS. 6-9 ), springs  404  (shown in  FIG. 10 ), and/or spindle  150  (shown in  FIGS. 4 and 5 ), for moving piston  638  toward or away from lower sliding surface  620 . More specifically, activation device  636  forces piston  638  in a first direction toward lower sliding surface  620  to apply a braking force to ring gear  602 . Activation device  636  forces piston  638  in a second direction away from lower sliding surface  620  to lessen the brake force to enable supporting ring  604  to move rotationally with respect to ring gear  602 . In the exemplary embodiment, control system  36  (shown in  FIG. 2 ) is coupled in communication with, such as in operational control communication with, motor  646  for controlling a movement of activation device  636 . 
     During operation of wind turbine  10  (shown in  FIGS. 1 and 2 ), a yaw direction of nacelle assembly  16  is controlled using yaw motors  66  (shown in  FIG. 2 ) and yaw bearing assembly  600 . More specifically, to change the yaw direction of nacelle assembly  16 , activation device  636  is activated to move piston  638  away from lower sliding surface  620 . As such, shoe  640  and lower sliding pad  642  are moved away from lower sliding surface  620  with lowing sliding pad  642  maintaining contact with lower sliding surface  620 . By moving lower sliding pad  642  away from lower sliding surface  620 , a braking force against ring gear  602  is reduced to allow movement between ring gear  602  and supporting ring  604 . Yaw motors  66  are activated to rotate base  58  with respect to tower  12  to position nacelle assembly  16  at a desired yaw direction and then are deactivated. 
     To maintain the yaw direction of nacelle assembly  16 , motor  646  activates activation device  636  to move piston  638  toward lower sliding surface  620 . As such, shoe  640  and lower sliding pad  642  are moved toward lower sliding surface  620  to increase the braking force applied to lower sliding surface  620 . Accordingly, the ability of supporting ring  604  to move with respect to ring gear  602  is reduced such that the yaw direction of nacelle assembly  16  is maintained. 
       FIG. 13  is a schematic cross-sectional view of a sixth alternative exemplary yaw bearing assembly  700  that may be used with nacelle assembly  16  (shown in  FIG. 2 ) as yaw bearing assembly  72  (shown in  FIG. 2 ). Similar components shown in  FIGS. 1 ,  2 , and  13  are numbered similarly. 
     In the exemplary embodiment, yaw bearing assembly  700  includes a ring gear  702  coupled to tower  12  and a supporting ring  704  coupled to base  58 . Ring gear  702  includes a substantially vertically-aligned tooth face  706 , a substantially horizontally-aligned upper face  708 , and a lower face  710  having a substantially horizontal portion  712  and a tapered portion  714 . A substantially vertically-aligned inner face  716  extends between upper face  708  and lower face  710 . Tooth face  706  includes a plurality of teeth  78  configured to engage pinion gear  68  (shown in  FIG. 2 ). Upper face  708  includes an upper sliding surface  718 , and tapered portion  714  of lower face  710  includes a lower sliding surface  720 . Horizontal portion  712  of lower face  710  is coupled to adjacent to tower  12 . 
     Supporting ring  704  is coupled to base  58 . Alternatively, supporting ring  704  includes an upper portion coupled to base  58  and a lower portion coupled to the upper portion, as shown and described with respect to  FIGS. 4 and 5 . In the exemplary embodiment, base  58  and supporting ring  704  define a groove  722  that is shaped to correspond to a shape of ring gear  702 . More specifically, in the exemplary embodiment, base  58  includes substantially horizontally-oriented lower face  724  that corresponds to upper face  708  of ring gear  702 , and supporting ring  704  includes an angled upper face  726  that corresponds to tapered portion  714  of lower face  710 . Lower face  724  is positioned adjacent upper sliding surface  718 , and upper face  726  is positioned adjacent to lower sliding surface  720 . A lower sliding pad  728  is coupled to upper face  726  of supporting ring  704  adjacent to lower sliding surface  720  of ring gear  702 . In the exemplary embodiment, lower sliding pad  728  extends circumferentially about upper face  726  and is in contact with lower sliding surface  720 . Lower sliding pad  728  can be segmented to facilitate maintenance and/or repair. Alternatively, lower sliding pad  728  includes a plurality of pads that are spaced circumferentially about upper face  726 . 
     In the exemplary embodiment, a brake assembly  730  is coupled to base  58  and at least partially extends through lower face  724 . Brake assembly  730  includes an activation device  732 , a piston  734  coupled to activation device  732 , a shoe  736  coupled to piston  734 , and an upper sliding pad  738  coupled to shoe  736 . It should be understood that brake assembly  730  includes any suitable number of pistons  734  coupled to activation device  732 . In an alternative embodiment, brake assembly  730  does not include shoe  736  and sliding pad  738  is coupled to piston  734 . In the exemplary embodiment, at least a portion of piston  734  and/or shoe  736  is positioned within an opening  740  defined within lower face  724  of base  58 . Upper sliding pad  738  is positioned to extend outwardly from lower face  724  to contact upper sliding surface  718 . In the exemplary embodiment, upper sliding pad  738  and lower sliding pad  728  are formed from any suitable material that enables supporting ring  704  to rotate with respect to ring gear  702 . More specifically, materials for upper sliding pad  738  and lower sliding pad  728  and/or material for upper sliding surface  718  and/or lower sliding surface  720  are selected to enable yaw bearing assembly  700  to function as described herein. In a particular embodiment, upper sliding surface  718  and lower sliding surface  720  are steel and upper sliding pad  738  and lower sliding pad  728  are a conventional sliding material. 
     Activation device  732  is directly or indirectly coupled to piston  734  for controlling movement of piston  734  toward and away from upper sliding surface  718 . Activation device  732  is any of activation device  148  (shown in  FIGS. 4 and 5 ), activation device  202  (shown in  FIGS. 6 and 7 ), activation device  302  (shown in  FIGS. 8 and 9 ), and/or activation device  402  (shown in  FIG. 10 ). A motor  742  is coupled to activation device  732  for control of the components thereof, such as cylinder  204  and/or cylinder  314  (shown in  FIGS. 6-9 ), springs  404  (shown in  FIG. 10 ), and/or spindle  150  (shown in  FIGS. 4 and 5 ), for moving piston  734  toward or away from upper sliding surface  718 . More specifically, activation device  732  forces piston  734  in a first direction toward upper sliding surface  718  to apply a braking force to ring gear  702 . Activation device  732  forces piston  734  in a second direction away from upper sliding surface  718  to lessen the brake force to enable supporting ring  704  to move rotationally with respect to ring gear  702 . In the exemplary embodiment, control system  36  (shown in  FIG. 2 ) is coupled in communication with, such as in operational control communication with, motor  742  for controlling a movement of activation device  732 . 
     During operation of wind turbine  10  (shown in  FIGS. 1 and 2 ), a yaw direction of nacelle assembly  16  is controlled using yaw motors  66  (shown in  FIG. 2 ) and yaw bearing assembly  700 . More specifically, to change the yaw direction of nacelle assembly  16 , activation device  732  is activated to move piston  734  away from upper sliding surface  718 . As such, shoe  736  and upper sliding pad  738  are moved away from upper sliding surface  718  with upper sliding pad  738  maintaining contact with upper sliding surface  718 . By moving upper sliding pad  738  away from upper sliding surface  718 , a braking force against ring gear  702  is reduced to allow movement among ring gear  702 , base  58 , and/or supporting ring  704 . Yaw motors  66  are activated to rotate base  58  with respect to tower  12  to position nacelle assembly  16  at a desired yaw direction and then are deactivated. 
     To maintain the yaw direction of nacelle assembly  16 , motor  742  activates activation device  732  to move piston  734  toward upper sliding surface  718 . As such, shoe  736  and upper sliding pad  738  are moved toward upper sliding surface  718  to increase the braking force applied to upper sliding surface  718 . Accordingly, the ability of supporting ring  704  and/or base  58  to move with respect to ring gear  702  is reduced such that the yaw direction of nacelle assembly  16  is maintained. 
       FIG. 14  is a schematic cross-sectional view of a seventh alternative exemplary yaw bearing assembly  800  that may be used with nacelle assembly  16  (shown in  FIG. 2 ) as yaw bearing assembly  72 .  FIG. 15  is a schematic cross-sectional view of yaw bearing assembly  800 . Similar components shown in  FIGS. 1 ,  2 ,  14 , and  15  are numbered similarly. In the exemplary embodiment, base  58  includes at least one aperture  802  defined therethrough. Aperture  802  is configured to receive a screw jack  804 , as described in more detail below. When base  58  includes more than one aperture  802 , apertures  802  are spaced circumferentially about base  58  adjacent an upper sliding pad  806 . 
     In the exemplary embodiment, yaw bearing assembly  800  includes a ring gear  808  coupled to tower  12  and a supporting ring  810  coupled to base  58 . Ring gear  808  includes a substantially vertically-aligned tooth face  812 , a substantially horizontally-aligned upper face  814 , and a lower face  816  having a substantially horizontal portion  818  and a tapered portion  820 . A substantially vertically-aligned inner face  822  extends between upper face  814  and lower face  816 . Tooth face  812  includes a plurality of teeth  78  configured to engage pinion gear  68  (shown in  FIG. 2 ). Upper face  814  includes an upper sliding surface  824 , and tapered portion  820  of lower face  816  includes a lower sliding surface  826 . Horizontal portion  818  of lower face  816  is coupled to adjacent to tower  12 . 
     Supporting ring  810  is coupled to base  58 . Alternatively, supporting ring  810  includes an upper portion coupled to base  58  and a lower portion coupled to the upper portion, as shown and described with respect to  FIGS. 4 and 5 . In the exemplary embodiment, base  58  and supporting ring  810  define a groove  828  that is shaped to correspond to a shape of ring gear  808 . More specifically, in the exemplary embodiment, base  58  includes a substantially horizontally-oriented lower face  830  that corresponds to upper face  814  of ring gear  808 , and supporting ring  810  includes an angled upper face  832  that corresponds to tapered portion  820  of lower face  816 . Lower face  830  is positioned adjacent upper sliding surface  824 , and upper face  832  is positioned adjacent to lower sliding surface  826 . Upper sliding pad  806  is coupled to lower face  830  of base  58  adjacent to upper sliding surface  824  of ring gear  808 . 
     In the exemplary embodiment, upper sliding pad  806  extends circumferentially about lower face  830  and is in contact with upper sliding surface  824 . Upper sliding pad  806  is segmented for maintenance and/or repair as described in more detail below. Alternatively, upper sliding pad  806  includes a plurality of pads that are spaced circumferentially about lower face  830 . Further, in the exemplary embodiment, upper sliding pad  806  does not cover a lower end  834  of aperture  802  through base  58 . For example, apertures  802  are located between segments  836  of upper sliding pad  806 . 
     In the exemplary embodiment, a brake assembly  838  is coupled to supporting ring  810  and at least partially extends through upper face  832 . Brake assembly  838  includes an activation device  840 , a piston  842  coupled to activation device  840 , a shoe  844  coupled to piston  842 , and a lower sliding pad  846  coupled to shoe  844 . It should be understood that brake assembly  838  includes any suitable number of pistons  842  coupled to activation device  840 . In an alternative embodiment, brake assembly  838  does not include shoe  844  and lower sliding pad  846  is coupled to piston  842 . In the exemplary embodiment, at least a portion of piston  842  and/or shoe  844  is positioned within an opening  848  defined through supporting ring  810 . Lower sliding pad  846  is positioned to extend outwardly from upper face  832  to contact lower sliding surface  826 . In the exemplary embodiment, upper sliding pad  806  and lower sliding pad  846  are formed from any suitable material that enables supporting ring  810  to rotate with respect to ring gear  808 . More specifically, materials for upper sliding pad  806  and lower sliding pad  846  and/or material for upper sliding surface  824  and/or lower sliding surface  826  are selected to enable yaw bearing assembly  800  to function as described herein. In a particular embodiment, upper sliding surface  824  and lower sliding surface  826  are steel and upper sliding pad  806  and lower sliding pad  846  are a conventional sliding material. 
     Activation device  840  is directly or indirectly coupled to piston  842  for controlling movement of piston  842  toward and away from lower sliding surface  826 . Activation device  840  is any of activation device  148  (shown in  FIGS. 4 and 5 ), activation device  202  (shown in  FIGS. 6 and 7 ), activation device  302  (shown in  FIGS. 8 and 9 ), and/or activation device  402  (shown in  FIG. 10 ). A motor  850  is coupled to activation device  840  for control of the components thereof, such as cylinder  204  and/or cylinder  314  (shown in  FIGS. 6-9 ), springs  404  (shown in  FIG. 10 ), and/or spindle  150  (shown in  FIGS. 4 and 5 ), for moving piston  842  toward or away from lower sliding surface  826 . More specifically, activation device  840  forces piston  842  in a first direction toward lower sliding surface  826  to apply a braking force to ring gear  808 . Activation device  840  forces piston  842  in a second direction away from lower sliding surface  826  to lessen the brake force to enable supporting ring  810  to move rotationally with respect to ring gear  808 . In the exemplary embodiment, control system  36  (shown in  FIG. 2 ) is coupled in communication with, such as in operational control communication with, motor  850  for controlling a movement of activation device  840 . 
     During operation of wind turbine  10  (shown in  FIGS. 1 and 2 ), a yaw direction of nacelle assembly  16  is controlled using yaw motors  66  (shown in  FIG. 2 ) and yaw bearing assembly  800 . More specifically, to change the yaw direction of nacelle assembly  16 , activation device  840  is activated to move piston  842  away from lower sliding surface  826 . As such, shoe  844  and lower sliding pad  846  are moved away from lower sliding surface  826  with lower sliding pad  846  maintaining contact with lower sliding surface  826 . By moving lower sliding pad  846  away from lower sliding surface  826 , a braking force against ring gear  808  is reduced to allow movement among ring gear  808 , base  58 , and/or supporting ring  810 . Yaw motors  66  are activated to rotate base  58  with respect to tower  12  to position nacelle assembly  16  at a desired yaw direction and then are deactivated. 
     To maintain the yaw direction of nacelle assembly  16 , motor  850  activates activation device  840  to move piston  842  toward lower sliding surface  826 . As such, shoe  844  and lower sliding pad  846  are moved toward lower sliding surface  826  to increase the braking force applied to lower sliding surface  826 . Accordingly, the ability of supporting ring  810  and/or base  58  to move with respect to ring gear  808  is reduced such that the yaw direction of nacelle assembly  16  is maintained. 
     To perform maintenance, repair, and/or replacement on yaw bearing assembly  800 , a screw jack  804  or other suitable device is inserted through at least one aperture  802  adjacent to a segment  836  of upper sliding pad  806  to be maintained. Screw jack  804  is then adjusted to remove weight of base  58  from segment  836  by, for example, creating and/or adjusting a gap between base  58  and upper sliding pad  806 . Segment  836  is then able to be removed from yaw bearing assembly  800  for repair and/or replacement. Segment  836  is replaced between base  58  and upper sliding surface  824 , and screw jack  804  is removed from aperture  802 . 
     The maintenance method described above can be used for yaw bearing assembly  100  (shown in  FIGS. 4 and 5 ), yaw bearing assembly  200  (shown in  FIGS. 6 and 7 ), yaw bearing assembly  300  (shown in  FIGS. 8 and 9 ), yaw bearing assembly  400  (shown in  FIG. 10 ), yaw bearing assembly  500  (shown in  FIG. 11 ), yaw bearing assembly  600  (shown in  FIG. 12 ), yaw bearing assembly  700  (shown in  FIG. 13 ), yaw bearing assembly  900  (shown in  FIG. 16 ), and/or yaw bearing assembly  1000  (shown in  FIG. 17 ) with slight modifications depending on the configuration of the yaw bearing assembly. 
       FIG. 16  is a schematic cross-sectional view of an eighth alternative exemplary yaw bearing assembly  900  that may be used with nacelle assembly  16  (shown in  FIG. 2 ) as yaw bearing assembly  72  (shown in  FIG. 2 ). Similar components shown in  FIGS. 1 ,  2 , and  16  are numbered similarly. 
     In the exemplary embodiment, yaw bearing assembly  900  includes a ring gear  902  coupled to tower  12  and a supporting ring  904  coupled to base  58 . Ring gear  902  includes a substantially vertically-aligned tooth face  906 , a substantially horizontally-aligned upper face  908 , and a substantially horizontally-aligned lower face  910 . Tooth face  906  includes a plurality of teeth  78  configured to engage pinion gear  68  (shown in  FIG. 2 ). An inner face  912  extends between upper face  908  and lower face  910  and includes a generally V-shaped groove  914  having an angled upper sliding surface  916  and an angled lower sliding surface  918 . 
     Supporting ring  904  includes an upper portion  920  coupled to base  58  and a wedge portion  922  extending from upper portion  920 . Wedge portion  922  is shaped to correspond to a shape of groove  914 . More specifically, in the exemplary embodiment, wedge portion  922  includes an angled lower face  924  that corresponds to lower sliding surface  918  and an angled upper face  926  that corresponds to upper sliding surface  916 . A brake assembly  928  includes a wedge-shaped piston  930  extends through a substantially horizontally-aligned opening  932  through wedge portion  922 . Piston  930  includes an angled upper face  934  having an upper sliding pad  936  coupled thereto and an angled lower face  938  having a lower sliding pad  940  coupled thereto. Upper face  934  and lower face  938  converge toward groove  914  to be at least partially received therein. In the exemplary embodiment, piston upper face  934  is substantially parallel to wedge portion upper face  926 , and piston lower face  938  is substantially parallel to wedge portion lower face  924 . Lower sliding pad  940  and upper sliding pad  936  are positioned to extend outwardly from an end  942  of opening  932  to contact lower sliding surface  918  and upper sliding surface  916 , respectively. In the exemplary embodiment, upper sliding pad  936  and lower sliding pad  940  are formed from any suitable material that enables supporting ring  904  to rotate with respect to ring gear  902 . More specifically, materials for upper sliding pad  936  and lower sliding pad  940  and/or material for upper sliding surface  916  and/or lower sliding surface  918  are selected to enable yaw bearing assembly  900  to function as described herein. In a particular embodiment, upper sliding surface  916  and lower sliding surface  918  are steel and upper sliding pad  936  and lower sliding pad  940  are a conventional sliding material. 
     In one embodiment, sliding pads are also coupled to wedge portion upper face  926  and wedge portion lower face  924  such that, when piston sliding pads  936  and  940  are moved away from sliding surfaces  916  and  918 , the wedge portion sliding pads are in contact with sliding surfaces  916  and/or  918  to facilitate reducing friction between ring gear  902  and supporting ring  904 . The sliding pads on wedge portion  922  may extend circumferentially about wedge portion  922  with openings at openings  932  or include a plurality of sliding pads that are spaced circumferentially about wedge portion  922 . The sliding pads on wedge portion  922  may be segmented to facilitate performing maintenance, replacement, and/or repair. 
     An activation device  944  is coupled to piston  930  for controlling movement of piston  930  toward and away from lower sliding surface  918  and upper sliding surface  916 . In the exemplary embodiment, lower sliding pad  940  is in contact with lower sliding surface  918 , and upper sliding pad  936  is in contact with upper sliding surface  916 . Further, activation device  944  is directly or indirectly coupled to piston  930  for controlling movement of piston  930  toward and away from upper sliding surface  916  and lower sliding surface  918 . Activation device  944  is any of activation device  148  (shown in  FIGS. 4 and 5 ), activation device  202  (shown in  FIGS. 6 and 7 ), activation device  302  (shown in  FIGS. 8 and 9 ), and/or activation device  402  (shown in  FIG. 10 ). A motor  850  is coupled to activation device  944  for control of the components thereof, such as cylinder  204  and/or cylinder  314  (shown in  FIGS. 6-9 ), springs  404  (shown in  FIG. 10 ), and/or spindle  150  (shown in  FIGS. 4 and 5 ), for moving piston  930  toward or away from lower sliding surface  918  and upper sliding surface  916 . 
     More specifically, activation device  944  forces piston  930  in a first substantially horizontal direction toward lower sliding surface  918  and upper sliding surface  916  to apply a braking force to ring gear  902 . Activation device  944  forces piston  930  in a second substantially horizontal direction away from lower sliding surface  918  and upper sliding surface  916  to lessen the brake force to enable supporting ring  904  to move rotationally with respect to ring gear  902 . In the exemplary embodiment, control system  36  (shown in  FIG. 2 ) is coupled in communication with, such as in operational control communication with, motor  946  for controlling a movement of activation device  944 . 
     During operation of wind turbine  10  (shown in  FIGS. 1 and 2 ), a yaw direction of nacelle assembly  16  is controlled using yaw motors  66  (shown in  FIG. 2 ) and yaw bearing assembly  900 . More specifically, to change the yaw direction of nacelle assembly  16 , activation device  944  is activated to move piston  930  away from lower sliding surface  918  and upper sliding surface  916 . As such, lower sliding pad  940  and upper sliding pad  936  are moved away from lower sliding surface  918  and upper sliding surface  916  with lower sliding pad  940  and/or upper sliding pad  936  maintaining contact with lower sliding surface  918  and/or upper sliding surface  916 , respectively. By moving lower sliding pad  940  and upper sliding pad  936  away from lower sliding surface  918  and upper sliding surface  916 , a braking force against ring gear  902  is reduced to allow movement between ring gear  902  and supporting ring  904 . Yaw motors  66  are activated to rotate base  58  with respect to tower  12  to position nacelle assembly  16  at a desired yaw direction and then are deactivated. 
     To maintain the yaw direction of nacelle assembly  16 , motor  946  activates activation device  944  to move piston  930  toward lower sliding surface  918  and upper sliding surface  916 . As such, lower sliding pad  940  and upper sliding pad  936  are moved toward lower sliding surface  918  and upper sliding surface  916  to increase the braking force applied to lower sliding surface  918  and upper sliding surface  916 . Accordingly, the ability of supporting ring  904  to move with respect to ring gear  902  is reduced such that the yaw direction of nacelle assembly  16  is maintained. 
       FIG. 17  is a schematic cross-sectional view of a ninth alternative exemplary yaw bearing assembly  1000  that may be used with nacelle assembly  16  (shown in  FIG. 2 ) as yaw bearing assembly  72  (shown in  FIG. 2 ). Similar components shown in  FIGS. 1 ,  2 , and  17  are numbered similarly. 
     In the exemplary embodiment, yaw bearing assembly  1000  includes a ring gear  1002  coupled to tower  12  and a supporting ring  1004  coupled to base  58 . Ring gear  1002  includes a substantially vertically-aligned tooth face  1006 , a substantially horizontally-aligned upper face  1008 , and a substantially horizontally-aligned lower face  1010 . Tooth face  1006  includes a plurality of teeth  78  configured to engage pinion gear  68  (shown in  FIG. 2 ). An inner face  1012  extends between upper face  1008  and lower face  1010  and includes a generally V-shaped groove  1014  having an angled upper sliding surface  1016  and an angled lower sliding surface  1018 . 
     Supporting ring  1004  includes an upper portion  1020  coupled to base  58  and a lower portion  1022  coupled to upper portion  1020 . Upper portion  1020  and lower portion  1022  define a wedge portion  1024  that is shaped to correspond to a shape of groove  1014 . More specifically, in the exemplary embodiment, lower portion  1022  includes an angled lower face  1026  that corresponds to lower sliding surface  1018 , and upper portion  1020  includes an angled upper face  1028  that corresponds to upper sliding surface  1016 . Upper face  1028  and lower face  1026  converge toward groove  1014  to be at least partially received therein. 
     In the exemplary embodiment, at a brake assembly  1030 , upper portion  1020  is coupled to lower portion  1022  via an activation device  1032 . Upper portion  1020  and lower portion  1022  at brake assembly  1030  is referred to herein as a piston  1034 . An upper face  1036  at brake assembly  1030  is substantially parallel to upper face  1028  of supporting ring  1004 , and a lower face  1038  at brake assembly  1030  is substantially parallel to lower face  1026  of supporting ring  1004 . A lower sliding pad  1040  is coupled to lower face  1038 , and an upper sliding pad  1042  is coupled to upper face  1036 . Lower sliding pad  1040  and upper sliding pad  1042  are positioned to contact lower sliding surface  1018  and upper sliding surface  1016 , respectively. In the exemplary embodiment, upper sliding pad  1042  and lower sliding pad  1040  are formed from any suitable material that enables supporting ring  1004  to rotate with respect to ring gear  1002 . More specifically, materials for upper sliding pad  1042  and lower sliding pad  1040  and/or material for upper sliding surface  1016  and/or lower sliding surface  1018  are selected to enable yaw bearing assembly  1000  to function as described herein. In a particular embodiment, upper sliding surface  1016  and lower sliding surface  1018  are steel and upper sliding pad  1042  and lower sliding pad  1040  are a conventional sliding material. 
     In one embodiment, upper and lower sliding pads are also coupled to upper face  1028  of supporting ring  1004  and/or lower face  1026  of supporting ring  1004  such that, when brake assembly sliding pads  1040  and  1042  are moved away from sliding surfaces  1016  and  1018 , the supporting ring sliding pads are in contact with sliding surfaces  1016  and/or  1018  to facilitate reducing friction between ring gear  1002  and supporting ring  1004 . The sliding pads on supporting ring  1004  may extend circumferentially about supporting ring  1004  with openings at brake assemblies  1030  or include a plurality of sliding pads that are spaced circumferentially about supporting ring  1004 . The sliding pads on supporting ring  1004  may be segmented to facilitate performing maintenance, replacement, and/or repair. 
     Activation device  1032  is coupled between upper portion  1020  and lower portion  1022  at brake assembly  1030  to control movement of lower sliding pad  1040  and upper sliding pad  1042  toward and away from lower sliding surface  1018  and upper sliding surface  1016 . In the exemplary embodiment, lower sliding pad  1040  is in contact with lower sliding surface  1018 , and upper sliding pad  1042  is in contact with upper sliding surface  1016 . Further, activation device  1032  is directly or indirectly coupled to piston  1034  for controlling movement of upper sliding pad  1042  and lower sliding pad  1040  toward and away from upper sliding surface  1016  and lower sliding surface  1018 . Activation device  1032  is any of activation device  148  (shown in  FIGS. 4 and 5 ), activation device  202  (shown in  FIGS. 6 and 7 ), activation device  302  (shown in  FIGS. 8 and 9 ), and/or activation device  402  (shown in  FIG. 10 ). A motor  1044  is coupled to activation device  1032  for control of the components thereof, such as cylinder  204  and/or cylinder  314  (shown in  FIGS. 6-9 ), springs  404  (shown in  FIG. 10 ), and/or spindle  150  (shown in  FIGS. 4 and 5 ), for moving lower sliding pad  1040  and upper sliding pad  1042  toward or away from lower sliding surface  1018  and upper sliding surface  1016 . 
     More specifically, activation device  1032  forces piston  1034  apart by moving lower portion  1022  downward in a first substantially vertical direction, thus moving lower sliding pad  1040  and upper sliding pad  1042  toward lower sliding surface  1018  and upper sliding surface  1016  to apply a braking force to ring gear  1002 . Activation device  1032  forces piston  1034  together by moving lower portion  1022  upward in a second substantially vertical direction, thus, moving lower sliding pad  1040  and upper sliding pad  1042  away from lower sliding surface  1018  and upper sliding surface  1016  to lessen the brake force to enable supporting ring  1004  to move rotationally with respect to ring gear  1002 . In the exemplary embodiment, control system  36  (shown in  FIG. 2 ) is coupled in communication with, such as in operational control communication with, motor  1044  for controlling a movement of activation device  1032 . 
     During operation of wind turbine  10  (shown in  FIGS. 1 and 2 ), a yaw direction of nacelle assembly  16  is controlled using yaw motors  66  (shown in  FIG. 2 ) and yaw bearing assembly  1000 . More specifically, to change the yaw direction of nacelle assembly  16 , activation device  1032  is activated to move piston  1034  away from lower sliding surface  1018  and upper sliding surface  1016 . As such, lower sliding pad  1040  and upper sliding pad  1042  are moved away from lower sliding surface  1018  and upper sliding surface  1016  with lower sliding pad  1040  and/or upper sliding pad  1042  maintaining contact with lower sliding surface  1018  and/or upper sliding surface  1016 , respectively. By moving lower sliding pad  1040  and upper sliding pad  1042  away from lower sliding surface  1018  and upper sliding surface  1016 , a braking force against ring gear  1002  is reduced to allow movement between ring gear  1002  and supporting ring  1004 . Yaw motors  66  are activated to rotate base  58  with respect to tower  12  to position nacelle assembly  16  at a desired yaw direction and then are deactivated. 
     To maintain the yaw direction of nacelle assembly  16 , motor  1044  activates activation device  1032  to move piston  1034  toward lower sliding surface  1018  and upper sliding surface  1016 . As such, lower sliding pad  1040  and upper sliding pad  1042  are moved toward lower sliding surface  1018  and upper sliding surface  1016  to increase the braking force applied to lower sliding surface  1018  and upper sliding surface  1016 . Accordingly, the ability of supporting ring  1004  to move with respect to ring gear  1002  is reduced such that the yaw direction of nacelle assembly  16  is maintained. 
     Any of the above-described embodiments can be used singly or in combination within wind turbine  10  to provide a yaw bearing and braking system. 
       FIG. 18  is a flowchart of an exemplary method  1100  that may be used with any of yaw bearing assemblies  72  (shown in  FIG. 2 ),  100  (shown in  FIGS. 4 and 5 ),  200  (shown in  FIGS. 6 and 7 ),  300  (shown in  FIGS. 8 and 9 ),  400  (shown in  FIG. 10 ),  500  (shown in  FIG. 11 ),  600  (shown in  FIG. 12 ),  700  (shown in  FIG. 13 ),  800  (shown in  FIGS. 14 and 15 ),  900  (shown in  FIG. 16 ), and/or  1000  (shown in  FIG. 17 ). For clarity of description, method  1100  will be described with respect to yaw bear assembly  100 . 
     By performing method  1100 , braking is performed within wind turbine  10  (shown in  FIG. 1 ). Method  1100  is performed by control system  36  (shown in  FIG. 1 ) sending commands and/or instructions to components of wind turbine  10 . Processor  40  (shown in  FIG. 1 ) within control system  36  is programmed with code segments configured to perform method  1100 . Alternatively, method  1100  is encoded on a computer-readable medium that is readable by control system  36 . In such an embodiment, control system  36  and/or processor  40  is configured to read computer-readable medium for performing method  1100 . In the exemplary embodiment, method  1100  is automatically performed continuously and/or at selected times. Alternatively, method  1100  is performed upon request of an operator of wind turbine  10  and/or when control system  36  determines method  1100  is to be performed. 
     Referring to  FIGS. 1 ,  2 ,  4 ,  5 , and  18 , in the exemplary embodiment, method  1100  includes rotating  1102  nacelle  16  with respect to tower  12 . More specifically, nacelle  16  is rotated  1102  using yaw bearing assembly  100  by, for example, operating yaw motor  66 . To prevent nacelle  16  from continuing to rotate  1102 , sliding pad  144  is moved  1104  toward first sliding surface  118  and/or second sliding surface  120  to apply a braking force to first sliding surface  118  and/or second sliding surface  120 . More specifically, activation device  148  moves  1104  piston  140  toward first sliding surface  118  and/or second sliding surface  120  to apply the braking force between nacelle  16  and tower  12 . Activation device  148  retains  1106  sliding pad  144  against first sliding surface  118  and/or second sliding surface  120  to maintain a yaw position of nacelle  16 . 
     To adjust the yaw position of nacelle  16 , sliding pad  144  is moved  1108  away from first sliding surface  118  and/or second sliding surface  120  to release and/or lessen the braking force. Once the braking force is sufficiently reduced, nacelle  16  is rotated  1102  with respect to tower  12 , as described above. 
     The embodiments described herein provide a yaw bearing assembly that includes a brake assembly having a sliding pad that acts against a sliding surface to facilitate maintaining a desired yaw position of a nacelle assembly. The above-described embodiments require less force to change the yaw direction of the nacelle assembly, as compared to brake assemblies that include friction pads and/or surfaces. Further, the above-described yaw bearing assemblies facilitate maintaining a desired yaw position of a nacelle with respect to a tower. The brake assemblies described herein include activation devices that apply a pre-loaded force to a ring gear to facilitate maintaining the yaw position of the nacelle. More specifically, the pre-loaded force can be adjusted by adjusting the position of a sliding pad of the braking system with respect to a sliding surface of the ring gear. Further, the embodiments described herein use one or more sliding bearings rather than ball bearings to facilitate rotating the nacelle with respect to the tower, which reduces the manufacturing coast and/or the maintenance cost of the yaw bearing assembly as compared to yaw bearing assemblies that include ball bearings. 
     Exemplary embodiments of yaw bearing assemblies and methods for operating the same are described above in detail. The assemblies are not limited to the specific embodiments described herein, but rather, assembly components of systems and/or steps of the methods may be utilized independently and separately from other components and/or steps described herein. For example, the assemblies may also be used in combination with other bearing systems and methods, and are not limited to practice with only the wind turbine systems and methods as described herein. Rather, the exemplary embodiment can be implemented and utilized in connection with many other bearing applications. 
     Although specific features of various embodiments of the invention may be shown in some drawings and not in others, this is for convenience only. In accordance with the principles of the invention, any feature of a drawing may be referenced and/or claimed in combination with any feature of any other drawing. 
     This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal language of the claims.