Split torque epicyclic gearing

[0002] This invention relates generally to a epicyclic gear arrangement for transmitting and combining torque from a plurality of sources or parallel power paths, and more particularly to a helicopter gearbox having redundant gearing.

[0003] In helicopters, power is commonly supplied by a pair of turbine engines running at 20,000 rpm or more. This power must be combined and delivered to a rotor at a speed that is reduced from the turbine speed by a factor of 50 to 150. It is desirable to split the drive train so that power is delivered within the transmission elements by two or more routes. In this way gear tooth loading is reduced, and operating safety is improved by the redundant gearing should one gear set fail during flight. Split torque gearing is known; for example, such arrangement of gears is disclosed in U.S. Pats. Nos. 5,135,442 and 5,802,918, the teachings of which are entirely incorporated herein by reference.

[0004] In the present invention, the main rotor of a helicopter is driven by an epicyclic gearbox comprising a rotor sun gear coaxial and co-rotational with the rotor, a stationary sun gear coaxial with the rotor sun gear, at least one planet having an axis parallel to and rotating about the sun gear axis, and making contact with both sun gears. The planet comprises first and second planetary coaxial gears mechanically constrained to rotate together. The first planetary gear is in meshing engagement with the stationary sun gear and the second planetary gear is in meshing engagement with the rotor sun gear. The first planetary gear is also in meshing engagement with the internal teeth of a first ring gear and the second planetary gear is also in meshing engagement with the internal teeth of a second ring gear. The first and second ring gears each have a second set of gear teeth (bull teeth) on the top, bottom or sides thereof for meshing engagement with one or more drive pinions. These bull teeth and may be external, face, or even internal. In a first embodiment, a first drive pinion drives the first ring gear and a second drive pinion drives the second ring gear in the same direction. Power derives from individual shaft connection with a pair of engines. In a second embodiment of the invention, input shaft power is further split prior to delivery to the epicyclic gearbox by a plurality of drive pinions.

[0005] The first planetary gear has a first planetary pitch diameter, and the second planetary gear has a second planetary pitch diameter, wherein the first planetary pitch diameter is different from the second planetary pitch diameter.

[0006] The present invention provides a plurality of power paths into the epicyclic gearing thereby reducing tooth loading and increasing the operational safety factor of the aircraft by adding drive redundancy.

[0007] It is therefore an object of the present invention to provide a epicyclic gearbox having parallel power input paths combined by at least one planetary gear.

[0008] It is another object of at least one embodiment of the invention to provide a gearbox having a plurality of power paths for driving the main rotor of a helicopter.

[0009] The above as well as other objects of the invention will become more apparent from the following detailed description of the preferred embodiments of the invention, when taken together with the accompanying drawings in which:

[0010] FIG. 1 is a perspective cutaway view of an epicyclic rotor drive having two input drive pinion perpendicular to the rotor, according to a preferred embodiment of the invention.

[0011] FIG. 1A is a perspective cutaway view of an epicyclic rotor drive having two input drive pinion perpendicular to the rotor, according to a preferred embodiment of the invention.

[0012] FIG. 1B is the perspective cutaway view of the epicyclic rotor drive shown in FIG. 1A, with alternative drive means replacing the input drive pinions.

[0013] FIG. 2 is a sectional view of an epicyclic rotor drive similar to that shown in FIG. 1

[0014] FIG. 3 is a perspective cutaway view of the right side of an epicyclic rotor drive such as that shown in FIG. 1, wherein the two input drive pinions are driven by a single input shaft.

[0015] FIG. 4 is a an perspective cutaway view of the night side of an epicyclic rotor drive such as that shown in FIG. 3, wherein the two input drive pinions are parallel to the rotor shaft.

[0016] FIG. 5 is an perspective cutaway view of the right side of an epicyclic rotor drive such as that shown in FIG. 4, wherein the two input drive pinions drive the same bull gear, according to a preferred embodiment of the invention.

[0017] Referring now to the drawings wherein like numerals refer to like parts, FIG. 1A illustrates an epicyclic gearbox (sans casing) generally indicated by numeral 1. Central rotor 2 rotates in a counter-clockwise direction, and is fixed to rotor sun gear 4, which is in meshing engagement with each of first planetary gears 6 of planets 8. Gears 6 are also in meshing engagement with first ring gear 10. First planetary gears 6 are rotationally fixed to second planetary gears 12, which are in meshing engagement with stationary sun gear 14, fixed to torque shaft 16 which is in turn fixed to a casing or other framework (not shown). Optionally, torque shaft 16 may be held irrotational by a brake, torque arm, compliant member, or by any other suitable means. Second planetary gears 12 are also in meshing engagement with second ring gear 18. First ring gear 10 is driven by first pinion 20 in meshing engagementwith face gear 22. Similarly, second ring gear 18 is driven by second pinion 24 in meshing engagement with face gear 26.

[0018] By employing opposing face gears 22, 26 as shown, the centerlines of shafts 28, 30 are positioned nearly in the same plane; however, other surfaces of ring gears 10, 18 may alternatively be used for mounting of the face gears. As shown in FIG. 1B, pulley 46 or sprocket 44 may replace face gears 22, 26 (FIG. 1A) and belt 46 or chain 43 may be used to drive the ring gears 10, 18 of epicyclic gearbox 1. Bevel gears may also be used. In any case, torque supplied by each of shafts 28, 30 (FIG. 1A) are combined by the planets, where the largest numbers of teeth are simultaneously in meshing contact.

[0019] First and second ring gears 10, 18 rotate at different speeds. First and second engines (not shown) driving pinions 20, 24 by way of shafts 28, 30 may operate with a speed ratio between them; alternatively, the ratios created by meshing pinion and ring gear pairs, 20, 22 and 24, 26, may be adjusted so as to bring the engine speeds into an equal or substantially equal relationship.

[0020] In FIG. 2, power is supplied to face gears 22, 26 by vertically offset driving pinions 20,24, mounted to shafts 28,30. Face gears 22,26 are fixed to carrier rings 31, 29, respectively. Internal ring gear 10 of carrier ring 31 is in meshing engagement with gear 6 of planet 8. Similarly, second ring gear 18 of carrier ring 29 is in meshing engagement with gear 12 of planet 8. Gear 6 and gear 12 are unified so as to rotate together. A plurality of planets 8 are each mounted to planet carrier rings 21, 23 by spacer tubes 27 and bolts 25. Gear 6 is in meshing engagement with rotor sun gear 4, rotationally fixed to rotor shaft 5 and rotor mounting flange 7. Bearing flange 9 is fixed to rotor shaft 5, and vertical loads are accommodated by roller bearings 11. Planet gear 12 is in meshing engagement with sun gear 18, which is fixed to flange member 13, bolted to lower casing element 17. Upper casing element 19 and casing ring 15 are fixed to lower casing element 17 to substantially complete the housing of the rotor drive assembly 3.

[0021] In a second embodiment illustrated in FIG. 3, in which half of a symmetrical gearbox 42 according to another embodiment of the present invention is shown, torque from an individual input shaft 32 may be split prior to delivery to the drive pinions 20, 24. Shaft 32 is shown to have two shaft pinions 34, 36 which are in meshing engagement with face gears 38, 40 respectively. Drive pinion 20 is in meshing engagement with face gear 22, and rotates in the opposite direction to drive pinion 24, which is in meshing engagement with face gear 26. In this embodiment and in other embodiments herein where at least one source of torque is split and re-combined prior to the planets, it is desirable that one or more gears in the split torque path be torsionally compliant. Such compliance is disclosed in U.S. Pats. Nos. 5,117,704, 4,831,897 and 4,674,351, for example, the teachings of which are entirely incorporated herein by reference. Any suitable means of achieving compliance whereby torque is shared substantially equally between parallel paths may be used.

[0022] In FIG. 3, a second engine (not shown) may supply gearbox 42 by way of a gear arrangement which is substantially the mirror image of that shown. Other gears may be used such as a take-off gear (not shown) to supply a tail-rotor. One example of such a gear is shown in FIG. 4 below.

[0023] In FIG. 4, an alternative gearing arrangement 48 is shown in which the torque supplied by engine 50 is delivered by pinion 52 to opposing face gears 54, 56, and thence to pinions 58, 60 to the external teeth of ring gears 62,64. Torque is further consolidated by planets 8 rotatably mounted in carrier 66.

[0024] In FIG. 5, a preferred embodiment of a gear arrangement 100 is shown in which torque supplied by engine 70 is delivered to shaft 74, and thence to a pair of pinions 76,78 driving face gears 8082, and pinions 86, 84 respectively. The torque from both pinions 86, 84 are consolidated by external ring gear 88, while engine 72 delivers torque to external ring gear 90, by means of a substantially mirror image gear arrangement (except that ring gear 90 is driven rather than ring gear 88). Torque from ring gears 88, 90 is further consolidated by planets 8. Face gear 92 fixed to planet carrier 66 drives take-off pinion 94 driving a tail rotor (not shown) by means of shaft 96. Optionally, pinion 94 may deliver power from a third source (not shown) to be combined by planets 8 with torque supplied by engines 70, 72. The following example is provided to illustrate the various ratios involved. Exemplary values are used for the various gears of the gear arrangement 100 as shown in FIG. 5.

[0025] The speed (Ω_pc) of the planet carrier 66 is given by the relation Ω_pc=Ω_rs/(1−ρ), where ρ=(N_ss*N_p1)/(N_rs*N_p2). If the speed of the rotor 2 is 200 rpm, Ω_pc≅730 rpm.

[0026] The speed of the second ring gear is given by the relation Ω_rg2=Ω_pc/(1+N_ss/N_rg2)≅1078 rpm.

[0027] The speed of the second drive pinions 86, 84 are equal to (Ω_rg2*N_fg2)/N_dp2≅6492 rpm, and the speed of the third drive pinions 76,78 and engine 70 are equal to (Ω_dp2*N_fg3)/N_dp3≅20,039 rpm, representing an overall speed reduction between the engine 70 and the rotor shaft 2 of about one hundred.

[0028] Although only a few exemplary embodiments of this invention have been described in detail above, those skilled in the art will readily appreciate that many modifications are possible in the exemplary embodiments without materially departing from the novel teachings and advantages of this invention. Accordingly, all such modifications are intended to be included within the scope of this invention as defined in the following claims. In the claims, means-plus-function clauses are intended to cover the structures described herein as performing the recited function and not only structural equivalents, but also equivalent structures.