GB2610212A - Double-helical device, method for manufacturing a double-helical device and a holding jig - Google Patents

Abstract

A double-helical gear device comprising a left-hand helical gear element 101 and a right-hand helical gear element 102. The two helical gear elements are connected coaxially through a connection element 110 positioned radially inwards or outwards from teethed surfaces 106 of the helical gear elements. A method for manufacturing a double-helical gear device, and a holding jig 120 to be used for the manufacturing of the double-helical gear device, are also disclosed.

Description

DOUBLE-HELICAL DEVICE, METHOD FOR MANUFACTURING A DOUBLE-HELICAL DEVICE AND A HOLDING JIG

Field

The invention relates to a double-helical device with the features of claim 1 and a method manufacturing a double-helical device with the features of claim 18 and a holding jig configured for the method with the features of claim 26.

Background

Double-helical devices are e.g. used in gearboxes, in particular planetary gearboxes in fanned turbo engines for aircrafts. Thrust loads on gears therein are canceled by the double-helical arrangement so that only symmetric radial loads are imposed on the associated bearings.

Double-helical gear devices are helical gears with both a left-and a right-hand helix. As the manufacturing of such devices is complex, the helices are generally present on the circumference of helical gear elements (i.e. a left-hand helical gear element and a right-hand helical gear element). The two helical gear elements can be manufactured onto a common component, but in this case a helix gap is normally required between the two helices. Alternatively, the helical gear elements can be joined after manufacturing the two elements separately. A special case of double helical gears is when there is no gap between the two helical gear elements, the double-helical device is then termed a herringbone device.

External double-helical devices have teeth on their geared surfaces pointing outwards, whereas internal double-helical devices have teeth on their geared surfaces pointing inwards.

To optimize power density, planetary gearboxes often use double-helical sun, planet and ring gears.

One known solution is to mill the tooth profile of the double-helical device either with a shaped milling cutter or a shaped small diameter grinding wheel. The small diameter of the tool gives challenges with tool deflection and high wear rates, both of which limit the accuracy of the tooth profile.

Another known solution is to manufacture the two helical gear elements separately and then bolt and pin the two halves together. Although this does give a weight reduction, there is still a weight penalty with the bolts and a significant risk of bolt or nut release during extended operation under extreme variations in temperature combined with high frequency vibration.

Therefore, solutions for double-helical devices and methods for assembly for double-helical devices are required which in particular in the instance where the helix gap is removed or reduced allow in particular for lower mass and higher accuracy.

Summary

The present invention provides a double-helical gear device, a method for manufacturing a double-helical gear device, and a holding jig, as set out in the appended claims.

In a first aspect there is provided a double-helical device that comprises a left-hand helical gear element and a right-hand helical gear element, wherein the two helical gear elements are connected coaxially through a connection element positioned radially inwards or outwards from teethed surfaces of the helical gear elements. The connection trough an inward or outward positioned connection element inter alia allows a robust connection of the two helical gear elements, in particular it is possible to fit the two helical gear elements without a gap. The helical gear elements can be parts of a sun gear, a planet gear or a ring gear in a planetary gearbox.

The two helical gear elements can be connected gap-free. But it is also possible to provide an interconnection element in the space between the helical gear elements. The interconnection element can take up some forces during the assembly of the two helical gear elements. In particular, the interconnection element can e.g. be configured as a relatively thin-walled cylindrical shell, having a radial thickness of 5 to 10% of the radial thickness of the metal material underneath the teeth.

If the connection element is inserted into the bores of the two helical gear elements, it is a hub element. Alternatively, the connection element is a sleeve element circumferentially enclosing the outside of the two helical gear elements.

There are different ways to interface the connection element with the helical gear element, the connection element can e.g. be connected to the helical gear elements through a press fit.

To save weight, but also to tailor the mechanical properties to specific load cases, it is possible, that the connection element comprises composite material or consists of composite material. The fibers in the composite can be aligned and / or distributed according to the load case. In one embodiment, the composite material comprises carbon fibers.

The connection element can also comprise a plurality of discs of composite material, each of the discs having different orientation of the fibers. Again, this allows the adaption of the connection device to specific load cases. The connection element can comprise a plurality of axial fibers, to provide some extra strength in that direction.

The connection element can also comprise preforms, which can have different zones with different fiber orientations and / or densities. This not only allows for specific load cases but also to gradual changes over different preforms.

To provide support for the helical gear elements, the connection element can axially extend towards the full width of the gear rim. This in particular supports the open sides of the helical gear elements.

In particular to save weight, the connection element comprises or consists of titanium, aluminum or is manufactured with an additive manufacturing process.

The interface between the connection element and the helical gear elements can e.g. comprise interface features such as a plain bore, a thread, a toothed profile and / or a set of keyways.

To prevent large-scale failures of the teeth of the helical gear elements, the helical gear elements comprise a hybrid metal-composite structure which helps that that in case of cracks, only a tooth might break off the helical gear element.

In a second aspect there is provided a method for manufacturing a double-helical gear device. In that method, first, a left-hand helical gear element and a right-hand helical gear element are positioned next to each other in a holding jig. Subsequently, the two helical gear elements are connected coaxially with a connection element positioned radially inwards or outwards from teethed surfaces of the helical gear elements. The holding jig allows for accurate alignment (axially and azimuthally) of the two helical gear elements.

The holding jig can comprise a clamping mechanism for exerting an axial force on the abutment faces of the two helical gear elements. The clamping mechanism can comprise a screw mechanism, a hydraulic mechanism and / or a pneumatic mechanism to exert the required force.

The bores in the helical gear elements can so be manufactured in-situ while the helical gear elements are positioned in the holding jig. Also the interface features between the connection element and the helical gear elements can be machined as plain bore, a thread, a toothed profile and / or as set of keyways Furthermore, a fitting step can be performed to fit the connection element into the bores the helical gear elements, in particular as a press fitting operation, a freeze fit operation, a molding operation or a bonding operation.

In some embodiments, a positioning system is used to automatically align the helical gear elements coaxially. The positioning system can e.g. determine datum teeth for optimally aligning the helical gear elements. The datum teeth can be determined by measuring the surface and comparing the measured data with design data. The teeth with the least deviation from the design data can be chosen as datum teeth.

In a third aspect there is provided a holding jig configured for the method of the second aspect.

In some embodiments, the holding jig comprises tooth locators for aligning the helical gear elements (101, 102).

It is self-evident to a person skilled in the art that a feature or parameter described in relation to any one of the above aspects may be applied to any other aspect, unless they are mutually exclusive. Furthermore, any feature or any parameter described here may be applied to any aspect and/or combined with any other feature or parameter described here, unless they are mutually exclusive.

Brief description of the drawings

Embodiments will now be described by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a sectional side view of a gas turbine engine; Figure 2 is a close up sectional side view of an upstream portion of a gas turbine engine; Figure 3 is a partially cut-away view of a gearbox for a gas turbine engine; Figure 4 schematically shows a sectional view before the assembly of an embodiment of a double-helical gear device with two helical gear elements and a radially inward located connection element as hub element; Figure 5 schematically shows a sectional view after the assembly of an embodiment of a double-helical gear device with two helical gear elements and a radially outward located connection element as sleeve element; Figure 6 schematically shows an embodiment of a holding jig for manufacturing a double-helical device; Figure 7 shows a sectional view A-A from Figure 6; Figure 8 shows a sectional view B-B from Figure 6; Figure 9 schematically shows the embodiment of the double-helical gear device after assembly, Figure 10 schematically shows a top view of a double-helical device illustrating apex 15 wander; Figure 11 schematically shows a cross-section through an embodiment of a double-helical device with a metallic interconnection element (e.g. mesh) between the helical gear elements, Figure 12 schematically shows an embodiment of a connection element with stacked composite discs; Figure 13 schematically shows a segmented approach to forming a composite component; Figure 14 schematically shows a view of an embodiment of a connection element with axial fiber reinforcement; and Figure 15 schematically shows an embodiment with a composite connection element comprising preforms.

The following table lists the reference numerals used in the drawings with the features to which they refer: Ref no. Feature A Core airflow B Bypass airflow F Pressing force PCD Touch point at pitch circle diameter R Rotational axis of double-helical gear device 9 Principal rotational axis Gas turbine engine 11 Engine core 12 Air intake 14 Low-pressure compressor High-pressure compressor 16 Combustion equipment 17 High-pressure turbine 18 Bypass exhaust nozzle 19 Low pressure turbine Core exhaust nozzle 21 Engine nacelle 22 Bypass duct 23 Fan 24 Stationary supporting structure 26 Shaft 27 Interconnecting shaft 28 Sun gear Epicyclic gearbox device 32 Planet gear 34 Planet carrier 36 Linkage 38 Ring gear Linkage Ref no. Feature 71 Fibers in first direction 72 Fibers in second direction Axis Double-helical gear device 101 Left-hand helical gear element 102 Right-hand helical gear element 103 Geared surface of helical gear element 104 Abutment faces Bore 106 Tooth of the gear 107 Interconnection element 108 Gap Connection element, hub element, sleeve element 111 First composite disc 112 Second composite disc 113 Third composite disc 114 Axial fibers First Preform 116 Second Preform 117 Third Preform Holding jig 121 Clamps 122 Clamping mechanism 123 Tooth locators 510 Annular layer 520 Sector 521 Sector end 522 Sector end

Detailed description

Aspects and embodiments of the present disclosure will now be discussed with reference to the accompanying figures. Further aspects and embodiments will be apparent to those skilled in the art.

Figure 1 illustrates a gas turbine engine 10 having a principal rotational axis 9. The engine comprises an air intake 12 and a propulsive fan 23 that generates two airflows: a core airflow A and a bypass airflow B. The gas turbine engine 10 comprises a core 11 that receives the core airflow A. The engine core 11 comprises, in axial flow series, a low pressure compressor 14, a high-pressure compressor 15, combustion equipment 16, a high-pressure turbine 17, a low pressure turbine 19 and a core exhaust nozzle 20. A nacelle 21 surrounds the gas turbine engine 10 and defines a bypass duct 22 and a bypass exhaust nozzle 18. The bypass airflow B flows through the bypass duct 22. The fan 23 is attached to and driven by the low pressure turbine 19 via a shaft 26 and an epicyclic gearbox device 30.

In use, the core airflow A is accelerated and compressed by the low pressure compressor 14 and directed into the high pressure compressor 15 where further compression takes place.

The compressed air exhausted from the high pressure compressor 15 is directed into the combustion equipment 16 where it is mixed with fuel and the mixture is combusted. The resultant hot combustion products then expand through, and thereby drive, the high pressure and low pressure turbines 17, 19 before being exhausted through the core exhaust nozzle 20 to provide some propulsive thrust. The high pressure turbine 17 drives the high pressure compressor 15 by a suitable interconnecting shaft 27. The fan 23 generally provides the majority of the propulsive thrust. The epicyclic gearbox device 30 is a reduction gearbox.

An exemplary arrangement for a geared fan gas turbine engine 10 is shown in Figure 2. The low pressure turbine 19 (see Figure 1) drives the shaft 26, which is coupled to a sun wheel, or sun gear, 28 of the epicyclic gear arrangement 30. Radially outwardly of the sun gear 28 and intermeshing therewith is a plurality of planet gears 32 that are coupled together by a planet carrier 34. The planet carrier 34 constrains the planet gears 32 to precess around the sun gear 28 in synchronicity whilst enabling each planet gear 32 to rotate about its own axis. The planet carrier 34 is coupled via linkages 36 to the fan 23 in order to drive its rotation about the engine axis 9. Radially outwardly of the planet gears 32 and intermeshing therewith is an annulus or ring gear 38 that is coupled, via linkages 40, to a stationary supporting structure 24.

Note that the terms "low pressure turbine" and "low pressure compressor" as used herein may be taken to mean the lowest pressure turbine stages and lowest pressure compressor stages (i.e. not including the fan 23) respectively and/or the turbine and compressor stages that are connected together by the interconnecting shaft 26 with the lowest rotational speed in the engine (i.e. not including the gearbox output shaft that drives the fan 23). In some literature, the "low pressure turbine" and "low pressure compressor" referred to herein may alternatively be known as the "intermediate pressure turbine" and "intermediate pressure compressor". Where such alternative nomenclature is used, the fan 23 may be referred to as a first, or lowest pressure, compression stage.

The epicyclic gearbox device 30 is shown by way of example in greater detail in Figure 3.

Each of the sun gear 28, planet gears 32 and ring gear 38 comprise teeth about their periphery to intermesh with the other gears. However, for clarity only exemplary portions of the teeth are illustrated in Figure 3. There are four planet gears 32 illustrated, although it will be apparent to the skilled reader that more or fewer planet gears 32 may be provided within the scope of the claimed invention. Practical applications of an epicyclic gearbox device 30 generally comprise at least three planet gears 32.

The epicyclic gearbox device 30 illustrated by way of example in Figures 2 and 3 is of the planetary type, in that the planet carrier 34 is coupled to an output shaft via linkages 36, with the ring gear 38 fixed. However, any other suitable type of epicyclic gearbox device 30 may be used. By way of further example, the epicyclic gearbox device 30 may be a star arrangement, in which the planet carrier 34 is held fixed, with the ring (or annulus) gear 38 allowed to rotate. In such an arrangement the fan 23 is driven by the ring gear 38. By way of further alternative example, the gearbox device 30 may be a differential gearbox in which the ring gear 38 and the planet carrier 34 are both allowed to rotate.

It will be appreciated that the arrangement shown in Figures 2 and 3 is by way of example only, and various alternatives are within the scope of the present disclosure. Purely by way of example, any suitable arrangement may be used for locating the gearbox device 30 in the engine 10 and/or for connecting the gearbox device 30 to the engine 10. By way of further example, the connections (such as the linkages 36, 40 in the Figure 2 example) between the gearbox device 30 and other parts of the engine 10 (such as the input shaft 26, the output shaft and the fixed structure 24) may have any desired degree of stiffness or flexibility. By way of further example, any suitable arrangement of the bearings between rotating and stationary parts of the engine (for example between the input and output shafts from the gearbox and the fixed structures, such as the gearbox casing) may be used, and the disclosure is not limited to the exemplary arrangement of Figure 2. For example, where the gearbox device 30 has a star arrangement (described above), the skilled person would readily understand that the arrangement of output and support linkages and bearing locations would typically be different to that shown by way of example in Figure 2.

Accordingly, the present disclosure extends to a gas turbine engine having any arrangement of gearbox styles (for example star or planetary), support structures, input and output shaft arrangement, and bearing locations.

Optionally, the gearbox may drive additional and/or alternative components (e.g. the intermediate pressure compressor and/or a booster compressor).

Other gas turbine engines to which the present disclosure may be applied may have alternative configurations. For example, such engines may have an alternative number of compressors and/or turbines and/or an alternative number of interconnecting shafts. By way of further example, the gas turbine engine shown in Figure 1 has a split flow nozzle 20, 22 meaning that the flow through the bypass duct 22 has its own nozzle that is separate to and radially outside the core exhaust nozzle 20. However, this is not limiting, and any aspect of the present disclosure may also apply to engines in which the flow through the bypass duct 22 and the flow through the core 11 are mixed, or combined, before (or upstream of) a single nozzle, which may be referred to as a mixed flow nozzle. One or both nozzles (whether mixed or split flow) may have a fixed or variable area. Whilst the described example relates to a turbofan engine, the disclosure may apply, for example, to any type of gas turbine engine, such as an open rotor (in which the fan stage is not surrounded by a nacelle) or turboprop engine, for example. In some arrangements, the gas turbine engine 10 may not comprise a gearbox device 30.

The geometry of the gas turbine engine 10, and components thereof, is defined by a conventional axis system, comprising an axial direction (which is aligned with the rotational axis 9), a radial direction (in the bottom-to-top direction in Figure 1), and a circumferential direction (perpendicular to the page in the Figure 1 view). The axial, radial and circumferential directions are mutually perpendicular.

In the context of Figures 2 and 3, a planetary gearbox 30 is shown in the context of a fanned turbofan engine for an aircraft. This planetary epicyclic gearbox is positioned between a turbine and the LP compressor (fan) 23 to lower the rotational speed for the fan 23 relative to the turbine.

The embodiments described in the following are to be understood only as one possible application for embodiments of a double-helical gear device 100 which is described in the following.

In Figure 4, an embodiment of a double-helical gear device 100, e.g. a planetary gear with a rotational axis R, is shown, in which two helical gear elements 101, 102 are connected with a connection element 110 which is located coaxially, radially inwards from the geared surfaces 103 with teeth 106. In this arrangement, the connection element 110 is operating as a hub element 110 as it is located radially inwards, i.e. it forms a hub for the helical gear elements 101, 102.

Rather than manufacturing a single gear with a double helix geared surface 103, each of the two helical gear elements 101, 102 is manufactured as a separate item without the additional material needed for a helix gap. This means that the helical gear elements 101, 102 abut each other at their respective abutment faces 104.

Each of the helical gear elements 101, 102 can be manufactured by a generally known method up to the point where the machining and processing of the teeth 106 has been completed. At this point, the hub bore 105 will contain excess material stock, but the rest of the gear will be in the finished condition.

Radially inwards from the geared surfaces 103, a bore 105 is provided into which the connection element 110 can be inserted. In Figure 4 it is exemplary shown that the connection element 110 is inserted from the right-hand side into the bore 105.

When fully assembled, the two helical gear elements 101, 102 and the connection element are coaxially joined around the rotational axis R and together they form the double-helical gear device 100. The connection between the helical gear elements 101, 102 and the connection element 110 is achieved e.g. through a press fit.

The connection element 110 preferably comprises composite material (such as e.g. carbon fiber material) or consists of composite material so that it has a reduced mass. A composite connection element 110 has the advantage that small amounts of fiber and resin can be progressively added and built up into a shaped form prior to curing. This allows fiber to be placed in the most advantageous orientation for the application. The fiber orientation can also be altered throughout the thickness of the component to further tailor its load carrying behavior.

Prior to assembly, the composite will be fully cured and machined to size including any metallic to composite interface features. It is also possible to use lightweight materials as titanium or aluminum for the connection element 110. The connection element 110 will also be manufactured in an additive manufacturing process from a polymer or metal base material.

In Figure 5, a variation of the embodiment depicted in Figure 4 is shown, so that the previous description applies.

Here, the two helical gear elements 101, 102 have inward facing geared surfaces 103 and the connection element 103 connects the two helical gear elements 101, 102 on their outer radial surface.

In the following, embodiments of a method for manufacturing a double-helical device 100 are described in connection with Figures 6 to 9.

After manufacturing the two helical gear elements 101, 102 they can be assembled into a holding jig 120 as shown in Figure 6.

At the axial ends of the holding jig 120 it comprises clamps 121 for pressing the two helical gear elements 101, 102 together against their abutment faces 104. The pressing force F exerted through the clamps 121 can be generated by a clamping mechanism 122, in Figure 6 shown as a screw mechanism, pressing the two clamps 121 together. In other embodiments, the clamping mechanism 122 can be hydraulic or pneumatic. Combinations of the different types of clamping mechanisms 122 can also be used.

With the holding jig 120, the pitch circle diameter (PCD) of the two helical gear elements 101, 102 can be accurately aligned. For the PCD, this can be achieved by co-location of either the helical gear element 101, 102 bores 105, the tooth tips or by using additional reference features on the helical gear elements 101, 102.

Using the holding jig 120 as a holding device, the hub bores 105 will be machined in-situ with respective interface features to accept the fitment of the lightweight connection device 110, most preferably comprising composite material.

The interface features could be a plain bore (e.g. providing a press fit), a thread, a toothed profile, a set of keyways or similar depending on the application. The holding jig 120 ensures that the interface features are coaxial across the two helices and tooth alignment is maintained.

Without removing the two helical gear elements 101, 102 from the holding jig 120, the lightweight connection element 110 is fitted into the bores 105 of the two helical gear elements 101, 102. The fitting process can be a press fitting operation, a freeze fit operation, a molding operation and / or a bonding operation, permanently fixing the two helical gear elements 101, 102 by virtue of fitting the connection element 110. The holding jig 120 can be removed and re-used.

To align the two helical gear elements 101, 102, a positioning system (not shown) will contain an algorithm that identifies a datum tooth for each helix on the helical gear elements 101, 102.

A coordinate measurement machine (CMM) algorithm of the positioning system measures the profile of each tooth 106 and calculates how far each tooth 106 deviates from the nominal profile and nominal position. The tooth 106 that shows the least deviation from nominal or the tooth 106 that best represents the average positional error of all teeth will be identified and used as the datum tooth for the holding jig 120. This process is typically referred to as a 'Best fitting'. This algorithm could be biased depending on the type of gear being measured. For example, a 'Sun' gear always transfers torque from one side of the tooth 106. In this case the algorithm will only 'best fit' the one side (flank) on each tooth 106. A 'Planet gear transfers torque from both flanks on each tooth 106, therefore, the algorithm will need to 'best fit' both flanks of each tooth.

The tooth 106 selected as the datum tooth will have an actual drive surface position that best represents the mean position of all the remaining teeth. The datum teeth are used by the holding jig 120 to circumferentially locate the two gear elements 101, 102 relative to each other. The holding jig 120 will use accurately positioned spherical locators aligned to the tooth POD that touch on the drive surface of the datum teeth thus restraining circumferential rotation and therefore providing accurate alignment of the two helical gear elements 101, 102.

In particular, they touch on one datum tooth from the left-hand helical gear element 101 and the right-hand helical gear element 102 and in doing so they align the helical gear elements 101, 102 together and restrain them during fitting of the connection element 110. The tip of the tooth locators 123 facing the geared surface 103 of the helical gear elements 101, 102 has a spherical shape.

In Figures 6 to 8 it is shown how tooth locators 123 are used in the alignment of the helical gear elements 101, 102. The tooth locators 123 are in a fixed location within the holding jig 120. The tooth locators 123 touch the teeth 106 on the geared surfaces 103 of the helical gear elements 101, 102.

Figure 7 is a sectional view along the line A-A in Figure 6. Figure 8 is a sectional view along the line B-B in Figure 6.

The tooth locators 123 are in line axially (see e.g. Figure 8). The contact points are the same radius and the same radial height (see Figure 7).

The optimal position for the tooth locators 123 is to touch the tooth 106 at the pitch circle diameter POD of the helical gear element 101, 102 (see Figures 7 and 8). The tooth locator 123 should be arranged to contact the tooth 106 in the same way for a 'Sun' or a Planet' gear. If necessary, a circumferential clamp (or a spring device) could be added to clamp the datum teeth against the tooth locators 123. This would ensure that the gear teeth 106 remain firmly in contact with the touch locators 123 during fitting of the lightweight connection element 110.

In the following, further embodiments are described in connection with Figures 10 to 15.

As in the embodiments described above, the metallic gear rim and teeth of the helical gear elements 101, 102 are supported by the separate connection element 110, preferably made from composite material In the embodiments, the connection element 110 made of composite material is structured in such a way as to give stiffness in the hoop direction (i.e. circumferentially), but some compliance in the axial direction. The connection element 110 is able to rigidly support the gear teeth on the helical gear elements 101, 102, maintaining roundness and giving a high accuracy mesh with the respective mating gear, whilst also allowing slight axial movement between helices. The axial compliance allows teeth 106 on the left and right helical gear element 101, 102 to self-align with the teeth on the mating gear(s). The self-aligning function absorbs meshing errors between the sun gear 28, planet gears 32 and ring gear 38 caused by tolerance stack up and variations in tooth machining.

To optimize power density, the gearbox 30 (see e.g. Figures 1 to 3) uses double-helical sun, planet and ring gears. Although the gears are maintained to grade 3-4 accuracy levels, the manufacturing method inevitably gives some tooth-to-tooth variation within each gear. This results in a misalignment between helices which is typically referred to as 'Apex wander' and is illustrated in Figure 10 for a system known in the art as it has a gap between the helical gear elements 101, 102. When meshed together, a set of two double-helical gears will axially self-align due to the opposing angles of the helices. Where two gears (as in Figure 10) are meshed together in a rotating system, the variation in apex position tooth-to-tooth on each gear will result in the gears shuffling side to side as the gears rotate.

In the epicyclic gearbox 30 described above, there are four planet gears 32 meshed with a sun gear 28 and a ring gear 38. Therefore, this system has eight constantly varying gear mesh positions which all form different axial alignments. An axial float built into the planet gear bearing supports can mitigate some of the motions between gear positions however it cannot completely resolve an eight-way conflict.

Embodiments described below give some level of axial flexibility for each tooth 106 to absorb the effects of apex wander and reduce the resulting gearbox vibration in operation. This differs from known solutions, which used e.g. double-helix gear devices manufactured from two welded helical gear elements. Primarily this allows a reduced helix gap to save weight, however it gives the added benefit of allowing some flexibility between helices. Heat induced distortion and shrinkage from the welding operation are factors which restrict the suitability of this solution to lower accuracy gears.

United States patent application US 2015/240931 Al describes a one-piece metallic solution that uses a gap between the helices to form a flexible connection which allows some level of helix self-alignment. The use of individual ligaments to provide compliance gives a variation in tooth support depending on whether the tooth inner edge is adjacent to a gap or to a ligament. This results in unevenly loaded teeth around the circumference.

A solution should provide some degree of stiffness in one (radial) direction whilst providing flexibility or compliance in another (axial) direction. As steel is an isotropic material having similar mechanical properties in all directions. Differing stiffness is achieved by geometry rather than material processing.

Figure 11 shows an embodiment of a double-helical device 100 that uses a composite connection element 110 to provide axial location and radial support for the metallic double helical gear elements 101, 102. In principle, reference can be made to the description of the embodiments described in the context of Figures 4 to 9.

The metallic helical gear elements 101, 102 are manufactured with a generally known method. The connection between the inner face of the bores 105 and the outer surface of the connection element 110 is formed by interface features, such as a plain bore, a thread, and a toothed profile, a set of keyways or similar features depending on the application. These interface features will be machined in place of a conventional gear bore and prior to fitting of the connection element 110.

In the central space (in axial direction) between the helical gear elements 101, 102 a thin metallic interconnection element 107 is placed surrounding the connection element 110. The interconnection element 107 may be separate or integral to one or both of the helical gear elements. The interconnection element 107 is thinner On radial direction) than a conventional double-helical gear and will be axially weaker than the supporting connection element 110.

The interconnection element 107 is configured to be weak enough to allow the connection element 110 to control the axial compliance whilst retaining enough strength to resist the forces generated during manufacturing and assembly.

In some embodiments, the composite hub connection element 110 is press fitted into the bore 105 of the helical gear elements 101, 102. The interface between the helical gear elements 101, 102 and the connection element 110 relies on friction from an interference fit only. In this situation, the metallic interconnection element 108 can prevent the helical gear elements 101, 102 from being forced together or forced apart under the axial load imparted by the torque load on the angled teeth. Therefore, the interconnection element 107 enables a better assembly of the double-helical gear device 100.

The metallic interconnection element 107 is configured as a relatively thin-walled cylindrical part and sized to resist the axial load only. Circumferential flexibility is permitted by the linear elastic deformation of the thin metallic interconnection element. One way of achieving this would be to make radial thickness of the interconnection element 107 about 5 to 10% of the total metal material underneath the teeth of the gear.

In other potential solutions, there is a mechanical axial fixing between the composite and the metallic. In these cases, the metallic interconnection element 107 is no longer required and the circumferential flexibility is provided by the composite connection element 110 only. A small helix gap would be needed to permit flexibility.

Figure 13 shows an embodiment of a composite design for the connection element 110. The connection element 110 comprises a stack of composite discs 111, 112, 113 (only cross-sections shown in Figure 13).

The composite discs 111, 112, 113 are made of either woven carbon fiber cut from a sheet or strips of composite material placed to form a circle. There are numerous methods of manufacture including pre-impregnated sheet, resin infusion, or resin transfer moulding. Once stacked together, they are then compressed and cured. The stacked composite layup places the fibers radially in line with the gear teeth 106. This gives maximum stiffness radially as the fibers carry a bigger proportion of the load than the resin. This provides maximum support for the teeth. There are no fibers placed axially which means that the resin carries a bigger proportion of the axial and circumferential load. Resin is less stiff than the fibers and therefore gives greater compliance.

The sheet may be woven from carbon fiber strands where the strands are orientated at 900 to each other to form the warp and weft. During layup, each composite disc 111, 112, 113 is stacked with its angular orientation staggered relative to the adjacent composite disc 111, 112, 113, as indicated by the coordinate systems (see Figure 12) indicating the respective weft and warp.

Alternatively, the sheet maybe made up of unidirectional fiber strands orientated to give a similar result. Whilst this is a more complex method of manufacture, there are mechanical advantages and material utilization is improved.

Figure 13 shows a segmented approach to forming a composite component (e.g. for a connection element 110), in which subsequent segment layers are rotated around to avoid weaknesses where the fiber joins between segments are. These segments can be made from either 2D woven material (as shown diagrammatically in Figure 13) or uni-directional sheet.

Figure 12 shows an embodiment of a subsurface layer 5 with an arrangement of fibers in the radial and circumferential directions. In this embodiment, the subsurface layer 5 is formed by a plurality of annular layers 510 which are arranged next to each other in the axial direction to form the subsurface layer 5. The annular layers 510 form thin slices extending in a plane perpendicular to the axis 90 of the subsurface layer 5.

Each annular layer 510 has the form of a ring which comprises a plurality of sectors 520 arranged next to each other in the circumferential direction, each sector 520 having an arrangement of fibers and sector ends 521, 522. The arrangement of fibers in each sector 520 is formed by a rectangular grid 7 of fibers, the grid 7 including fibers 71 extending in a first direction and fibers 72 extending in a second direction perpendicular to the first direction.

The fibers 71, 72 may be provided by a woven two-dimensional material.

However, any method to provide for a rectangular grid of fibers can be implemented.

It is provided that the fiber grid 7 is oriented such that for the fibers in the middle of a sector 520 (the middle being the middle between sector ends 521, 522) the first direction is the radial direction. This is illustrated in Figure 13 in which arrow A depicts a middle area of a sector 520 in which the fibers 71 that are oriented in the first direction run in the radial direction. The more the fibers 71 are located close to the border B to the subsequent sector 520, the more these fibers 71 deviate from the radial direction. Therefore, the more sectors 320 are provided, the better the orientation of the fibers 71 in the radial direction.

However, when orienting the carbon fibers 71, 72 in the subsurface layer 5, the actual fiber directions can deviate from the desired radial direction by a few degrees and still produce the required properties with minimal reduction. The reduction is approximated by one minus the cosine of the deviation angle, and hence sector angles of 20 degrees produce maximum deviations of 10 degrees and reductions of less than two percent.

In the embodiment of Figure 12, the different annular layers 510 may be arranged with a rotated pattern of sectors 520 in subsequent layers 510, to avoid that the joints between the sectors 520 are all located at the same circumferential position. This ensures that the outer regions of the sectors 520 are evenly spread over the many layers of composite, and hence produce a consistent coefficient of thermal expansion at all angles around the circumference.

The manufacture of the subsurface layer of Figure 13 may be by robot assembly, wherein a sheet of fibers is cut into sectors 520 and the sectors 520 are placed into a resin to produce one annular layer 510, and wherein the layers 520 are added sequentially. Other possible methods of manufacture include using dry carbon reinforcement, with a secondary step of introducing the resin before the curing process in a single sided (infusion) or completely encased tool (resin transfer moulding). The subsurface layer could also be manufactured using pre-impregnated fibers which have the resin already attached to the fibers before curing. Figure 6 describes one embodiment of the organic composite 41, 42. There are many ways to apply the fiber-reinforced plastic to the body 60, 320, i.e., the inner or outer race. In one embodiment, the inner and outer races are present when the fiber-reinforced plastic subsurface is assembled and cured or set. Such type of attachment is referred to as cobonding. In a further embodiment, the races are push-fit, where the races are inserted 30 into the fiber reinforced plastic subsurface with some preload. In a still further embodiment, the races have a textured or small mechanical feature to assist attachment to the fiber-reinforced plastic subsurface.

Composite stiffness is predominantly influenced by the fiber rather than the matrix (resin). Optimum composite stiffness is found where load is applied in line with the warp and weft fibers. The application of an off-axis load will result in reduced stiffness down to a minimum where load is applied at an angle of 45° to the warp and weft, i.e. midway between the two.

If the warp and weft were aligned with no stagger to their angular orientation, optimum stiffness would only occur at 0°, 900, 1800 and 2700. Varying stiffness around the circumference of the gear would cause vibration and uneven tooth wear. The angular stagger between discs therefore ensures uniform support from the warp and weft fibers under all gear teeth.

The steel used for gear construction is chosen based on the mechanical requirements of the gear teeth 106.

In the embodiments shown, the connection element 110 is relatively lightly loaded meaning that the steel is generally over-specified for the connection element 110. A connection element 110 constructed from composite material will give the required mechanical properties with a lower weight if compared to steel.

A steel gear typically has a recess on either side of the hub faces to reduce weight. This gives uneven radial support along the tooth length. A connection element 110, in particular a composite connection element 110, can axially extend to the full width of the gear rim (see e.g. Figure 9) with little weight penalty. This equalizes support along the tooth length thereby controlling tooth engagement lead and lag.

The use of composite material also furthers the tailoring of mechanical properties. Due to the method of construction, fiber orientation and therefore stiffness can be locally tailored independent of geometry. Stiffness and compliance can be built into the design of the connection element 110 where needed with a high degree of accuracy. Where an isotropic material such as steel is used, stiffness and compliance can only be locally altered with a change to geometry, i.e. thin sections, cut-outs (as e.g. US 2015/240931 Al), convolutions or bellows.

To further tailor the mechanical behavior of the composite, Through Thickness Reinforcement (TTR) can be used to locally enhance stiffness. In this case, fiber strands are inserted into the stack of fiber discs prior to curing.

Figure 14 shows an embodiment in which fibers 114 are inserted axially to a depth, directly under the gear teeth 106 (not shown here) in the composite connection element 110. These fibers 114 add axial stiffness under the helical gear elements 101, 102 and act to stabilize the composite to metallic interface.

The area of composite beneath helix gap 108 is free from TTR allowing greater axial compliance. Where beneficial, fibers 114 can be inserted at an angle to match the helix angle of the teeth 106 or can be inserted at an opposing angle such that a single fiber will bridge multiple teeth.

Figure 15 shows an embodiment with tailored preforms 115, 116, 117 in a connection element 110 comprising composite material.

Preforms 115, 116, 117 are uncured composite material shaped into three-dimensional structures prior to insertion into a composite molding tool. They are typically manufactured by robotic fiber placement or 3D weaving technologies. The fibers can be made to follow a contour that gives the most desirable combination of stiffness and compliance.

In the embodiment shown in Figure 15 three of the preforms 115, 116, 117 are used. The first preform 115 is located radially underneath the first helical gear element 101. The second preform 116 is located radially underneath the second helical gear element 102. Between the first and second preforms 115, 116 the third perform 117 is radially positioned underneath the gap 108 (as seen in the embodiment with the metallic interconnection element 107).

The first and second preforms 115, 116 comprise a pot-shaped fiber arrangement providing high axial stiffness under the helical gear elements 101, 102. The third preform comprises a stacked disc type layup to provide of a radial stiffness under the gap 108 between the helical gear elements 101, 102.

In places where the properties of the connection element 110 are less critical, sheets of randomly orientated chopped fiber can be used for the discs in place of woven material. In this application, multiple discs of chopped fiber are likely to be very close in performance to multiple discs of woven material with a staggered orientation. The advantage with chopped fiber is that it is more cost effective to use and lends itself to the use of a recycled material.

So that the metallic helix gap area does not restrict the compliance of the composite hub, this area could be machined to a bellows shape or a curved profile.

Considering Figure 11, a further embodiment of a double-helical gear device 100 will be described.

This embodiment relates to the mitigation of mechanical failures, i.e. losses of gear teeth 106 within a planetary gearbox 30. As mentioned above, planetary gearboxes 30, in particular in aircraft engines, need to perform at very high safety levels. A gearbox seizure can have severe implications, as e.g. the fan of the gas turbine engine 10 would suddenly become an aerodynamic obstacle.

One embodiment comprises helical gear elements 101, 102 comprising a hybrid of a metal gear rim with a composite backing. It is applicable to any external gears.

Generally, it is attempted in gear drives to ensure that any crack propagation trajectory from the position of high stress at the root of the gear teeth is across the tooth root and not into the bore of the gear. This way, only one tooth 106 rather than large pieces of e.g. a planet gear are lost.

Cracks in the root of the gear teeth propagate across the tooth, resulting in the release of one tooth which would be contained within the gearbox. The worst-case situation, with a conventional monolithic gear is, that the crack could propagate towards the gear bore which would result in failure of the complete gear via a bursting mechanism.

Typically, the gear would then split into three large sections which could have enough inertia to exit the gearbox and the engine as High Energy Debris (HED).

In the same situation with a hybrid metallic and composite gear, a crack propagating into the bore 105 would cause the metallic toothed rim to be stripped away, i.e., which would become completely separated from the composite hub. This is still a comparatively small metallic part (i.6. the rim) and the associated composite connection element 110 would be destroyed within the gearbox 30 and would remain contained within the engine without creating HED and thus avoiding gearbox seizure. Using this arrangement will produce an inherently safer gearbox system.

It will be understood that the invention is not limited to the embodiments above-described and various modifications and improvements can be made without departing from the concepts described herein. 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 features described herein.

Claims (27)

1. CLAIMS1. A double-helical gear device (100) comprising a left-hand helical gear element (101) and a right-hand helical gear element (102), wherein the two helical gear elements (101, 102) are connected coaxially through a connection element (110) positioned radially inwards or outwards from teethed surfaces (103) of the helical gear elements (101, 102).
2. 2. The double-helical gear device of claim 1, wherein the helical gear elements (101, 102) are parts of a sun gear (28), a planet gear or a ring gear (38) in a planetary gearbox (30).
3. 3. The double-helical gear device of claim 1 or 2, wherein in an axial direction the space between the helical gear elements (101, 102) is gap-free or the space comprises an interconnection element (107).
4. 4. The double-helical gear device of claim 3, wherein the interconnection element (107) is configured as a relatively thin-walled cylindrical shell, having a radial thickness of 5 to 10% of the radial thickness of the metal material underneath the teeth (106).
5. 5. The double-helical gear device of any preceding claim, wherein the connection element (110) is a hub element inserted in bores (105) in the two helical gear elements (101, 102).
6. 6. The double-helical gear device of any one of claims 1 to 4, wherein the connection element (110) is a sleeve element circumferentially enclosing the outside of the two helical gear elements (101, 102).
7. 7. The double-helical gear device of any preceding claim, wherein the connection element (110) is connected to the helical gear elements (101, 102) through a press fit.
8. 8. The double-helical gear device of any preceding claim, wherein the connection element (110) comprises composite material or consists of composite material.
9. 9. The double-helical gear device of claim 8, wherein the composite material comprises carbon fibers.
10. 10. The double-helical gear device of claim 8 or 9, wherein the connection device (110) comprises a plurality of discs (111, 112, 113) of composite material, each of the discs (111, 112, 113) having different orientation of the fibers.
11. 11. The double-helical gear device of claim 8 or 9, wherein the connection device (110) comprises composite material in fiber sheets with unidirectional fibers or 2D woven material.
12. 12. The double-helical gear device of any one of claims 8 to 11, wherein the connection device (110) comprises a plurality of axial fibers (114).
13. 13. The double-helical gear device of any one of claims 8 to 12, wherein connection element (110) comprises preforms.
14. 14. The double-helical gear device of any preceding claim, wherein the connection element (110) axially extends towards the full width of the gear rim.
15. 15. The double-helical gear device of any preceding claim, wherein the connection element (110) comprises or consists of titanium, aluminum or is manufactured with an additive manufacturing process.
16. 16. The double-helical gear device of any preceding claim, wherein interface features between the connection element (110) and the helical gear elements (101, 102) are a plain bore, a thread, a toothed profile and / or a set of keyways.
17. 17. The double-helical gear device of any preceding claim, wherein the helical gear elements (101, 102) comprise a hybrid metal-composite structure.
18. 18. A method for manufacturing a double-helical gear device, the method comprising the steps of: a) a left-hand helical gear element (101) and a right-hand helical gear element (102) are positioned next to each other in a holding jig (120); and b) the two helical gear elements (101, 102) are connected coaxially with a connection element (110) positioned radially inwards or outwards from teethed surfaces (103) of the helical gear elements (101, 102).
19. 19. The method of claim 18, wherein the holding jig (120) comprises a clamping mechanism (122) for exerting an axial force (F) on the abutment faces (104) of the two helical gear elements (101, 102).
20. 20. The method of claim 19, wherein the clamping mechanism comprises a screw mechanism, a hydraulic mechanism and / or a pneumatic mechanism.
21. 21. The method of any one of claims 18 to 20, wherein the bores (105) in the helical gear elements (101, 102) are manufactured in-situ while the helical gear elements (101, 102) are positioned in the holding jig (120).
22. 22. The method of any one of claims 18 to 21, wherein interface features between the connection element (110) and the helical gear elements (101, 102) are machined as plain bore, a thread, a toothed profile and / or as set of keyways
23. 23. The method of any one of claims 18 to 22, wherein a fitting step is performed to fit the connection element (110) into the bores (105) the helical gear elements (101, 102), in particular as a press fitting operation, a freeze fit operation, a molding operation or a bonding operation.
24. 24. The method of any one of claims 18 to 23, wherein a positioning system is used to automatically align the helical gear elements (101, 102) coaxially.
25. 25. The method of claim 24, wherein the positioning system determines datum teeth for optimally aligning the helical gear elements (101, 102).
26. 26. A holding jig (120) configured for the method of any one of claims 18 to 25.
27. 27. The holding jig of claim 26, comprising tooth locators (123) for aligning the helical gear elements (101, 102).