Epicyclic gear train with balanced carrier stiffness

An epicyclic gear train including a central sun gear, an outer ring gear, and a number of planet gears which are mounted to a planet carrier. The planet carrier includes a centrally disposed torque transfer coupling with a torque transmission point at an axial end thereof. First and second carrier plates extend radially from the torque transfer coupling and are axially spaced apart to support the planet gears therebetween at aligned gear mounting points. The first carrier plate is closer to the torque transfer point than the second carrier plate. The second carrier plate has a stiffness that is greater than that of the first carrier plate.

TECHNICAL FIELD

The present disclosure relates to gearboxes for gas turbine engines and, more particularly, to an epicyclic gear train having a planet gear carrier.

BACKGROUND

Epicyclic gear trains are frequently used in reduction gearboxes of gas turbine engines. The planet gear carriers of such epicyclic gear trains, however can be prone to torsional deflection wherein the planet carrier twists around its central axis under load, causing the individual axis of rotation of the planet gears to lose parallelism with the central axis of the carrier. This can negatively affect the efficiency and life span of the gear train.

Improvement is sought to provide a planet carrier that provides a more balanced stiffness, thereby allowing a more uniform load distribution on the two sides of each planet gear of the epicyclic reduction stage.

SUMMARY

There is accordingly provided an epicyclic gear train defining a central longitudinal axis, the epicyclic gear train comprising a central sun gear, an outer ring gear, and a number of planet gears which are mounted to a planet carrier, the planet gears rotating about respective planet gear axes, the planet gears disposed in meshed engagement with the central sun gear and the outer ring gear, the planet carrier including a centrally disposed torque transfer coupling concentric with the longitudinal axis and having a torque transmission point at an axial end of the torque transfer coupling, first and second carrier plates extending radially from the torque transfer coupling, the first and second carrier plates being parallel to each to each other and axially spaced apart to support the planet gears therebetween at aligned gear mounting points on each of the first and second carrier plates, the planet carrier having center links extending radially outward relative to the longitudinal axis and axially disposed between the carrier plates, the first carrier plate being closer to the torque transfer point than the second carrier plate, and the second carrier plate having a stiffness that is greater than that of the first carrier plate.

There is also provided an epicyclic gear train comprising a sun gear, an outer ring gear, and a number of planet gears which are mounted to a planet carrier and disposed radially between the sun gear and the outer ring gear in meshing engagement therewith to provide relative rotational motion at least between the planet carrier and the outer ring gear, the sun gear, the outer ring gear and the planet carrier being concentric with a central longitudinal axis, the planet carrier including a carrier body having a central tubular portion concentric with the longitudinal axis and first and second carrier plates extending radially outward from the central tubular portion, the first and second carrier plates axially spaced apart from each other by center links to support the planet gears therebetween, the central tubular portion defining torque transmission means at an axial end thereof, the first carrier plate being closer to the torque transmission means than the second carrier plate, and the second carrier plate having a greater stiffness than the first carrier plate.

There is further provided a gas turbine engine comprising an epicyclic gear train having at least one epicyclic reduction stage, the gear train including a central sun gear concentric with a longitudinal axis, an outer ring gear, and a number of planet gears which are mounted radially between, and are in meshed engagement with, the central sun gear and the planet gears, the central sun gear and the outer ring gear configured for relative rotation with respect to the planet gears, the planet gears rotatably mounted to a planet carrier including a torque transfer coupling centrally disposed concentrically with the longitudinal axis and carrier plates disposed radially outward from the torque transfer coupling, the carrier plates being axially spaced apart from each other and connected to the torque transfer coupling by center links, the center links disposed axially between the carrier plates and radially extending away from the torque transfer coupling, the center links having radially outer ends which terminate at a radially outer perimeter of the carrier plates such that the center links are entirely radially disposed within the radially outer perimeter of the carrier plates.

DETAILED DESCRIPTION

Referring toFIG. 1, a turboprop gas turbine engine10generally having a power plant14and a reduction gearbox (RGB)12. The engine power plant14includes a compressor section16, combustion chamber18, and a turbine section20. Air inlets22permit air to be drawn into the gas generator and, following power withdrawal by the turbine section, exhaust ducts24provide an engine exhaust gas outlet. While the exemplary turboprop gas turbine engine10as depicted inFIG. 1is of the type having an inversed configuration (i.e. air inlet towards the rear of the engine and the exhaust ducts24towards the front of the engine), it is to be understood that other configurations of the gas turbine engine10, and the power plant portion14in particular are within the scope of the present disclosure. The reduction gearbox12as described herein, and more particularly the epicyclic gear train27thereof, can be used in conjunction with any number of gas turbine engine types and configurations, including both turboprop and turboshaft engines. Further still, the epicyclic gear train as described herein can be used in a turbofan gas turbine engine, despite it not having a full reduction gear box as in turboshafts and turboprops.

Referring toFIG. 1, the operation of such an airborne gas turbine engine12is well known, and occurs generally as follows, by means of example only. Air enters the engine through the inlet17and is compressed by the compressor section16, in this case comprising axial flow compressors19and a centrifugal compressor21. The compressed air is then fed to the combustion chamber18where it is mixed with fuel and ignited. The hot gas then expands through the turbine section20, before being discharged to the atmosphere through exhaust ducts24. The turbine section20in this exemplary embodiment is comprised of a compressor turbine23and a power turbine25. The compressor turbine23drives the compressor18and the accessories through accessory gearbox15. The power turbine25, which is mechanically independent from the compressor turbine23and the reduction gearbox12, ultimately drives the propeller of the engine12via the propeller shaft29at the output of the gearbox12.

In the embodiment ofFIG. 1, the exemplary reduction gearbox12includes an epicyclic gear train27having two reductions stages, namely a first reduction stage26that receives input from the power plant14through power turbine output shaft34, and a second reduction stage28that receives power/torque from the first reduction stage26, thereby further reducing the rotational speed before transmitting torque to an output propeller shaft29. The output of the second reduction stage28of the epicyclic gear train27therefore drives a propeller (not shown), which is adapted to be fastened to a propeller flange30disposed at the forward end of the propeller shaft29. The gear train27is an epicyclic gear train, in that one or more of the reductions stages26,28thereof includes an the epicyclic gear train configuration as described herein.

While different configurations for reduction gearboxes and gear trains used therein exist, the reduction gearbox12and the gear train27of the present disclosure are respectively an epicyclic gearbox and an epicyclic gear train, in that they include one or more reduction stages that comprise an epicyclic configuration. While the terms “planetary” and “epicyclic” with respect to such gear trains and gearboxes are both used in the art and are generally understood interchangeably to refer to the same type of gear train and/or gearbox, the term “epicyclic” will be used herein.

The second reduction stage28will generally be described herein with reference to the epicyclic gear train27of the present disclosure, however it is to be understand that the features of the epicyclic gear train and the planet carrier thereof as described herein can similarly be employed as part of the first (or other) reduction stage of the RGB12. Similarly, as noted above, the epicyclic gear train configuration and the elements thereof as described herein can be employed in a turbofan gas turbine engine, despite it not comprising a full reduction gear box as in turboshafts and turboprops.

As will be seen, in the depicted embodiment, the second reduction stage28of the epicyclic gear train27within the RGB12of the gas turbine engine10is an epicyclic reduction stage that generally comprises a central sun gear, an outer ring gear, and at least two (but typically three or more) planet gears supported by a planetary carrier, all of which are described in further detail below.

Referring toFIGS. 2 and 3, the second epicyclic reduction stage28of the epicyclic gear train27includes generally a central sun gear32(best seen inFIG. 3), outer ring gear(s)36, and a plurality of (in this case, three) planet gears38which are in meshing engagement with both the sun gear32and the outer ring gear(s)36(36′). In the depicted embodiment, the outer ring gear36is a split gear, in that it includes two outer ring gear portions36′ which are axially spaced apart, each meshing with axially aligned gear teeth on axially opposite sides of the planet gears38, for better load distribution and torque transfer. The planet gears38are supported within the ring gear36by a torque-transferring planet carrier40, to which the planet gears38are rotatably mounted via journal bearings41for rotation about respective planet axes of rotation39.

As seen inFIG. 3, each planet gear38is rotatably mounted in the planetary carrier40about a planet axis39and is in meshing engagement with both the (radially inward) sun gear32and the (radially outward) ring gear(s)36(36′). The sun gear32, ring gear(s)36(36′), and planet carrier40are all concentric about a longitudinally-extending central axis37. Each planet gear38, which is mounted to and supported by the planet carrier40via journal bearings41, rotates about its own individual axis of rotation39. In one possible epicyclic gear train configuration, both the sun gear32and planet carrier40, in operation, rotate about this longitudinal axis37while the outer ring gear36remains rotatably fixed. In this configuration, the rotating planet gears38collectively rotate the planet carrier40about the central axis37, when driven by the sun gear32. In alternate epicyclic configurations, however, the outer ring gear36may not be fixed (i.e. the ring gear36may rotate about the longitudinal axis37), in which case either the planet carrier40or the sun gear32instead remains rotatably fixed, while the other rotates. As will be appreciated, for any epicyclic reduction stage, two of the three main components (i.e. sun gear, ring gear and planet carrier) rotate, while the third component is held rotationally fixed relative to the other two. In all cases, however, the planet gears38rotate about their respective axes39and torque is transferred through the planet carrier40.

Referring now toFIG. 3-4, the planet carrier40will be described in greater detail.

The planet carrier40of the present disclosure may be configured to better balance the stiffness along the torque paths extending therethrough, in order to provide a more uniform load distribution on the two sides of the planet gears38and thus on each of the plates48a,48bof the planet carrier40. Such a more uniform load distribution, permitted by the configuration of the planet carrier that enables stiffness balancing as will be described, may further optimize gear durability and thus reduce the probability of bearing touch down or other life span limiting occurrences. The present planet carrier may help to improve the power to weight ratio for the gear train27, and thus the gearbox12within which it is found.

The planet carrier40is monolithic, in that its body42is integrally formed as a one-piece component (i.e. the carrier body42is monolithic). The body42of the planet carrier40may, for example, be machined from a single piece of material, however other suitable manufacturing methods may also be used to form the planet carrier40as a single, one-piece, component (e.g. additive manufacturing, casting, molding, etc.).

The monolithic body42of the present planet carrier40includes generally a torque transfer coupling44, carrier plates48which in this case include first and second carrier plates48aand48b, and a number of center links50interconnecting the carrier plates48a,48band the torque transfer coupling44.

The torque transfer coupling44is centrally disposed within the body42of the planet carrier40, concentrically with the longitudinal center axis37. A central tubular portion43thereof defines a central bore45that axially extends at least partially therethrough. A coupling end54of the central tubular portion43, which provides the torque transmission means as described herein, is adapted to matingly receive therein, and rotatably engage via splines47formed within the opening to the central bore45, the propeller shaft29providing the output from the RGB12. In the case where the planet carrier40forms part of a first reduction stage26in a multi-stage gear train or gearbox, then the coupling end54of the central tubular portion43is configured to engage the input to the next reduction stage (such as a shaft driving a sun gear for another epicyclic reduction stage). Alternate embodiments are also possible, for example wherein the planet carrier40does not drive either the propeller shaft29or another downstream reduction stage, e.g. wherein the coupling end54serves as a torque input rather than a torque output, or wherein the coupling end54is engaged with a mating coupling shaft that prevents rotation of the planet carrier40. Regardless of the configuration, this coupling end54of the torque transfer coupling may also be referred to herein as the torque transmission point54of the planet carrier40, as the torque transmitted from (as torque output) or to (as torque input) will occur at this point of the planet carrier, with the mounting points57,59of the planet gears38forming the respective other torque input or torque output points to the carrier40.

As can be appreciated fromFIG. 4, the two carrier plates48aand48b, which respectively provide support for each axial side of the planet gears38, via their journal bearings41, differ from each other. More specifically, as can be seen inFIG. 4, the first carrier plate48a, which in the depicted embodiment is disposed on an axially forward (or front) end of the planet carrier40, is circumferentially split such that it includes several distinct radially extending finger segments55. The first carrier plate48ain this embodiment is therefore really a number (in this case, three) of finger segments5which lie in a common plane, and which are therefore circumferentially spaced about the planet carrier. Each of these finger segments55includes a planet gear axle opening49in a radially outer end thereof, which forms a forward attachment point57for one of the planet gears38. In contrast, the second carrier plate48b, which in the depicted embodiment is disposed on an axially rearward (or rear) end of the planet carrier40, is formed by a single, planar, plate that is substantially circumferentially uninterrupted (with the exception of the weight-saving openings63formed therein, as seen inFIG. 4) and includes a number (in this case, three) of planet gear axle opening49formed at circumferentially spaced apart locations therein in alignment with the corresponding openings49in the finger segments55forming the first carrier plate48a. The planet gear axle openings49in the second carrier plate48baccording form rear attachment points59for each of the respective planet gears38.

The carrier plates48aand48bboth however extend radially away form the central tubular portion43of the torque transfer coupling44, and are substantially perpendicular to the central axis37. The carrier plates48a,48bare axially spaced apart from each other to receive therebetween the planet gears38, which are supported on axially opposed ends by the carrier plates48aand48b, at the front and rear attachment points57and59thereon, respectively. The number of pairs of planet gear axle openings49which are aligned with each other in the carrier plates48a,48bcorresponding to the number of planet gears38(which in this case is three). As noted above, the openings49in opposed plates48aand48bare in radial and circumferential alignment with each other, concentrically disposed with, and defining, the individual axes of rotation39of the planet gears38.

The planet gear openings49and thus the planet gears38mounted therewithin are circumferentially equidistantly spaced about the body42of the planet carrier40, and are radially disposed a common distance relative to the central axis37. In the depicted embodiment, therefore, three planet gears38are provided and thus the individual axes of rotation39thereof, as defined by the planet gear openings49in the carrier plates48a,48b, are circumferentially spaced apart by 120 degrees about the central axis37. Each of the planet gears38is rotatably mounted to the carrier plates48a,48bby axles and/or bearings41, such as the journal bearings41for example, which extend through the aligned pairs of planet gear openings49to rotatably support each of the planet gears38.

The monolithic body42of the planet carrier40also includes a number of center links50which are integrally formed with, and interconnect, the first and second carrier plates48a,48band the torque transfer coupling44. More particularly, each of the center links50extends radially outwardly from the longitudinal axis37and extends axially between the carrier plates48a,48bto interconnect them. Each center link50is circumferentially disposed between two planet gears38, and therefore the center links50are circumferentially offset from the openings49in the carrier plates48within which the journal bearings41of the planet gears38are mounted. Each of the center links50terminate, at their radially outermost ends52, to form an integrally formed bridge71extending axially between the spaced apart first and second carrier plates48a,48b. In the depicted embodiment, the outer ends52of the center links50thus terminate at, and do not extend radially beyond, a radially outer perimeter60of the carrier plates48a,48b. Additionally, in one particular embodiment, the first and second carrier plates48a,48bare connected to the torque transfer coupling44(which may include the central tubular portion43) only by the center links50.

The center links50of the planet carrier40therefore define therethrough a torque path82(which will be described in further detail below) through which torque is transmitted during operation of the epicyclic gear train27between the splines47at the torque transmission point54of planet carrier and the second carrier plate48b, which is the furthest away from the coupling end. The center links50are centered axially between the carrier plates48a,48bsupporting the planet gears38, via axles or bearings (e.g. the journal bearings41) of the planet carrier40. The carrier plates48a,48bsupporting opposed sides of the planet gears38are “decoupled” from each other and from the torque transmitted through the body of the planet carrier during operation.

Referring still toFIG. 4, the monolithic body42of the planet carrier40defines two different torque paths80and82which extend respectively through the first and second carrier plates48aand48b, as will now be seen. The properties and/or geometric configuration of the planet carrier40are configured such that these two torque paths80and82are substantially balanced. This is at least partly made possible by providing each of the first and second carrier plates48aand48b(which in this embodiment are the front and rear carrier plates, respectively) with a different stiffness.

The torque paths80and82are depicted for understanding purposes by lines extending through the carrier40inFIG. 4, however it is to be understood that these torque path lines80and82are not visibly present on the carrier40but are shown inFIG. 4for the purposes of understanding the load and stiffness balance made possible by the configuration of the present planet carrier40. As shown inFIG. 4, a first torque path80of the planet carrier40extends between the torque transmission point54of the torque transfer coupling44and the first carrier plate48ahaving thereon the front planet attachment points57. A second torque path82of the planet carrier40extends between the coupling end54of the torque transfer coupling44and the second carrier plate48bhaving thereon the rear planet attachment points59. As can be appreciated fromFIG. 4, the first torque path80is shorter than the second torque path82, given that the second torque path82has a greater distance to cover between the torque transmission point54and its respective rear planet attachment points59. More specifically, the second torque path82extends first axially through the central tubular portion43of the torque transfer coupling44, then radially outward through the center links50, then axially through the bridges71formed at the radially outer ends52of the center links50, and then (given the circumferential offset between each of the center links50and the planet gear journal bearings41) circumferentially through the second carrier plate48bto the openings49forming the rear planet attachment points59. In contrast, the first torque path80extends first axially through the central tubular portion43of the torque transfer coupling44, then simply radially outward through the respective finger portions55forming first carrier plate48ato reach the openings49forming the front planet attachment points57. The first torque path80is therefore said to be a more direct torque path from the splines47at the torque transmission point54of the carrier to one side the planet gears38, whereas the second torque path82is a more indirect torque path from the splines47at the torque transmission point54of the carrier to the other side of the planet gears38.

In order to balance these two torque paths80and82within the planet carrier40, the first carrier plates48ais less stiff than the second carrier plate48b. Stated differently, the second carrier plate48bhas a higher stiffness than the first carrier plate48a. Accordingly, in this particular embodiment, the stiffness ratios of the two torque paths80and82is geometrically balanced so that the reaction forces on each side of the planet gears38, and therefore on each of the front gear mesh point90and the rear gear mesh point92(i.e. the respective front and rear sides of the meshing between the planet gears38and the outer ring gear36—seeFIG. 3) are substantially balanced. Because the torque transferred through the carrier will naturally tend to want to follow the shortest distance between the planet gear reactions points57,59and the spline47at the torque transmission point54(which provides the torque input or output to/from the carrier, depending on the configuration), the torque transferred through the shorter first torque path80will naturally transfer more load if the two carrier plates48aand48bwere of equal stiffness. However, by making the first carrier plate48a(which is closer to the torque transmission point54of the carrier40) of the present carrier40less stiff that the second carrier plate48b(which is further away from the torque transmission point54of the carrier40), the loads created within the two torque paths are more equally distributed and therefore any effects caused by the unequal-length torque paths80and82can be substantially neutralized. In short, by making the carrier plate exposed to higher torque (due to the shorter torque path) less stiff, and making the carrier plate exposed to lower torque (due to the longer torque path) more stiff, the overall loads imposed on the planet carrier can be more evenly distributed throughout the planet carrier40. The loads imposed on the front carrier plate48aand the rear carrier plate48bof the planet carrier40are therefore more evenly distributed, due to the relative different in stiffness between the two carrier plates48aand48b, and thus stress distribution within the planet carrier40, and the planet gears38supported thereby, is better optimized. Additionally, better alignment between the gears32,36,38of the epicyclic gear train27, and the journal bearings41on which the planet gears41rotate, may be possible.

Various geometric configurations may be used to achieve the above-described stiffness balance (i.e. a load balance via relative stiffness differential) between the first carrier plate48aand the second carrier plate48b. In the depicted example ofFIG. 4, for example, the first carrier plate48ais less stiff than the second carrier plate48bbecause the first carrier plate48ais circumferentially discontinuous, as noted above, being formed by the radially extending finger portions55, whereas the second carrier plate48bextends circumferentially fully about the planet carrier. The second carrier plate48bmay therefore said to be “larger”, or more massive (i.e. it occupies a greater overall volume in space) than the first carrier plate48a. Consequently, the shape and configuration of these finger portions55renders them collectively less stiff than the more massive, and circumferentially unified and reinforced (e.g. by the center links50) structure forming the second carrier plate48b. In an alternate embodiment, as will be described further below with reference toFIG. 5, the second carrier plate48b, defining the longer torque path82, may having a greater axial thickness than the first carrier plate48a, defining the shorter torque path80. Alternately, other structurally reinforcing features, such as gussets, ribs, etc., may be provided on the second carrier plate48b, in order to ensure that it is stiffer than the first carrier plate48a. Alternately still, the second carrier plate48bmay be provided with material properties such that it is stiffer that the first carrier plate48a. As will be appreciated, various geometric, material and structure properties may be used to ensure that the second carrier plate48bis stiffer than the first carrier plate48a.

Referring now toFIG. 5, an alternate planet carrier140of the present disclosure is shown. The planet carrier140is similar to the planet carrier40described above in many respects, however the planet carrier140has a carrier body142with an alternate configuration, as will now be described.

The planet carrier140similarly provides a stiffness balance (i.e. a load balance via relative stiffness differential) between a first carrier plate148a, that is located closest to the torque transmission point154of the planet carrier140, and the second carrier plate148b, that is located further away from the torque transmission point154than the first carrier plate148a. Accordingly, the torque path (not graphically shown inFIG. 5) extending through the second carrier plate148bwill be longer than the torque path extending through the first carrier plate148a. In this embodiment, however, the second carrier plate148bof the planet carrier140has an axial thickness T2that is greater than an axial thickness T1of the first carrier plate148a, thereby making the second carrier plate148bstiffer than the first carrier plate148a. In this configuration, therefore, the first and second carrier plates148a,148bare more geometrically (at least in perimeter shape) similar—but they have different thickness in order to ensure that the second carrier plate148bis thicker, and thus stiffer, that the first carrier plate148a. The first carrier plate148atherefore does not comprise the circumferentially discontinuous finger portions55of the carrier40, and instead is formed by a single, planar, plate that is substantially circumferentially uninterrupted (with the exception of the weight-saving openings163formed therein). In an end elevation view, therefore the first and second carrier plates148bmay appear substantially identical in shape and perimeter profile.

Much as per the planet carrier40, the planet carrier140also includes a number of center links150which are integrally formed with, and interconnect, the first and second carrier plates148a,148band the torque transfer coupling144. More particularly, each of the center links150extends radially outwardly from the longitudinal axis37and extends axially between the carrier plates148a,148bto interconnect them. Each center link150is circumferentially disposed between two planet gears, and therefore the center links150are circumferentially offset from the openings149in the carrier plates148a,148bwithin which the journal bearings of the planet gears are mounted. As can be seen inFIG. 5, the center links150of the planet carrier140are axially interrupted, in that an axial gap190is formed at an axial end of the radially outmost end152of the center links150. The axial gap190separates the first carrier plate148afrom the outermost end152of the center links150. This may more efficiently decouple the first and second carrier plates148a,148b.

The embodiments described above are intended to be exemplary only. For example, although an epicyclic configuration with three planet gears is described, any suitable number of planet gears can be employed. The planet carrier40and the epicyclic gear train27as described herein can be applicable to a gearbox and/or gear train having single reduction stage, a double reduction stage, or a gear train with more than two reduction stages. One skilled in the art will appreciate that the present gear train and gear box configuration described also has application well beyond the gas turbine engine example described.