Planet gearbox with cylindrical roller bearing with high density roller packing

A planet gearbox is provided for connection to a carrier of an epicyclic gearing arrangement with a single input and single output and including a sun gear, a ring gear and at least one double helix planet gear rotatable on a cylindrical roller bearing with a cage having a cross-web thickness of 15% to 25% of the diameter of the cylindrical rollers and an L/D ratio exceeding 1.0. A gas turbine engine includes a fan and LP shaft, which couples a compressor to a turbine. An epicyclic gearing arrangement has a single input from the LP shaft coupled to a sun gear, a single output coupled to the fan's shaft, and a planet bearing cage having a cross-web thickness measuring 15% to 25% of the roller's diameter.

FIELD OF THE INVENTION

The present subject matter relates generally to a cylindrical roller bearing, or more particularly to a cylindrical roller bearing for the planet gear in an epicyclic gearbox in a gas turbine engine.

BACKGROUND OF THE INVENTION

A gas turbine engine generally includes a fan and a core arranged in flow communication with one another with the core disposed downstream of the fan in the direction of the flow through the gas turbine. The core of the gas turbine engine generally includes, in serial flow order, a compressor section, a combustion section, a turbine section, and an exhaust section. With multi-shaft gas turbine engines, the compressor section can include a high pressure compressor (HP compressor) disposed downstream of a low pressure compressor (LP compressor), and the turbine section can similarly include a low pressure turbine (LP turbine) disposed downstream of a high pressure turbine (HP turbine). With such a configuration, the HP compressor is coupled with the HP turbine via a high pressure shaft (HP shaft), and the LP compressor is coupled with the LP turbine via a low pressure shaft (LP shaft).

In operation, at least a portion of air over the fan is provided to an inlet of the core. Such portion of the air is progressively compressed by the LP compressor and then by the HP compressor until the compressed air reaches the combustion section. Fuel is mixed with the compressed air and burned within the combustion section to provide combustion gases. The combustion gases are routed from the combustion section through the HP turbine and then through the LP turbine. The flow of combustion gasses through the turbine section drives the HP turbine and the LP turbine, each of which in turn drives a respective one of the HP compressor and the LP compressor via the HP shaft and the LP shaft. The combustion gases are then routed through the exhaust section, e.g., to atmosphere.

The LP turbine drives the LP shaft, which drives the LP compressor. In addition to driving the LP compressor, the LP shaft can drive the fan through a fan gearbox of an epicyclic gearing arrangement, which allows the fan to be rotated at fewer revolutions per unit of time than the rotational speed of the LP shaft for greater efficiency. The fan gearbox rotatably supports a sun gear that is disposed centrally with respect to a ring gear and a plurality of planet gears, which are disposed around the sun gear and engage between the sun gear and the ring gear. The LP shaft provides the input to the epicyclic gearing arrangement by being coupled to the sun gear, while the fan is coupled to rotate in unison with the carrier of the planet gears. Each planet gear meshes with the sun gear and with the ring gear, which is held stationary. The shaft of the fan is rotatable on its own bearing that is housed in a sun gearbox, which is also called the fan gearbox that is fixed to the rotationally central region of a carrier. Each planet gear is rotatable on its own bearing that is housed within a planet gearbox, which is fixed to the peripheral region of the carrier.

For any given gas turbine engine application, the planet gears are designed to provide a set reduction ratio between the rotational speed of the LP shaft and the rotational speed of the fan shaft. Because each planet gearbox that houses each planet gear is disposed within the flow path of the gas turbine engine, the challenge is to design on the one hand a reliable and robust planet gearbox that meets all flight conditions of the engine while on the other hand designing a planet gearbox that is compact sufficiently to fit inside the flow path in a way that does not require the entire engine size to be larger and heavier than otherwise would be needed in order to accommodate the planet gearbox.

Ceramic rolling elements are lighter in weight and known to provide a longer life than steel rollers, however ceramic rolling elements are used in the form of ball roller bearings or spherical roller bearings, which are not axially compliant and therefore not compatible with some helical gear configurations.

Accordingly, a gas turbine engine having one or more components for reducing the envelope required for the epicyclic gearing between the fan and the LP shaft would be useful. Specifically, a gas turbine engine having one or more components for reducing the envelope required for the planet gearboxes housing the planet gears of the planetary gearing would be particularly beneficial.

BRIEF DESCRIPTION OF THE INVENTION

In one exemplary embodiment of the present disclosure, a bearing for a planet gear of the power gearbox of a gas turbine engine is provided. The power gearbox includes an epicyclic gearing arrangement that has at least one planet bearing. The LP shaft of a turbofan engine provides the rotational input to the power gearbox, and the output from the power gearbox is provided to rotate the fan shaft of the turbofan engine. In one exemplary planetary embodiment, each planet gear has an outer race that includes a gearing surface that meshes with a sun gear input and a stationary ring gear to impart an output of reduced rotational speed to the carrier of the planet gears. In another exemplary star embodiment, each planet gear has an outer race that includes a gearing surface that meshes with a sun gear input while the carrier is held stationary to impart an output of reduced rotational speed to the ring gear.

The planet bearing is inner-race-guided, and the inner race desirably is a single piece having at least one roller track. For each respective roller track, a respective roller cage is disposed between the inner race and the outer race. The teeth on each of the planet gear, the sun gear and the ring gear desirably are arranged in a double helical pattern that restrains the planet gear axially to both the sun gear and the ring gear. The planet bearing uses a plurality of cylindrical rollers, which have outer cylindrical surfaces that rotatably contact both the inner race and the outer race, which is formed by the cylindrical inner surface of the planet gear. The roller cage is designed with a small clearance to the inner race and has a cross-web thickness of 15% to 25% of the diameter of the cylindrical rollers, which desirably have an L/D ratio exceeding 1.0 and desirably more than 1.3 times the roller diameter and up to and including 1.8 times the diameter.

In another exemplary embodiment of the present disclosure, a gas turbine engine is provided with a power gearbox that includes planet gears rotatably supported by a planet bearing. The gas turbine engine includes a compressor section having at least one compressor and a turbine section located downstream of the compressor section and including at least one turbine. The compressor section can include a low pressure compressor and a high pressure downstream of the low pressure compressor. The turbine section includes a high pressure (HP) turbine and a low pressure (LP) turbine downstream of the HP turbine. The gas turbine engine also includes a low pressure shaft mechanically coupling the low pressure compressor to the low pressure turbine via an epicyclic gearing arrangement, which includes one or more planet bearings as summarily described above and in more detail hereinafter.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

Referring now to the drawings, wherein identical numerals indicate the same elements throughout the drawings,FIG. 1is a schematic cross-sectional view of a gas turbine engine in accordance with an exemplary embodiment of the present disclosure. More particularly, for the embodiment ofFIG. 1, the gas turbine engine is a high-bypass turbofan jet engine10, referred to herein as “turbofan engine10.” As shown inFIG. 1, the turbofan engine10defines an axial direction A (extending parallel to a longitudinal centerline12provided for reference) and a radial direction R that is normal to the axial direction A. In general, the turbofan10includes a fan section14and a core turbine engine16disposed downstream from the fan section14.

The exemplary core turbine engine16depicted generally includes a substantially tubular outer casing18that defines an annular inlet20. As schematically shown inFIG. 1, the outer casing18encases, in serial flow relationship, a compressor section including a booster or low pressure (LP) compressor22followed downstream by a high pressure (HP) compressor24; a combustion section26; a turbine section including a high pressure (HP) turbine28followed downstream by a low pressure (LP) turbine30; and a jet exhaust nozzle section32. A high pressure (HP) shaft or spool34drivingly connects the HP turbine28to the HP compressor24to rotate them in unison. A low pressure (LP) shaft or spool36drivingly connects the LP turbine30to the LP compressor22to rotate them in unison. The compressor section, combustion section26, turbine section, and nozzle section32together define a core air flowpath.

For the embodiment depicted inFIG. 1, the fan section14includes a variable pitch fan38having a plurality of fan blades40coupled to a disk42in a spaced apart manner. As depicted inFIG. 1, the fan blades40extend outwardly from disk42generally along the radial direction R. Each fan blade40is rotatable relative to the disk42about a pitch axis P by virtue of the fan blades40being operatively coupled to a suitable actuation member44configured to collectively vary the pitch of the fan blades40in unison. The fan blades40, disk42, and actuation member44are together rotatable about the longitudinal axis12via a fan shaft45that is powered by the LP shaft36across a power gear box46. The power gear box46includes a plurality of gears for adjusting the rotational speed of the fan shaft45and thus the fan38relative to the LP shaft36to a more efficient rotational fan speed.

Referring still to the exemplary embodiment ofFIG. 1, the disk42is covered by a rotatable front hub48aerodynamically contoured to promote an airflow through the plurality of fan blades40. Additionally, the exemplary fan section14includes an annular fan casing or outer nacelle50that circumferentially surrounds the fan38and/or at least a portion of the core turbine engine16. It should be appreciated that the nacelle50may be configured to be supported relative to the core turbine engine16by a plurality of circumferentially-spaced outlet guide vanes52. Alternatively, the nacelle50also may be supported by struts of a structural fan frame. Moreover, a downstream section54of the nacelle50may extend over an outer portion of the core turbine engine16so as to define a bypass airflow passage56therebetween.

During operation of the turbofan engine10, a volume of air58enters the turbofan10through an associated inlet60of the nacelle50and/or fan section14. As the volume of air58passes across the fan blades40, a first portion of the air58as indicated by arrow62is directed or routed into the bypass airflow passage56, and a second portion of the air58as indicated by arrow64is directed or routed into the upstream section of the core air flowpath, or more specifically into the inlet20of the LP compressor22. The ratio between the first portion of air62and the second portion of air64is commonly known as a bypass ratio. The pressure of the second portion of air64is then increased as it is routed through the high pressure (HP) compressor24and into the combustion section26, where the highly pressurized air is mixed with fuel and burned to provide combustion gases66.

The combustion gases66are routed into and expand through the HP turbine28where a portion of thermal and/or kinetic energy from the combustion gases66is extracted via sequential stages of HP turbine stator vanes68that are coupled to the outer casing18and HP turbine rotor blades70that are coupled to the HP shaft or spool34, thus causing the HP shaft or spool34to rotate, thereby supporting operation of the HP compressor24. The combustion gases66are then routed into and expand through the LP turbine30where a second portion of thermal and kinetic energy is extracted from the combustion gases66via sequential stages of LP turbine stator vanes72that are coupled to the outer casing18and LP turbine rotor blades74that are coupled to the LP shaft or spool36, thus causing the LP shaft or spool36to rotate, thereby supporting operation of the LP compressor22and rotation of the fan38via the power gearbox46.

It should be appreciated, however, that the exemplary turbofan engine10depicted inFIG. 1is by way of example only, and that in other exemplary embodiments, the turbofan engine10may have any other suitable configuration. For example, in other exemplary embodiments, the fan38may be configured in any other suitable manner (e.g., as a fixed pitch fan) and further may be supported using any other suitable fan frame configuration. Moreover, it also should be appreciated that in other exemplary embodiments, any other suitable LP compressor22configuration may be utilized. It also should be appreciated, that in still other exemplary embodiments, aspects of the present disclosure may be incorporated into any other suitable gas turbine engine. For example, in other exemplary embodiments, aspects of the present disclosure may be incorporated into, e.g., a turboshaft engine, turboprop engine, turbocore engine, turbojet engine, etc.

FIG. 2depicts portions of the power gearbox46constructed according to an aspect of the present disclosure. For purposes of illustrating features of the planet bearing that rotatably supports each planet gear,FIG. 2illustrates a view rendered partly in perspective and partly in cross-section and focused on the planet bearing portions of a one quarter section of an exemplary embodiment of a planetary gearbox configuration that desirably serves as a component of the power gearbox46. The power gearbox46is an epicyclic type and has a central axis of rotation that is coincident with the longitudinal axis12shown inFIG. 1.

As schematically shown inFIG. 2for example, the power gearbox46includes a centrally-located sun gear80that is rotatable about the longitudinal axis12shown inFIG. 1. The bearing that rotationally supports the sun gear80has been omitted from the drawings, as the bearing for the sun gear80is not the focus of the present disclosure. The sun gear80desirably has a double-helical pattern of gear teeth81. A carrier surrounds the sun gear80, which is rotatable with respect to the carrier. The carrier carries at least one planet gear84and desirably an annular array of planet gears84. In the illustrated example there are four planet gears84but varying numbers of planet gears84may be used. As shown inFIGS. 5 and 7, each planet gear84desirably has a double helical pattern of gear teeth85that are configured to mesh with the gear teeth81of the sun gear80.

As schematically shown inFIGS. 2 and 7for example, the power gearbox46desirably is an epicyclic gearing arrangement having a stationary ring gear86that is disposed circumferentially around the sun gear80and the planet gears84. In one exemplary embodiment, the ring gear86that surrounds the sun gear80and the planet gears84is rendered stationary by being coupled to the outer casing18in a manner that is not illustrated in the drawings, as this particular arrangement can be performed in any of a number of conventional manners, any one of which being suitable for purposes of illustrating exemplary embodiments of the present disclosure. For example, the ring gear86can be fixed (as by being mechanically bolted or welded) to the outer casing18via a central circumferential flange88that is drilled with a plurality of axial holes89therethrough as shown inFIG. 2. In an alternative exemplary embodiment employing a star configuration of an epicyclic gearing arrangement, it is the carrier that is coupled to the outer casing18, and the specifics of this coupling also are not needed for the explanation of the desired aspects of the present invention. However, in both embodiments, and as schematically shown inFIG. 7for example, the ring gear86is rotatably enmeshed with each planet gear84, which also is rotatably enmeshed with the sun gear80, and thus the ring gear86also desirably has a double helical pattern of gear teeth87configured to mesh with the teeth85of the planet gear84.

Collectively the sun gear80, the planet gears84, and the ring gear86constitute a gear train. InFIG. 7for example, there are schematic representations of one complete planet gear84, portions of two other planet gears84and a portion of the sun gear80and the ring gear86. The dashed line with the smaller radius of curvature schematically represents the apexes of the teeth87of the ring gear86, while the dashed line with the larger radius of curvature schematically represents the troughs of the teeth87of the ring gear86. Similarly, the dashed line with the larger radius of curvature schematically represents the apexes of the teeth85of the planet gear84, while the dashed line with the smaller radius of curvature schematically represents the troughs of the teeth85of the planet gear84. InFIG. 7, the dashed line with the larger radius of curvature schematically represents the apexes of the teeth81of the sun gear80, while the dashed line with the smaller radius of curvature schematically represents the troughs of the teeth81of the sun gear80. Each of the planet gears84meshes with both the sun gear80and the ring gear86.

The sun gear80, planet gears84, and ring gear86may be made from steel alloys. One embodiment of the epicyclic gearing arrangement contemplated herein desirably is a planetary configuration that has only a single input and a single output, and the ring gear86is held stationary. In operation, the sun gear80is turned by an input that is the LP shaft, while the carrier that carries the planet gearboxes is coupled to a mechanical load that is the fan shaft45shown inFIG. 1. Thus, the power gearbox46is effective to reduce the rotational speed of the sun gear80in a known manner to a rotational speed appropriate for the load coupled to the carrier, namely, rotation of the fan shaft45.

Each of the planet gears84is rotatably carried by a bearing that in turn is carried by a planet gearbox that in turn is carried by the carrier. The construction and mounting of the bearing for one planet gear84will be described with the understanding that each of the planet gears84is constructed and mounted identically, though to different points on the carrier.

As schematically shown inFIGS. 2 and 4for example, the carrier includes a forward wall90and an aft wall92spaced axially apart from the forward wall90and together forming part of the carrier of each planet gearbox. Each of the forward wall90and the aft wall92respectively defines therethrough a respective coaxial bore91and93. The carrier desirably includes a plurality of sidewalls94that extend axially between and connect the forward and aft walls90,92of the carrier. Desirably, pairs of the sidewalls94are disposed on opposite sides of the coaxial bores91,93defined respectively in the forward and aft walls90,92of the carrier.

In one exemplary embodiment employing a planetary configuration of an epicyclic gearing arrangement, the carrier is non-rotatably coupled to the fan shaft45in a conventional manner so that they rotate in unison at the same speed, but the manner of this coupling is not critical to an understanding of the present disclosure and thus need not be further discussed. In an alternative embodiment employing a star configuration of an epicyclic gearing arrangement, it is the ring gear86that is non-rotatably coupled to the fan shaft45in a conventional manner so that they rotate in unison at the same speed, but the manner of this coupling is not critical to an understanding of the present disclosure and thus need not be further discussed.

As shown inFIGS. 2-4 and 6for example, a support pin96is hollow, generally cylindrical, and has forward and aft ends. The support pin96is provided to mount the bearing of the planet gear84to the carrier and thus is configured to be fixed to the carrier. As shown inFIG. 2for example, each opposite end of the support pin96is received in one of the bores91and93defined in the carrier. As shown inFIGS. 2 and 4for example, the forward end of the support pin96includes a threaded, reduced-diameter surface97, while the aft end includes an annular, radially-outwardly-extending flange98. A retainer99(in this example a threaded locknut) engages the reduced-diameter surface97at the forward end to secure the support pin96in position against rearward axial movement.

As shown inFIGS. 3 and 4for example, the support pin96defines a cylindrical outer surface101. As shown inFIG. 2, the cylindrical outer surface101of the support pin96is disposed radially equidistant from a central axis106that extends in an axial direction through the support pin96. This central axis106also defines the axis of rotation for the planet gear84.

The support pin96desirably includes a plurality of feed holes formed therein and extending radially therethrough, but as the number and placement of these feed holes is conventional as far as the present disclosure is concerned, none of them is shown in the drawings herein. In operation, oil is fed through the opening at the aft end of the support pin96and into the interior of the hollow support pin96from whence the oil flows through such feed holes to an inner race102, providing both cooling and lubrication.

As shown inFIG. 3for example, the planet bearing includes an inner race102that defines a cylindrical inner surface112that is non-rotatably connected to a cylindrical outer surface101of the support pin96. Desirably, the inner cylindrical surface112of the inner race102is press-fitted to the cylindrical outer surface101of the support pin96.

Desirably, the planet bearing is inner race guided, and accordingly the inner race102desirably is a single integral component having disposed opposite the inner surface112an outer surface that defines at least one roller track that defines a roller raceway. Each respective track is defined by a pair of guiderails108, which are spaced apart from each other in the axial direction and extend circumferentially around the inner race102and provide respective guiding surfaces to each respective roller cage118(described more fully below). As contemplated herein, the inner race102can include a single track or a plurality of tracks such as a dual track inner race102or a triple track inner race102, etc. However, explanation of the structure and operation of the planet gearbox herein will use the specific example of a dual track inner race102, thus informing how additional tracks would be accommodated or a single track would remain after the elimination of one of the dual tracks.

Accordingly, in a dual track embodiment, the outer surface of the inner race102incorporates two pairs of guiderails108, which extend continuously in the circumferential direction around the inner race102and define a pair of annular raceways, a forward raceway107and an aft raceway109, respectively, axially spaced apart from each other. The use of a single inner race102with dual raceways107,109spaced axially apart from each other provides for good concentricity between sets of rollers104, but two separate inner races102could be used as well. The axial dimension of the inner race102desirably is sized so that the inner race102cannot move axially relative to the opposing and axially spaced apart walls90,92of the carrier.

As shown inFIG. 4for example, each of the pair of tracks extends circumferentially around the outer surface of the inner race102. Each of the pair of tracks is separated in the axial direction from the other pair of tracks. Each of the pair of tracks is disposed parallel in the circumferential direction with respect to the other pair of tracks. Each of the pair of tracks defines a surface in the form of a raceway107or109that extends circumferentially and concentrically with respect to the cylindrical inner surface of the112of the inner race102.

Each of the pair of tracks in the inner race102is configured to receive and rotatably guide therein a respective plurality of cylindrical rollers104, which are free to rotate relative to both the inner race102and the outer race of the planet bearing. Thus, the raceways107,109of the inner race102receive rollers104, in two tandem rings. A first plurality of cylindrical rollers104is rotatably disposed on the forward raceway107within a first one of the pair of tracks of the inner race102. Similarly, a second plurality of cylindrical rollers104is rotatably disposed on the aft raceway109within a second one of the pair of tracks of the inner race102. Thus, the raceways107,109of the inner race102contact a portion of each of the cylindrical outer surfaces114of the cylindrical rollers104disposed in the respective track.

Leaving aside for the moment the usual rounded corners and crown radius at each opposite end thereof, as schematically shown inFIGS. 2, 5 and 6for example, in profile view each of the rollers104has a uniform cylindrical shape. As shown inFIG. 5for example, each cylindrical roller104defines a cylindrical outer surface114that is disposed with a central axis of rotation that extends in the axial direction of the roller104and extends in a direction that is parallel to the rotational axis106of the rotationally supported planet gear84. Desirably, at least a central section of the cylindrical outer surface114of each cylindrical roller104is disposed uniformly equidistant from the roller's central axis of rotation along a central section of the axial length of the cylindrical roller104. As schematically shown inFIG. 6for example, each opposite end of each roller104will have the usual rounded corners and a conventionally commensurate crown radius, both of which features serving to diminish the diameter of the cylindrical outer surface114of the roller104at those end locations. It is the central section of the cylindrical outer surface114of each roller104that is bounded at each opposite end by the rounded corners and crown radius, and it is this central section of the cylindrical outer surface114that is the surface that will come into contact with the respective raceway107,109of the inner race102during operation of the planet bearing.

As shown inFIG. 3andFIG. 4for example, the cylindrical outer surface114of each cylindrical roller104is defined by a diameter D that is taken at the midpoint of the roller104and extends through the central axis of rotation of the roller104along a direction that is normal to the central axis of rotation of the roller104. As shown inFIG. 6for example, the outer cylindrical surface114of each cylindrical roller104defines a length L in the direction parallel to the axis of rotation of the cylindrical roller104. The ratio of each cylindrical roller's length L to each cylindrical roller's diameter D is greater than one. Desirably, the ratio of each cylindrical roller's length to each cylindrical roller's diameter is greater than 1.3. For example, a roller having a diameter D of 29 mm desirably has a length L of 43 mm. Desirably, each cylindrical roller104has a length-to-diameter ratio (L / D) that falls within the range from 1.3 to 1.8, inclusive. For example, a roller having a diameter D of 1.25 inches desirably has a length L of 1.6875 inches. The cylindrical rollers104can comprise a ceramic material of a known composition, for example silicon nitride (Si.sub.3Ni.sub.4).

As shown inFIGS. 3 and 6for example, the outer race of the planet bearing is formed by the cylindrical interior surface103of the planet gear84. Thus, the outer race84of the planet bearing defines an outer cylindrical surface that defines a gearing surface85that is configured to mesh with both the gearing surface81of the sun gear80and the gearing surface87of the ring gear86. Desirably, as shown inFIG. 5for example, the gearing surface85of each cylindrical outer race84is patterned with a double helical gearing surface with the bias of each one of the two double helical gearing surfaces of the outer race84being disposed nonparallel with the other one of the two double helical gearing surfaces of the outer race84.

Because each of the gear meshes (sun-to-planet and planet-to-ring) has a double-helical gear tooth profile, there is no relative movement possible between the sun gear80and the planet gears84in a direction that is parallel to the axis A. Nor is there any movement in this direction between the planet gears84and the ring gear86. The double helical pattern restrains the planet gear84axially to both the sun gear80and the ring gear86, and the planet gears84are mounted to provide an axial degree of freedom to the carrier.

As shown inFIG. 3for example, a plurality of cylindrical rollers104is disposed between the inner race102and the cylindrical interior surface103of the planet gear84that serves as the outer race of the planet bearing. As shown inFIG. 4for example, the inner cylindrical surface103of the outer race84of the planet bearing rotatably contacts both the first plurality of cylindrical rollers104and the second plurality of cylindrical rollers104.

As shown inFIGS. 3-5for example, the planet gearbox desirably includes a pair of roller cages118disposed between the inner race102and the outer race84and free to rotate with respect to both, but at a different speed than the speed of rotation of the outer race84. In the embodiment shown inFIGS. 3 and 5for example, because the inner race102has side-by-side dual tracks, a separate roller cage118is provided over each of the dual tracks. As shown inFIG. 5for example, a first roller cage118defines a first circumferential row, and a second roller cage118defines a second circumferential row separated in the axial direction from the first circumferential row. The circumferential row of each roller cage118is disposed above a respective track of the pair of tracks of the inner race102. Each roller cage118is configured with circumferentially extending shoulder elements119and axially extending web elements120to maintain in each respective track with its respective raceway107,109of the inner race102, a respective separation in the circumferential direction between each respective cylindrical roller104in each pair of circumferentially adjacent cylindrical rollers104in that respective track.

Each circumferential row in each roller cage118defines a plurality of generally cylindrical openings. Each generally cylindrical opening of the roller cage118is defined by a major axis in the axial direction and a minor axis in the circumferential direction. As shown inFIG. 5for example, each generally cylindrical opening of the roller cage118is bounded by a pair of opposing and spaced apart web elements120that elongate in the axial direction and by a pair of opposing and spaced apart shoulder elements119that elongate in the circumferential direction. The major axis of each generally cylindrical opening is configured to accommodate the length L of an individual roller104, while the minor axis of each generally cylindrical opening is configured to accommodate the diameter D of an individual roller104. As shown inFIGS. 3 and 5for example, the openings in each row are spaced equidistantly apart circumferentially around the roller cage118with the number of openings in each row being equal to the number of cylindrical rollers104disposed in the respective one of the pair of tracks disposed beneath the respective row of the roller cage118. Accordingly, as shown inFIG. 5for example, each respective cylindrical roller104is disposed with its cylindrical outer surface114extending through a respective opening defined by the roller cage118.

Desirably, each roller cage118can be provided in the form of a circumferentially split cage, which is achieved by having one of the webs120split in half along an axial cut. As schematically shown inFIG. 5for example, a split web130is split in half axially along its axial centerline with a small cut that leaves a very slight gap between the opposing edges that are formed in the split web130as a result of the cut that axially extends completely through the split web130. The circumferentially split roller cage118so provided, serves to reduce hoop stress in the cage118.

As shown inFIG. 4for example, each respective shoulder element119of each roller cage118is disposed above a respective guiderail108of the inner race102with a small clearance between the two respective opposing surfaces of the shoulder element119and the guiderail108. Because the planet bearing is inner-race-guided, the roller cage118is designed with a small clearance between the cylindrically-shaped, circumferential inner surface defined by the shoulder elements119of the cage118and the cylindrically-shaped, circumferential outer surfaces of the guiderails108of the inner race102, and this small clearance desirably is on the order of 0.005 to 0.050 inches inclusive.

As shown in circumferential cross-section inFIGS. 3 and 6and in axial cross-section inFIG. 4for example, respective web elements120of each roller cage118are disposed to extend axially between the opposing shoulder elements119of the roller cage118. Each of these web elements120defines a web120of the roller cage118that is shown in cross-section inFIG. 6, which for the sake of avoiding undue complexity in the drawing, does not have any cross-hatching that normally would be found in a cross-sectional view of a metal component such as the web120of the roller cage118or the cylindrical rollers104. All of the webs120of both roller cages118are identically configured and dimensioned.

As shown inFIG. 6, the transverse, cross-sectional outline of each web120resembles a trapezoid with the non-parallel side legs of the trapezoid outlining the sides of the web120. Each opposite side of each web120will contact the outer cylindrical surface114of an adjacent cylindrical roller104at a particular point on the side leg of the trapezoid, and two times the distance from the centerline106(FIG. 5) of the bearing to this point where the cylindrical roller104contacts the web120is called the “roller contact diameter” of the web120.

Also shown inFIG. 6, is a dashed line that designates what is known as the “pitch circle” of the planet bearing. The long curved dashed line inFIG. 6is a virtual line that is drawn through the central axis of rotation of each of the cylindrical rollers104and would form a complete circle if all of the rollers were shown inFIG. 6as the rollers104are schematically shown inFIG. 7. The diameter of the “pitch circle” of the bearing is the bearing's “pitch diameter”. The thickness of the web120of interest herein is the length of the portion of the pitch circle that lies between the two non-parallel side legs of the web120in the view shown inFIG. 6. This web thickness of interest herein can be calculated by measuring the angular ratio of the pitch circle that lies between the two non-parallel legs of the web, and multiplying it by the circumference of the pitch circle of the bearing.

Each of the openings defined by the roller cage118is spaced equidistantly apart circumferentially around the cage by a plurality of equidistantly spaced apart webs120, and each web120extends in the axial direction and defines a cross-web thickness in the circumferential direction. That cross-web thickness of each web120is in fact the ratio of the “web thickness” of the web120measured as described above divided by the roller diameter. In accordance with an aspect of the present invention, each web120of the respective roller cage118has a cross-web thickness that desirably measures 15% to 25% of the diameter of one of the cylindrical rollers104. In another embodiment, each web120of the respective roller cage118has a cross-web thickness that desirably measures 15% to 20% of the diameter of one of the cylindrical rollers104.

In some sense, these reductions in the cross-web thickness of each web120are rendered feasible due to the shape of the roller cage118. This desirable shape of the roller cage118in turn is made possible due to the relatively high ratio of the diameter of the outer race84to the diameter of the inner race102. The relatively high ratio of the diameter of the outer race84(measured at the inner surface103thereof) to the diameter of the inner race102(measured at the surface defining the raceway107,109thereof) results from a large diameter of the rollers104compared to the pitch diameter of the bearing. Because of this relatively high ratio between the roller's diameter and the bearing's pitch diameter, sufficient space exists between the rollers104radially outside the pitch circle (FIG. 6) to allow the thickness of the web120to increase markedly above the pitch circle, and this circumferential thickening of the web120radially outbound of the planet bearing's pitch circle in turn affords a reduction in the circumferential spacing between rollers104at the pitch line of the web120. Thus, in an exemplary embodiment in which the cross-web thickness measures 15% to 25% of the diameter of one of the cylindrical rollers104, this ratio would be on the order of 0.200 to 0.600 inclusive.

The planet gearbox with its planet bearing apparatus described herein has several advantages over the prior art. Briefly, the planet bearing apparatus described herein reduces the diameter of the planet gearbox that is required to transfer a given amount of power. The benefit in terms of cage thickness comes from having the cage outside of the pitch circle of the bearing, and with a large ratio of roller diameter to bearing pitch diameter (and therefore also a high ratio of outer race diameter to inner race diameter). The smaller that this web thickness of interest herein can be made, the more rollers of the same size can be fit inside a bearing with a fixed outer race diameter, which is the diameter that connects the center of the bearing and two points separated by 180 degrees on the cylindrical interior surface103of the planet gear84. For a fixed outer race diameter, the more rollers of the same size that are fit in the bearing, the greater the load carrying capacity of the bearing. Similarly, by being able to achieve a greater load carrying capacity for the same outer race diameter, it is possible to achieve a smaller bearing with an equivalent bearing load capacity by the same method of reducing the web thickness within the bearing. The smaller the diameter of the planet bearing, then the smaller the diameter of the planet gear84with that smaller diameter planet bearing. The smaller the diameter of the planet gear84, the more room becomes available for other components of the engine. Additionally, the smaller the diameter of the planet gear84translates into a greater reduction in the size and weight of the overall engine10.

For the embodiment depicted, the planet roller bearing may be formed of any suitable material. For example, in at least certain exemplary embodiments, the roller bearing may be formed of a suitable metal material, such as a chrome steel or a high carbon chrome steel. Alternatively, however in other exemplary embodiments the planet roller bearing may include one or more components formed of a suitable ceramic material.

The use of ceramic cylindrical rollers104allows the planet gears84to have a degree of freedom in the axial direction, simplifying the design. The ceramic rollers104are anticipated to provide at least a doubling in life compared to steel rollers, allowing the gearbox46to meet reliability targets. The ceramic rollers104also 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.

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.