Abstract:
A high ratio epicyclic gear train (A) with improved load carrying capability for transmitting power from a driven input shaft, such as may be driven by a turbine engine or engines, to a output shaft, as may be coupled to a rotor of a rotary wing aircraft. The compound epicyclic gear train incorporates a load sharing mechanism consisting of a drive sun gear ( 10 ), an idler sun gear ( 11 ), a ring gear ( 12 ), a set of drive planet gear assemblies ( 13 ), a set of idler planet gear assemblies ( 14 ), and a planet carrier assembly ( 15 ) coupled to provide at least two power pathways through said epicyclic gear train (A) between said driven input shaft and said output shaft to provide an improved overall power density of the transmission.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is related to, and claims priority from, U.S. Provisional Application Ser. No. 61/303,941 filed on Feb. 12, 2010 and International Application PCT/US2011/024446 filed Feb. 11, 2011 and published under International Publication No. WO 2011/100499, both of which are incorporated herein by reference. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH 
     Not Applicable. 
     BACKGROUND OF THE INVENTION 
     The present invention relates generally to an epicyclic gear train such as for use in rotary wing aircraft, and in particular, to a high-ratio epicyclic gear transmission incorporating idler sun and planet gears to provide an increased load carrying capability. 
     Rotary wing aircraft typically employ one or more high-speed turbine engines to drive a rotor providing lift and propulsion. A main gear transmission between the engine and the rotor is necessary to transmit engine power to the rotor while reducing the input engine speed to the appropriate output rotor speed. The main gear transmission is usually the heaviest subsystem in the drive train of the aircraft. Increasing power throughput and reducing the weight of the transmission is very desirable for modern rotary wing aircraft. 
     Owning to favorable power density values, epicyclic gear trains are the most popular gear system for the use in aircraft drive trains, being used virtually in all rotary wing aircraft main rotor drive transmission. A typical transmission configuration of a simple epicyclic gear train consists of a sun gear, a co-axially arranged ring gear, a planet carrier, and a set of planet gears (typically between three and six) supported on the planet carrier. The planet gears are spaced in the annular space between the sun gear and the ring gear. The sun gear is driven by an input shaft of the gear train and the planet carrier is operatively connected to an output shaft of the gear train. The ring gear is usually fixed to a transmission housing. The construction of the epicyclic gear train provides multiple load paths from the input shaft through the multiple planets to the output shaft, and the load is shared among the planet gears. However, as the speed ratio increases, the number of planet gears that can be packed into the epicyclic gear trains is limited, thereby limiting the load sharing capabilities. 
     Compound epicyclic gear trains have been utilized to improve the speed ratio of a simple epicyclic gear trains, such as shown in U.S. Pat. No. 6,966,865 to Drago et al., which discloses an epicyclic gear train employing multiple planet clusters. Each planet cluster in the compound epicyclic gear train includes a planet gear shaft, a large planet gear, and two small planet gears. The large planet gear and the two small planet gears are mounted on, and rotate with, the same planet gear shaft. The large planet gear meshes with a sun gear and the small planet gears each mesh with a fixed ring gear which includes two separate rings. To incorporate the maximum possible number of planet clusters into the gear train, two sets of planet clusters are employed. Each planet clusters is arranged with the large planets staggered axially, meshing with the sun gear, or with tilted-axis planet clusters, as shown in U.S. Pat. No. 7,507,180 to Robuck. 
     While such simple and compound epicyclic gear trains are suitable for use in the drive trains of rotary wing aircraft, further improvement in power density values for the drive trains is very desirable, as a reduction in weight and package size provides additional advantages over current epicyclic gear trains. Accordingly, it would be advantageous to provide an improved high-ratio epicyclical gear train having an increased load carrying capacity. 
     BRIEF SUMMARY OF THE INVENTION 
     Briefly stated, the present disclosure provides a high ratio epicyclic gear train with improved load carrying capability for transmitting power from a driven input shaft, such as may be driven by a turbine engine or engines, to a output shaft, as may be coupled to a rotor of a rotary wing aircraft. The compound epicyclic gear train incorporates a load sharing mechanism consisting of idler sun gear and idler planet gears to provide an improved overall power density of the transmission. 
     The foregoing features, and advantages set forth in the present disclosure as well as presently preferred embodiments will become more apparent from the reading of the following description in connection with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       In the accompanying drawings which form part of the specification: 
         FIG. 1A  is an exploded view of an epicyclic gear train of the present disclosure; 
         FIG. 1B  is a perspective sectional view of the epicyclic gear train of  FIG. 1A ; 
         FIG. 1C  is a sectional view of a planet cluster in the epicyclic gear train of  FIG. 1A ; 
         FIG. 2  is a perspective view of a drive planet gear utilized in the epicyclic gear train of  FIG. 1A ; 
         FIG. 3  is a perspective view of an idler planet gear utilized in the epicyclic gear train of  FIG. 1A ; 
         FIG. 4  is a perspective sectional view of an alternate configuration for an epicyclic gear train of the present disclosure incorporating floating small planet gears with external self-aligning bearings in the drive planet cluster; 
         FIG. 5A  is a perspective view of an alternate drive planet gear utilizing an Oldham coupling; 
         FIG. 5B  is an exploded view of the alternate drive planet gear of  FIG. 5A ; 
         FIG. 6  is a perspective sectional view of an epicyclic gear train embodiment utilizing drive planet gears of  FIG. 5A ; and 
         FIG. 7  is a sectional view of an alternate embodiment of the epicyclic gear train wherein a gear coupling is used to transmit torque from the larger planet gears to the small planet while allowing the small planet gears to float with respect to the large planet gears. 
     
    
    
     Corresponding reference numerals indicate corresponding parts throughout the several figures of the drawings. It is to be understood that the drawings are for illustrating the concepts set forth in the present disclosure and are not to scale. 
     Before any embodiments of the invention are explained in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the drawings. 
     DETAILED DESCRIPTION 
     The following detailed description illustrates the invention by way of example and not by way of limitation. The description enables one skilled in the art to make and use the present disclosure, and describes several embodiments, adaptations, variations, alternatives, and uses of the present disclosure, including what is presently believed to be the best mode of carrying out the present disclosure. 
     Turning to the Figures, and to  FIGS. 1A-1C  in particular, a first embodiment of the present disclosure is shown, wherein a high-ratio epicyclic gear train (A) of the present disclosure is shown comprising a drive sun gear  10 , an idler sun gear  11 , a ring gear  12 , a set of drive planet gear assemblies  13 , a set of idler planet gear assemblies  14 , and a planet gear carrier assembly  15 . The drive planet gear assembly  13  is a planet cluster, including as shown in  FIG. 2 , a large planet gear  13 A, at least one small planet gear  13 B and a planet shaft  13 C onto which both the large and small planet gears are secured in an axially spaced configuration. As seen in  FIG. 3 , each idler planet gear assembly  14  includes an idler planet gear  14 A and an idler planet shaft  14 B intervened by the idler planet gear  14 A. The planet gear carrier assembly  15 , best seen in  FIG. 1B , is comprised of a planet carrier base  15 A and a planet carrier plate  15 B. The planet carrier base  15 A and the planet carrier plate  15 B can be optionally formed into an integral planet carrier assembly  15 , or may be secured together by any suitable means. 
     The drive sun gear  10  and the idler sun gear  11  are co-axially arranged and spaced apart along a center axis of the epicyclic gear train (A), with the ring gear  12  located co-axially around the idler sun gear  11 . The drive planet gear assemblies  13  are each arranged in the annular space between the ring gear  12  and each of the sun gears  10  and  11 . Each drive planet gear assembly  13  is supported on the planet gear carrier assembly  15  by rolling element bearings (shown in  FIG. 1C ) at  16 , while the idler sun gear  11  is restricted axially on the planet gear carrier assembly  15  by a bearing or bearings (not shown) at  19 . Optionally, the bearings  19  can be replaced with thrust plates  11 A (shown in  FIG. 1C ) to axially secure the idler sun gear  11  in engagement with the small planet gears  13 B. 
     The idler planet gear assemblies  14  are arranged in the annular space between the ring gear  12  and the idler sun gear  11 , coplanar with the small planet gears  13 B of the drive planet gear assemblies  13 , and are supported on the planet carrier  15  by bearings (not shown) at  17  and  18 . The drive sun gear  10  meshes with, and drives, the large planet gears  13 A, while the idler sun gear  11  meshes with both the small planet gears  13 B and the idler planet gears  14 A. Similar to the idler sun gear  11 , the ring gear  12  meshes with both the small planet gears  13 B and the idler planet gears  14 A. 
     Input power from a turbine engine or other power source is delivered to the epicyclic gear train (A) through a first stage reduction gear system  200 . The first stage reduction gear system  200  preferably consists of a pair of bevel pinions  25 , which are driven by the power source and a bevel gear  26 . The pair of bevel pinions  25  are coupled to the bevel gear  26  which receives the combined driving force of the pair of bevel pinions  25 , and which in turn is coupled to the drive sun gear  10  by a shaft  27 . The power is then delivered to the drive sun gear  10  through drive shaft  7 . During operation, the drive sun gear  10  drives the large planet gears  13 A, splitting the power among each of the drive planet gear clusters  13 . A portion of the power is transmitted to, and combined at, the planet carrier  15  through the supporting bearings at  16 . The large planet gears  13 A drive the corresponding small planet gears  13 B, transmitting a portion of the power from the large planet gears  13 A to the small planet gears  13 B. The small planet gears  13 B in turn drive the idler sun gear  11  which subsequently drives the idler planet gear assemblies  14  through the idler planet gears  14 A. The remaining power is transmitted through the idler planet gear assemblies  14  and re-combined at the planet carrier  15 . The resulting output of the epicyclic gear train (A) is conveyed to an output shaft (not shown) coupled to the planet carrier  15 . 
     As described above, there are two power pathways through the epicyclic gear train (A) from the input shaft to the output shaft. The first power pathway is through the drive planet gear assemblies  13  directly to the planet carrier  15 , and the second is through the idler sun gear  11 , through the idler planet gear assemblies  14 , to the planet carrier  15 . To effectively utilize the idler planet gear assemblies  14  in torque and/or power transmission, and thus improve the epicyclic gear train (A) power density, the small planet gears  13 A in the drive planet gear assemblies  13  are purposely set to float in an annular space between the idler sun gear  11  and the ring gear  12 , in relation to the idler planet gears  14 A. 
     As can be appreciated, a greater number of load carrying planet gears ( 13 B and  14 A), due to their smaller size, can be packaged into the epicyclic gear train (A) as compared to the number which can be utilized in conventional simple and compound epicyclic gear trains. These load carrying planet gears are light in weight and significantly improved the load capacity, and thus power density, of the epicyclic gear train (A). 
     Those of ordinary skill will recognize that there are a number of possible design variations for the epicyclic gear train (A) of the present disclosure that allow for a floating small planet gear  13 A in the drive planet gear assembly  13 . For example,  FIGS. 10 and 4  show one variations, where an external self-aligning bearing  16 A is employed. The bearing is positioned between the large planet gear  13 A and the small planet gear  13 B. The self-aligning bearing  16 A is a cylindrical roller bearing. The outer diameter surface  16 B of an outer race ring  16 C of the bearing  16 A has a spherical configuration. The outer race ring  16 C is seated in a matching spherical bearing seat  16 D that is housed in the planet gear carrier assembly  15 . Other types of self-aligning bearings, such as spherical ball bearings or spherical roller bearings, can also be used for the bearing  16 . 
     An alternate configuration of the epicyclic gear train (A) utilizes an Oldham coupling between the larger planet gear  13 A and the small planet gear  13 B in each drive planet gear assembly  13 , as shown in  FIG. 5 . The planet shaft  13 C contains three segments. The first segment  13 D is fixed to the large planet gear  13 A at one end; the other end has a simple or dovetail-shaped slot  30  across it&#39;s diameter at the end face. Likewise, the second segment  13 F is fixed to the small planet gear  13 B at one end; the other end has a simple or dovetail-shaped slot  32  across its end face. The two slots  30 ,  32  are arranged perpendicular to each other. The third segment  13 E has two perpendicular ridges  34  made respectively at both end faces. Each ridge  34  mates with a respective slot  30 ,  32  on the segment  13 D and segment  13 F. The Oldham coupling permits torque to be transmitted from the large planet gear  13 A to the small planet gear  13 B while allowing the axis of small planet gear to be offset from the axis of the large planet gear  13 A, such that the small drive planet gear  13 B is free to float in the annular space between the ring gear  12  and the idler sun gear  11 . 
     Optional bearings  20  may be used to support the small drive planet gears  13 B. The bearings  20  provides a predetermined supporting stiffness in relation to the stiffness of idler planet gear support bearings  17  and  18 . Bearings  20  permits the small planet gears  13 B to float under a load, and thus to distribute the load in a predetermined proportion between the small planet gears  13 B and the idler planet gears  14 A. 
     A further design variation is shown in  FIG. 7 , where a drive planet gear assembly  13 G with a gear coupling is used to transmit torque from the larger planet gears  13 A, supported on a double row roller bearing  16 DR, to the small planet  13 B, while allowing the small planet gear  13 B to float with respect to the large planet gear  13 A. 
     Other design variations are possible without deviating from the sprit of current invention. The embodiment and design variations disclosed herein should be considered as ways for explaining or implementing current invention, not as ways to limit the scope of current invention. 
     Other usage of current invention is also possible, for example, the input shaft and output shaft to the epicyclic gear train (A) can be reversed with proper design alterations to allow the high-ratio epicyclic gear train (A) of the present disclosure to be used as a speed increaser, such as is needed for a wind energy generation main gear transmission system. 
     As various changes could be made in the above constructions without departing from the scope of the disclosure, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.