GB2544036A

GB2544036A - A planet carrier assembly for an epicyclic gearbox

Info

Publication number: GB2544036A
Application number: GB1517268.7A
Authority: GB
Inventors: Daniel Venter Gideon
Original assignee: Rolls Royce Deutschland Ltd and Co KG
Current assignee: Rolls Royce Deutschland Ltd and Co KG
Priority date: 2015-09-30
Filing date: 2015-09-30
Publication date: 2017-05-10
Also published as: GB201517268D0

Abstract

A planet carrier for an epicyclic gearbox comprising a planet gear cage having first and second separate end plates 422, 424 axially displaced along the principal axis, the first and second end plates providing a mechanical connection between a plurality of planet gears 416 when part of an assembled gear box. The first and second end plates have a plurality of radially outer walls extending therefrom towards the radially outer walls of the other of the first and second end plate, the plurality of radially outer walls being circumferentially separated around the perimeter of the respective first and second end plates to provide planet gear openings through which the planet gears are exposed for engagement with the ring gear when part of an assembled gear box. An attachment member includes a support ring having a plurality of arms 430 extending axially therefrom, the arms including an attachment portion 434 which is clamped between the radially outer walls of the first and second end plates. The planet carrier may find use in gas turbine engines (fig. 1).

Description

A Planet Carrier Assembly for an Epicyclic Gearbox Technical Field of Invention

This invention relates to a planet carrier assembly for an epicyclic gearbox. The planet carrier assembly and associated gearbox are preferentially used in high power gearboxes such as those used in gas turbine engines.

Background of Invention

Figure 1 shows a gas turbine engine in the form of a geared turbo fan engine. The engine includes a gas turbine engine generally indicated at 10, having a principal and rotational axis 11. The engine 10 comprises, in axial flow series, an air intake 12, a propulsive fan 13, a gear train 14, a low pressure compressor 15, a high-pressure compressor 16, a combustor 17, a high-pressure turbine 18, a low-pressure turbine 19 and an exhaust nozzle 20. A nacelle 21 generally surrounds the engine 10 and defines the intake 12.

The gas turbine engine 10 works in the conventional manner so that air entering the intake 12 is accelerated by the fan 13 to produce two air flows: a first air flow into the compressor and a second air flow which passes through a bypass duct 22 to provide propulsive thrust. The low- and high-pressure compressors 15,16 compress the air flow directed into it for combustion in the combustor 17.

The resultant hot combustion products expand through, and drive the high and low-pressure turbines 16,17 before being exhausted through the nozzle 18 to provide additional propulsive thrust. The high 16 and low 17 pressure turbines respectively drive the high pressure compressor 14 and the fan 13 via suitable shafting arrangements.

The fan 13 is driveably connected to a low pressure shaft via the gear train located driveably between the low pressure shaft and the fan 13. The gear train is a reduction gear train in that it is arranged to reduce the speed of the fan 13 relative to the speed of the low pressure turbine 18 and low pressure compressor 15. Such an arrangement allows for a higher speed and more efficient low pressure turbine, and a slow spinning larger fan which can provide a higher bypass ratio. This freedom allows the speed of the fan and low pressure turbine to be independently optimised, but at a potential weight penalty of resulting from the gear train.

The gear train may be an epicyclic gearbox arranged in a planetary or star configuration. As shown in Figure 1, the gear box is connected as a planetary gear box which provides a favourable gear ratio for large civil gas turbine engines. Smaller engines would more likely have a star configuration. It will be appreciated that some applications include differential or compound arrangements.

Other gas turbine engines to which the present disclosure may be applied may have alternative configurations. By way of example such engines may have an alternative number of interconnecting shafts (e.g. three) and/or an alternative number of compressors and/or turbines.

Irrespective of the style of epicyclic gearbox the torque of the respective components needs to be reacted either to the input or output drive or the static structure respectively. The objective is to transfer the torque through the centre of the respective components in order to avoid torsional deflection that affects the gear mesh alignment between the respective gear meshes.

Centralised torque reaction can easily be achieved for the sun and ring gear but to achieve this with the planet carrier is more of a challenge. For normal industrial applications where weight does not play a role the carrier can be stiffened by adding more material allowing the torque to be applied to or extracted from one side of the carrier. For aerospace applications, lightweight carriers are developed where load is extracted/applied on the centre line of the carrier by mounting of a flexible joint (Spherical bearing) either in the centre of the carrier or the planet gears. The flexible joint is integrated into flexible pins that protrudes from a side plate. The torsional deflection of flexible pins is isolated from the planet gears through the flexible joint.

If no spherical bearing attachments are used and the ring gear and/or carrier torque is reacted from the one end then there will be torsional deformation that will result in edge loading of the gear teeth causing excessive stresses that can lead to gear teeth and gear failures.

Any torque transfer drive attachment that is not positioned on the gearbox axial centre line will result in various degrees of torsional deflection that in turn will impact gear mesh and bearing alignment. To compensate for the effects of misalignment and to avoid localised overload effects, special gear flank profile correction and enlargement of gears will be required. Special gear flank profile correction is normally applied for a specific load case resulting in less optimal meshing conditions/flank loading at other load settings. That in turn require larger/wider gears to cope the specific loading conditions.

As far as the second alternative is concerned where the fan shaft is attached to the carrier axial centre line with spherical bearings which concentrate the stress on a localised point and is subject to wear. The attachment of the carrier support structure drive arms to the carrier is limited by the size of the planets. The size of the planets determines the available space between the planets that is required to structurally support the drive arms. The higher the reduction ratio, the larger the planet gears and the less space available to attach the drive arms and to react the required torque.

The use of spherical bearings to isolate carrier deflection will result in the wear and possible malfunction of the spherical bearing due to the relative movement and high vibratory loads.

Statements of Invention

The present invention provides a planet carrier and gearbox according to the appended claims.

The planet carrier for an epicyclic gearbox may have a sun gear, a ring gear and a plurality of planet gears located therebetween, the planet gears being connected by the planet carrier so as to be collectively rotatable about a principal axis, the planet carrier comprising: a planet gear cage having first and second separate end plates axially displaced along the principal axis, the first and second end plates providing a mechanical connection between a plurality of planet gears when part of an assembled gear box, each of the first and second end plates having a plurality of radially outer walls extending therefrom towards the radially outer walls of the other of the first and second end plate, the plurality of radially outer walls being circumferentially separated around the perimeter of the respective first and second end plates to provide planet gear openings through which the planet gears are exposed for engagement with the ring gear when part of an assembled gear box, and an attachment member including a support ring having a plurality of arms extending axially therefrom, the arms including an attachment portion which is clamped between the radially outer walls of the first and second end plates.

The epicyclic gearbox may be arranged in a star or planet configuration. Hence, the support ring may be configured to be attached to a static structure, or a rotating drive shaft. The radially outer walls of the first and second end plates may be located at the radial extreme of the respective end plate. The number of radially outer walls may be the same as the number of planet gears.

Each attachment portion may include a flange which is clamped between the radially outer walls of the first and second end plates, wherein the flange extends radially inwards with respect to the principal axis of rotation.

The radially extending flange may be predominantly radially extending. The radially extending flange may be in the normal plane of the principal axis of gearbox. Each of the radially extending flanges may be coplanar. The axial length of the radially outer walls may be axially coterminous. The axial length of the arms may coterminous.

The arms may have circumferential length in relation to the principal axis.

The circumferential length of the arms may correspond to the circumferential length of the radially outer walls. The circumferential edges of the radially outer walls and attachment portion walls which face the planet gear space may be curved. The radius of curvature may be concentric with the planet gear axis such that the circumferential walls are uniformly spaced from the planet gears in an assembled gear box.

The arms may be curved in the circumferential length, the radius of the curvature being concentric with the principal axis.

The radius of curvature may be the same for each of the arms. The arms may define the radial extreme of the portion of the planet carrier which is received within the ring gear when part of an assembled gear box. The arms may collectively provide a circumferentially segmented cylinder. The axial extent of the cylinder may be half the axial displacement of the first and second end plates.

Either or both of the radially outer walls of the first and second end plate may be rebated to receive the attachment portion of the arms.

The rebate may extend across the full circumferential extent of the radially outer walls. The radially outer walls may terminate in a flange. The flange may extend radially inwards. The flange may be normal to the principal axis of the gear box. The rebate may be partially located within an end facing surface of the flange of the radially outer walls. The rebate may not extend the full radial length of the flange.

Each of the first and second end plates may include radially inner walls which extend towards the other of the first and second end plate.

The corresponding circumferential ends of the radially outer walls and radially inner walls may be joined by radially extending webs which define a housing space in which the planet gears are located in an assembled gear box.

The support ring may be foremost or aftmost of both the first and second end plates.

The support ring may be full annular ring. The support ring may provide the axial extreme of the planet carrier. The support ring may be axially outside of the ring gear when in an assembled gear box. The support ring may include one or more features for attaching the support ring to an extraneous body. The extraneous body may be a drive arm or a grounding structure. The features may include one or more bolt receiving holes.

The first and second end plates may each have a plurality of bores which receive the planet gear spindles when part of a gear box. The planet gear spindles may be any suitable type known in the prior art.

The outer radial walls of the first end wall may have different axial lengths to the outer radial walls of the second end wall so that the attachment portion is clamped towards either of the first or second end wall.

Providing different axial lengths in the radially outer end plates moves the split line between the planet carrier cage halves from an axial mid portion towards one or other of the first and second end plates. The split may be axially towards the support ring.

An epicyclic gearbox may comprise: a plurality of planet gears connected by a planet carrier so as to be collectively rotatable about a principal axis of rotation of the gearbox; a sun gear; and, a ring gear, wherein the planet carrier includes a planet gear cage having first and second separate end plates axially displaced along the principal axis, the first and second end plates providing a mechanical connection between a plurality of planet gears when part of an assembled gear box, each of the first and second end plates having a plurality of radially outer walls extending therefrom towards the radially outer walls of the other of the first and second end plate, the plurality of outer walls being circumferentially separated around the perimeter of the respective first and second end plates to provide planet gear openings through which the planet gears are exposed for engagement with the ring gear when part of an assembled gear box, and an attachment member including a support ring having a plurality of arms extending axially therefrom, the arms including an attachment portion which is clamped between the radially outer walls of the first and second end plates.

The radially outer walls may extend from the first and second end plates are located radially within ring gear.

Each of the first and second end plates may include radially inner walls which extend towards the other of the first and second end plate, the radially inner walls being radially outwards of the sun gear.

The inner walls may be curved and the radius of curvature may be concentric with the principal axis.

The planet gears may be located within planet gear housings within the planet gear cage, the planet gear housings having walls radially outside of the planet gears with respect to the axis of rotation of the planet gears, wherein walls are curved and have a radius of curvature centred on the axis of rotation of the planet gear. A gas turbine engine may comprise the epicyclic gearbox in which the support ring may be attached to a drive arm. The drive arm may drive the fan of the gas turbine engine.

The support ring may be attached to a support structure.

Within the scope of this application it is expressly envisaged that the various aspects, embodiments, examples and alternatives, and in particular the individual features thereof, set out in the preceding paragraphs, in the claims and/or in the following description and drawings, may be taken independently or in any combination. For example features described in connection with one embodiment are applicable to all embodiments, unless such features are incompatible.

Description of Drawings

Embodiments of the invention will now be described with the aid of the following drawings of which:

Figure 1 shows a longitudinal section of a geared turbo fan engine.

Figure 2 shows a partial longitudinal section of a geared turbo fan and details a gearbox arranged in a star configuration.

Figure 3 shows a partial longitudinal section of a geared turbo fan and details a gearbox arranged in a planet configuration.

Figures 4a to 4e show an attachment member of planet carrier.

Figure 5a to 5e show one of a pair of end plates used to construct a planet carrier.

Figure 6 shows a longitudinal section of a radially outer portion of the planet carrier.

Figure 7 shows a longitudinal section of a radially outer portion of an alternative planet carrier.

Figure 8 shows a side view of an assembled planet carrier in which an attachment portion shown.

Detailed Description of Invention

Figure 2 shows a reduction gearbox 200 for a geared gas turbine engine. The gearbox 200 is located in the engine core downstream of the fan 212 and upstream of the low pressure compressor. The gearbox of Figure 2 is an epicyclic reduction gear train arranged in a star configuration. Thus, there is an inner sun gear 214, a circular array of planet gears 216 held by a carrier 218 and an outer ring 220 or star gear. The sun, carrier and star gear are concentrically nested and rotatable about the principal rotational axis 222 of the engine which is also the principal axis of the gear box.

The gearbox 200 is driven by the low pressure turbine via the low pressure shaft 224. The low pressure shaft is driveably engaged with the sun gear 214. The planet gear carrier 218 is attached to the engine casing 226 via an arm of the planet carrier 218. A suitable carrier is described in more detail in relation to Figures 4 to 7.

The drive for the fan 212 is taken from star (ring) gear 220 via a drive arm 228 which attaches to the fan shaft 230 via a coupling which is arranged to mechanically isolate the gearbox 200 from the fan shaft 230 to help reduce radial loading on the gearbox 200.

Bearings are provided for supporting the fan shaft. The bearings are axially separated along the fan shaft and include an aft thrust bearing 234 and a fore roller bearing 232. The outer bearing races are supported by a static structure in the form of a support cone which extends from the engine casing local to the compressor inlet. A further roller bearing 236 is provided downstream of the gearbox 200 for rotatably supporting the low pressure shaft 224. It will be appreciated that there will be at least one further low pressure shaft bearing, a thrust bearing, downstream of the partial engine section of Figure 2. It will also be appreciated that the roller bearing and thrust bearing may be interchanged where the engine architecture allows.

Figure 3 shows a similar arrangement that of Figure 2. However, in the arrangement of Figure 3, the reduction gearbox 300 is arranged in a planet configuration. Hence, the gearbox 300 includes a sun gear 314, a circular array of planet gears 316 held by a carrier 318 and an outer ring gear 320. The sun, carrier and ring gears are concentrically nested and rotatable about the principal axis 322 of the engine. However, here the ring gear is attached to a grounded or static structure via a static attachment in the form of an annular plate which extends from the ring gear to the engine casing 326. As shown, the static attachment 328 and support cone attach to the engine casing at a common location.

The fan 312 is driven by the planet carrier 318 via a coupling which transfers torque between the planet carrier 318 and fan shaft whilst helping to isolate radial loads from the fan shaft 330.

It will be appreciated that the shafting, support and bearing arrangements of Figure 3 will be the same as Figure 2.

Figures 4a-d and Figures 5a-d show the constituent parts of a planet carrier which may be used in the gear boxes of Figures 2 and 3. Figures 4e and 5e show the constituent components of the planet carrier with the ring 420, planet 416 and sun gears 414 to demonstrate the spaced relations therebetween.

The assembled carrier 418 includes a planet gear cage which has two separate end plates 422, 424 and an attachment member 426 as shown in the Figure 8. The attachment member 426 provides a drive arm when used in a planetary arrangement, or a grounding member when used in a star arrangement.

The attachment member 426 includes a support ring 428 and a plurality of arms 430. The arms 430 extend from a first end which is attached to the support ring 426. The support ring 428 serves to connect the respective attachment arms 430 to provide a single structure which can be mounted to a drive arm or static structure as required. Thus, the support ring 428 includes a plurality of fixture features in the form of bolt receiving holes 432 which are circumferentially distributed around the ring 428. The second ends of the arms 430 each include attachment portions 434 which provide a fixture to which the end plates 422, 424 are mounted.

In the example shown in Figures 4a-d, the support ring 428 is in the form of an annular plate which lies normal to the principal axis of the gearbox 437. The arms 430 extend from the radially inner edge of the support ring 428 and are evenly distributed around the circumference with a separating gap 436 therebetween. The separation of the arms provides openings or windows which expose the outer edge of planet gears 416 for driving engagement with the ring gear 420 when provided in an assembled gearbox.

The arms 430 are circumferentially curved with a radius which is concentric with the principal axis and ring gear 420. The attachment member can be thought of as being in the form a segmented cylindrical body in which a plurality of similarly sized arms 430 have separating gaps therebetween. The outer surface of the arms 430 provide the radially outer surface of the carrier 418 which is located within and proximate to the ring gear 420 in an assembled gear box. The outer surface of the arms are curved with a single radius so as to provide a uniform spacing radially inwards of the ring gear 420.

The attachment portions 434 are provided in the form of flanges which extend from the second end of the attachment arms 430 and in the normal plane of the principal axis 437. The attachment portions 434 are thus arcuate flanges which extend radially inwards in relation to the principal axis of the gearbox. Each flange may include a plurality of holes, five in the example shown, which are used to receive fixation devices in the form of bolts or dowels. It may be possible to provide flanges which are extend generally radially inwards but which are not in the normal plane.

The fixation devices which hold the end plates together may include sleeved bolts as shown in Figure 6 and described below, or any other suitable type of mechanical fixture.

The circumferential edges 438 of the arms 430 and attachment portion 434 which define the separating gap 436 between the walls are curved when viewed from an axial end. The radius of curvature is concentric with the axis of the planet gears and provides for a uniform separation between the planet gears and arms and plates.

The end plates of the carrier 418 are shown in Figures 5a-d. Only one carrier end plate 424 is shown but it will be appreciated the planet carrier 418 is made up from two similar structures which are presented to one another in opposing abutment. The two halves of the planet gear cage may have different axial lengths such the axial split line is shifted to one side or the other of the axial midline of the carrier 418 as shown in Figure 7.

The carrier cage includes an end plate 424 and a plurality of axially extending walls 440. In the described example as shown in the Figures, there is a plurality of radially outer end plates 442, a plurality of radially inner end plates 444, and interconnecting webs 446 which extend between corresponding radially and inner end plates. The walls collectively form housing spaces for the planet gears and the sun gear. It will be appreciated that some of these walls may be optional with only the outer radial wall being required for the three part carrier 418 to be assembled.

The end plate 424 is in the form of a circular plate or disc. The plate is generally planar and resides in the plane normal to the principal axis of the engine and has axial thickness. The plate includes several through-holes or apertures 448 which receive the mountings for the planet gears 416 and low pressure shaft which is driveably connected to the sun gear 414. The mountings may be any suitable mountings known in the art. For example, the mountings may be spindles which include bearings about which the planet gears can rotate in use.

The central bore is concentric with the plate and principal axis of the engine. The five planet gear bores are evenly distributed around the end plates and centre on a common radius from the principal axis of the engine.

The radially outer walls 442 extend in an axial direction from the radially outer edge of the end plate. The walls 442 are arcuate having a circumferential length which extends around the circumference of the end wall outer radius. The radius of the radially outer surface of the wall 442 is the same as the radius of the end plate, although this may not be the case in some examples. Axially extending through-holes 450 are provided along the circumferential length of the attachment wall 442. The through-holes 450 receive fixtures in the forms of bolts when the carrier 418 is assembled. As can be seen from Figures 6 and 7, the through-holes may terminate with recesses which receive bolt heads in use.

The radially inner walls 444 of the planet gear housing are arcuate, each having common radius such that the walls collectively provide a segmented annular housing in which the sun gear resides in use. The separation between each of the planet gear housings radially inner walls provides openings which expose the planet gears 416 to the sun gear for driveable engagement. The curved webs extend between the circumferential ends of the radially inner 444 and outer walls 442 and define the housing space for the planet gears 416. Thus the curvature of the curved webs 446 is uniform and centred coaxially with the rotational axis with the planet gears.

The distal end of the planet housing walls 442, 444, 446 provide an abutment surface for contacting the opposing end cap of the carrier 418. In the example shown in the Figures, the planet carrier housings terminate in a common plane which is axially removed from the plane of the end plate 426.

The radially outer walls provide a clamp into which the attachment portion is received and retained. To provide a stronger joint, the clamping zone of the radially outer walls include a rebated portion 452. The rebated portion 452 is provided in the facing or abutment surface of the radially outer walls 442. The rebate is of uniform axial depth and extends through the full circumferential length of the arcuate wall 442. The rebate extends radially inwards from the extreme outer surface of the wall 442 and terminates in a shoulder. The shoulder is circumferentially curved to match the outer surface of the wall 442. The rebate 452 is provided to receive the attachment flanges of the attachment member. The depth of the rebate of the example shown is half of the attachment portion flange thickness, but this may not be the case and the rebate may be axially deeper in one or other of the end plate walls. There may be some examples in which the rebate is only provided in one or the other of first and second end plates. When assembled, the radially inner edge of the attachment portion flange abuts the shoulder and increases the rigidity of the assembled carrier.

As can be seen from Figure 4e, when assembled, the ring gear 420 is located radially outside of the carrier 418 and more specifically outside of the carrier arms 430. The ring gear 420 interfaces with the planet gears 416 which have a chordal exposure between the arms 430 of the carrier attachment member. Thus, the radius of the radially outer surface of the arms 430 is less than the ring gear 420 but greater than the rotational axes or centres of the planet gears 416 so as to provide a clearance space between the ring gear 420 and arms 430. The clearance space is preserved in use by the engagement of the planet gears and ring gear. The radius of the circumferential edge surfaces 438 of the arms and attachment portion flanges are larger than the radii of the planet gear. There are a corresponding number of planet gears and arms.

Figure 5e shows one of the end plates with the planet gears 416, ring gear 420 and sun gear 414 in place. Thus, the ring gear 420 is radially outside of the radially outer walls 442 of the end plate 422 and separated at least by the thickness of the attachment arms such that they can be located therebetween. The web portions 446 of the planet housing walls have a radius of curvature greater than that of the planet gears 416 such that the planet gears 416 are free to rotate therein. The radius of the radially inner walls is greater than the sun gear to allow the sun gear to rotate therein. It will be appreciated, the planet gears, ring and sun gears all include teeth to allow for respective gears to be driveably engaged with one another.

Figure 6 shows a partial longitudinal section of the assembled planet carrier 418. Thus there is shown the carrier 418, attachment member 426 having a support ring 428, a first end plate 422 and a second end plate 424. The attachment portion is in the form of a flange which projects radially inwards normal to the principal axis. A fixture in the form of a bolt 454 is shown in section. The fixture passes through the first end plate 422, the attachment portion 434 and the second end plate 424 and is tightened to provide a clamping force on the attachment portion 434.

The bolt 454 is received within a sleeve 456 which has an interference fit within the carrier main components. Thus, there is an interference fit with each of the end plates and attachment portion holes. The interference fit of the sleeve 456 ensures that any torsional load is carried by the sleeve 456 and not by the bolts. Thus, the bolts merely provide the clamping force to keep the parts together.

It will also be noted from Figure 6 that the support ring 428 is placed axially beyond the first (or second) end plate and includes a plurality of holes 432 for fixing the support ring 428 to the drive or static structure.

Thus, the attachment member is radially outside of the end plates with at least one of the end plates being nested within the arms of the attachment member.

Figure 7 shows another example of carrier 718 in which the attachment portion 734 of the carrier is not centrally mounted between the two planet cages. This is achieved by having the planet cage halves at different lengths as previously described.

To manufacture the planet carrier 418, the clamping holes in the outer rim of the end plates and torque crown are match drilled and marked as a set to ensure that they are always fitted together in the correct angular orientation. Hence, items 422, 424 and 428 are clamped together and machined as an assembly. Then the angular orientation of the parts as it was machined are then marked to ensure correct alignment during assembly. This will ensure the best load share between the respective bolts that clamps the 3 parts together.

It will be understood that the invention is not limited to the described examples and embodiments and various modifications and improvements can be made without departing from the concepts described herein and the scope of the claims. Except where mutually exclusive, any of the features may be employed separately or in combination with any other features and the disclosure extends to and includes all combinations and sub-combinations of one or more described features.

Claims

Claims:

1. A planet carrier for an epicyclic gearbox having a sun gear, a ring gear and a plurality of planet gears located therebetween, the planet gears being connected by the planet carrier so as to be collectively rotatable about a principal axis, the planet carrier comprising: a planet gear cage having first and second separate end plates axially displaced along the principal axis, the first and second end plates providing a mechanical connection between a plurality of planet gears when part of an assembled gear box, each of the first and second end plates having a plurality of radially outer walls extending therefrom towards the radially outer walls of the other of the first and second end plate, the plurality of radially outer walls being circumferentially separated around the perimeter of the respective first and second end plates to provide planet gear openings through which the planet gears are exposed for engagement with the ring gear when part of an assembled gear box, and an attachment member including a support ring having a plurality of arms extending axially therefrom, the arms including an attachment portion which is clamped between the radially outer walls of the first and second end plates.

2. A planet carrier as claimed in claim 1, wherein each attachment portion includes a flange which is clamped between the radially outer walls of the first and second end plates, wherein the flange extends radially inwards with respect to the principal axis of rotation.

3. A planet carrier as claimed in claims 1 or 2 wherein the arms have circumferential length in relation to the principal axis.

4. A planet carrier as claimed in claim 3, wherein the arms are curved in the circumferential length, the radius of the curvature being concentric with the principal axis.

5. A planet carrier as claimed in any preceding claim, wherein either or both of the radially outer walls of the first and second end plate are rebated to receive the attachment portion of the arms.

6. A planet carrier as claimed in any preceding claim, wherein each of the first and second end plates include radially inner walls which extend towards the other of the first and second end plate.

7. A planet carrier as claimed in claim 6 when dependent on claim 3, wherein the corresponding circumferential ends of the radially outer walls and radially inner walls are joined by radially extending webs which define a housing space in which the planet gears are located in an assembled gear box.

8. A planet carrier as claimed in any preceding claim, wherein the support ring is foremost or aftmost of both the first and second end plates.

9. A planet carrier as claimed in any preceding claim wherein the first and second end plates each have a plurality of bores which receive the planet gear spindles when part of a gear box.

10. A planet carrier as claimed in any preceding claim, wherein outer radial walls of the first end wall have different axial lengths to the outer radial walls of the second end wall so that the attachment portion is clamped towards either of the first or second end wall.

11. An epicyclic gearbox, comprising: a plurality of planet gears connected by a planet carrier so as to be collectively rotatable about a principal axis of rotation of the gearbox; a sun gear; and, a ring gear, wherein the planet carrier includes a planet gear cage having first and second separate end plates axially displaced along the principal axis, the first and second end plates providing a mechanical connection between a plurality of planet gears when part of an assembled gear box, each of the first and second end plates having a plurality of radially outer walls extending therefrom towards the radially outer walls of the other of the first and second end plate, the plurality of outer walls being circumferentially separated around the perimeter of the respective first and second end plates to provide planet gear openings through which the planet gears are exposed for engagement with the ring gear when part of an assembled gear box, and an attachment member including a support ring having a plurality of arms extending axially therefrom, the arms including an attachment portion which is clamped between the radially outer walls of the first and second end plates.

12. An epicyclic gearbox as claimed in claim 11, wherein the radially outer walls which extend from the first and second end plates are located radially within ring gear.

13. An epicyclic gearbox as claimed in claim 12, wherein each of the first and second end plates include radially inner walls which extend towards the other of the first and second end plate, the radially inner walls being radially outwards of the sun gear.

14. An epicyclic gearbox as claimed in claim 13, wherein the inner walls are curved and the radius of curvature is concentric with the principal axis.

15. An epicyclic gearbox as claimed in any of claims 11 to 14, wherein the planet gears are located within planet gear housings within the planet gear cage, the planet gear housings having walls radially outside of the planet gears with respect to the axis of rotation of the planet gears, wherein walls are curved and have a radius of curvature centred on the axis of rotation of the planet gear.

16. A gas turbine engine comprising the epicyclic gearbox of any of claims 11 to 15, wherein the support ring is attached to a drive arm, the drive arm driving the fan of the gas turbine engine.

17. A gas turbine engine comprising the epicyclic gearbox of any of claims 11 to 15, wherein the support ring is attached to a support structure.