Abstract:
A planet gear journal for an epicyclic gear system is asymmetrically formed to compensate for carrier twist. The asymmetry may be between a pair of opposed outwardly open annular channels inboard of a bearing surface of the journal. A particular application is the planetary gear system of a geared turbofan engine.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    (1) Field of the Invention  
           [0002]    This invention relates to transmissions, and more particularly to geared turbofan engines.  
           [0003]    (2) Description of the Related Art  
           [0004]    In high bypass turbofan engines it is advantageous that the turbine drive the fan through a reduction gearing system. This permits the turbine to operate at the relatively high speeds at which it is efficient while the much larger diameter fan operates at the relatively lower speeds at which it is efficient. Possible gearing systems include epicyclic systems. In an exemplary planetary implementation, the turbine shaft directly drives a sun gear. A number of planet gears are enmeshed between the sun gear and a ring gear that is non-rotating relative to an engine nacelle or other environmental structure in which the engine is mounted. The fan is directly driven by a carrier cage holding the planet gears. Bearings are typically provided: (a) for supporting the turbine shaft and sun gear relative to the environment; and (b) for rotatably supporting the planetary gears relative to the carrier. Whereas the former are often rolling-element bearings, the latter are advantageously journal bearings. Journal bearings may offer an advantageous balance of compactness and load-carrying ability.  
           [0005]    In a common configuration, first and second end portions (front and rear for ease of reference) of each journal are mounted in front and rear portions of the carrier. In a basic configuration, the front and rear portions are separate plates coupled to each other by the journals and the front plate is, in turn, coupled to the fan. Under load, there may be a torsional twist of the front plate relative to the rear plate causing a journal axes to twist out of parallel with the engine axis. This misalignment can damage the gears as is noted in U.S. Pat. No. 5,466,198, the disclosure of which is incorporated by reference herein in its entirety as if set forth at length. The &#39;198 patent teaches use of a torque frame system to prevent carrier twist. Alternatively, U.S. Pat. No. 5,102,379 teaches the presence of undercuts or annular compartments in front and rear faces of the journal to permit flexing of distal portions of the journal bearing surface relative to a journal centerline to at least partially accommodate radial loading on journals.  
         BRIEF SUMMARY OF THE INVENTION  
         [0006]    Accordingly one aspect of the present invention involves an epicyclic transmission wherein a planet gear carrier has first and second portions respectively supporting planet gear journals on first and second sides. Each journal has an outer portion extending between first and second outer rims and an inner portion extending between first and second ends. A transition portion joins the inner and outer portions and has first and second ends respectively forming bases of first and second compartments open in opposite first and second directions. The compartments are asymmetric to each other. Advantageously, the asymmetry serves to control journal lubricant film thickness in response to carrier twist.  
           [0007]    The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]    [0008]FIG. 1 is a schematic longitudinal view of a geared turbofan engine.  
         [0009]    [0009]FIG. 2 is a transverse schematic cutaway view of the transmission of the engine of FIG. 1.  
         [0010]    [0010]FIG. 3 is a longitudinal semi-schematic sectional view of a journal of the transmission of FIG. 2.  
         [0011]    [0011]FIG. 4 is a deformed longitudinal radial sectional view of a journal and carrier under load.  
     
    
       [0012]    Like reference numbers and designations in the various drawings indicate like elements.  
       DETAILED DESCRIPTION  
       [0013]    [0013]FIG. 1 shows a geared turbofan engine  20  having a main housing  22  containing a rotor shaft assembly  24 . An exemplary engine is a high-bypass turbofan. In such an engine, the normal cruise condition ratio of air mass flowing outside the core (e.g., the compressor sections and combustor) to air mass passing through the core (the bypass ratio) is typically in excess of 4.0 and, more narrowly, typically between 4.0 and 8.0. Via high and low shaft portions of the shaft assembly  24 , high and low turbine sections  25  and  26  drive high and low compressor sections  28  and  30 . The engine extends along a longitudinal axis  500  from a fore end to an aft end. Adjacent the fore end, a shroud  40  encircles a fan  42  and is supported by vanes  44 . The low shaft portion of the rotor shaft assembly  24  drives the fan  42  through a reduction transmission  46 . An exemplary reduction transmission is a planetary gear system.  
         [0014]    [0014]FIG. 2 schematically shows details of the transmission  46 . A forward end of the rotor shaft assembly is coupled to a sun gear  52 . The externally-toothed sun gear is encircled by an internally-toothed ring gear  54  which is substantially irrotatably mounted relative to the housing  22 . A number of externally-toothed planet gears  56  are positioned between and enmeshed with the planet gear and ring gear.  
         [0015]    A cage or planet carrier assembly  60  carries the planet gears via associated journals  62 . The journals have circumferential surface portions  64  closely accommodated within internal bore surfaces  66  of the associated planet gears. The speed reduction ratio is determined by the ratio of diameters of the ring gear to the sun gear. This ratio will substantially determine the maximum number of planet gears in a given ring. The actual number of planet gears will be determined by stability and stress/load sharing considerations. An exemplary reduction is between 2:1 and 13:1. An exemplary number of planet gears is between 3 and 11. FIG. 2 shows the equally-spaced positioning of five planet gears  56  about the sun gear  52 .  
         [0016]    [0016]FIGS. 3 and 4 show additional structural details of an exemplary carrier  60  and journal or pin  62 . The exemplary journal comprises the unitary combination of a central section  70  and shaft sections  72  and  74  extending from ends of the central section. The surface portion  64  is formed along the central section  70 . A central bore  80  may extend inward from the outboard ends of the shaft sections  72  and  74 . Distal end portions of the shaft sections  72  and  74  are secured to carrier plates  82  and  84  via nuts  86  and  88 . FIG. 4 shows the journal and carrier under load (an unloaded condition being shown in broken lines). Alternatively described, the shaft sections and inboard portion of the central section form an inner portion of the journal extending between distal ends of the shaft section. An outer portion of the central section extending to the surface portion  64  forms an outer portion of the journal and a radially intermediate portion of the central section forms a transition portion between inner and outer portions. FIG. 4 further shows the carrier plates  82  and  84  joined by one of a plurality of webs  90  alternatingly positioned between journals. The webs along with the journals provide torsional coupling between the carrier plates. In the exemplary embodiment, the first carrier plate  82  is the main or loaded plate which, in the exemplary engine embodiment is coupled to the fan. The second plate  84  is free or unloaded. Accordingly, the first plate  82  is generally more robust and provided with a feature  92  for coupling to the fan.  
         [0017]    Returning to FIG. 3, the central section  70  and its surface portion  64  extend from a first rim  94  to a second rim  96  at respective first and second ends of the central section. First and second annular compartments or channels  100  and  102  extend longitudinally inward from the first and second ends between respective collar (wing when viewed in section) portions  104  and  106  of the central section  70  and the respective shaft sections  72  and  74 . The channels have longitudinally inboardmost portions or bases  108  and  110  defining channel depths L 1  and L 2  relative to their associated rims. In the exemplary embodiment, the channels have radially inboard surfaces substantially formed by generally cylindrical portions of the associated shafts having diameters D 1  and D 2  . Radially outboard surfaces  120  and  122  are generally frustoconical, having concave transitions to the inboard surfaces at the channel bases. The outboard surfaces are at exemplary angles θ 1  and θ 2  relative to the inboard surfaces and central longitudinal axis  510 . For further reference, the channel bases are at respective distances L 3  and L 4  from a transverse centerplane  512  of the central section  70 . In the exemplary embodiment, the channel bases  108  and  110  have respective sectional radii of curvature R 1  and R 2 . These radii may be constant or vary.  
         [0018]    In the exemplary embodiment, the channels are asymmetric. This may involve asymmetries in one or more of the identified parameters or other shape factors. In the illustrated embodiment, the depth L 1  is substantially larger than the depth L 2 . An exemplary ratio of these depths is between 1.3:1 and 5:1 (more narrowly, between 1.5:1 and 3:1). In an exemplary implementation, the diameters D 1  and D 2  are 1.42 inch and the central bore diameter is 0.68 inch. The bearing surface  64  is at a radius of 1.11 inch and has a length of 2.36 inches. The channel depths L 1  and L 2  are 0.81 inch and 0.43 inch, respectively. The radii of curvature R 1  and R 2  are 0.10 and 0.15 inch, respectively. Inboard radii of the surfaces  94  and  96  are 1.11 inch and 1.07 inch, respectively. The angle θ 2  is approximately a constant 12° whereas the angle θ 1  is approximately halfway divided between an inboard 12° portion and an outboard 20° portion so that the surface  120  is substantially bi-frustoconical. These asymmetries in channel depth and the thickness profile of the collar portions permit control of load performance (e.g., flexibility) of portions of the systems including the portions of the shaft sections within the channels and the collar portions of the central section. In view of (or in combination with) carrier and planet gear properties, the asymmetries may be optimized to provide advantageous performance parameters. Notable parameters involve performance under load, especially high max. power (e.g., takeoff) loads normally associated with carrier twist.  
         [0019]    [0019]FIG. 4 shows the deformation of the radial (relative to the axis  500 ) longitudinal section of the journal and carrier under load. The load on the journal represents a complex combination of forces. These are often discussed as including a beam-bending effect caused by local film pressure from the planet gear and gear forces associated with torque transmission. The presence of two symmetric channels in the prior art tends to address the former whereas the latter is typically addressed via use of a torque frame. The present asymmetries can be utilized in the absence of a torque frame to address torsion. In the illustrated embodiment, the greater depth of the first channel tends to concentrate twisting along the first shaft section  72 . A desired result of the asymmetry is to maintain a minimum lubricant film thickness between the surface  64  and mating planet gear surface above a targeted critical value under the presented operating conditions (whether transient or steady state). Accordingly, as discussed above the properties of both the planet gears and the carrier may be relevant to the optimization of journal properties.  
         [0020]    Diverse journal structures are known and may be developed, and the principles of the present invention may be applied to various such journals. For example, journals engineered according to principles of the present invention may be used as drop-in replacements for existing journals, in which cases their configuration may be influenced by the remainder of the existing transmission.  
         [0021]    The journal and gear are typically made of steel. Both may be made of the same steel, which is typically case-hardened by carburization. Commonly used steels include AMS 6265 and AMS 6308. In an exemplary embodiment, the surfaces  66  are formed by or covered with a bearing material such as a soft metal coating. In the exemplary embodiment, the surfaces  64  are covered with a lubricous coating. Advantageously the coating is of a solid film coating material.  
         [0022]    The optimization of the parameters to achieve a desired deformation may be iteratively resolved on an embodiment of the transmission&#39;s design. Such embodiment may be a physical embodiment such as an actual transmission or engine, a partial transmission, or a model appropriately scaled for simulation purposes, or may be in the form of a computer simulation. In such an iterative design process, for a given journal geometry under the anticipated conditions the resulting effect on film thickness and/or other factors are observed. The parameters of channel asymmetry may be varied and the simulation repeated until the factors are within specific desired ranges.  
         [0023]    One or more embodiments of the present invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. For example, assemblies may replace individual parts or vice versa. The principles may be applied both to various existing engines and transmissions and engines and transmissions yet to be developed. Accordingly, other embodiments are within the scope of the following claims.