Abstract:
A method of constructing an integral coupling system containing a sun gear is disclosed. An integral sun gear is provided that has a sun gear component and a connection shaft. A first straight flat inner edge is machined on a first diaphragm to use as a datum point, and then the straight flat edge is fixtured. A tapered outer edge is machined on the first diaphragm, and the first diaphragm is welded to the integral sun gear. The flat inner edge is generally perpendicular to a central axis of the connection shaft.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional application of U.S. patent application Ser. No. 11/391,764 by Loc Duong, Michael E. McCune, and Louis J. Dobek, entitled “EPICYCLIC GEAR TRAIN INTEGRAL SUN GEAR COUPLING DESIGN,” filed on Mar. 22, 2006, now U.S. Pat. No. 7,591,754, which is herby incorporated by reference. Further, U.S. patent application Ser. No. 12/536,642 by Loc Duong, Michael E. McCune, and Louis J. Dobek, entitled “INTEGRAL SUN GEAR COUPLING,” filed on even date with this application, now U.S. Pat. 7,780,568, is a divisional application of U.S. patent application Ser. No. 11/391,764 by Loc Duong, Michael E. McCune, and Louis J. Dobek, entitled “EPICYCLIC GEAR TRAIN INTEGRAL SUN GEAR COUPLING DESIGN,” filed on Mar. 22, 2006. 
    
    
     BACKGROUND 
     This invention relates to planetary gear trains. More particularly, the invention relates to a coupling system for flexibly connecting a sun gear to a rotating shaft so that the reliability and durability of the gear system components are improved. The invention is useful in aircraft engines where reliability, durability and simplicity are highly desirable. 
     Planetary gear trains are mechanical structures for reducing or increasing the rotational speed between two rotating shafts. The compactness of planetary gear trains makes them appealing for use in aircraft engines where space is at a premium. 
     The forces and torque transferred through a planetary gear train place tremendous stresses on the gear train components, making them susceptible to breakage and wear, even under ideal conditions. In practice, conditions are often less than ideal and place additional stresses on the gear components. For example, the longitudinal axes of a sun gear, a planet carrier, and a ring gear are ideally coaxial with the longitudinal axis of an external shaft that rotates the sun gear. Perfect or ideal coaxial alignment, however, is rare due to numerous factors including imbalances in rotating hardware, manufacturing imperfections, and transient flexure of shafts and support frames due to aircraft maneuvers. The resulting parallel and angular misalignments impose moments and forces on the gear teeth, the bearings which support the planet gears in their carrier, and the carrier itself. The imposed forces and moments accelerate gear component wear and increase the likelihood of component failure in service. Thus, accelerated component wear necessitates frequent inspections and part replacements which can render the engine and aircraft uneconomical to operate. 
     The risk of component breakage can be reduced by making the gear train components larger and therefore stronger. Increased size may also reduce wear by distributing the transmitted forces over correspondingly larger surfaces. However, increased size offsets the compactness that makes planetary gear trains appealing for use in aircraft engines, and the corresponding weight increase is similarly undesirable. The use of high strength materials and wear resistant coatings can also be beneficial, but escalates the cost of the gear train and therefore reduces its desirability. 
     Stresses due to misalignments can also be reduced by the use of flexible couplings to connect the gear train to external devices such as rotating shafts or nonrotating supports. For example, a flexible coupling connecting a sun gear to a drive shaft flexes so that the sun gear remains near its ideal orientation with respect to the mating planet gears, even though the axis of the shaft is oblique or displaced with respect to a perfectly aligned orientation. Many prior art couplings, however, contain multiple parts which require lubrication and are themselves susceptible to wear. Prior art couplings may also lack adequate rigidity and strength, with respect to torsion about a longitudinal axis, to be useful in high torque applications. Misalignment can also be accommodated by a splined connection. However the motion that occurs between the contacting spline teeth in a splined connection creates friction which is highly variable and causes the flexibility of such a connection to also be variable. 
     In view of these shortcomings, a simple, reliable, coupling system for connecting components of a planetary gear train to external devices while accommodating misalignment there between is sought. 
     SUMMARY 
     A method of constructing an integral coupling system containing a sun gear is disclosed. An integral sun gear is provided that has a sun gear component and a connection shaft. A first straight flat inner edge is machined on a first diaphragm to use as a datum point, and then the straight flat edge is fixtured. A tapered outer edge is machined on the first diaphragm, and the first diaphragm is welded to the integral sun gear. The flat inner edge is generally perpendicular to a central axis of the connection shaft. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic cross-sectional side elevation view of a turbine engine containing a planetary gear train. 
         FIG. 2  is a cross-sectional elevation view of a coupling system for the planetary gear system of the present invention. 
         FIG. 3  is a cross-sectional elevation view of an undulant flexible section of the present invention. [Insert text] 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows a typical turbine engine  10  known in the art, which includes as its principal components one or more compressors  12 ,  14 , one or more turbines  16 ,  18  for powering compressors  12 ,  14 , combustion chamber  20 , fan  22 , primary exhaust  24  and fan exhaust nozzle  26 . A power train such as shafts  28 ,  30  extends from each turbine  16 ,  18  to drive the corresponding compressor  12 ,  14 . The rotary motion of one of compressors  12 ,  14  is conveyed to fan  22  by way of planetary gear train  32 . Planetary gear train  32  reduces the rotational speed of a compressor to a speed more suitable for the efficient operation of fan  22 . The principal engine components are ideally concentric with central longitudinal axis  34 . 
       FIG. 2  is a cross-sectional elevation view of an integral coupling system  40  for planetary gear system  32  of  FIG. 1 , and its relationship to engine  10 . Integral coupling system  40  comprises inflexible spindle  42  and at least one undulant flexible section  44  which rotate about central longitudinal axis  34 . 
     Also illustrated are compressor drive shaft  28 , planet gear  46 , ring gear  48 , ring gear housing  50 , ring gear coupling  52 , and integral sun gear  56 . The forward end of compressor drive shaft  28  is joined by splines  54  to the aft end of integral coupling system  40 . Integral sun gear  56  engages planet gear  46  through mesh  58 . Rotary motion of drive shaft  28  is thus transferred to sun gear  56 , which meshes with multiple planet gears  46 . Each planet gear  46  is rotatably mounted in planet carrier  62  by journal pin  64  or other suitable bearing so that the rotary motion of integral sun gear  56  urges each planet gear  46  to rotate about longitudinal axis  66 . Each planet gear  46  also meshes with ring gear  48  through mesh  60 . The ring gear  48  is mounted in ring gear housing  52 . 
     In one embodiment, ring gear coupling  52  joins ring gear housing  50  to a mechanical ground to prevent rotating of the ring gear. Since planet gears  46  mesh with both a nonrotating ring gear  48  and rotating integral sun gear  56 , planet gears  46  not only rotate about axes  66  but also orbit integral sun gear  56  causing planet carrier  62  to rotate about axis  34 . This is commonly referred to as a planetary gear system. Planet carrier  62  motion is conveyed to fan  22  (of  FIG. 1 ) by any suitable means, not illustrated. 
     In an alternate embodiment, the ring gear  48  is allowed to rotate, while the planet gears  46  remain is a set position and only rotate about their respective individual central axes. The rotating of the integral sun gear  56  about the planet gears  46  results in rotary motion of the ring gear  48 . The motion of the ring gear is conveyed to fan  22  (of  FIG. 1 ) by any suitable means not illustrated. This configuration is referred to a star gear system. Sun gear coupling  40  of the current invention may be present in either the star gear system or the planetary gear system. 
     Sun gear coupling  40  comprises inflexible spindle  42  and at least one undulant flexible section  44 . Flexible section  44  includes cylindrical ring  68 , which has a diameter greater than that of spindle  42 , and is joined to spindle  42  by longitudinally spaced diaphragms  70  and  72 . Junctures  74  between diaphragms  70 ,  72  and spindle  42 , as well as junctures  76  between diaphragms  70 ,  72  and ring  68 , have a curved cross sectional profile to improve the flexibility of coupling  42  and minimize stress concentrations at junctures  74 ,  76 . A single flexible section  44  is adequate for accommodating angular misalignment between integral sun gear  56  and shaft  28 . 
     Two or more longitudinally spaced apart flexible sections  44  are used for accommodation of parallel misalignment or a combination of angular and parallel misalignment. Splines  54  at the end of coupling  40  does not contribute materially to the flexibility of integral coupling  40 ; rather integral coupling  40  derives its flexibility primarily from undulant sections  44 . The torsional rigidity of ring  68  and spindle  42  make coupling  40  rigid with respect to torsion about longitudinal axis  34 . In addition, the undulant character of flexible section  44  makes coupling  40  compliant with respect to torsion about vertical and lateral axes (i.e. with respect to angular misalignments in a horizontal plane and in a vertical plane parallel to axis  34 ) and with respect to translation about all three axes. Accordingly, integral coupling  40  transmits high torque while isolating gear train  32  from forces and moments arising from misalignments between integral sun gear  56  and external shaft  28 . 
     Integral coupling system  40  includes integral sun gear  56  comprising sun gear component  78  and connection shaft  80 . Integral sun gear  56  is fabricated as single piece from steel or other appropriate material known to those of skill in the art. Connection shaft  80  will rotate about central longitudinal axis  34  as the integral sun gear  56  rotates and act as part of spindle  42 . Integral sun gear  56  is connected to flexible sections  44  to create a unitary integral coupling system  40  such as by welds  82 ,  84 , and  86 . Although illustrated as three relatively short length sections, spindle  42  comprises various sized sections which allow joining the integral sun gear  56  to the drive shaft  28  between the respective sun gear  56 , flexible sections  44 , and shaft  28 , all of which are made from steel, or similar material. Integral sun gear  56  eliminates the manufacturing of separate parts of a splined gear and corresponding splined coupling shaft present in the prior art, thus reducing the complexity and cost of the system. A reduction in maintenance cost is also achieved as the reliability of the gear train is increased due to the prevention of spline wear and a reduced risk of gear train component breakage resulting from the misalignment thereof. The unitary construction reduces the previous five piece multipart design to one piece. The continuous nature of integral coupling system  40  provides greater flexibility than could be obtained with bolted flanges at the rings  68  or similar structures in an equivalent radial space. 
     Integral gear coupling  40  may include a flexible tubular insert  90  having inlet  92  and outlet  94  which acts as a conduit to deliver oil for lubrication of the system. Oil, not shown, is supplied to inlet  92  and is centrifuged radially outward by the rotation of integral coupling  40  and insert  90 , so that the oil forms a film on the inner surface  96  of insert  90 . Forward and aft standoffs  98  and  100  each form a ring around the circumference of insert  90  to support it radially within integral coupling  40 . Insert  90  is retained longitudinally in place by a snap ring (not illustrated) or other suitable means for securing insert  90  with respect to coupling  40 . Surfaces  102  and  104  of standoffs  98  and  100 , respectively, are spherical to promote rolling motion along inner wall  106  of integral coupling  40  and not resist the flexure thereof. Each spherical surface also contains groove within which seal rings are disposed to prevent oil leakage into undulant sections  44  (not illustrated). 
     A group of elbows  108  associated with each undulant flexible section  44  extends through the wall of insert  90  so that the interior of each undulant section  44  can be readily inspected with a flexible optical instrument, not illustrated. The optical instrument is inserted longitudinally along the coupling, and into mouth  110  of elbow  108 . Further insertion of the instrument causes it to follow the contour of elbow  108  and bend radially outward so that the interior of diaphragms  70 ,  72  and ring  68  can be easily viewed. Mouth  110  of each elbow  108  is radially spaced from inner surface  96  of insert  90 . This ensures that the oil film will not be captured by mouth  110  and centrifuged into the interior of undulant section  44  where it can cause a rotary imbalance. In one embodiment, three elbows  108  are used at each undulant section  44 , however any number of elbows greater than or equal to two can be used provided they are equally distributed around the circumference of the insert to preclude imbalance. 
       FIG. 3  is a cross-sectional elevation view of an undulant flexible section  44  of the present invention. Flexible section  44  again includes cylindrical ring  68  which has a diameter greater than that of spindle  42 , and is joined to spindle  42  by longitudinally spaced diaphragms  70  and  72 . The diaphragms  70 ,  72  contain a flat inner straight edge  112  and  114 , respectively, and non-symmetric hyberbolic outer edges  116  and  118 , respectively. Straight edges  112 ,  114  reduce the cost of manufacturing the coupling. In the prior art, diaphragms  70 ,  72  are manufactured by simultaneously and equally removing material from both sides of the diaphragms  70 ,  72  to create a symmetric and hyperbolic profile. Typically, a specialized machine is used such as those made by Bendix Corporation. With the inventive integral coupling  56 , the specialized machines or tooling are not required, and conventional machining processes are used to fabricate the diaphragms  70 ,  72 . In manufacturing the flexible section  44 , the flat edge  112  or  114  will be machined first. Flat edge  112  or  114  will then be used a datum reference for fixturing the diaphragm section for subsequent machining operations, including the tapering of the outeredges  116 ,  118 . 
     In one embodiment, diaphragms  70  and  72  are mirrored images of one another resulting in the standardization of parts to further reduce costs. Diaphragms  70 ,  72  are joined together by weld  84 . Weld  84  is formed by a process such as electron beam welding or similar process known to those in the art. Preferably weld  84  is made on the outside of ring  68  to improve the strength of the ring  68 . 
     In the embodiment illustrated, ring  68  contains a flat inner surface  120  and hyberbolic outer surface  122 . In a lubricated system, drain holes  124  are distributed around the circumference of each ring  68 . Drain holes  124  distributed around the circumference of each ring  68  allow any oil which inadvertently leaks into the interior of the undulant section  44  to escape, thus preventing the oil from accumulating therein and cause a rotary imbalance of the integral coupling  40 . A group of elbows (not illustrated) may also be associated with each undulant flexible section  44  as previously described. The elbows allow the interior of each undulant section  44  can be readily inspected with a flexible optical instrument to inspect welds or search for stress fractures or similar signs of defects. 
     The hyberbolic outer surface  122  contains balancing rim  126 . Balancing rim  126  is an excess of material that can be removed by a process such as machining that allows for balancing of the integral coupling system  40 . In an alternate embodiment (not illustrated), inner surface  120  also contains a hyperbolic profile or any similar shape to allow the removal of material from the inner surface for balancing the coupling system  40 . 
     The undulant flexible section  44  contains the following design parameter variables shown in  FIG. 3 : 
     Ra: Diaphragm outer radius 
     Rb: Diaphragm inner radius 
     ta: Outer radius thickness 
     tb: Inner radius thickness 
     ro: Outer ring radius 
     ri: Inner ring radius 
     Rs: Shaft/diaphragm fillet radius 
     In one embodiment, the above variables are used to manufacture the undulant flexible section  44  with the following design parameters: 
     Rb/Ra&lt;0.6 
     2&lt;tb/ta&lt;3.5 
     ri&gt;2*ta 
     ro&gt;2*ri 
     Rs&gt;2*tb 
     The above design parameters allow integral coupling system  40  of an epicyclic gear train the ability to accommodate the combined axial, lateral, and angular misalignments common for such systems while simultaneously allowing for the transfer of torque in the system. Integral coupling system  40 , through the non-symmetric tapered contour profile, isolates the spline system from the helical sun gear misalignment. The integral coupling system  40  primary control design parameters are the thickness and radii ratios as listed. The set of diaphragms as illustrated in  FIG. 2  can be of different radial dimensions to isolate the spline system from the epicyclic gear train excursion and shaft system misalignment. Thus, the overall system is improved in reliability because the propensity for spline wear to occur is remote, and the epicyclic gearbox has the ability to operate under misalignment. 
     Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.