Patent Publication Number: US-7223197-B2

Title: Reduced twist carrier

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is a divisional of U.S. patent application Ser. No. 10/318,220, filed on Dec. 13, 2002 now U.S. Pat. No. 6,855,089, which is a continuation-in-part of U.S. patent application Ser. No. 10/017,152 filed Dec. 14, 2001 which issued on Dec. 16, 2003 as U.S. Pat. No. 6,663,530, the entire contents of which are incorporated herein by reference. 

   TECHNICAL FIELD 
   The present invention relates to epicyclic gearboxes, and more particularly, to a gear carrier in an epicyclic gearbox. 
   BACKGROUND OF THE INVENTION 
   Epicyclic gearboxes are frequently used in gas turbine engines for their compact designs and efficient high gear reduction capabilities. Planetary and star gear trains are well known, and are generally comprised of three gear train elements: a central sun gear, an outer ring gear with internal gear teeth, and a plurality of planet gears supported by a planet carrier between and in meshing engagement with both the sun gear and the ring gear. All three gear train elements share a common longitudinal central axis, about which at least two of them rotate. An advantage of epicyclic gear trains is their versatility. A rotary input can be connected to any one of the three elements. Holding one of the remaining two elements stationary with respect to the other two, permits the third to serve as an output. 
   In gas turbine engine applications, where a speed reduction transmission is required, the central sun gear generally provides rotary input from the powerplant. In planetary gear trains, the outer ring gear is generally held stationary and the planet gear carrier therefore rotates in the same direction as the sun gear and provides torque output at a reduced rotational speed. In star gear trains, the gear carrier is held stationary and the output shaft is driven by the ring gear in a direction opposite that of the sun gear. 
   However, certain shortcomings do exist with epicyclic drive trains. For example, as with many mechanical elements that transfer torque, a small but nevertheless significant amount of torsional deflection commonly occurs due to the elasticity of the material of the carrier, as a result of twist between upstream and downstream plates of the gear carrier, when the gear train is under load. The gear carrier twists around its central axis, causing the individual axis of rotation of the gears to lose parallelism with the central axis of the gear carrier. This torsional deflection results in misalignment at gear train journal bearings and at the gear teeth mesh, which leads to efficiency losses and reduced life of the parts. Additionally, increased oil flow is required to the journal bearings to compensate for the misalignments caused by torsional deflections of the gear carrier plates. 
   Attempts to address this problem of planetary carrier torsional deflection are known. U.S. Pat. No. 5,466,198 issued Nov. 14, 1995 to McKibbin et al, for example, clearly sets out the problem and proposes a planetary gear train drive system which isolates the planetary carrier from torsional deflections. A torque frame or torque transfer structure is connected to a rotating load, such as a bladed propulsor. Pivotal joints, circumferentially disposed with respect to the carrier, each pivotable about a radial axis, connect axially extending arms of a torque frame to the planetary carrier. The pivotal joints, which are vital to the invention of McKibbin et al, permit the planetary carrier to be isolated from torsional deflections. While McKibbin et al do provide a device that eliminates planetary carrier torsional deflections, the planetary carrier system disclosed is of significant complexity. Both a low number of parts and low weight are characteristics vital in aircraft applications. Also, added parts, especially involving pivotable joints, increases the possibility of reliability problems. 
   Therefore, there remains a need for a simple, compact, device capable of transferring torque while eliminating twist within a planetary carrier. 
   SUMMARY OF THE INVENTION 
   It is an object of the present invention to provide an improved epicyclic gear train. 
   It is an object of the present invention to provide a torque transfer device for use in a epicyclic gear train. 
   It is another object of the present invention to provide a gear carrier capable of torque transfer while incurring minimal twist between upstream and downstream plates of a gear carrier. 
   Therefore, in accordance with one aspect of the present invention, there is provided a torque transfer assembly adapted for use in an epicyclic gear train, the gear train including a sun gear rotatable about an axially extending central axis, a concentric outer ring gear, and a plurality of gears mechanically intermediate said sun gear and said ring gear and in meshing engagement therewith, said plurality of gears being adapted for receiving torque input from said sun gear, said torque transfer assembly comprising: an epicyclic carrier, rotatable about said axially extending central axis and adapted to rotatably support said plurality of gears on a plurality of axles between first and second axle ends, said first and second axle ends defining first and second planes respectively, said plurality of axles being parallel to said central axis and said first and second planes being perpendicular to said central axis, said plurality of gears being circumferentially located on the epicyclic carrier about said central axis, said epicyclic carrier having a first connecting member extending therefrom; and a torque transfer coupling adapter, disposed concentrically with said epicyclic carrier and rotatable therewith, said torque transfer coupling adapter having a central torque output element and a second connecting member extending therefrom, said second connecting member being adapted to be engaged with said first connecting member to structurally join said coupling adapter and said epicyclic carrier, said first and second connecting members being structurally joined together between said first and second planes. 
   There is also provided, in accordance with a second aspect of the present invention, an epicyclic carrier assembly adapted for use in an epicyclic gear train, said gear train including a sun gear rotatable about a central axis, a concentric outer ring gear, and a plurality of gears circumferentially spaced relative to, and mechanically intermediate of, said sun gear and said ring gear and in meshing engagement therewith, said plurality of gears being adapted for receiving torque input from said sun gear, said planetary carrier assembly comprising: a split gear carrier, comprising a first half and a second half for rotatably supporting respective first and second ends of a plurality of axles of said plurality of gears, said plurality of axles being parallel to said central axis, said plurality of gears being circumferentially located about said central axis and axially disposed between said first and second halves of said split gear carrier, each of said first and second halves having a corresponding first attachment member; and a carrier support torque frame disposed concentrically with said central axis and axially located between said first half and said second half of said split gear carrier, said carrier support torque frame having an annular mounting flange adapted for fixed engagement with an outer housing and comprising a second attachment member, said second attachment member being mounted to said first attachment members to structurally connect said first and second halves of said split gear carrier to said carrier support torque frame, said first and second attachment members being mounted to one another in a plane between said first and second ends of said plurality of axles of said plurality of gears. 
   There is further provided, in accordance with another aspect of the present invention, an epicyclic carrier assembly adapted for use in an epicyclic gear train, said epicyclic gear train including a sun gear rotatable about a central axis, a concentric outer ring gear, and a plurality of gears circumferentially spaced relative to, and mechanically intermediate of, said sun gear and said ring gear and in meshing engagement therewith, said plurality of gears being adapted for receiving torque input from said sun gear, said epicyclic carrier assembly comprising: a gear support member, concentric with said central axis, and adapted to rotatably support said plurality of gears on a plurality of axles between first and second axle ends, said plurality of axles being parallel to said central axis, said plurality of gears being circumferentially located on said epicyclic carrier about said central axis, said gear support member having at least a first connecting member; and a coupling adapter disposed concentrically with said gear support member, said coupling adapter having and a second connecting member, said second connecting member being engaged with said first connecting member to structurally join said coupling adapter and said gear support member, said first and second connecting members being mounted to one another in a plane between said first and second axle ends. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Further features and advantages of the present invention will become apparent from the following detailed description, taken in combination with the appended drawings, in which: 
       FIG. 1  shows a schematic view of a gas turbine engine having a multi-stage planetary gearbox incorporating the present invention. 
       FIG. 2  shows a cross-sectional detail view of the planetary gearbox in  FIG. 1 . 
       FIG. 3  shows a perspective view of the torque transfer device according to the present invention. 
       FIG. 4   a  shows a perspective view of a planetary carrier for use in the torque transfer device of  FIG. 3 . 
       FIG. 4   b  shows a front elevation view of the planetary carrier of  FIG. 4   a.    
       FIG. 5  shows a perspective view of the coupling adapter element of the torque transfer device of  FIG. 3 . 
       FIG. 6  shows a perspective exploded view of the torque transfer assembly of the present invention. 
       FIG. 7  shows a partial cross-sectional view of a turbofan engine incorporating an epicyclic gear set according to a second aspect of the present invention; 
       FIG. 8  shows an enlarged cross-sectional view of the embodiment of  FIG. 7 . 
       FIG. 9  shows a perspective view of the annular carrier support flange of  FIG. 8 . 
       FIG. 10  shows a perspective view of both halves of the split gear carrier of  FIG. 8 . 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Referring to  FIG. 1 , a turboprop gas turbine engine  10  generally having a power plant  14  and a reduction gearbox  12 . The engine power plant  14  includes a compressor section  16 , combustion chamber  18 , and a turbine section  20 . Air inlets  22  permit air to be drawn into the gas generator and, following power withdrawal by the turbine section, exhaust ducts  24  provide an engine exhaust gas outlet. 
   The operation of such a gas turbine engine is well known, and occurs generally as follows by means of example only. Air enters the engine through the inlet  17  and is compressed by the compressor section  16 , in this case comprising axial flow compressors  19  and a centrifugal compressor  21 . The compressed air is then fed to the combustion chamber  18  where it is mixed with fuel and ignited. The hot gas then expands through the turbine section  20 , comprised of a compressor turbine  23  which drives the compressor  18  and the accessories through accessory gearbox  15 , and the power turbine section  25 , which is mechanically independent from the compressor turbine  23 , drives the propeller shaft  29  by means of the planetary reduction gearbox  12 . Planetary or epicyclic gearboxes are well known in turboprop applications, and generally comprise a sun gear, a ring gear, and at least two planet gears supported by a planetary carrier, all of which are described in further detail below. The hot gas is then discharged to the atmosphere through exhaust ducts  24 . 
   In the exemplary embodiment, the planetary reduction gearbox  12  includes a first reduction stage  26  and a second reduction stage  28  which drive a propeller (not shown), fastened to propeller flange  30 , through propeller shaft  29 . 
   Referring now to  FIG. 2 , the reduction gearbox  12  will now be described in more detail. The first reduction stage  26  receives input from the power plant through power turbine output shaft  34  which drives the first stage sun gear  32 . The first stage outer ring gear  36  is held stationary within the gearbox casing, and a plurality of planet gears  38  are supported within ring gear  36  by a torque transfer planetary carrier assembly  40 , comprised of a first stage planetary carrier  42  and coupling adapter  44 . Each planet gear  38  is rotatably mounted in the planetary carrier  42  about an axis  39 , as describe further below, and is in meshing engagement with both the sun gear  32  and the outer ring gear  36 . The drive shaft  34 , sun gear  32 , ring gear  36 , and planetary carrier  42  are all concentric about, and both the sun gear  32  and planetary carrier  42  are adapted to rotate about, a central axis  37 . Each planet gear  38  has its own individual axis of rotation  39 , about which each rotates, and are thereby adapted to rotate the planetary carrier  42  about the central axis  37  when driven by shaft  34  through sun gear  32 . 
   The coupling adapter  44  is fastened to, and is therefore adapted to rotate with, the first stage carrier  42  and serves to transfer torque to the second reduction stage  28  of the gearbox as described below. The second stage  28  operates substantially as per the first stage described above, with certain modifications which will be apparent to those skilled in the art, and thus is only described briefly here. The second stage  28  similarly comprises a central second stage sun gear  56  supported within the adapter  44 , which is in meshing engagement with a plurality of second stage planet gears  60 , which rotate within a stationary second stage outer ring gear  58 . The revolving second stage planet gears  60  rotate a second stage planetary carrier  62  which provides output torque to the propeller shaft  29 . The second stage sun gear  56  and planetary carrier  62  also rotate about the central axis  37  of the reduction gearbox, and second stage planet gears  60  rotate about their individual axes of rotation  59 . 
   Referring now to  FIGS. 3 ,  4   a ,  4   b ,  5  and  6 , the torque transfer planetary carrier assembly  40  is generally comprised of the first stage planetary carrier  42  and the coupling adapter  44 . The planet gears  38  are each rotatably mounted in the planetary carrier  42  on axles  41  between planet gear brackets  46  defined in two radially extending carrier plates  48   a  and  48   b , perpendicular to central axis  37  and having axle openings  49  therein. The carrier plates comprise an upstream plate  48   a  and a downstream plate  48   b , preferably integrally joined to one another. The planet gear axle openings  49  and the individual axes of rotation  39  are preferably radially an circumferentially equidistantly spaced about central axis  37 . Therefore, in a preferred embodiment having three planet gears  38 , the individual axes of rotation  39  are spaced 120° apart around central axis  37 . 
   A plurality of mounting pads  50  extending from the planetary carrier  42  preferably intermediate each planet gear individual axis of rotation  39 . The mounting pads  50  are preferably axially located between the upstream and downstream plates,  48   a  and  48   b  respectively, of the planetary carrier  40 . The coupling adapter  44  has an equal number of legs  52  extending therefrom and adapted to correspond to and be mated with the mounting pads  50  of the carrier  42 . Mating holes  53  are provided for connection, and the two elements are preferably mounted together using press fit pins  57  and a threaded nut, though any other connection means is possible. In the exemplary embodiment, the coupling adapter  44  also comprises a first stage output spline  54  having internal gear teeth  55  adapted to mesh with and transfer torque to another splined component, which in this case, as shown in  FIG. 2 , is a second stage sun gear  56 . In a single stage planetary gearbox, this splined component receiving torque output can be almost any output shaft, such as, for example, a propeller shaft. 
   In use, drive shaft  34  rotates sun gear  32  to drive planet gears  38 . As planet gears  38  rotate within stationary ring gear  36 , the planetary carrier  42  is driven via a load transfer through the planet axles  41  to plates  48   a  and  48   b . Pins  57  pass the load from carrier pads  50  to adapter legs  52  to rotatingly drive the coupling adapter  42  at a reduced speed relative to shaft input drive  34 . Further speed reduction is achieved through the second reduction stage  28 . 
   The configuration of the link between the carrier and the coupling adapter is such that no substantially relative twist between the upstream and downstream plates  48   a  and  48   b  of the planetary carrier occurs. Therefore, no torsional deflection of the planetary carrier occurs, as the torque input is transferred directly to the adapter  44  by the pads on carrier  42 . Thus, a differential torsional load across the planet gear axles  41 , is avoided. The location of the interface between the pads  50  of the carrier  42  and the legs  52  of the adapter  44 , intermediate the ends of axles  41  of the planet gears  38 , further assists in removing differentially torque loading across the gear axles, and therefore twist in the planetary carrier  42 , thereby improving gear alignment. 
   Turning now to  FIGS. 7–10 , a second aspect of the present invention is disclosed. This aspect is particularly directed to star gear sets, in which the gear carrier is stationary and the ring and sun gears rotate. Such star gear systems, like that disclosed below, are particularly beneficial for use in low transmission ratio scenarios where the use of planetary gears is not as effective or perhaps not even possible. One such examples is the geared turbofan engine depicted in  FIG. 7 . A geared turbofan engine generally features a gear set between the fan and the low pressure turbine, which acts as a speed reducer enabling both elements to run at their respective optimal speed, thereby improving the efficiency of the engine. Considering the high power and low transmission ratio requirements, a single stage star epicyclic gear is most suitable. 
   Referring to  FIG. 8 , in the embodiment of  FIGS. 7 to 10  a star carrier assembly  70  substantially reduces torsional deflection in the star carrier, while further significantly reducing the overall weight of the assembly. The star carrier assembly  70  includes a split star gear carrier, having a first half  72  and a second half  74  respectively providing downstream and upstream support for the integral star gear axles, and a central carrier support torque frame  76 . The central carrier support torque frame  76  is concentrically disposed with the split carrier halves  72  and  74 , and is axially sandwiched therebetween. Unlike the first embodiment, the star carrier assembly  70  does not rotate, but instead is fixed within the outer gearbox housing  71  via splines  80  on a radially outer flange of the carrier support torque frame  76  which mate with inner splines  81  on an outer gearbox casing  71 , while an outer ring gear  77  is free to rotate. A central sun gear  73  is driven by output shaft  85  via sun gear coupling  79 , and rotates about a longitudinally extending central axis  69 . The plurality of circumferentially spaced star gears  75  is radially intermediate the sun gear  73  and the outer concentric ring gear  77  and are in meshing engagement therewith. As the carrier assembly  70  is fixed relative to the outer housing  71 , and the star gears  75  rotate about their individual axes  87  when driven by the central sun gear  73 , the rotating outer ring gear  77  thereby provides torque output to either a gearbox output shaft or a subsequent stage of a gear train, via a ring gear coupling  89  fixed to the ring gear  77 . 
   Referring to  FIG. 9 , the central carrier support torque frame  76  comprises an outer flange  78  having a spline  80  on the outer diameter of the flange  78 , for engagement with the corresponding spline  81  within the outer gearbox housing  71 . The carrier support torque frame  76  is thereby fixed in place within the outer housing  71 , and provided with a mounting arrangement that reacts against torque transmitted through the star gears  75 . The carrier support torque frame  76  extends radially inwards from the outer flange  78  and comprises spoke-like webs  82 , which are circumferentially spaced and extend between the spaces where each star gear  75  is disposed. Attachment boss members  84  are integrally formed with the spoke-like webs  82 , and are preferably press fitted with sleeve dowels  86 . Thus a plurality of first attachment members  83  are provided, permitting the two carrier halves  72 , 74  to be bolted or otherwise fastened to the central carrier support torque frame  76 . 
   Referring to  FIG. 10  showing the split gear carrier halves  72 , 74  in more detail, the downstream first carrier half  72  and the upstream second carrier half  74  each have a corresponding plurality of gear support bores  88 , within which roller bearings  92  (as seen in  FIG. 8 ) that support the axles of the star gears  75  are preferably disposed. Between each star gear mounting bore  88  is an attachment member  90  which comprises a bore or sleeve  93  corresponding to the sleeves  86  of the annular carrier support torque frame  76 , such that the first gear carrier half  72  and the second gear carrier half  74  can be engaged together by bolts  94  (see  FIG. 8 ) extending completely therethrough. Preferably, the two carrier halves  72 , 74  are fastened onto either side of the first attachment members  83  of the central support torque frame  76  by the hollow bolts  94 , disposed through the bores  93  of the circumferential spaced attachment members  90 . 
   Each of the gear support bores  88 , within the first and second carrier halves  72  and  74 , preferably receives a roller bearing  92 , as shown in  FIG. 7 , for supporting an opposing axle end of each star gear  75 . Although the use of journal bearings is equivalently possible, roller bearings are preferred as they generally provide improved reliability. The term gear “axles” as used herein is intended to include both integrally-formed and independent axle elements. 
   This star carrier assembly  70  is similar to the carrier assembly  40  according to the first embodiment of the present invention, in that torsional deflection of the carrier is substantially reduced as a result of the attachments of the fixed support torque frame  76  with the carrier halves  72 , 74  very close to the plane of action of the resultant gear forces of the star gear train. However, the star carrier assembly  70  provides a more lightweight assembly compared to prior art such as U.S. Pat. No. 5,466,198 (McKibbin et al). The lightweight structure is possible largely as a result of the carrier support torque frame  76  being engaged between the carrier halves  72 , 74  rather than at the back of the down stream half, as in U.S. Pat. No. 5,466,198. The distance between the mounting plane and the plane of action is thereby greatly reduced. Thus, the location of the annular mounting plane of the carrier support torque frame  76  with the outer housing  71 , very close to the plane of action of the resultant gear forces at the midsection of the gear train, significantly reduces bending and twisting of the spoke-like webs  82 , to which the split carrier halves  72 , 74  are fastened, permitting a light carrier assembly structure. The carrier halves  72 , 74 , with bearing outer rings having gear mounting bores  88  and retaining bolts  94 , form a more rigid box structure that twists and warps much less than the spokes of the torque frame, thereby reducing the misalignment of the star gear axle and its effects on the operation of gears and bearings. Additionally, the splined joint between the support torque frame  76  and the outer gearbox housing  71  eliminates the need for traditionally used fitted bolts or dowel pins, which are commonly subject to fretting (especially in light alloy housings) and add to the overall weight of the assembly. U.S. Pat. No. 5,466,198 also includes a torque frame and two carrier halves. However, the carrier support is mounted about 3.5 times farther from the load plane, requiring a fairly robust, and consequently rather heavy, structure. The side by side carrier and carrier support arrangement is much disadvantaged to the present invention, in which the carrier support is sandwiched between the two carrier halves. 
   As mentioned above, the embodiment of  FIGS. 7–10  is particularly adapted for use in a geared turbofan engine. Referring again to  FIGS. 7 and 8 , when the carrier assembly  70  is employed in a single stage epicyclic gear set of a geared fan engine, the engine output shaft  85  can be driven directly by the low pressure turbine, and the ring gear coupling  89  engages a downstream fan output shaft. Such a single stage epicyclic gear set meets the low transmission ratio and relatively high power requirements necessary for such an application. 
   The embodiments of the invention described above are intended to be exemplary only. For example, in the preferred embodiments three planet and five star gears are used, however a different plurality of planet gears can be employed. Additionally, the torque transfer assembly can also be applicable in a single or a multiple reduction stage gearbox or gear train. One skilled in the art will appreciate that the present invention also has application well beyond the gas turbine engine examples described. The scope of the invention is therefore intended to be limited solely by the scope of the appended claims.