Abstract:
A gearbox support apparatus includes a gearbox carrier having a central axis, the carrier configured to mount one or more rotating gears therein, the carrier including spaced-apart forward and aft walls, and a pin disposed between the forward and aft walls. The gearbox support apparatus includes an inner race connected to the pin as a single integral component incorporating two pairs of raised guides defining a forward raceway and an aft raceway.

Description:
PRIORITY INFORMATION 
       [0001]    The present application claims priority to, and is a continuation of, U.S. patent application Ser. No. 13/835,687 titled “Gearbox and Support Apparatus for Gearbox Carrier” of van der Merwe, et al. filed on Mar. 15, 2013, which claims priority to U.S. Provisional Patent Application Ser. No. 61/622,592 filed on Apr. 11, 2012 and to U.S. Provisional Patent Application Ser. No. 61/666,532 filed on Jun. 29, 2012; the disclosures of which are incorporated by reference herein. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    This invention relates generally to epicyclic gearboxes, and more specifically to carrier support apparatus and bearings of an epicyclic gearbox. 
         [0003]    Epicyclic gearboxes are often used in aircraft engines to transmit power, for example to drive a propeller or fan from a power turbine. Gearboxes for aircraft applications must be lightweight, capable of transmitting high torque loads, and highly reliable. The system level reliability of the gearbox is the biggest hurdle from a technical perspective. 
         [0004]    In operation, the planets in the gearbox transfer large loads into the carrier, which cause deflections and misalignment in the bearings and gears of the gearbox. In order to have a commercially long life, these deflections and misalignments must be minimized. 
         [0005]    It is known to support a gearbox carrier centrally using spherical bearings to transfer load at an axial midpoint in carrier. This isolates the torque fingers that couple the gearbox carrier to adjacent structures from bending moments. However, the working spherical joints with moving parts are subject to wearing and looseness, and their presence increases the complexity of the gearbox. 
         [0006]    Furthermore, the use of traditional steel bearings (e.g. M50 steel alloy or similar) will yield a low system level life due to bearing count in the gearbox. 
         [0007]    Ceramic rolling elements are known to provide a longer life than steel rollers, however they are used in the form of ball or spherical roller bearings which are not axially compliant and therefore not compatible with some helical gear configurations. 
         [0008]    Accordingly, there is a need for a gearbox with a durable, compliant carrier mounting configuration, and a durable, axially-compliant bearing configuration. 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0009]    This need is addressed by the present invention, which provides an epicyclic gearbox having a carrier attached to adjacent structure through a plate that is flexible enough to allow for the torque to be absorbed as strain energy. The present invention also provides a an epicyclic gearbox having planet gears with a herringbone or double helical gear pattern. The planet gears are mounted for rotation by tandem cylindrical roller bearings made from a ceramic material. 
         [0010]    According to one aspect of the invention, an apparatus for supporting a gearbox includes: a gearbox carrier having a central axis, the carrier configured to mount one or more rotating gears therein, the carrier including spaced-apart forward and aft walls, and a flexible center plate structure disposed between the forward and aft walls; an annular support ring disposed axially adjacent to the carrier; and a plurality of axially-extending torque fingers interconnecting the mounting ring and the center plate. 
         [0011]    According to another aspect of the invention, a gearbox carrier is provided having a central axis, the carrier configured to mount one or more rotating gears therein. The carrier includes: spaced-apart forward and aft walls with respective forward and aft coaxial bores; a pin with forward and aft ends received in the forward and aft bores, respectively, the pin secured against axial movement relative to the carrier; an inner race mounted on the pin between the forward and aft ends, the inner race including raised guides that define an annular raceway, the inner race secured against axial movement relative to the carrier; a plurality of generally cylindrical rollers made of a ceramic material disposed in the raceway; and a planet gear mounted for rotation about the pin such than an cylindrical interior surface of the planet gear defines an outer race surrounding the rollers. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    The invention may be best understood by reference to the following description taken in conjunction with the accompanying drawing figures in which: 
           [0013]      FIG. 1  is a cross-sectional view of an epicyclic gearbox constructed in accordance with an aspect of the present invention; 
           [0014]      FIG. 2  is a side view of a bearing roller of the gearbox of  FIG. 1 . 
           [0015]      FIG. 3  is a partially-sectioned view of a portion of a carrier of the gearbox of  FIG. 1 ; 
           [0016]      FIG. 4  is a view taken along lines  4 - 4  of  FIG. 3 ; and 
           [0017]      FIG. 5  is a partially-sectioned perspective view of a portion of the gearbox of  FIG. 1 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0018]    Referring to the drawings wherein identical reference numerals denote the same elements throughout the various views,  FIG. 1  depicts a gearbox  10  constructed according to an aspect of the present invention. The gearbox  10  is an epicyclic type and has a central axis “A”. The gearbox  10  includes a centrally-located sun gear  12 . The sun gear  12  has a double-helical or “herringbone” pattern of gear teeth  14 . A carrier  16  surrounds the sun gear  12  and carries an annular array of planet gears  18 . In the illustrated example there are four planet gears  18  but varying numbers of planet gears  18  may be used. Each planet gear  18  has a herringbone pattern of gear teeth  20 . A ring gear  22  surrounds the planet gears  18  and also has a herringbone pattern of gear teeth  24 . Collectively the sun gear  12 , the planet gears  18 , and the ring gear  22  constitute a gear train. Each of the planet gears  18  meshes with both the sun gear  12  and the ring gear  22 . The sun gear  12 , planet gears  18 , and ring gear  22  may be made from steel alloys. In operation, the sun gear  12  is turned by an input (for example, a rotor shaft, not shown) while the ring gear  22  is coupled to a mechanical load (such as a fan, not shown). The gearbox  10  is effective to reduce the rotational speed of the sun  12  to a rotational speed appropriate for the load coupled to the ring gear  22 , in a known manner. 
         [0019]    Because each of the gear meshes (sun-to-planet and planet-to-ring) has a double-helical or “herringbone” gear tooth profile, there is no relative movement possible parallel to the axis A between the sun gear  12  and the planet gears  18 , or the planet gears  18  and the ring gear  22 , or in other words there is no axial compliance between these elements. 
         [0020]    The planet gears  18  are therefore selected and mounted in a manner to provide axial compliance between the carrier  16  and the planet gears  18 . 
         [0021]    The mounting of one planet gear  18  will be described with the understanding that all of the planet gears  18  are mounted identically. The carrier  16  includes a forward wall  26  and an aft wall  28 , with coaxial bores  30  and  32 , respectively. A pin  34  is received in the bores  30  and  32 . The pin  34  is hollow, generally cylindrical, and has forward and aft ends. The forward end includes a threaded, reduced-diameter surface  36  while the aft end includes an annular, radially-outwardly-extending flange  38 . A retainer  40  (in this example a threaded locknut) engages the reduced-diameter forward surface  36  to secure the pin  34  in position against rearward axial movement. The pin  34  has a plurality of feed holes  42  formed therein. In operation, oil is fed to the interior of the hollow pin  34  and flows through the feed holes to an inner race  44 , providing both cooling and lubrication. Roller bearings  52  are disposed between the inner race  44  and the interior surface of the planet gear  18 . 
         [0022]    In the illustrated example, the inner race  44  is a single integral component incorporating pairs of raised guides  46  which define annular forward and aft raceways  48  and  50 . The flange  38  of the pin  34  bears against the inner race  44  which in turn bears against the interior face of the front wall  26  of the carrier  16 . This secures the pin  24  against forward axial movement. The use of a single inner race provides for good concentricity between roller sets, but two separate inner races could be used as well. The inner race  44  is sized so that it cannot move axially relative to the carrier  16 . 
         [0023]    The channels  48  and  50  receive rollers  52 , in two tandem rings. The rollers  52  comprise a ceramic material of a known composition, for example silicon nitride (Si 3 Ni 4 ). The rollers  52  are configured as cylindrical rollers. As seen in  FIG. 2 , in profile view the rollers  52  have a barrel-like shape with a central crown  53  of maximum diameter, with end portions  55  that taper off in a convex curve to a smaller diameter (the shaping is exaggerated for illustration in  FIG. 2 ). Careful selection of the shape and dimensions of the crown  53  and end portions  55  in accordance with known practices will provide the longest life for the rollers  52 . 
         [0024]    Referring back to  FIG. 1 , the cylindrical interior surface of the planet gear  18  defines the outer race  58  for the rollers  52 . In operation, axial sliding of the rollers  52  can occur relative to the outer race  58 , which in turn permits limited axial movement of the carrier  16  relative to the planet gears  18 . This allows for tolerance and thermal stackup in the carrier  16 . 
         [0025]    The carrier  16  is also supported in a manner to prevent misalignment in the gears and bearings of the gearbox  10  during operation, as illustrated in  FIGS. 3-5 . 
         [0026]    The forward and aft walls  26  and  28  of the carrier  16  are interconnected by axially-extending sidewalls  54  (see  FIG. 3 ). Pairs of the sidewalls  54  are disposed on opposite lateral sides of each of the planet gears  18 . Collectively, the forward wall  26 , aft wall  28 , and the sidewalls  54  define a plurality of lobes or arms  56  of the carrier  16 , with spaces therebetween. Each planet gear  18  is enclosed within one lobe  56 . The carrier  16  also includes a center plate structure as an integral part of its structure. As seen in  FIGS. 3 and 4 , the center plate structure is segmented into a plurality of individual center plates  58 . Each center plate  58  takes the form of an arc-shaped portion of a circular disk, and lies substantially in a radial plane (i.e. is parallel to the forward and aft walls  26  and  28 ). Each center plate  58  spans the space between a pair of the lobes  56 , connecting to adjacent ones of the sidewalls  54 . In the illustrated example, the center plates  58  are located approximately halfway between the forward and aft walls  26  and  28 . The axial location of the center plates  58  may be adjusted to suit a particular application. In particular, through careful placement of the axial location of the center plates  58 , misalignment in the carrier  16  can be controlled. 
         [0027]    An annular support ring  60  (see  FIG. 1 ) is disposed axially adjacent to the aft wall  28 . The support ring  60  is provided with means such as bolt holes (not shown) to secure it to a torque ring structure (not shown). A plurality of torque fingers  68  extend axially between the support ring  60  and the center plate structure. One torque finger  68  is provided for each of the center plates  58 . The torque finger  68  is functionally integral with the center plate  58  and the support ring  60 . It may be constructed as part of an integral (i.e. monolithic) component with the center plate  58  and the support ring  60 , or it may be a separate component which is assembled to center plate  58  and the support ring  60 . The center plate structure as well as the surrounding structures may be constructed from a suitable metallic alloy such as an iron-, nickel-, or titanium-based alloy. The torque finger  68  has a first cross-sectional area at its aft end  70  ( FIG. 1 ) and tapers to a smaller cross-sectional area at its forward end  72  ( FIG. 4 ). Its cross-sectional width in the tangential direction is generally greater than its cross-sectional thickness in a radial direction. The forward and aft ends  72  and  70  taper smoothly into the center plate  58  and the support ring  60  through concave-curved fillets. 
         [0028]    In operation, the planet gears  18  transfer large tangential forces into the carrier  16 , causing the carrier  16  to tend to rotate relative to the support ring  60  (see the relative direction marked by the arrows “R” in  FIG. 4 ). This results in elastic bending of the torque fingers  68  in the tangential direction (shown by arrow “B”). The center plates  58  will deflect (arrows “D”) to accommodate the bending of the of the torque fingers  68 . Their effect is to absorb the torque as strain energy and isolate the movement of the torque fingers  68  from the carrier  16 . This avoids distortion of the carrier  16  and consequent misalignment of the bearings and changing of gear operating clearances. The center plates  58  are sized and shaped such that the stresses in them will remain in the elastic range for the expected operating loads. 
         [0029]    The gearbox support apparatus described herein has several advantages over the prior art. It eliminates several separate parts as compared to a prior art gearbox. No lubrication of joints is required. The low misalignment provided by this apparatus is enabling technology for use of a gearbox embedded in a gas turbine engine. In particular, low misalignment will result in gear and bearing life that meets system level requirements. 
         [0030]    Furthermore, the use of ceramic cylindrical rolling elements allows the planet gears  18  to have a degree of freedom in the axial direction, simplifying the design. The ceramic rolling elements are anticipated to provide at least a doubling in life compared to steel rollers, allowing the gearbox  10  to meet reliability targets. The ceramic rolling elements also bring excellent oil-off performance, low oil flow requirements, low heat generation, and light weight design as additional benefits. Commercially the design will have a long life, which will minimize the cost of replacement over the life of the product. 
         [0031]    The foregoing has described a gearbox carrier support apparatus, a gearbox, and a bearing arrangement therefore. While specific embodiments of the present invention have been described, it will be apparent to those skilled in the art that various modifications thereto can be made without departing from the spirit and scope of the invention. Accordingly, the foregoing description of the preferred embodiment of the invention and the best mode for practicing the invention are provided for the purpose of illustration only and not for the purpose of limitation.