Patent Publication Number: US-2012028754-A1

Title: Liquid equalized gear system and method for using same

Description:
FIELD 
     The invention relates to a gear system that incorporates planet and sun gears, and more particularly, to a gear system with a balanced load between the planet gears. 
     BACKGROUND 
     Planetary gear systems are known. Such systems  10 , as shown schematically in  FIGS. 1 and 3 , include a carrier  11  which houses a centralized sun gear  14  and a plurality of planet gears  16 . The planet gears  16  engage the rotating sun gear  14  and a stationary ring gear  12  encircling the carrier  11 . 
     Planetary gear systems, such as system  10 , find use in applications such as wind turbines. Other potential applications can be found in mill operations, the oil and gas industry, and the aviation industry. 
     For planetary gear systems utilizing multiple planet gears, a problem that has developed is the creation of an unbalanced load between the planet gears. As a rotating member—depending upon the gear system, the ring gear, the carrier, or the sun gear may supply an input to the gearbox—rotates, it places a force, or a load, on the planet gears. The load factor K γ  of a planetary gear system may be defined as: 
     
       
      
       K 
       γ 
       =T 
       Branch 
       N 
       CP 
       /T 
       Nom  
      
     
     Where T Branch  is the torque for the gear with the heaviest load, N CP  is the number of planets, and T Nom  is the total nominal torque for the system. Ideally, the force should be the same on each planet gear, i.e., K γ =1.0, thereby creating a balanced load. However, for a variety of reasons planetary gear systems often suffer from unbalanced loads. 
     One reason is that the gear teeth  20  ( FIG. 2 ) of the planetary gears are manufactured with a normal variance for such teeth. For example, the thickness of the gear teeth G W  may vary to an extent expected of tolerances for gear teeth. Additionally, the pitch—the distance between adjacent gear teeth  20 —also may vary. 
     Another reason is due to an inherent difficulty in properly position openings  17  in the carrier  11  that become the anchoring points for the planet gears  16 . This difficulty is especially true for large carriers  11 , such as those used in wind turbine gear systems. The equipment used to form openings, or planet pin bore holes,  17  within large carriers  11  does not have the accuracy to form and locate the openings  17  with the amount of precision and cost desired. 
     For planetary gear systems that utilize a number of planet gears greater than three, such as four planet gears, it is desired to anchor the planet gears along a midline M C  of the carrier  11 . As shown in  FIG. 3 , the opening  17  formed in the carrier  11  which will become an anchoring point for a planet gear  16  is offset from the midline M C  of the carrier  11 . By offset is meant that the midline of the opening  17  does not pass through the midline M C  of the carrier  11 . When other openings  17  or formed in the carrier and the planet gears are anchored thereto, the true positions  25  of the planet gears are offset from the desired positions  30  of the planet gears. This offset can lead to a load imbalance between the planet gears, as will be further described with reference to  FIGS. 4-7 . 
     The ring gear  12  includes gear teeth  13  for engaging with the gear teeth of the planet gears  16 . The carrier  11  and ring gear  12  are shown in  FIGS. 4-7  linearly, as though each were folded out into a single plane. As shown  FIG. 4 , there is an offset M O  between the midline M C  of the carrier  11  and the midline M P  of a first planet gear  16 . The midline M C  of the carrier  11  is aligned with where the first planet gear  16  would be located if it were in a true position. The other planet gears are shown to be aligned with their true positions. 
     When the ring gear  12  engages with the planet gears and begins to rotate in direction D R , a force F 1  is exerted from the first planet gear  16  toward the gear tooth  13  ( FIG. 5 ). Further, the offset M O  between the midline M C  of the carrier  11  and the midline M P  of a first planet gear  16  is lessened. This is due to the inherent stiffness in the carrier  11 . 
     As the ring gear  12 , or carrier  11 , continues to rotate in direction D R , as shown in  FIG. 6 , the force F 1  increases and the offset M O  between the midline M C  of the carrier  11  and the midline M P  of a first planet gear  16  continues to diminish. Finally, as shown in  FIG. 7 , the inherent stiffness of the carrier  11  and the rotation of the ring gear  12  result in forces F 2 , F 3  and F 4  to be exerted from, respectively, the second, third and fourth planet gears  16 . As illustrated, the forces F 1 -F 4  are not balanced between the four planet gears  16 . 
     Known attempts at creating a planetary gear system with a balanced load between individual planet gears have focused on reducing the stiffness of the carrier, thereby reducing the stiffness of the planetary gear system as a whole.  FIG. 8  illustrates graphically how the increase in stiffness of the carrier causes an increase in stiffness in the planetary gear system as a whole. Thus, attempts to lessen the carrier stiffness would concomitantly reduce the overall system stiffness. Such known attempts may be found in U.S. Pat. Pub. 2008/0194378 and U.S. Pat. Nos. 7,056,259 and 7,297,086. 
     Under normal manufacturing practices, the carrier bore hole when drilled will end up away from its centric true position. This is due to manufacturing tolerance limitations, complexity of the machined part, measuring capability, and human error. This scenario causes each planet gear to carry a load different from what they are designed for. Also, under normal loading conditions, the carrier may twist slightly. This twist may contribute to the misalignment between the planet gears and the ring gear/sun gear assembly. Depending on the number of planets and their respective tolerances, loads experienced by any single planet can increase dramatically, as much as 2× or more. 
     Reducing the load factor K γ  on a gear system will allow smaller system components to be utilized or allow greater loads on system components than are currently placed. 
     With some of these concerns in mind, an improved planetary gear system would be welcome in the art. 
     SUMMARY 
     An embodiment of the invention includes a carrier for use in a planetary gear system and having a plurality of openings or slots offset from a midline thereof. The carrier includes alignment mechanisms positioned in each of the plurality of openings or slots to substantially align each planet gear about the midline of the carrier. 
     An embodiment of the invention includes a planetary gear system. The system includes a carrier having a plurality of openings or slots offset from a midline thereof; a stationary central gear; a plurality of peripheral planet gears, each said planet gear in a meshed relationship with the stationary central gear; and an alignment mechanism for substantially aligning each planet gear about the midline of the carrier. 
     An embodiment of the invention includes a method for balancing a load on a planetary gear system between the planet gears. The method includes forming oversized openings or slots within a carrier, the openings or slots being offset from a midline of the carrier; positioning alignment means within the oversized openings or slots, each alignment means having an opening which, when the alignment means are positioned, are substantially bisected by the midline of the carrier; securing the alignment means within the oversized openings or slots; and anchoring the planet gears about the alignment means openings. 
     These and other features, aspects and advantages of the present invention may be further understood and/or illustrated when the following detailed description is considered along with the attached drawings. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic view of a conventional carrier illustrating unbalanced loads on planet gears. 
         FIG. 2  is a schematic view illustrating manufacturing variances of conventional gear teeth of a planet gear. 
         FIG. 3  is a schematic view of the carrier of  FIG. 1  illustrating the effect the unbalanced load has on the true positions of the planet gears. 
         FIGS. 4-7  are schematic views illustrating the effect that rotational movement of a conventional carrier, or ring gear, has on the balance of loads between four planet gears within a carrier. 
         FIG. 8  is a graph illustrating the influence of carrier stiffness on the overall stiffness of a gear system. 
         FIG. 9  is a schematic view of a carrier and an eccentric disk in accordance with an embodiment of the invention. 
         FIG. 10  is a schematic view of the carrier of  FIG. 9  with the eccentric disk in place in accordance with an embodiment of the invention. 
         FIG. 11  is a perspective view of a carrier with eccentric disks in accordance with an embodiment of the invention. 
         FIG. 12  is another perspective view of the carrier of  FIG. 11 . 
         FIGS. 13  (A) and (B) are cut-away views showing anchoring techniques for the eccentric disk in accordance with embodiments of the invention. 
         FIG. 14  is a perspective view of a carrier with inserts in accordance with an embodiment of the invention. 
         FIG. 15  is a schematic view of the carrier and insert of  FIG. 14 . 
         FIG. 16  is a schematic view of the insert positioned within a slot of the carrier of  FIG. 14 . 
         FIG. 17  is a schematic view of the insert anchored into position in a slot of the carrier of  FIG. 14 . 
         FIG. 18  illustrates a process for balancing a load on a planetary gear system between the individual planet gears. 
     
    
    
     DETAILED DESCRIPTION 
     The present specification provides certain definitions and methods to better define the embodiments and aspects of the invention and to guide those of ordinary skill in the art in the practice of its fabrication. Provision, or lack of the provision, of a definition for a particular term or phrase is not meant to imply any particular importance, or lack thereof; rather, and unless otherwise noted, terms are to be understood according to conventional usage by those of ordinary skill in the relevant art. 
     Unless defined otherwise, technical and scientific terms used herein have the same meaning as is commonly understood by one of skill in the art to which this invention belongs. The terms “first”, “second”, and the like, as used herein do not denote any order, quantity, or importance, but rather are used to distinguish one element from another. Also, the terms “a” and “an” do not denote a limitation of quantity, but rather denote the presence of at least one of the referenced item, and the terms “front”, “back”, “bottom”, and/or “top”, unless otherwise noted, are merely used for convenience of description, and are not limited to any one position or spatial orientation. If ranges are disclosed, the endpoints of all ranges directed to the same component or property are inclusive and independently combinable (e.g., ranges of “up to about μwt. %, or, more specifically, about 5 wt. % to about 20 wt. %,” is inclusive of the endpoints and all intermediate values of the ranges of “about 5 wt. % to about 25 wt. %,” etc.). 
     The modifier “about” used in connection with a quantity is inclusive of the stated value and has the meaning dictated by the context (e.g., includes the degree of error associated with measurement of the particular quantity). Reference throughout the specification to “one embodiment”, “another embodiment”, “an embodiment”, and so forth, means that a particular element (e.g., feature, structure, and/or characteristic) described in connection with the embodiment is included in at least one embodiment described herein, and may or may not be present in other embodiments. In addition, it is to be understood that the described inventive features may be combined in any suitable manner in the various embodiments. 
       FIGS. 9-11  illustrate a planetary gear system  100  including a carrier  110 . The carrier, as schematically shown in  FIGS. 9 and 10 , has an opening  117  formed therein. It is to be understood that a multiple of openings  117  are to be formed in the carrier  110  for a planetary gear system having multiple planet gears. Opening  117  differs from opening  17  in that it is larger. The larger openings  117  may be formed using the same equipment used to form openings  17 , thereby allowing the larger openings  117  to maintain the same true position tolerance as the openings  17  have. 
     The carrier  110  includes a midline M C  which bisects the carrier  110 . As shown, the midline M C  does not bisect the opening  117 , as the opening is offset from the midline M C . An eccentric disk  125  is provided with an eccentric opening  127 . The disk  125  includes a midline M D . The opening  127  is offset from the midline M D . 
     It is envisioned that batches of eccentric disks  125  with a particular offset can be fabricated. For example, a batch of disks  125  can be formulated to handle an offset of, for example, 0.1 millimeters, 0.075 millimeters, 0.05 millimeters, or 0.025 millimeters. So, if for example, an opening  117  is offset from the midline M C  of the carrier  110  by 0.088 millimeters, a disk  125  formulated to handle a 0.1 millimeter offset would be used, thus positioning a planet gear having only a 0.012 offset from the midline M C  of the carrier  110 . Any offset below 0.0125 millimeters is to be considered substantially aligned, and such substantial alignment is capable of reducing the overall load imbalance on the planet gears by as much as eight to ten percent. 
     The eccentric disk  125  is positioned within the opening  117  ( FIG. 10 ) so as to render the disk non-rotatable. Further, the disk  125  is positioned such that the midline M D  coincides with the midline M C . To ensure that the disk  125  is properly positioned, both the disk  125  and the carrier  110  in the vicinity of the opening  117  are marked so that the mark on the disk  125  matches the mark on the carrier opening  117 . One method for positioning the disk  125  in the opening  117  is to cool the disk  125  down, allowing it to contract in size, positioning it in the opening  117 , and allowing it to warm up and expand in size to the point where it has a friction fit with the opening  117 . 
     To further ensure non-rotation of the disk  125 , the disk can be cooled down and an adhesive material can be added to the disk. Once the disk  125  is positioned in the opening  117  and allowed to warm up and expand in size, the disk can adhere to the surface of the opening  117 .  FIG. 11  illustrates a plurality of disks  125  positioned in a carrier  110 . 
       FIG. 12  illustrates pins  131  positioned within the disks  125 . The pins  131  serve as anchoring points for the planet gears. 
       FIGS. 13  (A) and (B) illustrate alternative methods for securing the pins  131  within the carrier  110 . As shown in  FIG. 13  (A), a bolt  133  can be added to one side of the carrier  110  concentric with the disk  125 . As shown in  FIG. 13  (B), a peg and slot assembly  135  is provided in the bolt  133 . The bolt has a slot into which a peg is located to prevent rotation of the bolt  133  and the disk  125 . It is to be understood that other methods for rendering the disks  125  non-rotatable may be employed. For example, the disks  125  may be coated with friction-enhancing coatings. Further, the openings  117  may include one or more indents into which respective knurls located on the disks  125  fit. Alternatively, the disks  125  may include a key that would fit within a notch found in the openings  117 . 
     The eccentric disks  125  serve to properly align the planet gears, such as planet gears  16 , both axially and radially. Through such alignment of all the planet gears  16  in their true positions, the load placed on any single planet gear can be balanced with the load placed on the remaining planet gears. 
       FIG. 14  illustrates a planetary gear system  200  including a carrier  210 . The carrier  210  includes a plurality of inserts  240 . Each insert includes an opening  244  and a flange  246 . Although illustrated as curved, the flange  246  alternatively may be straight.  FIG. 15  illustrates a slot  242  formed in a carrier, such as carrier  210 . It is to be understood that multiple slots  242  are formed in the carrier  210  to accommodate multiple inserts and, ultimately, multiple planet gears. 
     The slot  242  has a width S W  and a midline M S  bisecting the width. The midline M S  is offset M O ′ from the midline M C  of the carrier  210 . The insert  240  has a width I W  and includes an opening  244 . The width I W  of the insert  240  is shorter than the width S W  of the slot  242  ( FIG. 16 ), leading to a gap  248  between the insert  240  and the slot wall  242 . The insert  240  is positioned within the slot  242  such that the opening  244  is bisected by the midline M C  of the carrier  210 . 
     A shim  250  may be placed within the gap  248  to immobilize the insert  240  into place within the slot  242  ( FIG. 17 ). As illustrated, a single shim  250  may be located within the gap  248 , forcing the insert into an abutting relationship with a wall of the slot  242 . Alternatively, and based upon the positioning of a midline of the opening  244  coinciding with the midline M C  of the carrier, two shims  250  of different sizes may be positioned within gaps  248  on either side of the insert  240 . 
     An adhesive material may also be used to immobilize the insert  240  within the slot  242 . Further, attachment means  252  may be inserted through the flange  246  into the carrier  210  to ensure immobilization of the insert  240 . The attachment means may take any suitable form, such as, for example, screws, nails, pins, brads, staples, adhesive material, clamps, or any other like means. 
     Planet gears, such as planet gears  16 , are to be anchored, via pins positioned in the openings  244  of the inserts  240 . In this way, the planet gears may be tangentially aligned to better balance the load between planet gears. The use of inserts  240  ensures no rotational movement of the planet gears about the midline M C  of the carrier  210 . 
     The openings  244  are sized to receive pins  260 , upon which the planet gears are anchored. Alternatively, the openings  244  may be formed oversized and eccentric disks, such as disks  125 , can be placed within the openings  244  as described previously, with the pins  260  being received in the openings of the eccentric disks. In this way, the planet gears can be maintained axially and radially about the midline M C  of the carrier  210 . 
     Next, and with particular reference to  FIG. 18 , is described a method for balancing a load placed upon a planetary gear system, such as a planetary gear system used in a wind turbine. 
     At Step  300 , a carrier is provided. For a planetary gear system having four planet gears, four openings or slots are formed in the carrier. The openings, such as openings  117 , or the slots, such as slots  242 , are formed oversized. Further, the central line of the openings or slots may be offset somewhat from a midline of the carrier. The openings may be formed through a boring technique, while the slots may be formed through a machining technique. 
     At Step  305 , alignment means are positioned within the openings or slots. The alignment means for openings are eccentric disks, such as disks  125 . The disks  125  have openings  127  that are offset from a midline of the disks  125 . To ensure that the eccentric disk is properly positioned, both the disk and the carrier in the vicinity of the oversized opening are marked so that the mark on the disk matches the mark on the carrier opening. 
     The alignment means for slots are inserts, such as inserts  240 . The slots have a width and a midline which bisects the width. The slot midlines are offset from the midline of the carrier. The inserts have a width and each includes an opening. The width of the insert is shorter than the width of the slot, leading to a gap between each insert and the slot walls. The inserts are positioned within the slots such that the insert openings are bisected by the midline of the carrier. 
     At Step  310 , the alignment means are secured within the openings or slots. One method for securing the eccentric disk in position in the opening is to pre-cool the disk, allowing it to contract in size, prior to positioning it in the opening. Once positioned in the opening, the eccentric disk is allowed to be warmed up and expands in size to the point where it has a friction fit with the opening. 
     To further ensure non-rotation of the disk, it can be cooled down and an adhesive material can be added to the disk. Once the disk is positioned in the opening and allowed to warm up and expand in size, the disk can adhere to the surface of the opening. 
     A shim may be placed within the gap to secure the inserts into place within the slots. A single shim may be located within each gap between an insert and a slot wall, forcing the insert into an abutting relationship with a wall of the slot. Alternatively, and based upon the positioning of a midline of the openings coinciding with the midline of the carrier, two shims of different sizes may be positioned within a pair of gaps on either side of the inserts in each slot. 
     The inserts may be immobilized within the slots with an adhesive material. Further, attachment means may be inserted through curved flanges on the inserts into the carrier to ensure immobilization of the inserts. The attachment means may take any suitable form, such as, for example, screws, nails, pins, brads, staples, adhesive material, clamps, or any other like means. 
     The embodiments of the invention described herein will enable an increase in torque capacity, as well as an improved load distribution, for a planetary gearbox. Further, these enhancements are enabled with a minimum of overall mass impact on the gearbox. 
     While the invention has been described in detail in connection with only a limited number of embodiments, it should be readily understood that the invention is not limited to such disclosed embodiments. Rather, the invention can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described, but which are commensurate with the spirit and scope of the invention. For example, while embodiments have been described in terms that may initially connote singularity, it should be appreciated that multiple components may be utilized. Additionally, while various embodiments of the invention have been described, it is to be understood that aspects of the invention may include only some of the described embodiments. Accordingly, the invention is not to be seen as limited by the foregoing description, but is only limited by the scope of the appended claims.