Composite gear

Abstract

A composite gear (200) comprising a gear body having a plurality of teeth (202) extending in a radial direction and defining a gear mesh surface, wherein the gear mesh surface is at least partially covered by a plurality of discontinuous fibre-reinforced polymer portions (210) having fibres orientated in a direction parallel to the gear mesh surface.

Description

COMPOSITE GEARBACKGROUND

[0001] Gears for the transmission of rotation and / or power are well known and are widely used in a range of different applications, for example in automotive and aerospace applications, electric bicycles, and in robots. Metal gears and plastic gears are both well known. Types of gears include parallel axis gears, crossed axis gears, and involute gears.

[0002] Figure 1A shows a known gear 100 made of metal, having a plurality of teeth 102. Gears made of metal are able to withstand high torque loading forces but are significantly heavier and generate more noise when they mesh together. To address this issue, plastic gears such as the known example shown in Figure IB have been developed which benefit from being lighter in weight and generate less noise. However, plastic gears are not able to withstand high torque loading forces to the same extent as metal gears, and are often susceptible to wear and therefore need replacing more frequently.

[0003] It is desirable to provide a gear with increased performance of strength while being lightweight and which generates less noise during operation. It is an object of the present invention to provide a composite gear with a good balance of performance, weight and noise. It is another object of the present invention to provide a composite gear suitable for use in a wider variety of applications.SUMMARY OF THE INVENTION

[0004] The present invention is defined in the appended claims.

[0005] According to one aspect, there is provided a composite gear comprising a gear body having a plurality of teeth extending in a radial direction and defining a gear mesh surface, wherein the gear mesh surface is at least partially covered by a plurality of discontinuous fibre- reinforced polymer portions having fibres orientated in a direction parallel to the gear mesh surface.

[0006] Preferably, the gear mesh surface is partially covered by said discontinuous fibre- reinforced polymer portions. Preferably, said polymer portions cover less than 100% of the gear mesh surface. Preferably, said portions cover less than 90% of said surface, preferably between 25% and 90% of said surface.

[0007] Thus, by at least partially covering the gear mesh surface with a plurality of discontinuous fibre-reinforced polymer portions having fibres orientated parallel to the gear mesh surface, the strength of the gear is improved. The orientation of the fibres in the discontinuous fibre-reinforced polymer portions parallel to the gear mesh surface aligns the fibre matrix and leads to a high resistance to the loading applied to the gear, increasing the strength of the gear. The gear mesh surface is subject to stresses when gears mesh together, so it is advantageous to reinforce these surfaces using the fibre-reinforced polymer portions. An increase in torque capability is provided without significantly increasing the weight of the gear and since the composite gear does not have teeth that are made of metal, the meshing of gears does not generate significant noise. Additionally, providing the fibre-reinforcement as a plurality of discontinuous fibre-reinforced polymer portions improves the manufacturability and longevity of the composite gear. The composite gear of the present invention therefore has improved manufacturability and provides a good balance of performance, weight and noise, and can be used in a wider variety of applications.

[0008] In examples, the discontinuous fibre-reinforced polymer portions are carbon fibre- reinforced polymer portions.

[0009] In examples, the discontinuous fibre-reinforced polymer portions are fused into or onto the gear body to form the gear mesh surface.

[0010] In examples, the discontinuous fibre-reinforced polymer portions at least partially cover a gear root of the gear mesh surface. By providing discontinuous fibre-reinforced polymer portions that cover a gear root of the gear mesh surface, the area subject to high stresses is reinforced, improving the strength and reducing the wear of the gear. In examples, the discontinuous fibre-reinforced polymer portions cover every gear root of the gear mesh surface.

[0011] In examples, the discontinuous fibre-reinforced polymer portions comprise at least one U-shaped segment. In examples, the U-shaped segment spans between two adjacent gear flanks of the gear mesh surface. In examples, the U-shaped segment extends across the gear root between the two adjacent gear flanks of the gear mesh surface.

[0012] In examples, the discontinuous fibre-reinforced polymer portions comprise at least one S-shaped segment. In examples, the S-shaped segment extends between a gear tip and a gear root of the gear mesh surface.

[0013] In examples, a portion of the gear body at least partially extends over an end of the fibre-reinforced polymer portions to retain the fibre-reinforced polymer portions in place.

[0014] In examples, the ends of at least one fibre-reinforced polymer portions are tapered.

[0015] In examples, the ends of at least one fibre-reinforced polymer portions are nonparallel.

[0016] In examples, at least one spacing between adjacent discontinuous fibre-reinforced polymer portions. In examples, the spacing is located at a gear tip of the gear mesh surface. In examples, the gear tip is free from the discontinuous fibre-reinforced polymer portions.

[0017] In other examples, at least two of the discontinuous fibre-reinforced polymer portions are in abutment on the gear mesh surface. In examples, the at least two discontinuous fibre- reinforced polymer portions are in abutment at a gear root of the gear mesh surface. In examples, the at least two discontinuous fibre-reinforced polymer portions are in abutment at a gear tip of the gear mesh surface. In examples, the at least two discontinuous fibre-reinforced polymer portions are in abutment at a gear flank of the gear mesh surface.

[0018] In examples, the discontinuous fibre-reinforced polymer portions at least partially cover a tooth face and / or tooth flank of the gear mesh surface.

[0019] In examples, the gear is comprised of a plastic and fibres. The gear may comprise a plastic matrix with fibers embedded within or it may comprise regions of a plastic matrix with fibers embedded within. The plastic may be a thermosetting plastic or a thermoplastic or a mixture thereof. Preferably, the plastic is a thermoplastic. A thermoplastic is advantageous as it is easier to manufacture; results in higher toughness, lower void content and more preferable fibre orientation which are all factors in gear performance.

[0020] In one embodiment, the thermoplastic may be selected from polyaryletherketones (PAEK), such as polyetheretherketone (PEEK) or polyetherketoneketone (PEKK), polyphenylene sulphide (PPS), polyetherimide (PEI), polycarbonate (PC), polyethylene terephthalate (PET), polyamide (PA), poly(methyl methacrylate) or mixtures thereof.

[0021] Preferably, the plastic is a thermoplastic that comprises a polyaryletherketone (PAEK), such as polyetheretherketone (PEEK). PAEKs are advantageous because of their high temperature and chemical resistance, and low permeability.

[0022] In particular, the PAEK may comprise a repeat unit of formula:

[0023] wherein tl and wl independently represent 0 or 1 and vl represents 0, 1 or 2.

[0024] Preferred PAEKs have a repeat unit wherein tl=l, vl=0 and wl=0; tl=0, vl=0 and wl=0; tl=0, wl=l, vl=2; or tl=0, vl=l and wl=0. More preferred PAEKs have a repeat unit wherein tl=l, vl=0 and wl-0; or tl=0, vl=0 and wl=0. The most preferred has a repeat unit wherein tl=l, vl=0 and wl=0.

[0025] In preferred embodiments, the PAEK is selected from polyetheretherketone, polyetherketone, polyetherketoneetherketoneketone and / or polyetherketoneketone, and / or a copolymer including polyetheretherketone and polyetherdiphenyletherketone. In a more preferred embodiment, the PAEK is selected from polyetherketone and / or polyetheretherketone, and / or a copolymer including polyetheretherketone and polyetherdiphenyletherketone. In an especially preferred embodiment, the PAEK is selected from polyetheretherketone, and / or a copolymer including polyetheretherketone and poly etherdiphenyletherketone.

[0026] The PAEK suitably includes at least 50 mol%, (e.g. 50-99.8 mol%), preferably at least 60 mol% (e.g. 60-100 mol%), more preferably at least 68 mol% (e.g. 68 to 100 mol%), of repeat units of formula I, especially such units where tl=l, vl=0 and wl=0. In an especially preferred embodiment, the PAEK includes at least 90 mol%, preferably at least 95 mol%, more preferably at least 98 mol%, especially at least 99 mol% of repeat units of formula I, especially repeat units of formula I wherein tl=l, vl=0 and wl=0. Other repeat units in the PAEK may be different repeat units of formula I or may include -Ph-Ph- moieties where Ph represents an unsubstituted phenylene moiety (especially wherein both -Ph- moieties are linked to each other and to adjacent repeat units at the 4,4’ positions-). Other repeat units may include Ph moieties bonded to two moieties selected from carbonyl moieties and ether moieties and -Ph-Ph- moieties bonded to two ether moieties.

[0027] The PAEK may be a copolymer which comprises a first moiety of formula I and a second moiety which includes -Ph-Ph- moieties where Ph represents an unsubstituted phenylene moiety (which suitably includes 4,4’ bonds to adjacent moieties).

[0028] In one embodiment, the PAEK may be selected from: a polymer comprising at least 98 mol% of a repeat unit of formula I, especially such units wherein tl=l, vl=0 and wl=0; and a copolymer which includes a repeat unit of formula:-O-Ph-O-Ph-CO-Ph- II and a repeat unit of formula:-O-Ph-Ph-O-Ph-CO-Ph- III

[0029] wherein Ph represents a phenylene moiety. Preferably the repeat units of the copolymer consist essentially of the repeat units II and III.

[0030] In a preferred embodiment, the PAEK is homopolymer polyetheretherketone, PEEK, with repeat units consisting of formula II:

[0031] -O-Ph-O-Ph-CO-Ph- II

[0032] or is a copolymer with repeat units consisting repeat units of formula II and repeat units of formula III:

[0033] -O-Ph-Ph-O-Ph-CO-Ph- III.

[0034] The ends of the polymer may be provided by the same monomers as the monomers making up the repeat units or may be provided by other compounds specifically added to provide end-capping.

[0035] The PAEK may preferably comprise at least 98 mole% (e.g. 98 to 99.9 mole%) of a repeat unit of formula I or a copolymer which includes repeat units of formulae II and III.

[0036] In the copolymer, the repeat units II and III are preferably in the relative molar proportions VI: VII of from 50:50 to 95:5, more preferably from 60:40 to 95:5, even more preferably from 65:35 to 95:5.

[0037] The phenylene moieties (Ph) in each repeat unit II and III may independently have 1,4- para linkages to atoms to which they are bonded or 1,3- meta linkages. Where a phenylene moiety includes 1,3- linkages, the moiety will be in the amorphous phase of the polymer.Crystalline phases will include phenylene moieties with 1,4- linkages. It is generally preferred for the PAEK or PEEK to be semi-crystalline, for instance having a crystallinity of about 25 to 35% and, accordingly, the PAEK or PEEK preferably includes high levels of phenylene moieties with 1,4- linkages.

[0038] In a preferred embodiment, at least 95%, preferably at least 99%, of the number of phenylene moieties (Ph) in the repeat unit of formula II have 1,4-linkages to moieties to whichthey are bonded. It is especially preferred that each phenylene moiety in the repeat unit of formula II has 1,4-linkages to moieties to which it is bonded.

[0039] In a preferred embodiment, at least 95%, preferably at least 99%, of the number of phenylene moieties (Ph) in the repeat unit of formula III have 1,4-linkages to moieties to which they are bonded. It is especially preferred that each phenylene moiety in the repeat unit of formula III has 1,4- linkages to moieties to which it is bonded.

[0040] Preferably, the phenylene moieties in the repeat unit of formula II are unsubstituted. Preferably, the phenylene moieties in the repeat unit of formula III are unsubstituted.

[0041] The repeat unit of formula II preferably has the structure:

[0042] The repeat unit of formula III preferably has the structure:

[0043] The copolymer may include at least 50 mol%, preferably at least 60 mol% of repeat units of formula IV. Particular advantageous copolymers may include at least 62mol%, or, especially, at least 64 mol% of repeat units of formula IV. The copolymer may include less than 90 mol%, suitably 82mol% or less of repeat units of formula IV. The copolymer may include 58 to 82 mol%, preferably 60 to 80 mol%, more preferably 62 to 77 mol% of units of formula IV.

[0044] The copolymer may include at least 10 mol%, preferably at least 18 mol%, of repeat units of formula V. The copolymer may include less than 42 mol%, preferably less than 39 mol% of repeat units of formula V. Particularly advantageous copolymers may include 38 mol% or less; or 36 mol% or less of repeat units of formula V. The copolymer may include 18 to 42 mol%, preferably 20 to 40 mol%, more preferably 23 to 38 mol% of units of formula V.

[0045] The sum of the mol% of units of formula IV and V in the copolymer is suitably at least 95 mol%, is preferably at least 98 mol%, is more preferably at least 99 mol%.

[0046] In examples, polyaryletherketone such as polyetheretherketone is compounded with up to 30 wt % of glass fibres or carbon fibres. Such materials have been commercially available for many years from Victrex Pic under the names VICTREX PEEK 150GL30 and 150CA30 respectively.

[0047] In examples, the composite gear is any one of a: spur gear, bevel gear, helical gear, worm gear, miter gear, screw gear, double helical gear, herringbone gear, rack and pinion, spiral gear, hypoid gear, planetary gear, strain wave gear or cycloidal gear.

[0048] According to another aspect, there is provided a method of making a composite gear, comprising the steps of: forming a gear body, the gear body having a plurality of teeth extending in a radial direction and defining a gear mesh surface; and at least partially covering the gear mesh surface with a plurality of discontinuous fibre-reinforced polymer portions having fibres orientated in a direction parallel to the gear mesh surface.

[0049] In examples, the discontinuous fibre-reinforced polymer portions are carbon fibre- reinforced polymer portions.

[0050] In examples, the method comprises positioning a plurality of discontinuous fibre- reinforced polymer portions in a mould of an injection moulding machine, and injection moulding the gear body behind the fibre-reinforcement portions such that the plurality of discontinuous fibre-reinforced polymer portions are positioned on and / or within the gear mesh surface of the injection moulded gear body. In examples, the method further comprises fusing the discontinuous fibre-reinforced polymer portions onto the gear body to form the gear mesh surface in the mould of the injection moulding machine.

[0051] In examples, the method further comprises preforming the discontinuous fibre- reinforced polymer portions into shaped segments before positioning the shaped segments in the mould. In examples, the discontinuous fibre-reinforced polymer portions are preformed as U-shaped segments, or as S-shaped segments.BRIEF DESCRIPTION OF THE DRAWINGS

[0052] Embodiments of the invention are now described, by way of example only, hereinafter with reference to the accompanying drawings.

[0053] FIG. 1 A (PRIOR ART) illustrates a known gear made of metal.

[0054] FIG. IB (PRIOR ART) illustrates a known gear made of plastic.

[0055] FIG. 2A illustrates perspective view of an example composite gear.

[0056] FIG. 2B illustrates close up view of a composite gear showing the arrangement of fibres in a fibre-reinforced polymer portion.

[0057] FIG. 3A illustrates a schematic representation showing different teeth of a composite gear having different arrangements of fibre-reinforced polymer portions.

[0058] FIG. 3B shows an enlarged view of a tooth of the composite gear of FIG. 3 A.

[0059] FIG. 4 illustrates a schematic representation of a gear tooth subject to a load.

[0060] FIG. 5 illustrates a schematic representation of a three-point bend test used to approximate the loading of a composite gear tooth.

[0061] FIG. 6 is a graph showing initial testing data of a three-point bend test.

[0062] FIG. 7 is a graph showing testing data that is corrected and replotted.

[0063] FIG. 8 shows the graph of Figure 7, indicating the elastic region of the data.

[0064] FIG. 9 is a graph comparing results for natural PEEK to composite test specimens.

[0065] FIG. 10 is a graph comparing results for specimens having a 48mm span and specimens having a 120mm span, indicating the points of interest for specimens having a 48mm span.

[0066] FIG. 11 is a graph comparing results for specimens having a 48mm span and specimens having a 120mm span, indicating the points of interest for specimens having a 120mm span.

[0067] FIG. 12 illustrates an example of a fibre-reinforced polymer portion provided as a single concentric piece.

[0068] FIG. 13 illustrates a part of an injection moulding tool used to form a composite gear.DETAILED DESCRIPTION

[0069] Certain terminology is used in the following description for convenience only and is not limiting. The words ‘right’, ‘left’, ‘lower’, ‘upper’, ‘front’, ‘rear’, ‘upward’, ‘down’ and ‘downward’ designate directions in the drawings to which reference is made and are with respect to the described component when assembled and mounted. The words ‘inner’, ‘inwardly’, ‘outer’ and ‘outwardly’ refer to directions toward and away from, respectively, adesignated centreline or a geometric centre of an element being described (e.g. central axis), the particular meaning being readily apparent from the context of the description.

[0070] Further, as used herein, the terms ‘connected’, ‘attached’, ‘coupled’ and ‘mounted’ are intended to include direct connections between two members without any other members interposed therebetween, as well as, indirect connections between members in which one or more other members are interposed therebetween. The terminology includes the words specifically mentioned above, derivatives thereof, and words of similar import.

[0071] Further, unless otherwise specified, the use of ordinal adjectives, such as, “first”, “second” and “third” etc. merely indicate that different instances of like objects are being referred to and are not intended to imply that the objects so described must be in a given sequence, either temporally, spatially, in ranking or in any other manner.

[0072] Like reference numerals are used to depict like features throughout.

[0073] Referring now to Figure 2A, there is shown a composite gear 200. The gear 200 includes a gear body having a plurality of teeth 202 and a central opening 218. The opening 218 is provided on an interface 212 for supporting a shaft (not shown) that is inserted through the gear 200 during operation of the gear 200. It will be appreciated that in some other examples the composite gear 200 may have a different feature for mounting to a shaft or flange or other, for example fastener holes. In this example, the gear 200 is a spur gear. However, it is envisaged that in other examples, the gear 200 may be a bevel gear, helical gear, worm gear, miter gear, screw gear, double helical gear, herringbone gear, rack and pinion, spiral gear, hypoid gear, planetary gear, strain wave gear or cycloidal gear.

[0074] The teeth 202 of the gear 200 protrude outwardly in a radial direction and each have a gear root 204, gear tip 206 and gear face 208. The teeth 202 define a gear mesh surface that is radially outward facing and covered by a plurality of discontinuous fibre-reinforced polymer portions 210. The gear mesh surface meshes with another gear during use and load is transferred from one gear to the other through the gear mesh surface. The gear mesh surface includes the gear root 204, gear tip 206 and gear face 208. The fibre-reinforced polymer portions 210 contain fibres (216, Figure 2B) that are orientated in a direction parallel to the gear mesh surface, as best illustrated in Figure 2B. The fibre-reinforced polymer portions 210 are carbon fibre-reinforced portions. The carbon fibre-reinforced polymer portions 210 may be provided in the form of a unidirectional tape separated into individual strips. In this particular example, each of the fibre-reinforced polymer portions 210 is provided as a U-shaped segment.The U-shaped segments extend across the gear mesh surface from adjacent to one gear tip 206, over the gear root 204, to adjacent to the next gear tip 206. Although in this example, the fibre- reinforced polymer portions 210 are provided as U-shaped portions, it is envisaged that the fibre-reinforced polymer portions 210 may instead be provided as S-shaped portions. For example, the U-shaped portions may instead be provided with a gap at the gear root 204 to form two separate S-shaped portions, each provided on a gear face 208 of adjacent gear teeth 202.

[0075] The gear 200 is made of polyaryletherketone (PAEK) in this example. However, other materials are envisaged such as another polymer such as polyether ketone ketone (PEKK) or a co-polymer such as LMPAEK™ material made by Victrex Manufacturing Limited. The fibre- reinforced polymer portions 210 are provided separate from one another. Accordingly, the fibre-reinforced polymer portions 210 are discontinuous. The fibres between the fibre- reinforced polymer portions 210 are also separate from and discontinuous from one another. The fibre-reinforced polymer portions 210 cover a gear root 204 of the teeth 202, and extend to additionally cover a gear face 208 of the teeth 202, so that the fibre-reinforced polymer portions 210 span between a gear face 208 of two adjacent gear teeth 202. The gear face 208 forms a part of the gear mesh surface that engages with a tooth 202 of an adjacent gear during operation. The gear face 208 may be termed a gear flank. Adjacent gear faces 208 face one another. In this example, the gear tips 206 of the teeth 202 are free from the fibre-reinforced polymer portions 210. That is, the ends of the fibre-reinforced polymer portions 210 are spaced from each other at the gear tips 206. The fibre-reinforced polymer portions 210 are fused onto the gear mesh surface of the teeth 202. Although the fibre-reinforced polymer portions 210 cover all of the gear roots 204 in this example, it is envisaged that some examples, the fibre- reinforced polymer portions 210 cover one or some of the gear roots 204.

[0076] As described, although the fibre-reinforced polymer portions 210 are provided as U- shaped segments in Figure 2A, the fibre-reinforced polymer portions 210 may alternatively be provided as S-shaped segments. The S-shaped segments may be arranged on a portion of the gear mesh surface of the teeth 202, with each fibre-reinforced polymer portion 210 extending between a gear tip 206 and a gear root 204. In some examples, the S-shaped segments may be arranged to extend between and cover the gear tip 206 and the gear root 204. The gear tip 206 and the gear root 204 may be on the same gear tooth 202.

[0077] Figure 2B shows a close-up view of a tooth 202 of the composite gear 200 at the gear root 204. The surface of the tooth 202 is covered by a fibre-reinforced polymer portion 210, which in this example is a U-shaped segment that covers the gear root 204. The fibre-reinforced polymer portion 210 has a plurality of fibres 216 that are orientated in a direction parallel to the surface of the gear tooth 202. The alignment of the fibres 216 provides a high resistance to the loading applied to the gear, which increases the strength of the gear. In particular, the teeth 202 of the composite gear 200 are stiffened, to expand the torque capability of the gear 200. In addition, the direction of the fibres 216 parallel to the surface of the gear tooth 202 may reduce friction between the gear 200 and the other gear during use. In this example, in addition to fibres 216 provided in a direction parallel to the surface the gear tooth 202, perpendicular fibres 214 are also provided in a direction perpendicular to the surface of the gear tooth 202. The perpendicular fibres 214 are provided close to an outer layer of the fibre-reinforced polymer portion 210 to improve wear characteristics of the gear.

[0078] To manufacture the composite gear 200, the fibre-reinforced polymer portions 210 are preformed into the desired shape, which may be a U-shape or a S-shape, for example. The fibre-reinforced polymer portions 210 are placed into a gear tool of an injection moulding machine, and a polymer is injected behind the fibre-reinforced polymer portions 210 to form the gear 200 with the fibre-reinforced polymer portions 210 in place. The fibre-reinforced polymer portions 210 may be fused onto the polymer during the injection moulding, for example by the heat of the injection moulding process or by subsequent heating of the mould. The polymer may be PEEK, a co-polymer such as Victrex LMPAEK™ material, or PEKK, for example. The fibre-reinforced polymer portions 210 may be preformed into a number of segments equal to the number of teeth 202 on the gear 200 (e.g. for U-shaped segments), or twice the number of teeth 200 on the gear 200 (e.g. for S-shaped segments).

[0079] Figure 3 A shows a gear having different fibre-reinforced polymer portions 310 (i.e. 310a, 310b, 310c, 31 Od) covering the teeth 302. Figure 3 A illustrates each tooth provided with a different profile of a fibre-reinforced polymer portion 310. In each example the fibre- reinforced polymer portion 310 is U-shaped. It is envisaged that each tooth 302 may be provided with any one of the profiles of fibre-reinforced polymer portions 310 shown. In these examples, advantageously, the gear roots 304 of the teeth 302 are covered by the fibre- reinforced polymer portions 310 as these areas are subject to high stress during use of the gear. Covering the gear roots 304 with fibre-reinforced polymer portions 310 reduces the stress atthese areas. The gear tips 306 of the teeth 302 are free from the fibre-reinforced polymer portions 310. This does not impact the performance of the gear 300 as the tips 306 are subject to low stresses and typically do not contact the meshing gear during use. The gear faces 308 of the teeth 302 covered by the fibre-reinforced polymer portions 310 are subject to higher loads during use because these are contact areas engaged by the meshing gear. Covering the gear faces 308 reduces stresses at these areas.

[0080] The fibre-reinforced polymer portions 310 may be provided in a number of different profiles on the gear teeth 302. As shown, the fibre-reinforced polymer portions 310 may be provided symmetrically on the gear faces 308 of the teeth 302. In other examples, the fibre- reinforced polymer portions 310 may be provided asymmetrically on the gear faces 308 of the teeth 302. In some examples, the end of the fibre-reinforced polymer portions 310 may be perpendicular to the gear tip 306. In some examples, the fibre-reinforced polymer portions 310 may terminate at the gear tip 306. In other examples, the fibre-reinforced polymer portions 310 may terminate at the gear face 308 before it reaches the gear tip 306.

[0081] The first example fibre-reinforced polymer portion 310a of FIG. 3 A extends to the gear tip 306, but does not extend over it. The shaped ends of adjacent fibre-reinforced polymer portions 310a are spaced from each other. The apex of the gear tip 306 is therefore free from the fibre-reinforced polymer portions 310a. This spacing prevents contact between the adjacent fibre-reinforced polymer portions 310a, which may cause them to detach from the gear tooth 302 either during manufacture (e.g., in moulding, or in post-moulding shrinkage) or during use as loads are applied to the fibre-reinforced polymer portions 310a.

[0082] The second example fibre-reinforced polymer portion 310b of FIG. 3 A has ends that are spaced from the gear tip 306. The gear tip 306 partially extends over an end of the fibre- reinforced polymer portion 310b and acts to retain the fibre-reinforced polymer portion 310b in place (the fibre-reinforce portion 310b is wedged between the gear tips 306 of adjacent gear teeth 302). During manufacture, the body of the gear 300, which is made of a polymer, is moulded partly over and / or around the fibre-reinforced polymer portion 310b so that the fibre- reinforced polymer portion 310b is retained after moulding.

[0083] The third example fibre-reinforced polymer portion 310c of FIG. 3 A has tapered parallel ends, so that adjacent fibre-reinforced polymer portions 310c have ends that are parallel with one another. The end of the fibre-reinforced polymer portion 310c may be perpendicular to the surface of the gear tip 306.

[0084] The fourth example fibre-reinforced polymer portion 310d of FIG. 3 A has tapered nonparallel ends, as best shown in Figure 3B. Adjacent fibre-reinforced polymer portions 310d have ends that are non-parallel with one another and diverge towards the gear tip 306. The gear tip 306 is therefore free from the fibre-reinforced polymer portions 310d and a spacing is formed in between the ends of the fibre-reinforced polymer portions 310d. The gear tip 306 partially extends over an end of the fibre-reinforced polymer portion 31 Od to retain the fibre- reinforced polymer portion 31 Od in place. The divergence between the ends of the fibre- reinforced portions 31 Od increases the retention of the fibre-reinforced portions 31 Od by the moulded gear tip 306. During manufacture, the body of the gear 300 is moulded partly over and / or around the fibre-reinforced polymer portion 310d so that the fibre-reinforced polymer portion 31 Od is retained after moulding.

[0085] Experiment

[0086] Figures 4 and 5 shows the approximation of a gear tooth 402 as a rectangular beam for testing under a load. Figure 4 shows a tooth 402 of a gear 400 that is subject to a load F in a direction denoted ‘d’. The gear tooth 402 is composite gear tooth of a composite gear 400 with a fibre-reinforced polymer portion that is approximated as a rectangular beam in a three-point bend test.

[0087] Figure 5 shows a three-point bend test of a composite rectangular beam subject to a load F. The rectangular beam has a polymer part 511 at the core and fibre-reinforced polymer portions 510 covering either side of the polymer part 511. In some examples, the fibre- reinforced polymer portion 510 may be at the core and the polymer part 511 on either side. The rectangular beam is supported on the sides by supports 530. The three-point bend test method is as described in the ASTM D790 standard, dated 1 July 2017. The test is performed at room temperature with two different span-to-depth ratios: 16: 1 (48mm span) and 40: 1 (120mm span). A span-to-depth ratio of 16:1 is recommended for most materials tested with the ASTM D790 standard. A span-to-depth ratio of 40: 1 is recommended when the test specimen has composite reinforced due to low in-plane shear strength and relatively high tensile strength in the outer layers.

[0088] Figure 6 shows a stress-strain curve for the deformation under load of the beams with a span-to-depth ratio of 16: 1 (48mm span) and 40: 1 (120mm span).

[0089] The original testing data was reviewed and replotted such that all stress values had a Y intercept of 1 OOMPa. This method was used to get rid of the toe in stress as set out in appendixAl of the ASTM D790 standard. The modulus of elasticity was calculated in the elastic portion of the graph. The slope was calculated between stress values of 100 and 800 Mpa. This resulted in a more consistent value for the modulus, specifically in the test span of the span-to-depth ratio 16:1.

[0090] Table 1 sets out the calculated modulus for each of the original and corrected data for a span-to-depth ratio of 40: 1. The difference in the modulus values are negligible because the toe region in the curve is insignificant.Table 1: Comparison of 40:1 span calculation

[0091] Table 2 sets out the calculated modulus for each of the original and corrected data for a span-to-depth-ratio of 16: 1. The difference in calculations are greater than the previous 40:1 span data, which is likely due a more significant toe region in the curve for a span-to-depth ratio of 16: 1.Table 2: Comparison of 16:1 span calculation

[0092] Figure 7 shows a stress-strain curve with the corrected data for the deformation under load of the beams with a span-to-depth ratio of 16:1 (48mm span) and 40: 1 (120mm span). Thedata is corrected for initial displacement to set the Y intercept to 1 OOMpa. The strain is set to zero where the stress reached 1 OOMpa, so that all data points are shifted to the left.

[0093] Literature describes that typically, the flexural modulus increases with span length to an extent, and then decreases so the difference in stiffness response is expected when span length is varied. Table 3 shows a comparison between the flexural modulus and flexural strength of a 16: 1 span-to-depth ratio (48 mm span) composite solution such as a co-polymer (e.g. Victrex LMPAEK™), and material grades of Victrex high-performance gears (HPG) 140 and 240. Victrex HPG 140 and Victrex HPG 240 are grades of gears made from PEEK. The data is taken at room temperature.Table 3: Material comparison between the 000463 16:1 span ratio composite solution and grades of HPG PEEK at room temperature

[0094] Figure 8 highlights the elastic region of the stress-strain curve with a dashed rectangle. Assessment for the modulus were done in this region, from 100 to 800 MPa, and horizontal reference lines are shown to indicate where assessments were made.

[0095] Figure 9 shows a stress-strain curve for natural PEEK HPG140 and the 000463 composite test specimens in flexural testing.

[0096] FIG. 10 is a graph comparing results for specimens having a 48mm span and specimens having a 120mm span, indicating the points of interest for specimens having a 48mm span. FIG. 11 is the same graph, but indicating the points of interest for specimens having a 120mm span. The observed failure modes differed between specimens having different span lengths. The failure modes of the 120mm span specimens were a through- thickness shear failure and delamination (interply failure). The failure mode of the 48mm span specimens was, in addition to through-thickness shear failure and delamination (interply failure), transverse shear failure of the PEEK core.

[0097] The stress-strain curve shown in Figure 10 and Figure 11 shows that the 40: 1 span tests fail at lower flexural strain levels than the 16: 1 span tests. The bottom composite layers inthe 40: 1 specimens exhibited more through-thickness shear failure, instead of the delamination- dominated bottom layer failure modes shown in the 16: 1 specimens.

[0098] The results of the experiment show that a composite gear provides a high energy density gear solution and transmits torques currently not possible in natural PEEK gear solutions.

[0099] Examples for composite gear manufacture

[0100] Referring to Figure 12, there is shown a fibre-reinforced polymer portion 610 that is preformed as one concentric piece. In this example, the fibre-reinforced polymer portion 610 is a carbon fibre-reinforced polymer portion. The fibre-reinforced polymer portion 610 is preformed by bending and fusing it into a single gear shaped outer piece. To manufacture a composite gear, polymer is injection moulded behind the preformed fibre-reinforced polymer portion 610 to hold it in place. Fusion of the fibre-reinforced polymer portion 610 may take place inside the injection moulding tool or prior to injection moulding.

[0101] Alternatively, instead of preforming the fibre-reinforced polymer portion 610 by bending and fusing it into a single gear shaped outer piece, the fibre-reinforced polymer portion 610 may be preformed by 3D printing the shape of the perimeter of a gear.

[0102] Figure 13 shows part of an injection moulding tool 732 with a gear cavity 736. The gear 300 shown in Figure 3A can be injection moulded using the injection moulding tool 732. Discrete fibre-reinforced polymer portions are preformed into the desired shape, e.g. U-shape or a S-shape, and are placed into cavities 736 within the injection moulding tool 732. A polymer is injected behind the fibre-reinforced polymer portions to form a gear 700 that is covered with fibre-reinforced polymer portions. The fibre-reinforced polymer portions may be fused onto the polymer during the injection moulding, for example by the heat of the injection moulding process or by subsequent heating of the mould. The polymer may be PEEK, a copolymer such as Victrex LMPAEK™, or PEKK, for example. The number of discrete segments of the fibre-reinforced polymer portions may be equal to the number of teeth on the gear 700 (e.g. for U-shaped segments), or twice the number of teeth on the gear 700 (e.g. for S-shaped segments). A venting line 738 to provided for gas to escape.

[0103] Another way of creating the fibre-reinforced polymer portion is to take a composite material and weave it into a tube with the appropriate layers of laminate. The outer diameter of the composite would allow for it to expand into the shape of the gear inside the injection moulding tool 732. The woven composite material is pressed into place along the gear tipprofiles in the injection moulding tool 732, and polymer is injection moulded behind the composite to hold it into place. The composite layer could fuse onto the polymer gear inside the injection moulding tool 732 or prior to injection moulding. This creates a gear 700 where the teeth are covered by the fibre-reinforced polymer portion.

[0104] It will be appreciated by persons skilled in the art that the above detailed examples have been described by way of example only and not in any limitative sense, and that various alterations and modifications are possible without departing from the scope of the invention as defined by the appended claims. Various modifications to the detailed examples described above are possible.

[0105] Through the description and claims of this specification, the words “comprise” and “contain” and variations of them mean “including but not limited to”, and they are not intended to (and do not) exclude other moieties, additives, components, integers or steps. Throughout the description and claims of this specification, the singular encompasses the plural unless the context otherwise requires. In particular, where the indefinite article is used, the specification is to be understood as contemplating plurality as well as singularity, unless the context requires otherwise.

[0106] Features, integers, characteristics, compounds, chemical moieties or groups described in conjunction with a particular aspect, embodiment or example of the invention are to be understood to be applicable to any other aspect, embodiment or example described herein unless incompatible therewith. All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and / or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and / or steps are mutually exclusive. The invention is not restricted to the details of any foregoing embodiments. The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract or drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.

[0107] It will be appreciated by persons skilled in the art that the above embodiment(s) have been described by way of example only and not in any limitative sense, and that various alterations and modifications are possible without departing from the scope of the invention as defined by the appended claims. Various modifications to the detailed designs as described above are possible.

Claims

CLAIMS1. A composite gear comprising a gear body having a plurality of teeth extending in a radial direction and defining a gear mesh surface, wherein the gear mesh surface is at least partially covered by a plurality of discontinuous fibre-reinforced polymer portions having fibres orientated in a direction parallel to the gear mesh surface.

2. A composite gear of claim 1, wherein the discontinuous fibre-reinforced polymer portions are carbon fibre-reinforced polymer portions.

3. A composite gear of claim 1 or 2, wherein the discontinuous fibre-reinforced polymer portions are fused onto the gear mesh surface.

4. A composite gear of any one of claims 1 to 3, wherein the discontinuous fibre-reinforced polymer portions at least partially cover a gear root of the gear mesh surface.

5. A composite gear of claim 4, wherein the discontinuous fibre-reinforced polymer portions cover every gear root of the gear mesh surface.

6. A composite gear of any one of claims 1 to 5, wherein the discontinuous fibre-reinforced polymer portions comprise at least one U-shaped segment.

7. A composite gear of claim 6, wherein the U-shaped segment spans between two adjacent gear flanks of the gear mesh surface.

8. A composite gear of claim 7, wherein the U-shaped segment extends across the gear root between the two adjacent gear flanks of the gear mesh surface.

9. A composite gear of any one of claims 1 to 5, wherein the discontinuous fibre-reinforced polymer portions comprise at least one S-shaped segment.

10. A composite gear of claim 9, wherein the S-shaped segment extends between a gear tip and a gear root of the gear mesh surface.

11. A composite gear of any one of claims 1 to 10, wherein a portion of the gear body at least partially extends over an end of the discontinuous fibre-reinforced polymer portions to retain the discontinuous fibre-reinforced polymer portions in place.

12. A composite gear of any one of the preceding claims, wherein the gear body is made of a polymer material comprising polyaryletherketone, PAEK, preferably a co-polymer.

13. A method of making a composite gear, comprising the steps of: forming a gear body, the gear body having a plurality of teeth extending in a radial direction and defining a gear mesh surface; and at least partially covering the gear mesh surface with a plurality of discontinuous fibre- reinforced polymer portions having fibres orientated in a direction parallel to the gear mesh surface.

14. The method of claim 13, wherein the discontinuous fibre-reinforced polymer portions are carbon fibre-reinforced polymer portions.

15. The method of clam 14, wherein the polymer comprises polyaryletherk etone, PAEK, preferably a co-polymer.

Images