Drive sprocket

Abstract

A drive sprocket (2) comprising a plurality of teeth (6, 8) for engaging with a drive member (20) to transmit rotary motion, the drive member (20) including a plurality of engagement pockets (22) for receiving the teeth (6, 8) of the drive sprocket (2). The plurality of teeth (6, 8) comprises a plurality of dual engagement teeth (6) and a pair of alternation teeth (8). Each dual engagement tooth (6) has a tooth profile defined by a first side (11) comprising a first engagement surface (13) and an opposite second side (12) comprising a second engagement surface (14), the engagement surfaces (13, 14) being configured such that, when a dual engagement tooth (6) is fully engaged with a respective engagement pocket (22), both engagement surfaces (13, 14) contact the engagement pocket (22). Adjacent dual engagement teeth (6) engage with every second engagement pocket (22). The pair of alternation teeth (8) are located adjacent to one another on the drive sprocket (2) and configured to engage with adjacent engagement pockets (22).

Description

[0001] DRIVE SPROCKET

[0002] This invention relates to a drive sprocket forming part of a transmission system that also comprises a drive member.

[0003] BACKGROUND TO THE INVENTION

[0004] Drive sprockets, or pulleys, having a plurality of teeth for use with drive members such as power transmission chains or belts are well known, and often take the form of a substantially circular sprocket having a plurality of teeth spaced apart around an outer circumference of the sprocket.

[0005] A variety of different drive members may be used with such drive sprockets.

[0006] One type of known drive member is a power transmission chain in the form of a roller chain. The roller chain has a plurality of engaging formations for enabling engagement with the drive sprocket. The engaging formations are in the form of receiving formations, for receiving the teeth of the drive sprocket. An example of a use of a roller chain is for a bicycle. The roller chain for a bicycle passes around a front drive sprocket in the form of a crank drive sprocket, and it also passes around a rear drive sprocket in the form of a gear wheel. Known roller chains may also be used in many other different types of apparatus including, for example, tricycles, motorcycles, chain saws and conveyor chains. Conveyor chains may be used to convey items or materials on conveyor lines.

[0007] Known drive sprockets do not transmit power to known drive members as efficiently as would be desired. More specifically, the drive members invariably make contact with the drive sprockets under significant loads, and in such situations, the drive members frequently tend to move relative to the teeth of the drive sprocket whilst maintaining contact under this high loading. The result is that known power transmission systems do not work efficiently. Known power transmission systems are also not as durable as desired because both the drive sprockets and the drive members suffer wear that lowers the lifespan of the components.

[0008] The listing or discussion of an apparently prior-published document in this specification should not necessarily be taken as an acknowledgement that the document is part of the state of the art or is common general knowledge.SUMMARY OF THE INVENTION

[0009] According to a first aspect of the invention, there is provided a drive sprocket comprising a plurality of teeth for engaging with a drive member to transmit rotary motion, the drive member including a plurality of engagement pockets for receiving the teeth of the drive sprocket, wherein the plurality of teeth comprises:

[0010] a plurality of dual engagement teeth wherein:

[0011] each dual engagement tooth has a tooth profile defined by a first side comprising a first engagement surface and an opposite second side comprising a second engagement surface, the engagement surfaces being configured such that, when a dual engagement tooth is fully engaged with a respective engagement pocket, both the first engagement surface and the second engagement surface contact the engagement pocket, and

[0012] adjacent dual engagement teeth do not engage with adjacent engagement pocket of the drive member; and,

[0013] a pair of alternation teeth located adjacent to one another on the drive sprocket and configured to engage with adjacent engagement pockets of the drive member.

[0014] The simultaneous contact between both engagement surfaces of a dual engagement tooth and the respective engagement pocket when fully engaged (or "meshed") may be generally referred to in the context of this disclosure as 'dual engagement'. The process of dual engagement results in the drive member gripping the tooth with stable contact points and reducing the relative movement between the drive member and drive sprocket during use. This, in turn, reduces the wear and tear of both the drive member and the drive sprocket. In particular, although contact loading between the engagement surfaces of a dual engagement tooth and respective engagement pocket is slightly increased, there is a reduction of the contact loading internally within the drive member. Reducing the internal contact loading is a key element of reducing wear of the drive member. In addition, frictional losses are reduced, which increases transmission efficiency.

[0015] The fact that the dual engagement teeth do not engage with every adjacent engagement pocket (instead they may engage with e.g. every second engagement pocket) of the drive member also means that articulation angles in the drive member vary as the drive member elongates over time. This further helps to reduce the internal wear in the drive member, increasing its lifespan before requiring replacement.In some embodiments, the adjacent dual engagement teeth engage with every second engagement pocket of the drive member.

[0016] In some embodiments, a majority of the plurality of teeth consists of the plurality of dual engagement teeth and one or more pairs of alternation teeth. In In other words, the sprocket may include other types of teeth, but predominantly consists of dual engagement teeth and alternation teeth.

[0017] In some embodiments, the plurality of teeth may consist only of the plurality of dual engagement teeth and the pairs of alternation teeth. In other words, the sprocket may not include any other types of teeth.

[0018] In some embodiments, the plurality of teeth comprises an odd number of pairs of alternation teeth. In some embodiments, the odd number of pairs of alternation teeth may be only one pair of alternation teeth.

[0019] Alternatively in other embodiments, the plurality of teeth may comprise an even number of pairs of alternation teeth, the teeth of each pair of alternation teeth being spaced apart with an angular spacing. The drive sprocket may further comprise an odd number of lengthened separation gaps, each lengthened separation gap having an angular length equal to m times the angular spacing between the teeth of each pair of alternation teeth, where m is an odd integer greater than or equal to 3. In some embodiments, the drive sprocket may consist of 1 lengthened separation gap. In some embodiments, m may be equal to 3. The or one or more of the lengthened separation gap(s) may be positioned separating one of the alternation teeth from an adjacent dual engagement tooth.

[0020] The drive member may comprise a plurality of spaced apart transverse members, each pair of adjacent transverse members forming a respective engagement pocket. Preferably, the drive sprocket may be configured such that two transverse members are positioned between adjacent dual engagement teeth when fully engaged with respective engagement pockets.

[0021] Conversely, the pair (or one or more of the pairs) of alternation teeth may be configured so that only one transverse member is positioned between the pair of alternation teeth when fully engaged with respective engagement pockets.The plurality of spaced apart transverse members may be connected together by links that alternate between inner links and outer links. Correspondingly, the engagement pockets may alternate between inner engagement pockets formed by inner links of the drive member, and outer engagement pockets formed by outer links of the drive member. In use, the pair of alternation teeth may cause all dual engagement teeth to alternate between engagement with inner and outer engagement pockets with every revolution of the sprocket for a drive member consisting of an even number of links.

[0022] The first engagement surface may be defined at least partially by a first arc, and the second engagement surface may be defined at least partially by a second arc. In some embodiments of the invention, the distance between the centre of the first arc and the centre of the second arc of each dual engagement tooth may be substantially the same as the distance between the centre of the first arc of a first dual engagement tooth and the centre of the second arc of an adjacent dual engagement tooth.

[0023] The drive member may be considered as having a pitch p. The 'pitch' is the distance from the centre of one transverse member to the centre of an adjacent transverse member. In some embodiments of the invention, the distance between the centre of the first arc and the centre of the second arc of each dual engagement tooth may be substantially equal to p. Similarly, the distance between the centre of the first arc of a first dual engagement tooth and the centre of the second arc of an adjacent dual engagement tooth may be substantially equal to p.

[0024] Preferably, the first and second sides of each dual engagement tooth may be symmetrical relative to a radial axis extending centrally through the tooth.

[0025] Additionally, the pair of alternation teeth may be symmetrical relative to a radial axis extending radially between the two alternation teeth. In other words, the two alternation teeth may be symmetrical to one another.

[0026] If each dual engagement tooth and the pair of alternation teeth have the above symmetrical characteristics, it may be possible for the drive sprocket to rotate in either direction with no difference in performance. The drive sprocket may also be well suited for applications having one drive direction but the potential for torque to be reversed during operation. Thus, the symmetrical characteristics make the drive sprocket suitable for a variety of different uses.Each tooth may extend along a radial axis. Preferably, the angular spacing between the radial axes of any two adjacent dual engagement teeth is substantially double the angular spacing between the radial axes of the pair of alternation teeth.

[0027] The drive member may be a chain drive member formed of a plurality of articulated chain links and the sprocket may have a size, n, corresponding to the minimum number of articulated chain links required to wrap around the drive sprocket to form a closed loop. In such embodiments, the plurality of dual engagement teeth may be regularly spaced apart with an angular spacing of 2a rad and the pair of alternation teeth may be spaced apart with an angular spacing of a rad, wherein a is equal to 2TT divided by n.

[0028] The minimum number of articulated chain links required to wrap around the drive sprocket to form a closed loop may correspond to a regular polygon with n sides having a side-length equal to the pitch p (mm) wherein the regular polygon is inscribed in a pitch circle, the pitch circle having a radius, rp(mm), equal top(m™

[0029] n

[0030] The drive member may be a roller chain wherein each transverse member is a roller. Preferably, the radius of each roller forming the roller chain may be substantially equal to, or slightly smaller than, the radius of each arc forming the first and second faced arcs.

[0031] According to a second aspect of the invention, there is provided a drive sprocket comprising a plurality of teeth for engaging with a drive member to transmit rotary motion, the drive member comprising a roller chain having a plurality of spaced apart rollers wherein each adjacent pair of rollers defines an engagement pocket for receiving a tooth of the drive sprocket, wherein the plurality of teeth comprises:

[0032] a plurality of dual engagement teeth wherein:

[0033] each dual engagement tooth has a tooth profile defined by a first side comprising a first engagement surface and an opposite second side comprising a second engagement surface, the engagement surfaces being configured such that, when a dual engagement tooth is fully engaged with a respective engagement pocket, the first engagement surface contacts one roller defining the engagement pocket and the second engagement surface contacts the other roller defining the engagement pocket, and

[0034] (at least, in some examples) two rollers are positioned between adjacent dual engagement teeth when the adjacent dual engagement teeth are fully engaged with respective engagement pockets; and,a pair of alternation teeth located adjacent to one another on the drive sprocket and configured so that only one roller is positioned between the pair of alternation teeth when the pair of alternation is fully engaged with respective engagement pockets.

[0035] The features and advantages of the first aspect of the invention and its embodiments apply mutatis mutandis to this aspect of the invention and its embodiments.

[0036] According to a third aspect of the invention drive sprocket comprising a plurality of teeth for engaging with a chain drive member formed of a plurality of articulated chain links, the drive sprocket having a size, n, corresponding to the minimum number of articulated chain links required to wrap around the drive sprocket to form a closed loop, the plurality of teeth comprising:

[0037] a plurality of dual engagement teeth regularly spaced apart with an angular spacing of 2a rad; and,

[0038] a pair of alternation teeth located adjacent to one another on the drive sprocket and spaced apart with an angular spacing of a rad, wherein a is equal to 2TT divided by n.

[0039] In some embodiments, there may comprise an odd number of pairs of alternation teeth, the alternation teeth of each pair being located adjacent to one another on the drive sprocket.

[0040] The features and advantages of the first and second aspects of the invention and their embodiments apply mutatis mutandis to this aspect of the invention and its embodiments.

[0041] The chain drive member has a pitch p (mm). The minimum number of articulated chain links required to wrap around the drive sprocket to form a closed loop corresponds to a regular polygon with n sides having a side-length equal to the pitch wherein the regular polygon is inscribed in a pitch circle having a radius, rp(mm), equal

[0042]

[0043] Preferably, each dual engagement tooth may have a tooth profile defined by a first side comprising a first engagement surface and an opposite second side comprising a second engagement surface, the engagement surfaces being configured such that when a dual engagement tooth is fully engaged with a respective engagement pocket, both the first engagement surface and the second engagement surface contact the engagement pocket.In embodiments of the invention, adjacent dual engagement teeth may engage with every second engagement pocket.

[0044] The chain drive member may comprise a plurality of spaced apart transverse members, each pair of adjacent transverse members forming a respective engagement pocket. The drive sprocket may be configured such that (at least, in some examples) two transverse members are positioned between adjacent dual engagement teeth when said adjacent dual engagement teeth are fully engaged with respective engagement pockets.

[0045] The pair of alternation teeth may be configured to engage with adjacent engagement pockets. In other words, the pair of alternation teeth may be configured so that only one transverse member is positioned between the pair of alternation teeth when the pair of alternation teeth is fully engaged with respective engagement pockets.

[0046] The plurality of spaced apart transverse members may be connected together by links that alternate between inner links and outer links. Correspondingly, the engagement pockets may alternate between inner engagement pockets formed by inner links of the drive member, and outer engagement pockets formed by outer links of the drive member. In use, the pair of alternation teeth may cause all dual engagement teeth to alternate between engagement with inner and outer engagement pockets with every revolution of the sprocket for a drive member consisting of an even number of links.

[0047] BRIEF DESCRIPTION OF THE DRAWINGS

[0048] Embodiments of the invention will now be further described by way of example only with reference to the accompanying drawings in which:

[0049] Figures 1 and 2 show a transmission system comprising a drive sprocket and a drive member, the drive sprocket shown at different angles of rotation in Figures 1 and 2 respectively;

[0050] Figure 3 shows the drive sprocket of Figures 1 and 2 with a drive member wrapped entirely around the drive sprocket;

[0051] Figure 4 shows the drive sprocket of preceding figures in isolation;

[0052] Figure 5 shows two dual engagement teeth of the drive sprocket shown in preceding figures;Figure 6 shows the angular spacing between teeth of the drive sprocket shown in preceding figures; and

[0053] Figure 7 shows another example of a drive sprocket.

[0054] DETAILED DESCRIPTION

[0055] Figures 1 and 2 show a drive sprocket 2 comprising a plurality of teeth 6, 8. The drive sprocket 2 forms part of a transmission system that also comprises a drive member 20 having a plurality of engagement pockets 22 which engage (or mesh with) the teeth 6, 8 as the drive sprocket 2 rotates. In this example, the drive member 20 is a roller chain comprising a plurality of linked rollers 24 that are spaced apart from one another by a plurality of articulated links (not shown). In other examples, the drive member may be any type of drive member suitable for engaging with a drive sprocket for power transmission. In such examples, the rollers 24 may be replaced by pins, bushes, flanges or equivalent transverse members configured to engage with the teeth of the drive sprocket.

[0056] The plurality of teeth 6, 8 includes a plurality of dual engagement teeth 6 and a pair of alternation teeth 8.

[0057] Each dual engagement tooth 6 has a tooth profile defined by a first side 11 comprising a first engagement surface 13 and an opposite second side 12 comprising a second engagement surface 14. The engagement surfaces 13, 14 are configured such that, when a dual engagement tooth 6 is fully engaged with a respective engagement pocket 22, both the first engagement surface 13 and the second engagement surface 14 contact the engagement pocket 22. In other words, the engagement surfaces 13, 14 of each dual engagement tooth 6 are configured so that the tooth exhibits 'dual engagement' with the roller chain 20.

[0058] This dual engagement reduces the relative movement between the roller chain 20 and drive sprocket 2 during use, thereby reducing wear and tear on both the roller chain 20 and the drive sprocket 2. In addition, frictional losses are reduced, thereby increasing transmission efficiency.

[0059] In order that each dual engagement tooth 6 achieves simultaneous contact with two adjacent rollers 24a, 24b, the dual engagement teeth 6 are spaced apart so that, when meshed with a roller chain 20, two rollers 24a, 24b are located between each pair of adjacent dual engagement teeth 6. In other words, adjacent dual engagement teeth6 will engage with every second engagement pocket 22 present in a roller chain 20. This results in a ratio of engagement pockets 22 to dual engagement teeth 6 of 2: 1.

[0060] In some applications, certain dimensions of a drive member are standardised across the industry. For example, in the bicycle industry the 'pitch' of any roller chain is standardised to 1 / 2 inch (12.7 mm). (The 'pitch' is the distance from the centre of one roller to the centre of an adjacent roller.) Accordingly, in such applications, the size of a drive sprocket configured to engage with a standardised drive member is dependent on the number of articulated links of the drive member that can be accommodated by the drive sprocket at any one time. Thus, a drive sprocket may be considered as having a size, n, corresponding to the minimum number of articulated links required in a drive member for that drive member to wrap entirely around the drive sprocket to form a closed loop.

[0061] As two engagement pockets (i.e. two articulated links) are required for each dual engagement tooth 6, a sprocket having only dual engagement teeth equally spaced around the sprocket must always have an 'even' size. In other words, the minimum number of articulated links required to wrap entirely around the drive sprocket to form a closed loop would always need to be even. This requirement is somewhat limiting, particularly in applications where gear ratios play an important factor in the selection of drive sprockets, such as in cycling.

[0062] The pair of alternation teeth 8 engages with a total of three rollers 24c, 24d, 24e, which is one fewer than a pair of dual engagement teeth would engage with. This means that the ratio of engagement pockets 22 to alternation teeth is 3:2 rather than 2:1. Therefore, by including the pair of alternation teeth 8, the drive sprocket 2 is provided with an 'odd' size rather than an 'even' size.

[0063] Figure 3 more clearly demonstrates the odd size of the drive sprocket 2.

[0064] Specifically, Figure 3 shows a roller chain 120 wrapped entirely around the drive sprocket 2 using the minimum number of articulated links (for demonstrative purposes and not representative of how the drive sprocket 2 would be used in practice). The articulated links forming a loop around the drive sprocket 2 are represented as a regular polygon 28 wherein each side of the polygon 28 is equivalent to one link and has a side-length equal to the pitch, p, of the roller chain 120. In this example, the polygon 28 has 15 sides in total and, thus, the size of the drive sprocket, n, is 15.A pitch circle 26 may be defined as a circle with its circumference plotted along the vertices of the polygon 28, such that the polygon 28 is inscribed in the pitch circle 26. The pitch circle 26 has a radius, rp(mm), given as:

[0065]

[0066] Although the pair of alternation teeth 8 enable the drive sprocket 2 to have an odd size, it will be appreciated that this is at the expense of achieving complete dual engagement. By virtue of the pair of alternation teeth 8 having three adjacent rollers 24c, 24d, 24e seated around them, it is not possible for the middle roller 24d to simultaneously contact both alternation teeth 8. Therefore, at least one of the alternation teeth 8 will fail to achieve dual engagement.

[0067] However, the inclusion of the pair of alternation teeth 8 will have a minimal, or even negligible, effect on the overall performance of the drive sprocket 2. The drive sprocket 2 therefore still benefits from advantages of dual engagement (i.e. reduced wear, for example) that are not achieved by known drive sprockets, while also having an odd size that is incompatible with 'complete' dual engagement drive sprockets.

[0068] It is also theorised that the odd size of the drive sprocket 2, enabled by the pair of alternation teeth 8, may have a beneficial effect in extending the lifespan of the associated roller chain by more evenly distributing wear across the various components of the roller chain.

[0069] In a conventional roller chain, the various rollers are connected together by links that alternate between inner links and outer links. In other words, if one engagement pocket is formed by two rollers connected together by an inner link, the adjacent engagement pockets will be formed by two rollers connected together by an outer link. The odd size of the drive sprocket 2 results in the dual engagement teeth 6 alternating between engagement with engagement pockets having inner links and engagement pockets having outer links. Assuming the roller chain has an even number of links (as is typically the case), this alternation would not occur on an evenly sized sprocket. Hence, it is theorised that the inclusion of the alternation teeth 8 may further improve the performance of the drive sprocket 2 in comparison to a conventional drive sprocket.

[0070] A roller chain may therefore be considered as having two types of engagement pocket - inner engagement pockets formed by inner links of the chain, and outer engagement pockets formed by outer links of the chain.In some examples, the pair of alternation teeth may be defined according to its role of alternating the engagement of the dual engagement teeth between meshing with the inner engagement pockets and meshing with the outer engagement pockets.

[0071] For the example shown in Figures 1 and 2, the dual engagement teeth 6 will alternate between meshing with the inner engagement pockets and meshing with the outer engagement pockets after each revolution of the drive sprocket 2. More specifically, the alternation will occur across the pair of alternation teeth 8, with one alternation tooth 8 meshing into one type of engagement pocket 22 and the other alternation tooth 8 meshing into the other type of engagement pocket 22.

[0072] In use, each tooth 6, 8 of the sprocket 2 meshes with the engagement pockets 22 of the roller chain sequentially as the sprocket 2 rotates. On this basis, relative to a first tooth, a second tooth that meshes with the roller chain subsequently to the first tooth can be defined as a "subsequent tooth". The pair of alternation teeth 8 may be considered as alternating the type of engagement pocket that the subsequent dual engagement teeth 6 mesh with.

[0073] In the example shown in Figures 1 and 2, the pair of alternation teeth 8 alternate the type of engagement pocket that all subsequent dual engagement teeth 6 mesh with. In other words, the pair of alternation teeth cause all dual engagement teeth 6 to alternate between engagement with inner and outer engagement pockets with every revolution of the sprocket 2 for a drive member consisting of an even number of links.

[0074] Figure 4 shows the drive sprocket 2 independently from any drive member.

[0075] The first side 11 and second side 12 of each dual engagement tooth 6 are symmetrical relative to a radial axis 18 extending centrally through the dual engagement tooth 6. The pair of alternation teeth 8 are also symmetrical relative to a radial axis 19 extending radially between the two alternation teeth 8. In other words, the two alternation teeth are symmetrical to one another. An advantage of the symmetrical nature of each dual engagement tooth 6 and the pair of alternation teeth 8 is that it is possible for the drive sprocket 2 to rotate in either direction with no difference in performance. The symmetric characteristics also enable applications where there is only one drive direction but potential for torque to be reversed during operation. Thus, the drive sprocket 2 is adapted for a variety of different uses.Figure 5 shows two dual engagement teeth 6 of the drive sprocket 2. In particular, Figure 5 shows the first and second engagement surfaces 13, 14 of the dual engagement teeth 6.

[0076] The first engagement surface 13 is defined at least partially by a first arc and the second engagement surface 14 is defined at least partially by a second arc, wherein each arc is centred about a respective vertex of the regular polygon 28 (shown in full in Figure 3) and having a radius represented as rs.

[0077] The distance between the centre of the first arc and the centre of the second arc of each dual engagement tooth is substantially the same as the distance between the centre of the first arc of a first dual engagement tooth and the centre of the second arc of an adjacent dual engagement tooth. The distances between the first and second arcs are also substantially equal to p.

[0078] Figure 6 shows the angular spacing between the teeth 6, 8 of the drive sprocket 2. In this disclosure, references to angular spacing between teeth is to be understood as angular spacing between radial axes that extend centrally through the respective teeth, irrespective of whether they are dual engagement teeth or alternation teeth.

[0079] In brief, the angular spacing between adjacent dual engagement teeth 6 is double the angular spacing between the pair of alternation teeth 8. More specifically, the plurality of dual engagement teeth 6 are regularly spaced apart with an angular spacing of 2o rad whereas the pair of alternation teeth are spaced apart with an angular spacing of a rad, wherein:

[0080]

[0081] In this example, where n = 15, the pair of alternation teeth 8 are spaced by 0.419 rad (24.0°) and the dual engagement teeth 6 are spaced by 0.838 rad (48.0°).

[0082] It may be noted that the angular spacing between an alternation tooth 8 and its adjacent dual engagement tooth 6 is the same as between any two dual engagement teeth 6, i.e. 2o rad. This allows the dual engagement teeth 6 adjacent to the pair of alternative teeth 8 to achieve the desired dual engagement function despite their proximity to the pair of alternation teeth 8.Figure 7 shows a drive sprocket 202 that is larger than the drive sprocket 2 shown in the preceding figures. Specifically, the drive sprocket 202 has a size of 23 (i.e. a roller chain would need 23 articulated chain links to wrap around the drive sprocket 202 to form a closed loop). The larger size of the drive sprocket 202 means that the angular spacing between teeth 6, 8 is reduced in comparison to that of the drive sprocket shown in the preceding figures. In this example, the pair of alternation teeth 8 are spaced by 0.273 rad (15.7°) and the dual engagement teeth 6 are spaced by 0.546 rad (31.3°).

[0083] It will be appreciated that, although the size of the drive sprockets 2, 202 shown in Figures 1 to 7 has a bearing on the angular spacing between teeth 6, 8, the size of the drive sprockets 2, 202 has no impact on whether the inclusion of the pair of alternation teeth 8 is effective in providing the drive sprocket with an odd size.

[0084] It will also be appreciated that more than one pair of alternation teeth 8 may be included in a single drive sprocket. In Figure 7 and the other examples discussed above, an odd number of pairs of alternation teeth 8 are provided to provide the drive sprocket with an odd size.

[0085] Provided that there is an odd number of pairs of alternation teeth 8, the pairs of alternation teeth 8 will always cause all dual engagement teeth 6 to alternate between inner and outer engagement pockets with every revolution of the sprocket for a drive member consisting of an even number of links.

[0086] However, a drive sprocket with an odd size can also be formed using an even number of pairs of alternation teeth. Figure 8 shows an example of such a drive sprocket 204. As with Figure 3, Figure 8 shows a roller chain 120 wrapped entirely around the drive sprocket 204 using the minimum number of articulated links (for demonstrative purposes and not representative of how the drive sprocket 204 would be used in practice). The articulated links forming a loop around the drive sprocket 204 are represented as a regular polygon 28 wherein each side of the polygon 28 is equivalent to one link and has a side-length equal to the pitch, p, of the roller chain 120. In this example, the polygon 28 has 17 sides in total and, thus, the size of the drive sprocket, n, is 17 and the sprocket 204 is an odd size.

[0087] The sprocket 204 comprises dual engagement teeth 6 and a plurality of pairs of alternation teeth 8. However rather than consisting of an odd number of pairs of alternation teeth 8, the sprocket 204 in this example consists of an even number ofpairs of engagement teeth 8 (specifically, 2 pairs). To achieve the desired odd size of the sprocket 204, one or more lengthened separation gaps 10 are provided (specifically, a single lengthened separation gap 10 in this example). The lengthened separation gap 10 in this example separates an alternation tooth 8 from an adjacent dual engagement tooth 6. In other examples, the or each lengthened separation gap 10 may be positioned elsewhere, for instance between a pair of adjacent dual engagement teeth 8. The "lengthened separation gap" is so named because the angular length of the separation gap 10 is greater than the normal angular separation between dual engagement teeth 6.

[0088] The lengthened separation gap 10 has an angular length equal to m times the angular spacing between each pair of alternation teeth, where m is an odd integer greater than or equal to 3. Since m is odd, the size of the drive sprocket is odd when there are an even number of pairs of alternation teeth 8 and an odd number of lengthened separation gaps 10. In the example shown, the angular separation between the alternation teeth is a rad, and the angular length of the lengthened separation gap is 3o rad, i.e. m = 3.

[0089] Figure 8 represents the simplest example of this embodiment, with 2 pairs of alternation teeth 8 and a single lengthened separation gap 10. However, it will be appreciated that the number of pairs of alternation teeth 8 may be increased (e.g. to 4, 6, 8, etc). Alternatively or additionally, the number of lengthened separation gaps 10 may be increased. Where the number of pairs of alternation teeth 8 is even, the number of lengthened separation gaps 10 must be odd.

[0090] Preferences and options for a given aspect, feature or parameter of the invention should, unless the context indicates otherwise, be regarded as having been disclosed in combination with any and all preferences and options for all other aspects, features and parameters of the invention.

Claims

CLAIMS1. A drive sprocket comprising a plurality of teeth for engaging with a drive member to transmit rotary motion, the drive member including a plurality of engagement pockets for receiving the teeth of the drive sprocket, wherein the plurality of teeth comprises:a plurality of dual engagement teeth wherein:each dual engagement tooth has a tooth profile defined by a first side comprising a first engagement surface and an opposite second side comprising a second engagement surface, the engagement surfaces being configured such that, when a dual engagement tooth is fully engaged with a respective engagement pocket, both the first engagement surface and the second engagement surface contact the engagement pocket, andadjacent dual engagement teeth do not engage with adjacent engagement pockets of the drive member; and,a pair of alternation teeth located adjacent to one another on the drive sprocket and configured to engage with adjacent engagement pockets of the drive member.

2. The drive sprocket of claim 1, wherein the plurality of teeth comprises an odd number of pairs of alternation teeth,and preferably wherein drive sprocket consists of only one pair of alternation teeth.

3. The drive sprocket of claim 1, wherein the plurality of teeth comprises an even number of pairs of alternation teeth, the teeth of each pair of alternation teeth being spaced apart with an angular spacing,and the drive sprocket further comprises an odd number of lengthened separation gaps, each lengthened separation gap separating a pair of adjacent teeth and having an angular length equal to m times the angular spacing between the teeth of each pair of alternation teeth, where m is an odd integer greater than or equal to 3, and preferably wherein the drive sprocket consists of 1 lengthened separation gap, and m is equal to 3.

4. The drive sprocket of any preceding claim, wherein a majority of the plurality of teeth consists of the plurality of dual engagement teeth and one or more pairs of alternation teeth,and preferably wherein the plurality of teeth consists only of the plurality of dual engagement teeth and one or more pairs of alternation teeth.

5. The drive sprocket of any preceding claim, wherein:the drive member comprises a plurality of spaced apart transverse members, each pair of adjacent transverse members forming a respective engagement pocket.

6. The drive sprocket of claim 5, wherein the drive sprocket is configured such that at least two transverse members are positioned between adjacent dual engagement teeth when said adjacent dual engagement teeth are fully engaged with respective engagement pockets.

7. The drive sprocket of claim 5 or 6, wherein the pair of alternation teeth is configured so that only one transverse member is positioned between the pair of alternation teeth when the pair of alternation teeth are fully engaged with respective engagement pockets.

8. The drive sprocket of any one of claims 5 to 7, wherein:the plurality of spaced apart transverse members are connected together by links that alternate between inner links and outer links, the drive member consisting of an even number of links;the engagement pockets correspondingly alternate between inner engagement pockets formed by inner links of the drive member, and outer engagement pockets formed by outer links of the drive member; andthe pair of alternation teeth cause all dual engagement teeth to alternate between inner and outer engagement pockets with every revolution of the sprocket.

9. The drive sprocket of any preceding claim, wherein the first engagement surface is defined at least partially by a first arc, and the second engagement surface is defined at least partially by a second arc, wherein the distance between the centre of the first arc and the centre of the second arc of each dual engagement tooth is substantially the same as the distance between the centre of the first arc of a first dual engagement tooth and the centre of the second arc of an adjacent dual engagement tooth.

10. The drive sprocket of claim 9, wherein the drive member has a pitch p and the distance between the centre of the first arc and the centre of the second arc of each dual engagement tooth is substantially equal to p.

11. The drive sprocket of any preceding claim, wherein the first and second sides of each dual engagement tooth are symmetrical relative to a radial axis extending centrally through the tooth.

12. The drive sprocket of any preceding claim, wherein each tooth extends along a radial axis and the angular spacing between the radial axes of any two adjacent dual engagement teeth is substantially double the angular spacing between the radial axes of the pair of alternation teeth.

13. The drive sprocket of any one of claims 10 to 12, wherein:the drive member is a chain drive member formed of a plurality of articulated chain links;the sprocket has a size, n, corresponding to the minimum number of articulated chain links required to wrap around the drive sprocket to form a closed loop;the plurality of dual engagement teeth is regularly spaced apart with an angular spacing of 2a rad; and,the pair of alternation teeth are spaced apart with an angular spacing of a rad, wherein a is equal to 2rr divided by n.

14. The drive sprocket of any one of claims 10 to 12, wherein the minimum number of articulated chain links required to wrap around the drive sprocket to form a closed loop corresponds to a regular polygon with n sides having a side-length equal to the pitch p (mm) wherein the regular polygon is inscribed in a pitch circle, the pitch circle having a radius, rp(mm), equal15. The drive sprocket of any one of claims 10 to 14, wherein the drive member is a roller chain and the radius of each roller forming the roller chain is substantially equal to, or slightly smaller than, the radius of each arc forming the first and second faced arcs.

16. The drive sprocket of any preceding claim, wherein the adjacent dual engagement teeth engage with every second engagement pocket of the drive member.

17. A drive sprocket comprising a plurality of teeth for engaging with a drive member to transmit rotary motion, the drive member comprising a roller chain having a plurality of spaced apart rollers wherein each adjacent pair of rollers defines an engagement pocket for receiving a tooth of the drive sprocket, wherein the plurality of teeth comprises:a plurality of dual engagement teeth wherein:each dual engagement tooth has a tooth profile defined by a first side comprising a first engagement surface and an opposite second side comprising a second engagement surface, the engagement surfaces being configured such that, when a dual engagement tooth is fully engaged with a respective engagement pocket, the first engagement surface contacts one roller defining the engagement pocket and the second engagement surface contacts the other roller defining the engagement pocket, andat least two rollers are positioned between adjacent dual engagement teeth when the adjacent dual engagement teeth are fully engaged with respective engagement pockets; and,a pair of alternation teeth located adjacent to one another on the drive sprocket and configured so that only one roller is positioned between the pair of alternation teeth when the pair of alternation is fully engaged with respective engagement pockets.

18. A drive sprocket comprising a plurality of teeth for engaging with a chain drive member formed of a plurality of articulated chain links, the drive sprocket having a size, n, corresponding to the minimum number of articulated chain links required to wrap around the drive sprocket to form a closed loop, the plurality of teeth comprising:a plurality of dual engagement teeth regularly spaced apart with an angular spacing of 2a rad; and,a pair of alternation teeth located adjacent to one another on the drive sprocket and spaced apart with an angular spacing of a rad, wherein a is equal to 2TT divided by n.

19. The drive sprocket of claim 18, wherein the chain drive member has a pitch p (mm), the minimum number of articulated chain links required to wrap around the drive sprocket to form a closed loop corresponds to a regular polygon with n sides having a side-length equal to the pitch wherein the regular polygon is inscribed in a pitch circle having a radius, rp(mm), equal top(m™20. The drive sprocket of claim 18 or 19, wherein each dual engagement tooth has a tooth profile defined by a first side comprising a first engagement surface and an opposite second side comprising a second engagement surface, the engagement surfaces being configured such that when a dual engagement tooth is fully engaged with a respective engagement pocket, both the first engagement surface and the second engagement surface contact the engagement pocket.

21. The drive sprocket of any one of claims 18 to 20, wherein adjacent dual engagement teeth engage with every second engagement pocket.

22. The drive sprocket of claim 21, wherein:the drive sprocket is configured to engage with a chain drive member comprising a plurality of spaced apart transverse members, each pair of adjacent transverse members forming a respective engagement pocket.

23. The drive sprocket of claim 22, wherein the drive sprocket is configured such that at least two transverse members are positioned between adjacent dual engagement teeth when said adjacent dual engagement teeth are fully engaged with respective engagement pockets.

24. The drive sprocket of claim 22 or 23, wherein the pair of alternation teeth is configured to engage with adjacent engagement pockets.

25. The drive sprocket of any one of claims 22 to 24, wherein the pair of alternation teeth is configured so that only one transverse member is positioned between the pair of alternation teeth when the pair of alternation teeth is fully engaged with respective engagement pockets.

26. The drive sprocket of any one of claims 22 to 25, wherein:the plurality of spaced apart transverse members are connected together by links that alternate between inner links and outer links;the engagement pockets correspondingly alternate between inner engagement pockets formed by inner links of the drive member, and outer engagement pockets formed by outer links of the chain drive member; andthe pair of alternation teeth cause all dual engagement teeth to alternate between engagement with inner and outer engagement pockets with every revolution of the sprocket.

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