This invention relates to gears, and in particular to gears manufactured, at least in part, of composite materials.

Composite materials are in increasingly common use in a wide range of applications as they are typically of high strength and good load transmitting properties whilst being of low weight. Accordingly, in weight critical applications such as in aerospace applications or certain automotive applications where increased weight can lead to reduced fuel efficiency or reduced responsiveness to changing control inputs, the use of composite materials is thought to be advantageous.

Whilst composite materials may be of benefit in that they are of good strength and low weight, problems can be faced in attaching composite material components to other materials.

In the case of gears, it may be desired for the gear teeth to be of metallic form, for example provided on a metallic material annular component, and for the composite material to be used to provide a load or torque transmitting path between the metallic material component and, for example, a drive shaft or to another part of the gear, depending upon the application in which the gear is used. Providing a strong load transmitting connection between the composite material element and the metallic material component can be difficult to achieve.

Where the teeth are provided upon a metallic material component, in order to maximise the opportunity to make weight savings, it is desirable for the metallic component to be of thin form. However, components of such a form are relatively flexible and this may lead to undesirable movement between the gear teeth and the body of the gear. It is an object of the invention to provide a gear manufactured or fabricated, at least in part, from a composite material in which at least some of the disadvantages associated with known gears of this type are overcome or are of reduced effect.

According to a first aspect of the invention there is provided a gear comprising a first component of a first material, and a second, composite material component of a second, different material, the composite material component including an engagement section bearing against the first component, wherein the engagement section is of a composite material designed such that the coefficients of thermal expansion of the composite material and the material of the first component substantially match one another in the axial direction of the first component.

The composite material may be of braided form. Alternatively, it could be of wound fibre form. Where of wound fibre form, a winding angle of the wound fibre material of the engagement section may be chosen relative to the coefficients of thermal expansion of the fibre material and the material of the first component so as to substantially match thermal expansion of the engagement section in the axial direction to that of the first component.

The surface of the first component that engages the engagement section is preferably provided with a microspline formation. It will be appreciated that in order to maintain the integrity of the microsplined connection between the toothed component and the engagement section, it is important to ensure that relative dimensional changes that may occur, in use, as a result of temperature changes are kept to an acceptably low level as failure to do so could result in the gear being incapable of bearing or transmitting the torque loadings applied thereto, in use. The effective coefficient of thermal expansion of the engagement section is dependent upon the nature of the reinforcing fibre material and also the winding angle thereof. By appropriate selection of the winding angle, the coefficient of thermal expansion of the engagement section may be substantially matched to that of the first component, for example they may be within, say, 5% of one another, with the result that the integrity of the connection therebetween may be maintained over a wide range of operating temperatures. By way of example, the winding angle may be in the region of 40-60°, preferably in the region of 45-55°, and more preferably in the region of 52°. The first component may be of a metallic material. It may be formed with a series of gear teeth. The gear teeth may be formed after assembly of the first component to the second component.

The engagement section is preferably of relatively great wall thickness, so as to be able to transmit loads around the gear.

According to a second aspect of the invention there is provided a gear comprising a first component of a first material, and a second, composite material component of a second, different material, the composite material component including an engagement section bearing against the first component, wherein the engagement section is of wound fibre reinforced composite material form, wherein a winding angle of the wound fibre material of the engagement section is chosen relative to the coefficients of thermal expansion of the fibre material and the material of the first component so as to achieve a peak compressive load between the engagement section and the first component at a selected operating temperature of the gear.

According to a third aspect of the invention there is provided a gear comprising a first component of a first material, and a second, composite material component of a second, different material, the composite material component including an engagement section bearing against the first component, wherein the engagement section is of composite material form, wherein face of the first section engaged by the engagement section is of non-uniform diameter along its length, and the face of the engagement section is correspondingly shaped.

The composite material may be of wound fibre form. Alternatively, it could be of braided form.

As the gear may be of a significant axial length, press fitting the first component onto the composite material component may require the loads applied during assembly to be high, and the spline teeth may become blunt before completion of the assembly process, again resulting in the required assembly loads being increased. By appropriate selection of the shapes of the faces of the first component and the engagement section, these disadvantages may be overcome.

By way of example, the interengaging faces of the first component and the engagement section may be of tapering and/or stepped form.

If the required toothed form is of herringbone design, two (or more) separate toothed first components each with a single toothed profile may be assembled onto a common composite material component.

The gear may comprise a planet gear, in which case the second, composite material component may be shaped to carry the outer race of a bearing, or to form the outer race of a bearing whereby the gear is rotatably mounted upon a support or carrier. Where the composite material component is shaped to carry the outer race of a bearing, the outer race may be press fitted to the composite material component.

Alternatively, the gear may comprise a ring gear or a sun gear. Where serving as a ring gear, it will be appreciated that the first component is located radially within the second component. A reinforcing ring may be required to apply a load to the second component to compress it against the first component.

According to another aspect of the invention there is provided a gear comprising a metallic material component of annular form, and a composite material component, wherein the composite material component includes an outer layer or ply of reinforcing material bound with a resin material, the outer layer or ply being of relatively thick form. By way of example, the outer layer or ply may be of thickness in the region of at least 3 to 5 times the diameter of the reinforcing material. By providing an outer layer or ply of this form, increased bundling or bunching of the reinforcing material within the outer layer or ply may occur, at least some of the reinforcing material extending non-tangentially. A consequence of this is that the composite material component may be better able to withstand substantially radially directed loads, and as a result may be better able to support the metallic component against flexing. The risk of relative movement between the gear teeth and the body of the gear is thus reduced.

It will be appreciated that as a result, the use of metallic components of reduced wall thickness, and hence of lighter weight may be possible.

The metallic component is preferably a press fit with the composite material component. Preferably, the metallic component is provided, on a surface thereof which, in use, cooperates with the composite material component, with a series of spline teeth formations. The spline teeth are of small radial height, for example of less than 0.5mm and preferably of less than 0.25mm height. Such spline teeth are referred to herein as microsplines. During assembly, the microsplines dig into the outer surface of the composite material component, and it is thought that the interaction between the microsplines of the metallic component provides further resistance to bending or deformation of the metallic component. Again, this may permit reduced wall thickness metallic components to be used, leading to additional weight savings.

The composite material may be of electrically conductive form, and there is a risk, as a consequence, of galvanic corrosion occurring at the interface between the composite material component and the metallic component. In order to reduce this risk, a non-conductive material coating is preferably applied to the surface of the metallic component that, in use, engages with the composite material component. The coating should be of a material sufficiently resilient as to be able to withstand abrasion during the press fitting operation so as to maintain a sufficiently good level of electrical insulation between the components. By way of example, certain ceramic materials or polymers such as PEEK could be used for this purpose. The composite material component may comprise layers of woven fibrous material, adjacent layers being orientated such that the threads or fibres of one of the layers are angularly offset relative to the threads or fibres of the adjacent layer. Each layer takes the form of an annular disc. The layers of fibrous material are impregnated with a suitable resin. Preferably, reinforcing fibres are stitched to the layers, or to individual ones of the layers. Some of the stitched reinforcing fibres are preferably arranged annularly adjacent the outer peripheries of the layers. Others of the stitched reinforcing fibres are preferably arranged annularly adjacent the inner peripheries of the layers. The stitched reinforcing fibres adjacent the outer periphery of the layers may form the aforementioned relatively thick outer layer.

Such an arrangement is advantageous in that it allows the formation of gears of relatively large diameters in a relatively simple and convenient fashion.

According to another aspect of the invention there is provided a gear comprising a metallic material component of annular form, and a composite material web component, and a composite material retainer to secure the metallic material component so the composite material web component to reduce or avoid relative movement therebetween.

The composite material retainer may comprise, for example, one or more annular components of a composite material that may be press fitted to the metallic material component to strengthen and/or compress at least part of the metallic material component, thereby reducing the risk of deformation of the metallic material component, in use.

Alternatively, the composite material retainer may take the form of fibres of a reinforcing material that are wound across a face of the gear between a first point on a periphery thereof and a second point on the periphery thereof spaced from the first point. The first and second points may be located substantially diametrically opposite one another, in which case the composite material retainer may serve primarily to compress the metallic material component onto the composite material web component, reducing the risk of flexing thereof. Alternatively, the first and second points may be at a reduced angular spacing and the composite material retainer may further serve to transmit loads around the periphery of the gear.

According to yet another aspect of the invention there is provided a gear comprising a metallic material component and a composite material component, wherein a peripheral surface of the component material component engaged by the metallic material component is of toothed form, and the metallic material component is of thin walled form.

In such an arrangement, the metallic material component serves as a wear surface, protecting the teeth of the composite material component from wear, whilst the composite material component provides the required load carrying capacity. The metallic material component may comprise a coating applied to the composite material component.

To provide the required level of strength, the composite material component preferably includes fibre ends exposed at the flanks of the teeth. The composite material component may be fabricated as an annular component that is subsequently machined to form the teeth thereon.

According to another aspect of the invention there is provided a gear comprising a metallic material component of annular form, and a composite material component, the composite material component comprising a laminate made up of a plurality of layers of woven fibrous material, at least some adjacent layers being orientated such that the threads or fibres of one of the layers are angularly offset relative to the threads or fibres of the adjacent layer, each layer having an outer periphery of circular shape. Preferably, reinforcing fibres are stitched to the layers, or to individual ones of the layers. Some of the stitched reinforcing fibres are preferably extend annularly, being arranged adjacent the outer peripheries of the layers. Others of the stitched reinforcing fibres are preferably arranged adjacent the inner peripheries of the layers. The stitched reinforcing fibres adjacent the outer periphery of the layers may form a relatively thick outer layer for engagement with the metallic component, or may have such a layer provided thereon.

The invention will further be described, by way of example, with reference to the accompanying drawings, in which:

Figure 1 is a diagrammatic sectional view illustrating a gear in accordance with an embodiment of the invention, the gear serving, in use, as a planet gear;

Figure 2 illustrates an alternative embodiment;

Figure 3 illustrates a further embodiment;

Figure 4 illustrates an embodiment of the invention in which the gear takes the form of a ring gear;

Figure 5 is a diagrammatic view illustrating part of a gear in accordance with another embodiment of the invention; and

Figure 6 to 10 are diagrammatic views illustrating parts of gears in accordance with other embodiments of the invention. Referring firstly to Figure 1, a gear 10 is illustrated. The gear 10 is intended to be used as a planet gear in a planetary gear arrangement, the planet gear being mounted upon a rotatable support or carrier and being in meshing cooperation with a sun gear and an outer ring gear. Whilst the gear 10 illustrated in the accompanying drawing is intended for use in such an application, it will be appreciated that the invention is not restricted in this regard and that the gear may be used in other applications, for example as a pinion gear, a ring gear or a sun gear.

The gear 10 comprises an outer first component 12 of annular form, the first component 12 being of metallic form. The first component 12 is milled or otherwise formed, on its outer periphery, with a series of gear tooth formations 12a. The formation of the gear teeth is conveniently undertaken towards the end of the manufacturing process, after assembly of the various part of the gear 10. However, this need not always be the case and arrangements are possible in which the gear teeth formations are formed before assembly of the gear 10. Forming the gear teeth formations towards the end of the assembly process has the advantage that the risk of subsequent steps in the assembly process resulting in damage to the teeth formations is reduced.

The gear 10 further comprises a second, composite material component 14. The second component 14 is, in this embodiment, of wound fibre reinforced composite material form, and includes a radially inner part 16, a radially outer engagement section 18, and an interconnecting part 20 interconnecting the inner part 16 and the engagement section 18 so as to support the engagement section 18 at a fixed distance from the inner part 16, providing a rigid structure, in use. The various parts of the second component 14 are preferably formed as part of a single manufacturing operation which involves winding the reinforcing fibre material of the second component 14 onto a shaft or mandrel, impregnating the material with a suitable resin material, locating the resin impregnated fibre material within a mould or the like, and curing the resin material or allowing the resin material to cure to result in the formation of a component of a desired shape. The resin impregnation step may be undertaken before or after winding the fibre material onto the shaft or mandrel. Whilst the description hereinbefore is of a wound fibre composite material, it will be appreciated that the invention is not restricted in this regard and is also applicable to other composite materials, for example to braided materials. Where used as a planet gear, the primary function of the interconnecting part 20 is to properly support the engagement section 18 at a fixed distance from the inner part 16, pushing the carrier upon which the planet gear is mounted for movement, in use, rather than to transmit significant torque loadings therebetween, and the interconnecting part 20 can be designed accordingly. The first component 12 and the engagement section 18 serve to transmit rotary motion of the gear 10 between the opposing points at which the gear 10 mates with the outer ring gear and the inner sun gear, and the connection therebetween will need to withstand the transmission of a shearing load to perform this function.

In the arrangement shown, the outer periphery of the engagement section 18 is of cylindrical form, and the face of the first component 12 which, in use, cooperates with the engagement section 18 is also of cylindrical form, but is conveniently formed with a series of small spline teeth, referred to herein as microspline formations. By way of example, the microspline formations may have a height of 2mm or less, more preferably 1mm or less, for example they may be of height in the region of 0.5mm.

The first and second components 12, 14 are assembled to one another by press fitting the first component 12 onto the second component 14, the microspline formations digging into the surface of the engagement section 18. The engagement between the first component 12 and the second component 14 is such that significant shearing loadings can be transmitted therebetween. It will be appreciated that if the invention is employed in other applications, then the transmission of torque loadings between the various parts of the gear may be of greater importance and the gear design may be modified accordingly.

Where the gear 10 is used in applications that are subject to significant variations in temperature, then thermal expansion of the first and second components 12, 14 can be expected to occur. If the axial length of the engagement section 18 varies significantly relative to that of the first component 12, then this may impact upon the strength and effectiveness of the connection between the first and second components 12, 14, and may result in a reduction in the useful working life of the gear 10. By way of example, a reduction in contact area therebetween the components could occur, leading to a reduction or loss of load transmitting capability.

In order to reduce this, in accordance with the invention, the winding angle of the fibre material of the engagement section 18 is selected relative to the coefficients of thermal expansion of the fibre material and of the material of the first component 12 such that variations in the axial dimension of the outer diameter of the engagement section 18 with varying temperature substantially match the variations in the axial dimension of the inner diameter of the first component 12 under those temperature variations. It will be appreciated that, as a result, the effectiveness of the connection between the first and second components, and the working life of the gear, may be enhanced. Exactly matching the thermal expansion properties of the first and second components 12, 14 over a wide range of temperatures may be difficult or impossible to achieve, but by substantially matching them, with the result that differences in thermal expansion are restricted to, say 5% or less, and preferably lower than this, it will be appreciated that significant benefits may be attained. It is envisaged that using a winding angle in the region of 40 to 60°, more preferably in the region of 45 to 55°, conveniently around 52°, will achieve this effect where a carbon fibre material is used in the composite material. However, it will be appreciated that other materials such as glass fibre materials may be used, if desired, and these may require the use of other winding angles.

It is thought that variations in the relative diameters of the components of the gear with varying temperature will normally be accommodated through the nature of the press fit between the components. However, to minimise variations in the dimensions of the inner diameter of the first component 12 and the outer diameter of the second component 14 with varying temperature, then it may be desired to include at least some windings with a winding angle in the region of 30° within the structure.

Whilst matching or substantially matching the thermal expansion properties of the engagement section 18 and first component 12 may be desirable in many applications, if desired the winding angle could alternatively be selected so as to achieve a maximum compressive load between the first component 12 and the engagement section 18 at a preferred operating temperature such that the connection between the first component 12 and the engagement section 18 is optimal at that temperature.

The engagement section 18 is preferably of relatively thick form. Such an arrangement is advantageous, especially where the gear is used as a planet gear, as the loadings applied to a planet gear are applied from two opposing sides, as mentioned above, and the relatively thick engagement section 18 can transmit loadings between these parts of the structure without significant loadings needing to be transmitted therebetween via the interconnecting part 20 and the inner part 16.

The inner part 16 conveniently carries an outer bearing race component 22 that, in use, cooperates with an inner bearing race component and associated bearings to rotatably mount the gear 10 upon the carrier mentioned hereinbefore. The inner part 16 may be wound with the fibres thereof under high tension with result that this part of the gear 10 is preloaded, aiding in ensuring that the component 22 is firmly retained in position. Alternatively, the inner part 16 may be shaped to define the outer bearing race, in which case it may be desired to provide the inner part 16 with a metallic material coating to enhance the wear resistance thereof.

As the first component 12 and the engagement section 18 are of relatively great axial length, it will be appreciated that relatively high compressive loads may be required in order to complete the press fit assembly of these components. There is a risk that the application of such loads could damage parts of the gear 10, or may require them to be of greater thickness or the like in order to withstand the assembly loads, increasing the weight of the component. There is also a risk that the microspline formations may become damaged, blunt or worn as the assembly process progresses, again resulting in a need for the application of an increased assembly load.

Figure 2 illustrates a modification to the arrangement of Figure 1 in which these disadvantages are mitigated. In the arrangement of Figure 2, the cooperating faces of the engagement section 18 and the first component 12 are of slightly tapered form rather than being of uniform diameter. As a consequence, the loadings required to press fit the components to one another need only be applied over a relatively short distance. Consequently, the maximum required assembly load is reduced, and the risk of damage or blunting of the microspline formations is reduced.

Figure 3 illustrates another option. In this arrangement, the cooperating faces are of stepped form. Again, the assembly distance over which the load required to achieve press fitting is reduced, leading to a reduced maximum required applied load, and a reduction in damage or blunting of the microspline formations.

Of course, the tapering and stepped features may be used in combination, if desired.

In some applications, the teeth formations of the gear may be required to take a herringbone form. To achieve this, two first components preformed with the different angles of gear formations may be mounted upon a single second component 14.

As mentioned above, the gears 10 described hereinbefore take the form of planet gears for use in a planetary gear arrangement, but the invention is not restricted to such use. The invention could also be employed in the fabrication of the outer ring gear of such an arrangement, and Figure 4 illustrates such a gear using the same reference numerals to denote equivalent parts. In this arrangement, a reinforcing ring 24, or composite material, is provided to apply a compressive load to the engagement section of the second component 14 to compress it against the first component 12. Also, as mentioned above, the invention may be employed in pinion gears, or to sun gears.

It will be appreciated that a gear of the type outlined hereinbefore is advantageous in that the use of composite materials within parts of the gear can allow a reduction in weight to be achieved. The weight reduction may be achieved without significantly impacting upon the ability of the gear to carry the loads applied thereto, in use, and so the performance of the gear need not be negatively impacted through the use of the composite material.

Referring next to Figure 5, a gear 110 is illustrated comprising an outer metallic component 112 of annular form, and a composite material component 114. The metallic component 112 comprises a supporting ring 116 and a series of gear teeth 118 formed integrally with the ring 116. The ring 116 is of relatively thin walled form. Consequently, it is relatively flexible and there is a risk that the application of loadings to the teeth 118, in use, could cause undesired flexing of the ring 116. The composite material component 114 includes, adjacent its periphery, a composite material layer or ply 120 of relatively great thickness. By way of example, the thickness of this layer or ply 120 may be in the region of 3 to 5 times the diameter of the reinforcing fibres 122 contained therein. By containing the fibres 122 in a relatively thick layer or ply, it will be appreciated that increased bunching or bundling of the fibres 122 is permitted or encouraged and consequently in the layer 120 containing fibres 122 that extend in directions other than the tangential direction. Specifically, some of the fibres 122 may extend in a direction having a radial component. The effect of this is that the presence of the layer or ply 120 results in the composite material component 114 being of increased ability to withstand radially directed loads. The composite material component 114 is thus better able to support the metallic component 112, and in particular the ring 116 thereof, reducing the likelihood of flexing thereof.

It will be appreciated, therefore, that by providing the composite material component 114 with an outer layer or ply 120 of relatively great thickness, reduced thickness, and hence lighter metallic material components 112 may be used in the gear 110. Furthermore, in use, the outer layer 120 of the composite material component 114 may carry substantially all of the loads bourn by the composite material component 114, in use, and so the remainder of the component 114 may be designed in such a manner as to allow additional weight savings to be made.

The metallic component 112 and composite material component 114 are press fitted to one another and, as shown, the metallic component 112 is provided, on its inner periphery, with a series of spline teeth 124 that, during the assembly process, dig into the outer surface of the composite material component. The spline teeth 124 are of small radial height, preferably being of height less than 0.5mm, and more preferably being of height less than 0.25mm. Such formations are referred to herein as microsplines. It is thought that the interengagement between the spline teeth 124 and the composite material component 114 is such that flexing of the metallic component 112 is further reduced, as the spline teeth 124 would need to move relative to the surface of the composite material component 114 to permit such flexing, and the presence of the teeth 124 which are dug into the surface of the composite material component 114 resists such relative movement. Again, therefore, as the metallic component 112 is better supported, it is thought that the ring 116 may be of thinner wall thickness, and so the gear may be of reduced weight whilst maintaining a required level of strength.

As the composite material component 114 may be of an electrically conductive form, there is a risk of galvanic corrosion occurring at the interface between the composite material component 114 and the metallic material component 112. In order to reduce the risk of such corrosion, the surface of the metallic material component 112 that, in use, engages the composite material component 114 is preferably provided with a non-conductive material coating prior to assembly of the gear 110. The coating should be of a material capable of withstanding the abrasion and wear that will occur during the assembly process. By way of example, certain ceramic or polymer materials such as PEEK could be used.

Whilst the description hereinbefore relates to the outer periphery of a gear, and in particular to a planet gear, it will be appreciated that the features described may also or alternatively be applied at the inner periphery of a gear, for example if the gear takes the form of a ring gear. It will also be appreciated that the invention may be applied to other types of gear, for example to pinion gears, if desired.

Figure 6 illustrates an embodiment in which composite material reinforcing members 126 of annular, hoop wound form are provided. The members 126 are press fitted onto shoulders 116a of the support ring 116 and serve to provide an inwardly directed radial force, compressing the support ring 116 against the composite material component 114. In so doing, the risk of flexing of the metallic material component 112 as described hereinbefore is reduced, and so a gear of improved strength and load carrying ability is provided.

Figures 7 and 8 illustrate alternative forms of composite material reinforcing members 126. In each case, the reinforcing members 126 are of wound fibre form with the fibres wound in such a manner as to pass over a face of the gear between first and second locations on the periphery of the gear, over the periphery of the gear and across the reverse face of the gear. By appropriate angling of the windings, the fibres of the reinforcing members 126 may extend in a star-like configuration. In Figure 7, the first and second locations are spaced close to diametrically apart, and the reinforcing member 126 serves to apply a compressive load resisting lifting off and deflection or flexing of the metallic material component 112. In Figure 8, the first and second locations are more closely spaced, and the reinforcing member 126 serves, additionally, to transmit torque loads between the first and second locations. It will be appreciated that the positions of the first and second locations may be chosen depending upon the application in which the gear is to be used and the loads to be carried thereby.

If the first and second locations are truly diametrically opposite one another, then the reinforcing member 126 may extend across the axis of the gear. To allow the insertion of a hub or the like, the fibres may be pushed apart before curing, or machining after curing may be undertaken.

Turning to Figure 9, part of an alternative design of gear 130 is illustrated. The gear 130, like the gear 110, comprises a metallic material component 132 and a composite material component 134. In this embodiment, the composite material component 134 is, at least in part of laminated form and comprises a series of layers 136, each of which comprises an annular disc of a woven fibre material. The layers 136 are arranged so that the fibres of each of the discs are angled relative to the fibres of an adjacent one of the discs. The layers 136 are impregnated with a suitable resin material which is cured to form the composite material. Around the outer peripheries of the layers 136, prior to curing of the resin material, reinforcing fibres 138 are stitched to the layers 136. The reinforcing fibres 138 are stitched in such a fashion as to form a series of annular hoops extending around the layers 136, each of which is located at or close to the periphery of the composite material component 134. The reinforcing fibres 138 may form the fibres of a relatively thick outer layer or ply as described hereinbefore, or the laminated body may have the fibres of such a layer subsequently wound thereon. Another possibility is to use a braid rather than wound fibres to form the outer layer.

As before, whilst the description above is of the provision of the stitched reinforcing fibres 138 at or adjacent the outer periphery of the layers 136, reinforcing fibres may alternatively or additionally be located adjacent the inner periphery of layers 136, if desired.

The structure shown in Figure 9 is advantageous in that it permits the manufacture of a composite material component for a gear of relatively large diameter in a simple and convenient manner. It will be appreciated that where gears are required to be of a large diameter, the weight savings that can be made through the use of composite materials are significant.

Turning to Figure 10, an arrangement is illustrated in which a composite material component 140 is provided, an outer periphery thereof being formed with gear teeth formations 142. An outer layer 144 of a metallic material is applied over the outer periphery of the component material component 140. The outer layer 144 could take the form of a component pre-formed to the required shape that is press-fitted or the like to the composite material component. Alternatively, it could comprise a foil or the like applied to the composite material component 140, or it could comprise a coating applied thereto. In such an arrangement, the composite material component 140 provides the gear with the required load bearing and transmitting capacity, the metallic material layer 144 providing the required level of hardness and wear resistance.

The composite material component 140 may take the form of an annular element of wound fibre reinforced composite material which has been cured and, after cured, is machined to form the teeth formations 142 in the outer periphery thereof. Such a manufacturing technique has the advantage that the reinforcing fibres are exposed at the flanks of the gear teeth formations 142, aiding in achieving the required load bearing characteristics in the gear. However, other manufacturing techniques may be used. By way of example, the composite material may be wound in such a fashion as to take substantially the required shape, no subsequent machining (other than minor finishing) being required to form the gear teeth formations. The fibres within the composite material component 140 are preferably arranged at a range of winding angles to achieve the required level of strength and load bearing capacity.

It will be understood, as described hereinbefore, that gears incorporating composite materials in this manner are advantageous in that significant weight savings can be made without significantly reducing the strength or other characteristics of the gear, and the weight savings can lead to operating efficiencies, in use. Another benefit of such gears is that they are inherently more flexible than many metallic material gears, and so can provide additional damping, improving the fatigue life of the gears.

Although specific embodiments of the invention have been described hereinbefore, it will be appreciated that a wide range of modifications and alterations may be made thereto without departing from the scope of the invention as defined by the appended claims.