Patent Description:
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.

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

A first 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. The known roller chains are also able to be used in many other different types of apparatus including, for example, tricycles, motorcycles and chain saws.

A second type of known drive member is a power transmission chain comprising a silent chain. The silent chain also has a plurality of engaging formations for enabling engagement with the drive sprocket. The engaging formations are in the form of tooth formations for being received in receiving recesses formed between adjacent teeth on the drive sprocket. The silent chain is used for high torque applications which need high efficiency and the transfer of a lot of power.

Typical of such applications is the use of a silent chain as a timing chain for engines. The silent chain is also often referred to as a HY-VO chain.

A third type of know drive member is a belt which is adapted to engage with the teeth of a sprocket.

As is well known, a drive member enables transmission of power between drive sprockets. Known drive sprockets may drive the drive member as in the case of a front sprocket drive sprocket on a bicycle, or the drive sprockets may be driven by the drive member as in the case of rear gear drive sprockets on a bicycle.

It is known that power transmission chains are formed by chain links which are pivotally contacted together by pivots which extend transversely completely across the chain link.

The known drive members and known drive sprockets do not transmit power as efficiently as would be desired. More specifically, the known 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 sprocket whilst maintaining contact under this high loading. The result is that the known power transmission chains do not work efficiently on the drive sprockets.

Known power transmission drive members include power transmission chains or belts which are adapted to engage with the teeth of a drive sprocket or pulley.

For example, roller or bush chains, or hollow pins chains which are a variation of standard roller or bush chains are adapted to transmit rotational motion from one rotating shaft to another by meshing with the teeth of a sprocket attached to each of the shafts.

Standard bush chains comprise inner and outer links, where the inner links comprise two spaced apart inner plates connected by two bushes with press fits between plates and bushes. The outer links comprise two spaced apart outer plates connected by two pins with press fits between plates and pins. In standard roller chains, the bushes of the inner links pass through rollers which are free to rotate around the outer surface of the bushes and are contained within the inner link by the plates of the link. In both hollow pin chains and standard bush and roller chains, the links are connected by means of a pin of an outer link passing through the bush of an adjacent inner link. Adjacent outer and inner links are able to rotate relative to one another about this pin-bush interface whilst simultaneously carrying load. A chain of connected links is able to form a loop and articulate around multiple sprockets, transferring torque and rotary motion between the sprocket axes.

A hollow pin bush chain is similar to a standard bush chain except that the pins of the outer link are hollow. Such a configuration allows for attachments to be readily fitted to the chain, primarily for conveying purposes. Attachments may be fitted by inserting pins through the hollow pins of the chain. Hollow pins also allow the weight of the chain to be reduced whilst maintaining the stiffness of the components.

In known hollow pin bush chains, each tooth of a drive sprocket is received between two adjacent bushes. Each tooth makes contact with one of the two adjacent bushes and transfers load between the chain and the sprocket at this contact interface.

A disadvantage of such known power transmission chains is that power is not transmitted efficiently in many cases. More specifically, known power transmission drive members invariably make contact with drive sprockets under significant loads, and in such situations, the drive members frequently tend to move relative to the teeth of the sprockets whilst maintaining contact under this high loading. The result is that known power transmission members do not work efficiently on drive sprockets.

In addition, when the chain links of known drive members articulate at connecting pivots, friction leads to energy losses and component wear. This results in further efficiency losses and decreased drive lifetime.

European Patent application <CIT> describes a tooth wheel intended to co-act with a transmission member with transmission elements, which tooth wheel bears a number of protrusions arranged on the periphery which are each provided with at least one contact surface for co-action with the transmission elements, comprising self-adjusting adjusting means for adjusting the pitch of the contact surfaces, wherein each protrusion can swivel substantially in the main plane of the tooth wheel round at least one centre outside the connecting line between the centre line of the tooth wheel and the zone of engagement between the contact surface and a transmission element. It is an object of the invention to embody a tooth wheel such that the distribution of the forces over the protrusions is better in the case of individual variations of the relevant pitch distances. With a view to this object the tooth wheel according to the invention has the feature that during a swivelling of the relevant protrusion the contact surface of the transmission element co-acting therewith undergoes a substantial radial displacement in relation to the tooth wheel.

According to the present invention there is provided a drive sprocket comprising a plurality of teeth for meshing with a drive member to transmit rotary motion, the drive member including a plurality of engagement pockets engaging the teeth of the drive sprocket, wherein each 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, and characterized in that, said engagement surfaces are configured such that when driven, a tooth meshes to the engagement pocket at a first contact location on the first engagement surface and also at a second contact location on the second engagement surface, wherein the first contact location is radially offset from the second contact location.

By means of the present invention, during use of the drive sprocket, each tooth of the sprocket will engage with the drive member at two contact locations on opposite sides of each tooth. In addition, the first contact location will, during use be radially offset from the second contact location.

Such an arrangement reduces the stress on the sprocket during use thereby reducing the wear and tear on the drive sprocket as well as the frictional losses, thereby increasing transmission efficiency.

In addition, the radial offset of the first and second contact locations helps to prevent the engagement pocket of the drive member from becoming wedged, or stuck, on a tooth during use of the drive sprocket.

By means of the present invention, therefore, secure engagement of the pitch pocket with the tooth may be achieved as the drive member makes contact with the drive sprocket. In addition, the stress on the drive sprocket as the load is transferred between the drive sprocket and the drive member is distributed to reduce localised peak stresses. Further, disengagement of the pitch pocket from the tooth may be reliably achieved.

In embodiments of the invention, each tooth has a front face and a back face, the shape of which front and back faces being defined by the first and second sides, wherein the shape of each face is symmetrical about a radial axis of the tooth, and the sides of the faces are defined at least partially by two arcs. An advantage of having a tooth where the shape of the front and back faces is symmetrical, is that it is possible for the drive sprocket to rotates in both a forward and a reverse direction. A symmetric tooth also enables applications with only one drive direction to handle torque reversal during operation. This results in the drive sprocket being more adaptable to different uses.

Each arc defines a side of a tooth and has a radius of R, the centres of the arcs being at a distance x from one another, and at a perpendicular distance, y, from the centre of the drive sprocket, and wherein the centre of each arc is at +x/<NUM>,y.

In embodiments of the invention, adjacent teeth are spaced apart from one another by a connecting portion of the sprocket.

In such embodiments of the invention, the tolerance of a transmission system incorporating a drive sprocket according to embodiments of the invention to dimensional variations within the system's components will be increased.

There is provided a transmission system comprising a drive sprocket according to embodiments of the first aspect of the invention, and further comprising a drive member, which drive member is adapted to engage with the drive sprocket.

The drive member comprises a plurality of engagement pockets, each of which engagement pockets comprises a first engaging surface and a second engaging surface spaced apart from the first engaging surface, the first and second engaging surfaces forming an engagement surface pair, which pair is rotatable about a rotational axis, wherein adjacent engagement pockets are connected to one another by connecting members.

Adjacent engagement pockets are connected to one another by a primary link, which primary link is rotatable about the rotational axis of the engagement surface pair.

The drive member may engage with the teeth of the drive sprocket, such that each engagement pocket is adapted to receive a tooth of the drive sprocket and to engage with the tooth at first and second engaging surfaces. Because adjacent engagement pockets are connected to one another by a primary link which is rotatable about the rotational axis of the engagement surface pair, the tooth will thus mesh to the engagement pocket such that the first contact location engages with the first engaging surface, and the second contact location engages with the second engaging surface.

The tooth is thus securely held by the engagement pocket such that little or no movement of the tooth relative to the pocket is possible once the tooth has meshed to the engagement pocket. In addition, because the first and second contact locations are radially offset relative to one another during use of the transmission system, the tooth is less likely to become stuck, or wedged in the engagement pocket compared to when there is no radial offset.

Each primary link is rotatable about the rotational axis of each adjacent engagement pocket. This facilitates the articulation of the drive member.

The drive member comprises a plurality of first primary links which are coplanar with one another and are pivotally connected to one another at first and second pivot points, which pivot points are spaced apart from one another such that adjacent first primary links are pivotable about the axis of rotation of each adjacent engagement pocket.

Such an arrangement may be desirable when the drive member comprises a power transmission chain, for example.

The drive member comprises a plurality of second primary links coplanar with one another and pivotally connected to one another at first and second pivot points, which pivot points are spaced apart from one another such that adjacent second primary links are pivotable about the axis of rotation of each adjacent engagement pocket, wherein the first primary links are connected to the second primary links such the first and second primary links are substantially parallel to one another, and the first pivot points of the first links are coaxial with the second pivot points of the second links, and the second pivot points of the first links are coaxial with the first pivot points of the second links.

Each engagement pocket comprises first and second transverse members each having a first end and a second end, the first and second transverse members being spaced apart from one another, wherein the first and second engaging surfaces are formed on the first and second transverse members respectively.

In such embodiments, the secondary links may be parallel with the primary links, and the transverse members may be substantially perpendicular to the primary and secondary links.

Each engagement pocket comprises a first secondary link positioned at, or close to the first ends of the first and second transverse members, and a second secondary link positioned at, or close to the second ends of the transverse members, wherein the first and second secondary links are parallel with one another. In such embodiments of the invention, first and second secondary links may be positioned opposite one another with the first and second transverse members extending substantially parallel to one another and substantially perpendicularly to the first and second secondary links. Each engagement pocket is thus defined by the first and second secondary links and the first and second transverse members.

The first and second transverse members each have a radius r, wherein the distance between the first and second transverse members of an engagement pocket is p2, and the distance between first and second pivot points of a primary link is p.

The first and second transverse members comprise first and second rollers respectively, each of which first and second rollers may have a radius r and may be rotatable about their respective axes. In other embodiments, the first and second transverse members may comprise first and second pins respectively, each of which first and second pins may have a radius of r and may not be rotatable. In still other embodiments, the first and second transverse members comprise first and second curved surfaces each surface having a radius of curvature of r.

There is provided a drive member forming part of a transmission system according to embodiments of the invention.

There is provided a power transmission drive member adapted to mesh with a drive sprocket to transmit rotary motion, the drive member comprising a plurality of engaging mechanisms, each comprising an engaging body comprising an engagement pocket adapted to engage with the drive sprocket, each of which engagement pockets comprising a first engaging surface and a second engaging surface spaced apart from the first engaging surface, the first and second engaging surfaces forming an engaging surface pair, which pair is rotatable about an engaging mechanism rotational axis, wherein the power transmission drive member comprises a carrier, which carrier is articulated and is adapted to support the plurality of engaging mechanisms.

Each tooth of the drive sprocket will engage with an engaging body by contacting both the first engaging surface and the second engaging surface during use.

Such an arrangement reduces both the stress on the sprocket during use, and the relative movement between the chain and sprocket when engaged, thereby reducing wear and tear on the drive member as well as the drive sprocket. In addition, frictional losses are reduced thereby increasing transmission efficiency.

Because the carrier is articulated, the engaging bodies supported by the carrier are able to articulate around the drive sprocket during use.

The first and second engaging surfaces are positioned symmetrically relative to the rotational axis in respect of engaging bodies.

The engaging surfaces are configured such that when driven, a tooth of the sprocket meshes to the engagement pocket at a first contact location on the first engaging surface, and also at a second contact location on the second engaging surface.

The first contact location is radially offset from the second contact location during use.

This helps to prevent the engagement pocket of the drive member from becoming wedged or stuck on a tooth during use.

The first and second engaging surfaces are formed on first and second pins respectively.

The pins are formed integrally with the remainder of the engaging body, whilst in other embodiments the pins are formed separately to the remainder of the engaging body. In such embodiments, the pins may be attached to the remainder of the engaging body by any convenient method and may be attached to the attachment portion by means of a press fit, for example.

The first and second pins may be circular in cross-section. In other embodiments of the invention, one or both of the first and second pins may be partially circular in cross-section. For example, one of both of the first and second pins could have a semi-circular cross-sectional shape, and the respective engaging surface would be formed on a part of the pin that has a curved surface.

Each engaging mechanism comprises two engaging bodies, which engaging bodies are spaced apart from one another.

Each engaging mechanism comprises a connecting member having a first end and an opposite second end, and attachable to one engaging body at the first end, to the other engaging body at the second end, and extending colinearly with the rotational axis of the respective engaging mechanism wherein each engaging body of a respective engaging mechanism comprises a front face and an opposite back face, wherein the engaging surfaces of each engaging body extend from the front face of a respective engaging body, and the connecting member extends from the back face of each engaging body, which connecting member is adapted to enable connection of a respective engaging mechanism to the carrier.

Because the connecting member extends between the back faces of each engaging body, the connecting member may extend into the carrier in order to secure each engaging body to an opposite side of the carrier, with the engaging surfaces of each engaging body extending outwardly, away from the carrier.

The connecting member is attached to a respective engaging body by means of a press fit with the engaging body.

This means that each engaging body will rotate with the connecting member. The engaging bodies cannot rotate independently of rotation of the connecting member. This can be advantageous, since each engaging member of an engaging mechanism will rotate with the other engaging body forming the respective engaging mechanism.

The connecting member may be attached to a respective engaging body by means of a clearance fit.

The engaging body is free to rotate independently about the connecting member.

This can be advantageous in embodiments of the invention where the carrier is, for example, a bush chain. Such chains do not comprise hollow pins.

In embodiments of the invention comprising a connecting member, the connecting member may extend transversely through the carrier whereby a first engaging body may be positioned on a first side of the carrier and a second engaging body may be positioned on a second, opposite side of the carrier.

The engaging bodies may thus face outwardly from the carrier, with each engaging mechanism having a first engaging body on one side of the carrier, a second engaging body on an opposite side of the carrier, such that engagement with the teeth of a drive sprocket takes place externally to the carrier.

This is in contrast to known power transmission drive members such as bush chains where the chain engages with the teeth of a sprocket within the structure of the chain. In addition, because the connecting member is coaxial with the engaging mechanism rotational axis, each engaging body of an engaging mechanism is rotatable about the axis of the connecting member, and thus both engaging bodies rotate about the same axis.

The connecting member may take any convenient form, and may for example, comprise a pin.

The connecting member may be regarded as a central pin of the respective engaging mechanism.

Each engaging body may comprise a receiving portion adapted to receive the connecting member, which receiving portion comprises an aperture, the centre of which is coaxial with the rotational axis of a respective engaging mechanism.

By means of the aperture formed in each engaging body, it is possible to attach, or connect another component to the engaging body, whilst allowing rotation of the engaging body about the rotational axis.

The carrier comprises hollow pins extending transversely at least partially across the carrier at spaced apart intervals along the length of the carrier, wherein each connecting member extends through a hollow pin to thereby connect the engaging mechanisms to the carrier.

An engaging body may be fitted to each end of a connecting member so that one engaging body is on one side of the carrier, and the other engaging body is on the opposite side of the carrier, and both engaging bodies are external to the carrier with the engaging surfaces extending away from the carrier.

The carrier may comprise a hollow pin bush chain.

The connecting member may be attached to each engagement body press fit. This means that the engagement bodies will rotate with the connecting member.

The carrier may take different form and may not be a hollow pin bush chain. For example, the carrier could be a standard bush chain rather than a hollow pin bush chain.

The carrier comprises pins extending transversely at least partially across the carrier at spaced apart intervals along the length of the carrier, wherein each connecting member comprises a pin extending across the carrier between the engaging bodies of a respective engaging mechanism and through the aperture of each engaging body, wherein the pin is shaped to form an interference fit with the link plates of the bush chain, and a clearance fit with the apertures of each engaging body.

The engaging bodies are rotatable about the axis of a respective pin independently of the bush.

An advantage of the present invention is therefore, that a standard chain, such as a hollow pin bush chain may be adapted so that it engages with either two sprockets, or a single sprocket with two sets of teeth, whereby the teeth of the sprocket or sprockets mesh with engagement pockets positioned externally to the chain.

The planes of symmetry of both engaging bodies may be parallel to one another, such that the engaging surfaces of each engaging body are aligned with one another.

The power transmission drive member may be adapted to mesh with two drive sprockets, which drive sprockets are spaced apart from one another such that the teeth of a first drive sprocket engage with the engaging bodies on a first side of the carrier, and the teeth of a second drive sprocket engage with the engaging bodies on the second, opposite side of the carrier.

The carrier and the engaging mechanisms are adapted to articulate around the drive sprockets making contact via the engaging mechanisms. The two sprockets are positioned on either side of the carrier, with the teeth of one drive sprocket engaging with the engaging bodies on a first side of the carrier, and the teeth of a second sprocket engaging with the engaging bodies on a second, opposite side of the carrier.

The power transmission drive member comprises a single drive sprocket, which drive sprocket comprises two sets of teeth, which sets of teeth are spaced apart from one another.

The carrier and the engaging mechanisms may be adapted to articulate around the drive sprocket making contact via the engaging mechanisms. The two sets of teeth are positioned on either side of the carrier, with the first set of teeth engaging with the engaging bodies on a first side of the carrier, and the second set of teeth engaging with the engaging bodies on a second, opposite side of the carrier.

As mentioned above, in some embodiments of the invention, the carrier may comprise a standard hollow pin bush chain or a standard bush chain with solid pins. Such chains come in several predetermined sizes based on specific applications and international standards. The dimensions of these known chains are dependent on the sprocket with which a particular known chain is designed to engage. Key dimensions are the bush diameter and the inner width of the chain. The inner width of the chain is the distance between the inner surfaces of the two inner plates forming an inner link in the chain. Because the teeth of the drive sprocket, or drive sprockets engage with the engaging bodies externally to the chain, by means of the invention, there is no longer a need for the chain to interact with a sprocket tooth by conventional contact with a bush. This means that the width of the chain may be greatly reduced to the point that a sprocket tooth would not be able to fit within the remaining space.

Furthermore, the space in which a conventional roller chain tooth typically sits can be completely removed such that the inner link of the chain can be reduced to a single plate. Such a design reduces the number of components to the chain and allows the width of the chain to be drastically reduced, thereby reducing the width of the required sprocket and thus the entire system.

The inner link may be thicker than the outer link.

The inner link of the chain may comprise a composite inner link formed from a plurality of thinner link plates. An advantage of such an embodiment is that by manufacturing thinner link plates, it is possible to readily manufacture a composite link having a desired thickness by combining an appropriate number of the thinner link plates.

Each engaging mechanism comprises first and second extension members which extension members are spaced apart from, and coaxial with one another, and each have first and second end portions, wherein the extension members extend across the width of the engaging mechanism and through each engaging body such that the first and second end portions of each extension member extend from the first face of each engaging body, away from the carrier to form a pin, wherein the first engaging surfaces of each engaging body are formed on the first and second end portions respectively of the first extension member, and the second engaging surfaces of each engaging body are formed on the first and second end portions respectively of the second extension member,.

The first and second extension members serve to connect the two engaging bodies to one another; thus, parts of the engaging mechanism are integrally formed.

The power transmission drive member may be a chain formed from links, comprising a body portion and first and second legs extending from the body portion to define a space between the legs and the body portion, wherein each leg comprises a hollow pin receiving portion, wherein the hollow pin receiving portion of a first leg of a link is coaxial with the rotational axis of a first engaging mechanism, and the hollow pin receiving portion of the second leg of the link is coaxial with the rotational axis of a second, adjacent, engaging mechanism, and wherein each connecting member is adapted to extend through a respective hollow pin and engaging body, to thereby link the engaging bodies to the links, such that each engaging body is rotatable about its rotational axis, the space of each link providing space for such rotation.

There is an aperture, the hollow pin receiving portion of the first leg of a link will be coaxial with the aperture of a first engaging body, and the hollow pin receiving portion of the second leg of a link will be coaxial with the aperture of a second, adjacent engaging body, and the hollow pins will extend through the apertures of the engaging bodies.

By means of the present invention, when the chain is in tension, there is little, or no force transmitted to the central pin of the engaging mechanism. This means that the central pin is free to rotate about its axis regardless of the loading condition of the chain. This increases the efficiency of power transmission since during engagement, as the engaging mechanism rotates upon making contact with the tooth, it does so without significant load at its contact interface with the interior surface of the hollow pin chain or pin link, thereby greatly reducing the frictional losses.

On the other hand, it is possible that when travelling between sprockets, because the engaging mechanisms are essentially free to rotate without any resistance, they may adopt an undesirable orientation with respect to the teeth of the sprockets with which they are to engage. In other words, a position in which the orientation of the engaging body may cause it to get stuck on top of a tooth rather than adopting the correct position with engaging surfaces either side of the tooth may occur. If this situation arises, then it can either rectify itself by snapping into position when the chain tension increases sufficiently to pull it into position, causing undesirable vibrations and wear, or it may remain stuck in the incorrect position and disrupt the engagement of the following engaging mechanisms, thereby increasing the tension of the system and potentially causing the whole system to fail.

The carrier comprises angle of rotation limiters adapted to limit the rotation of the engaging mechanisms.

The angle of rotation limiters may comprise stops formed on the carrier, such as folded portions, punched portions, and punched and folded portions. Such portions provide physical stops to the rotational movement of the engaging mechanisms.

The angle of rotation limiters may be formed on the links of a chain forming the carrier.

Each engagement body comprises an engagement pocket adapted to engage with the drive sprocket, each of which engagement pockets comprising a first engaging surface and a second engaging surface spaced apart from the first engaging surface, the first and second engaging surfaces forming an engaging surface pair, which pair is rotatable about an engaging mechanism rotational axis, wherein the power transmission drive member comprises a carrier, which carrier is articulated and adapted to support the plurality of engaging bodies.

By means of present invention, each tooth of the drive sprocket will engage with an engaging body by contacting both the first engaging surface and the second engaging surface during use.

The engaging mechanisms forming part of the present invention are thus dual engaging mechanisms ensuring dual engagement of the teeth of a sprocket engaged with a power transmission drive member according to the first aspect of the invention.

There is provided an engaging mechanism forming part of a power transmission drive member according to the first aspect of the invention.

There is provided a power transmission system comprising a power transmission drive member according to the first aspect of the invention, and a drive sprocket, wherein the power transmission drive member is adapted to mesh with the drive sprocket to transmit rotary motion.

There is provided a drive sprocket comprising a plurality of teeth for meshing with a drive member to transmit rotary motion, the drive member including a plurality of engagement pockets engaging the teeth of the drive sprocket, wherein each 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, which engagement surfaces are configured such that when driven, a tooth meshes to the engagement pocket at a first contact location on the first engagement surface and also at a second contact location on the second engagement surface, the first contact location being radially offset from the second contact location, and wherein each tooth has a front face and a back face, the shape of which faces being defined by the first and second sides such that the shape of each face is symmetrical about a radial axis of the tooth, and the first side of each face is defined at least partially by a first face arc, and the second side of each face is defined at least partially by a second face arc, wherein the distance between the centre of the first face arc and the centre of the second face arc of each tooth is substantially the same as the distance between the centre of the first face arc of a first tooth and the centre of the second face arc of an adjacent tooth.

By means of the present invention therefore a drive sprocket is provided in which not only is each tooth symmetrical, and all teeth are shaped substantially the same, but the distance between adjacent teeth is defined by the radius of an arc forming the first face arc and the second face arc.

The first face arc forms a base portion of the first side of each tooth, and the second face arc forms a base portion of the second side of each tooth, wherein the first and second face arcs each comprise a roller seating curve.

The roller seating curve is adapted to receive a roller or other engaging part of the drive member which is adapted to mesh with the sprocket.

Each first and second side comprises a second portion comprising a convex arc extending from a respective roller seating curve towards a tip portion of a respective tooth.

The second portion comprising a convex arc may comprise a working curve. The convex arc shape of the working curve allows the drive member to articulate during engagement and disengagement without making contact with a tooth of the sprocket.

The drive sprocket may further comprise a supporting curve extending from the roller seating curve of a first tooth towards the roller seating curve of an adjacent tooth.

The supporting curve is adapted to receive a roller or other member of the drive member to support the roller or other member.

There is provided a transmission system comprising a drive sprocket according to embodiments of the first aspect of the invention, and further comprising a drive member, which drive member is adapted to mesh with the drive sprocket.

There is a plurality of engagement pockets, each of which engagement pockets comprising a first engaging surface and a second engaging surface spaced apart from the first engaging surface.

The drive member comprises a roller chain and the engagement pockets are defined between adjacent rollers forming the roller chain. An engaging pocket of a drive member is considered to be a pair of parallel cylindrical rollers at a fixed distance from one another, forming a space in which the teeth of the drive member are adapted to sit.

Where the drive member comprises a roller chain, an engagement pocket is defined between adjacent rollers of the roller chain.

When an engagement pocket is engaged with a tooth, it has a single degree of freedom only. This is the articulation of the engagement pocket about the centre of the respective roller.

The roller chain has a pitch p, and the distance between the centre of the first face arc and the centre of the second face arc of each tooth, and the distance between the centre of the first face arc of a first tooth and the centre of the second face arc of an adjacent tooth is substantially equal to p.

Two rollers will be positioned between adjacent teeth of the sprocket to form an engagement pair. This means that every other engagement pocket will engage with a tooth of the sprocket because only every other pair of rollers will be positioned around a tooth to form an engagement pair of rollers. The remaining pairs of rollers will be positioned between adjacent teeth of the sprocket and so the engagement pockets of these roller pairs will not be in contact with a tooth.

This is advantageous since only half of the rollers will be load bearing during articulation of the drive member on the sprocket. The other half will be supporting and will therefore have reduced contact load during their articulation. This in turn decreases some of the transmission system wear and frictional losses leading to higher transmission efficiency.

This is in sharp contrast to known sprockets for use with roller chain drive members where each roller is positioned between two adjacent teeth during use of the sprocket.

Where the drive member is a roller chain, the radius of each roller is substantially equal to, or slightly smaller than, the radius of each seating curve.

By means of the present invention, the rollers of the roller chain will be supported by the roller seating curve in such a way that the engagement pocket defined between adjacent rollers of the drive member will mesh with the tooth such that the engagement pocket meshes at two contact locations.

Due to the dimensions of the arc defining the roller seating curve, and due to the radius of each roller relative to that arc, during use of the transmission system, the roller chain will engage such that two rollers are positioned between adjacent teeth.

In addition, the radius of the first face arc and second face arc remains substantially the same regardless of the number of teeth on the sprocket.

This simplifies the production process of the sprocket.

During use of the transmission system, a first roller or other drive member engaging part, will be a load bearing roller or part, and a second roller or drive member part will serve as a supporting roller or part. When a roller or other engaging part is acting as a support it may be supported and received by the roller seating curve.

There is provided a transmission system comprising a drive sprocket and a drive member adapted to mesh with the drive sprocket, the drive sprocket comprising a plurality of teeth for meshing with the drive member to transmit rotary motion and the drive member comprising a plurality of engagement pockets adapted to engage the teeth of the drive sprocket,.

A transmission system is provided in which the articulation angle at a first roller that is in contact with a first tooth at a first contact point is different to the articulation angle at a second roller which is in contact with the same tooth on an opposite side of the tooth at a second contact point when a third roller is in contact with a second tooth adjacent to the first tooth on the first side of the first tooth, and a fourth roller is in contact with a third tooth adjacent to the first tooth on the second side the first tooth.

The drive member may be regarded as comprising a plurality of articulation points, and the articulation angles are defined at the articulation points.

The connecting members comprise links. In such embodiments of the invention, the links articulate about the articulation points, and the articulation angles define the degree of articulation between a first link and a second link.

A roller is situated on each articulation point such that each tooth is engaged by two rollers where:.

By having the magnitude of the first articulation angle not equal to the magnitude of the second articulation angle, the efficiency of the sprocket is improved. This is because in a conventional roller chain, alternate links articulate via two different types of articulation, known as bush articulations and pin articulations. During pin articulations, the pin of the articulating link rotates within the bush of the adjacent link which remains stationary relative to the sprocket. In bush articulations, the bush of the articulating link rotates within the roller and around the pin of the adjacent link which remains stationary relative to the sprocket. Bush articulations thus cause sliding at two surfaces during articulation, whilst pin articulations cause sliding at only one. This means that more energy is lost during a bush articulation than during a pin articulation. On a conventional roller chain, the articulation type alternates each articulation. By means of the present invention, the net energy losses of the drivetrain can be reduced by reducing the size of the articulation angle associated with the less efficient bush articulation and increasing the size of the articulation angle associated with the more efficient pin articulation.

The difference in articulation angle can also be employed to reduce the wear at the pin-bush interface that leads to chain elongation, known as chain stretch. The load at the pin-bush interface during bush articulations is less than that during pin articulations. This means that bush articulations lead to more wear than pin articulations. By means of the present invention, the net pin-bush wear in the drive member can be reduced by reducing the size of the articulation angle associated with the higher wearing pin articulation and increasing the size of the articulation angle associated with the lower wearing bush articulation.

The first roller is a load bearing roller, and the second roller is a supporting roller.

The magnitude of the first articulation angle is greater than the magnitude of the second articulation angle.

The magnitude of every other articulation angle is the same. In such embodiments, the articulation angle will therefore alternate between two values.

Where the first roller is a load bearing roller and the second roller is a support roller the first articulation angle at each load bearing roller will be the same, and the second articulation angle at each support roller will the same.

There may be a different variation between the articulation angles at articulation points around the sprocket. The magnitude of the articulation angles may be chosen to suit the prevailing conditions.

The shape of each tooth face is symmetrical about a radial axis of the tooth.

The first side of each face is defined at least partially by a first face arc, and the second side of each face is defined at least partially by a second face arc.

Each first and second side comprises a second portion comprising a convex arc extending from a respective roller seating curve towards the tip portion of a respective tooth.

The drive sprocket may further comprise a supporting curve extending from the roller seating curve of a first tooth towards a roller seating curve of an adjacent tooth.

The supporting curve is adapted to receive a roller to support the roller.

The roller chain comprises a plurality of inner links, each of which serves to connect two rollers to form a roller pair, and a plurality of outer links, each of which serves to connect roller pairs to one another to form the roller chain, such that a space is defined between inner surfaces of facing inner links, and also between inner surfaces of facing outer links wherein each tooth has a width which is the same as, or slightly less than the distance between inner surfaces of facing outer links, and greater than the distance between inner surfaces of facing inner links.

Because each tooth of the sprocket has a width which is the same as, or slightly less than the distance between inner surfaces of facing outer links, the tooth will fit between facing outer links with very little clearance between outside surfaces of the tooth and the inside surfaces of the facing outer links. The width of the tooth will also prevent the teeth from engaging between facing inner links, so the sprocket will be able to engage with the teeth between outer links only, and not between inner links. This helps to maintain the alignment of the roller chain during use.

This is in sharp contrast to the situation in known transmission systems, where, because the width of each tooth is less than the distance between inner surfaces of facing inner links, it is possible for the teeth of a known sprocket to engage with either the inner links or the outer links of the chain. This means that when the teeth of a known sprocket engage with the outer links, there will be significant clearance between the outside surfaces of the tooth and the inside surfaces of facing outer links.

Each tooth of the sprocket comprises a first width which is the same as or slightly less than the distance between inner surfaces of facing inner links, and a second width that is the same as or slightly less than the distance between the inner surfaces of facing outer links.

The portion of each tooth that has the first width prevents the inner links from interfering with the tooth when the tooth is engaged between facing outer links.

There is provided a sprocket forming part of a transmission system according to embodiments of the first aspect of the invention, and further comprising a drive member comprising a plurality of spaced apart rollers.

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

Referring initially to <FIG>, a drive sprocket according to an embodiment of the first aspect of invention is designated generally by the reference numeral <NUM>.

The sprocket <NUM> comprises a plurality of teeth <NUM> that are spaced apart from one another around an outer circumference <NUM> of the sprocket <NUM>.

Each tooth has a tooth profile defined by a first side <NUM> comprising a first engagement surface <NUM>, and an opposite second side <NUM> comprising a second engagement surface <NUM>. Each tooth further comprises a front face <NUM> and a back face <NUM>, the shape of which faces being defined by the first and second sides <NUM>, <NUM>, which in this embodiment comprise first and second engagement surfaces. The shape of each face is symmetrical about a radial axis <NUM> extending along the length of each tooth.

The shape of each side <NUM>, <NUM> is defined at least partially by an arc. Because each tooth <NUM> is symmetrical about the axis <NUM>, the dimensions of the arcs forming all sides is the same.

Referring now to <FIG>, it can be seen that in this embodiment, each arc defining a side of a tooth <NUM> has a radius of R. The centres of the arcs are at a distance x from one another, and at a perpendicular distance y, from the centre of the drive sprocket. The centre of each arc is at ±x/<NUM>,y. Adjacent teeth <NUM> are separated from one another to define a connecting portion <NUM>, as shown in <FIG>, for example. In the illustrated embodiment, the connecting portion is substantially flat. However, it is to be understood that in other embodiments of the invention the connecting portion may not be flat, or there may not be a connecting portion at all.

Turning now to <FIG>, a transmission system according to an embodiment of the second aspect of the invention is designated generally by the reference numeral <NUM>. The transmission system <NUM> comprises the sprocket <NUM> illustrated in <FIG> and described hereinabove. The transmission system <NUM> further comprises a power transmission chain <NUM>. The power transmission chain <NUM> is adapted to engage with the sprocket <NUM> as will be described hereinbelow in order to enable transmission of power between the drive sprocket <NUM> and another drive sprocket (not shown).

As shown particularly in <FIG>, <FIG> for example, the power transmission chain <NUM> comprises a plurality of engagement pockets <NUM> extending along the chain <NUM>.

Each engagement pocket <NUM> comprises a first engaging surface <NUM>, and a second engaging surface <NUM> which is spaced apart from the first engaging surface <NUM>. The first and second engaging surfaces <NUM>, <NUM> together form an engaging surface pair <NUM>.

Each engaging surface pair <NUM> is rotatable about a rotational axis <NUM>. Adjacent engagement pockets <NUM> are connected to one another by at least one primary link <NUM>, which primary link <NUM> is rotatable about the rotational axis <NUM>.

In this embodiment there is a set of first primary links <NUM> which are co-planar with one another, a set of second primary links <NUM> which are co-planar with one another, a set of third primary links <NUM> which are co-planar with one another and a set of fourth primary links <NUM> which are also co-planar with one another. Each set of primary links is substantially parallel with each other set of primary links.

The primary links <NUM> in a particular set, which are co-planar to one another, are also pivotally connected to one another. Each primary link <NUM> has a first pivot point <NUM>, and a second pivot point <NUM>, the first and second pivot points <NUM> and <NUM> being spaced apart from one another along each primary link <NUM>, such that adjacent primary links <NUM> are pivotable about the first and second primary pivot points <NUM>, <NUM>.

In the illustrated embodiment of the invention, the first <NUM> and second <NUM> primary links are connected to and abut one another such that the first pivot point <NUM> of a first primary link <NUM> is coaxial with the second pivot point <NUM> of a second primary link <NUM>, and vice versa.

Similarly, the third <NUM> and fourth <NUM> primary links are connected to and abut one another such that the first pivot point <NUM> of a third primary link <NUM> is coaxial with the second pivot point <NUM> of a fourth primary link <NUM>, and vice versa.

The power transmission chain <NUM> further comprises a plurality of secondary links <NUM> each of which secondary links is adapted to rotate substantially about the rotational axis <NUM> of the respective engagement pocket <NUM>. Each secondary link <NUM> is positioned to be substantially parallel with a respective primary link <NUM> such that the rotational axis <NUM> of a particular engagement pocket <NUM> is coaxial with the first <NUM>, or second <NUM>, pivot points of the corresponding primary links <NUM>. This in turn means that adjacent primary links <NUM> are pivotable about the axis of rotation <NUM>.

In this embodiment, the plurality of secondary links <NUM> comprises a plurality of first secondary links <NUM>, and a plurality of second secondary links <NUM>. Each first secondary link <NUM> abuts a second primary link <NUM>, and each second secondary link <NUM> abuts a third primary link <NUM>.

In this embodiment, two first secondary links <NUM> abut each second primary link <NUM>, and two second secondary links <NUM> abut each third primary link <NUM>.

In this embodiment of the invention, the first secondary links <NUM> are substantially coplanar with one another, and the second secondary links <NUM> are substantially coplanar with one another, the first secondary links <NUM> are spaced apart from the second secondary links <NUM> such that each first secondary link <NUM> faces a corresponding second secondary link <NUM> to form a pair of secondary links <NUM>.

In this embodiment, each engagement pocket <NUM> comprises first and second transverse members <NUM>, <NUM>, which are spaced apart from one another and on which the first and second engaging surfaces <NUM>, <NUM> respectively, are formed. The first and second transverse members <NUM>, <NUM> extend transversely between the corresponding first and second secondary links forming the pair. The transverse members <NUM>, <NUM> thus connect the secondary links together. In this embodiment, the first and second transverse members <NUM>, <NUM> each comprise a roller <NUM>. In other embodiments, each transverse member <NUM>, <NUM> may comprise a pin.

The space defined between the first and second transverse members of a secondary link forms an engagement pocket <NUM>. The engagement pockets <NUM> are shaped and positioned to receive and engage with a tooth <NUM> of the sprocket <NUM>, as shown in the Figures.

In use of the transmission system <NUM>, a tooth <NUM> of the drive sprocket <NUM> will mesh to the engagement pocket <NUM> at a first contact location <NUM> on the first engagement surface <NUM> and also at a second contact location <NUM> on the second engagement surface <NUM>, as shown in <FIG>, for example. Once engaged, the first contact location <NUM> will engage with the first engaging surface <NUM> of the engagement pocket <NUM>, and the second contact location <NUM> will engage with the second engaging surface <NUM> of the engagement pocket <NUM>.

Due to the inter-relationship between the primary links and secondary links as explained hereinabove together with the transverse members <NUM>, <NUM> and the features of the drive sprocket <NUM>, during use of the transmission system <NUM> the first contact location <NUM> is radially offset from the second contact location <NUM>.

This in turn results in the rollers <NUM> maintaining contact with the first and second engagement surfaces <NUM>, <NUM> of each corresponding tooth <NUM>, and each secondary link <NUM> sitting at an offset angle such that one roller <NUM> sits radially higher on one side the tooth <NUM> than the roller <NUM> on the other side of the tooth.

The geometry of the teeth <NUM> and of the transmission chain <NUM> will now be described in more detail with particular reference to <FIG>.

Referring initially to <FIG> a portion of the transmission chain <NUM> is illustrated schematically.

As can be seen, the distance between first and second p pivot points <NUM>, <NUM> of a primary link <NUM> is p, the distance between the axes of first and second transverse members <NUM>, <NUM> is p<NUM>, and the radius of each roller <NUM> is r.

Referring now to <FIG>, according to Cartesian coordinates where the origin is at the sprocket centre and the centreline of the tooth is parallel with the y axis:
A symmetrical tooth geometry is proposed with two arcs of radius R with arc centres at <MAT>, where, R, x, & y are defined such that when the chain is articulated around the sprocket and a load is applied to the chain:.

Furthermore, straight lines of length l extend above the tooth from the ends of the arcs towards the centreline at an angle, γ, relative to the tooth centreline such that as the chain wears, causing the pitch, p, of the chain to elongate and the corresponding pitch circle radius, rp, to increase:.

Referring now to <FIG> and <FIG>, the geometry of the sprocket will be considered in more detail.

Equations <NUM> & <NUM> below give α, the angle of articulation, and rp, the pitch circle radius, for an n-toothed sprocket of pitch p. <MAT> <MAT>.

The values of the arc parameters, R, x & y are given by the solutions to the set of simultaneous equations given by Equations <NUM> to <NUM>, where <MAT> <MAT> <MAT> <MAT> <MAT>.

Referring to <FIG>, and initially to <FIG>, a power transmission drive member according to an embodiment of the invention is designated generally by the reference numeral <NUM>. The drive member <NUM> is shown articulating around a drive sprocket <NUM>, shown in more detail in <FIG>. As can be seen, particularly from <FIG>, sprocket <NUM> comprises a first set of teeth <NUM> and a second set of teeth <NUM>. The sets of teeth <NUM>, <NUM> are spaced apart from one another by the sprocket body <NUM>. In other embodiments of the invention, the drive sprocket <NUM> may be replaced by two separate sprockets each having a single set of teeth and spaced apart from one another so that the teeth of both sprockets engage with the drive member.

In this embodiment of the invention, the drive member comprises a hollow pin bush chain <NUM> comprising inner links <NUM> and outer links <NUM>, the links <NUM>, <NUM> being connected together by hollow pins <NUM> as shown particularly in <FIG> and <FIG>, for example.

The drive member <NUM> further comprises engaging mechanisms <NUM>, as shown particularly in <FIG>. In this embodiment of the invention each engaging mechanism comprises two engaging bodies <NUM>. Each of the engaging bodies <NUM> comprises an engagement pocket <NUM> adapted to engage with the drive sprocket <NUM>. Each engagement pocket comprises a first engaging surface <NUM> and a second engaging surface <NUM> spaced apart from the first engaging surface <NUM>. Together the first and second engaging surfaces <NUM>, <NUM> form an engaging surface pair <NUM> which is rotatable about an engaging mechanism rotational axis shown by the dotted line <NUM> in <FIG>.

When the drive member <NUM> articulates with the sprocket <NUM>, each tooth <NUM> of the drive sprocket <NUM> will engage with an engaging body <NUM> by contacting both the first engaging surface <NUM> and the second engaging surface <NUM> of the engaging body <NUM>. In other words the engaging mechanisms <NUM> are adapted to engage with each tooth <NUM> of the sprocket <NUM> using the principle of dual engagement, whereby contact is made on both sides of each tooth <NUM> to enable a secure engagement that is energetically efficient and able to distribute the load of the chain over a larger number of teeth of the sprocket <NUM>.

The first and second engaging surfaces <NUM>, <NUM> are configured such that when driven, a tooth <NUM> of the sprocket <NUM> meshes to the engagement pocket <NUM> of an engaging mechanism <NUM> at a first contact location <NUM> on the first engaging surface <NUM>, and also at a second contact location <NUM> on the second engaging surface <NUM>.

During use, the first contact location <NUM> is radially offset from the second contact location <NUM>. This helps to prevent the engagement pockets <NUM> from becoming wedged or stuck on a tooth <NUM> during use.

In this embodiment of the invention, the first and second engaging surfaces <NUM>, <NUM> are formed on first and second pins <NUM>, <NUM> respectively.

The pins <NUM>, <NUM> may be integrally formed with the remainder of the engagement body <NUM>.

In another embodiment of the invention, the pins <NUM>, <NUM> may be formed separately from the remainder of the engagement body <NUM> as shown in <FIG>. In this embodiment, the engaging body <NUM> comprises pin apertures <NUM> shaped such that the pins <NUM>, <NUM> may be press fitted into the pin apertures <NUM>.

In another embodiment of the invention, the pins <NUM>, <NUM> have a semi-circular cross-section, with the engaging surfaces being formed on the curved portion of the pins <NUM>, <NUM>.

In another embodiment, the first and second engaging surfaces <NUM>, <NUM> are formed from folded sheet material. Alternatively, the engaging body <NUM> is shaped to optimise engagement with a sprocket tooth <NUM>.

Each of the engaging bodies <NUM> comprises an aperture <NUM>, the centre of which is coaxial with the engaging mechanism rotational axis <NUM>.

The engaging mechanisms <NUM> each further comprise a connecting member <NUM>, having a first end <NUM> and a second end <NUM>. The connecting member <NUM> is attachable to a first engaging body <NUM> at its first end <NUM> and to a second engaging body <NUM> at its second end <NUM>, such that it extends coaxially with the rotational axis of the respective engaging mechanism.

In this embodiment of the invention, the first and second ends <NUM>, <NUM> of the connecting member <NUM> each fit into an aperture <NUM> of an engaging body <NUM> such that both engaging bodies <NUM> of an engaging mechanism <NUM> rotate about the rotational axis <NUM> with the connecting member <NUM>. In other words, the engaging bodies <NUM> are not able to rotate independently of the connecting member. The aperture <NUM> thus serves as a receiving portion adapted to receive the connecting member <NUM>.

In some embodiments of the invention, the aperture <NUM> is profiled. This may aid orientation of the engaging body <NUM> relative to the connecting member <NUM>.

In this embodiment of the invention, each connecting member <NUM> extends through a hollow pin <NUM>, thereby connecting the engaging mechanisms <NUM> to the chain <NUM>, such that a first engaging body <NUM> is on one side of the chain <NUM>, and the other engaging body <NUM> is on the other side of the chain. Both of the engaging bodies <NUM> are thus external to the chain <NUM>, with the engaging surfaces extending away from the chain, and the connecting member <NUM> extending transversely across the chain. In addition, both engaging bodies <NUM> rotate about the rotation axis <NUM>.

By means of the invention, therefore, a standard hollow pin bush chain may be readily adapted so that it can engage with either two sprockets, or, as is the case in this embodiment, it can engage with a single sprocket <NUM> having two sets of teeth <NUM>, <NUM>, whereby the teeth <NUM> of the sprocket <NUM> mesh with engagement pockets <NUM> positioned externally to the chain.

Referring now to <FIG>, a power transmission drive member <NUM> according to another embodiment of the invention is shown articulating around a drive sprocket <NUM>, having teeth <NUM>.

In this embodiment of the invention, the power transmission drive member <NUM> comprises a hollow pin chain <NUM> which is narrower than a conventional hollow pin chain of the type shown in <FIG> for example. The chain <NUM> comprises inner links <NUM>, and outer links <NUM> which are similar to the links <NUM> and <NUM> of the chain <NUM> of <FIG>, except that the width of the chain <NUM> no longer has to be wide enough to accommodate sprocket teeth. This is because the teeth <NUM> of sprocket <NUM> engage externally of the chain <NUM> in the same way as described herein above with respect to the embodiment illustrated in <FIG>.

Because the width of the chain <NUM> is narrower than that of chain <NUM>, the space between the two sets of teeth of sprocket <NUM> is correspondingly narrower than the space between the two sets of teeth of sprocket <NUM>.

In an alternative embodiment illustrated in <FIG> and <FIG>, the inner links <NUM> are replaced by a single plate <NUM>, which plate comprises first and second hollow pin receiving portions adapted to receive a hollow pin in a similar manner to the previous embodiments described above.

In all other respects, the power transmission drive member <NUM> contains corresponding parts and operates in the same way as power transmission drive member <NUM>.

Turning now to <FIG> a further embodiment of the invention is shown. In this embodiment, the inner link <NUM> has been replaced by a plurality of thinner link plates <NUM> forming a composite inner link. This can be advantageous from a manufacturing point of view, and also means that by having a plurality of link plates <NUM>, the thickness of the composite link can be varied according to suit the application.

Turning now to <FIG> and <FIG>, another embodiment of a power transmission drive chain according to an embodiment of the invention is shown.

In this embodiment of the invention, the power transmission drive member comprises a bush chain <NUM> comprising solid pins <NUM> which extend across the width of the chain <NUM>.

Each of the pins <NUM> has a pin extension <NUM> at either end of each pin <NUM>. Each of the pins <NUM> passes through apertures in the outer link plates <NUM> and the inner link plates <NUM> as well as bushes <NUM>. The pins are sized and shaped so that there is an interference fit between each pin and a respective outer link plate <NUM>. Each pin extends between respective engaging bodies <NUM>, and each pin extension <NUM> is adapted to pass through the aperture <NUM> of each engaging body <NUM>. Each pin extension is sized and shaped such that there is a clearance fit between each pin extension <NUM> and a respective engaging body <NUM>.

In such embodiments of the invention each engaging body <NUM> is able to rotate independently about a respective pin extension <NUM>.

Each pin <NUM> may have a head formed at each end thereof in order to prevent each engaging body <NUM> from becoming detached from a respective pin <NUM>.

Referring now to <FIG> part of a power transmission drive member <NUM> according to another embodiment of the invention is shown.

In this embodiment, each engaging mechanism <NUM> comprises two engaging bodies <NUM> which are spaced apart from one another. Each engaging mechanism further comprises first and second extension members <NUM>, <NUM>, which extend through each engaging body and serve to connect two engaging bodies <NUM> to one another.

Each extension member <NUM>, <NUM> extends through the engaging bodies <NUM> to form pins <NUM>, <NUM> on which the first and second engaging surfaces <NUM>, <NUM> are formed. The first and second engaging surfaces of both engaging bodies <NUM> are thus integrally formed.

The drive member <NUM> comprises a chain <NUM>, part of which is shown particularly in <FIG>. The chain comprises outer links <NUM>, and inner links <NUM> connected together by a hollow pin <NUM>.

Each link <NUM>, <NUM> comprises a body portion <NUM>, and first and second legs <NUM>, <NUM> integrally formed with the body portion <NUM>, and extend from the body portion <NUM> to define a space <NUM> between the legs <NUM>, <NUM> and the body portion <NUM>. Each leg <NUM>, <NUM> comprises a hollow pin receiving portion <NUM>, and each link <NUM>, <NUM> is positionable on the engaging bodies <NUM> such that the hollow pin receiving portion <NUM> of a first leg <NUM> of a link is coaxial with the rotational axis of a first engaging mechanism, and the hollow pin receiving portion <NUM> of the second leg of the link is coaxial with the rotational axis of a second, adjacent engaging mechanism. This means that the hollow pin receiving portions <NUM> are coaxial with the apertures <NUM> of the engaging bodies <NUM>.

Each engaging mechanism <NUM> further comprises a central pin <NUM> which passes through a respective hollow pin <NUM>.

Each hollow pin <NUM> fits through the hollow pin receiving portion <NUM> of a respective inner link <NUM>. The central pin <NUM> extends through the hollow pin <NUM> and the respective apertures <NUM> of both engaging bodies <NUM>, with the engaging bodies <NUM> positioned on either side of the outer link <NUM>.

This arrangement enables the engaging mechanisms to rotate about their respective rotational axes. The space <NUM> provides space for the rotation.

The engaging mechanism <NUM> and the engaging bodies <NUM> are equivalent to the engaging mechanism <NUM> and the engaging bodies <NUM> and function in the same way. In particular, the first and second engaging surfaces <NUM>, <NUM> form an engagement pocket <NUM> which is equivalent to engagement pocket <NUM> and therefore results in dual engagement of the tooth of a sprocket in the pitch pocket as described with reference to the previous embodiment.

Turning now to <FIG> a further embodiment of an outer link plate <NUM> is shown including angle of rotation limiters. These limiters are designed to prevent over rotation of the engaging bodies during use.

In the embodiment shown in <FIG>, the outer link plate <NUM> comprises limiters <NUM> formed from bent sections of the outer link plate <NUM>. The angle limiters <NUM> limit the rotational movement of the engaging bodies <NUM> and thus reduce the likelihood that the engaging bodies will become stuck.

By means of the present invention, and as described above, each tooth of a drive sprocket will engage with an engaging body by contacting both a first engaging surface and a second engaging surface. This dual engagement reduces the stress on the sprocket as well as relative movement between the chain and sprocket during use thereby reducing wear and tear on the drive member as well as the drive sprocket. In addition, frictional losses are reduced, thereby increasing transmission efficiency.

Referring now to <FIG> a transmission system according to an embodiment of the invention is designated generally by the reference numeral <NUM>. The transmission system comprises a sprocket <NUM> and a drive member comprising a roller chain <NUM>.

In this embodiment of the invention the roller chain <NUM> is a standard roller chain comprising a plurality of rollers <NUM> which extend transversely across the transmission member and are spaced apart along the length of the drive member to form the chain. The rollers are connected to one another by links <NUM> in a known manner. The roller chain <NUM> is able to articulate between adjacent rollers <NUM>. An engagement pocket <NUM> is defined between adjacent rollers <NUM>. Each engagement pocket <NUM> is adapted to engage with a tooth <NUM> as will be described in more detail below.

By means of the present invention, however, only every other engagement pocket <NUM> will engage with a tooth during use of the transmission system <NUM>. The remaining every other engagement pockets <NUM> will effectively engage with the space between adjacent teeth <NUM>.

Turning now to <FIG>, the sprocket <NUM> is shown in more detail.

The sprocket <NUM> comprises a plurality of teeth <NUM> which are all shaped substantially identically to one another. Each tooth has a tooth face or profile <NUM> which is symmetrical about a radial axis R of the sprocket <NUM>.

The tooth profile <NUM> is defined by a first side <NUM> comprising a first engagement surface <NUM>, and a second side <NUM> defining a second engagement surface <NUM>. Each of the first and second sides <NUM>,<NUM> comprises a base portion <NUM> which forms a roller seating curve <NUM>. Each side further comprises a portion <NUM> extending from the roller seating curve towards a tip <NUM> of the tooth. The portion <NUM> is convex and defines a working curve <NUM>.

The sprocket <NUM> comprises a further curve <NUM> forming a supporting curve <NUM> which extends between adjacent teeth.

As shown in <FIG> and <FIG> particularly, in use of the transmission system <NUM>, every other engagement pocket <NUM> will engage with a respective tooth <NUM> whilst the remaining every other engagement pocket <NUM> will not engage a tooth. This is because, due to the dimensions of the sprocket, and particularly the profile of the tooth, relative to the dimensions of the rollers <NUM>, when the roller chain <NUM> is engaged with the sprocket <NUM> there will be two rollers <NUM> positioned between adjacent teeth. This in turn means that every other engagement pocket <NUM> will engage with a tooth <NUM>, with every other engagement pocket effectively engaging with spaces between adjacent teeth <NUM> of the sprocket.

Referring to <FIG> the manner in which the rollers <NUM> engage with the sprocket <NUM> during use of the transmission system <NUM> is shown schematically.

When considering a pair of rollers <NUM> positioned on either side of a tooth <NUM>, one roller <NUM> will be a load bearing roller, and the second roller <NUM> will be a supporting roller <NUM>.

The roller seating curve <NUM> provides an initial seating position for the engaged rollers <NUM> of the roller chain <NUM>. For both load bearing and supporting rollers, this curve helps to distribute the contact load over a larger area reducing material stresses, at least initially when the chain wear is low. The roller seating curve <NUM> enables rollers to easily transition between supporting and load bearing positions if the drive direction is ever reversed.

The load bearing roller <NUM> will engage with the tooth <NUM> on a first engagement surface <NUM>, and the support roller <NUM> will engage with the tooth at a second engagement surface <NUM>.

The first and second engagement surfaces <NUM>,<NUM> are radially offset from one another. This enables the pair of rollers <NUM> engaging the tooth <NUM> to engage with dual engagement, since the roller chain makes contact with the sprocket teeth <NUM> at two contact points <NUM>, <NUM> on engagement surfaces <NUM>, <NUM> in each tooth of the sprocket.

The two contact points <NUM>, <NUM> are thus on opposing sides of the tooth relative to its radial centreline R, and are radially offset from one another and therefore not symmetric relative to the radial centreline R.

The combination of these features leads to a secure engagement of the drive sprocket tooth by the roller chain <NUM> and ensures that the rollers <NUM> do not become wedged on the tooth. In addition, there is little to no relative movement between the tooth and the rollers <NUM> whilst in contact.

The first contact point <NUM> is load bearing and transfers the load between the roller chain <NUM> and the tooth <NUM>. The second contact point <NUM> is supporting and thus stabilises the roller chain <NUM> on the sprocket <NUM> and increases the load distribution over the sprocket teeth <NUM>.

As shown in <FIG>, each tooth <NUM> further comprises a working curve <NUM> that extends from the roller seating curve towards the tip <NUM> of the tooth.

The working curve <NUM> is convex, and the convex arc forming the working curve <NUM> curves towards the tooth centreline R. The surface of working curve <NUM> makes contact with the load bearing roller <NUM>, enabling torque transfer between the roller chain <NUM> and the sprocket <NUM>. As the chain pitch elongates due to internal wear, this surface also accommodates the climbing of the load-bearing roller as shown in <FIG>.

The tip <NUM> of each tooth does not need to have a pointed profile. This is because when an engagement pocket <NUM> is at the point of engagement with the tooth <NUM> it has a single degree of freedom only which is the articulation of the engagement pocket about the centre of the roller.

The working curve is the primary load bearing contact surface situated on an upper portion of the sides of each tooth. It is this surface that makes contact with the load bearing roller <NUM>, enabling torque transfer between chain and sprocket. As the chain pitch elongates due to internal wear, this surface also accommodates the climbing of the load bearing roller ensuring that the sprocket is able to transfer load through the entire lifetime of the chain.

Turning again to <FIG>, the sprocket further comprises a supporting curve <NUM> which extends between the roller seating curves of adjacent teeth.

The supporting curve is designed to accommodate the supporting roller <NUM>. The supporting curve may also accommodate some movement of the supporting roller <NUM> over the lifetime of the roller chain <NUM>, as the worn chain adopts an altered position on the sprocket.

Referring specifically to <FIG> and <FIG>, the engaging pockets <NUM> are represented by lines <NUM>.

The line <NUM> of each engagement pocket sits with endpoints situated on a circle <NUM> known as the pitch circle. The pitch circle defines the centre point of all the roller seating curves <NUM>.

In this embodiment of the invention the radius of each roller seating curve is slightly larger than the radius of each roller. This results in the engagement pockets <NUM> sitting marginally off the pitch circle <NUM>. This in turn ensures that the rollers adopt their respective load bearing and supporting positions and prevents the engagement pockets from getting stuck on the teeth.

Referring to <FIG>, the dimensions of the roller chain <NUM> are shown in more detail.

As can be seen from <FIG>, the distance between adjacent rollers, known as the chain pitch, may be represented by the letter p, and the diameter of each roller may be represented by dr.

Referring now to <FIG>, a schematic representation of part of the transmission system <NUM> of <FIG> is shown. The circle radius rp represents the pitch circle. This is the circle which passes through all of vertices of a regular polygon of n sides, for a sprocket <NUM> which has n/<NUM> teeth. Each side of the regular polygon has a length ρ. <FIG> shows three of the sides of the regular polygon showing the length as ρ.

The radius of the arc forming the first face arc and the second face arc may be represented by rs. The centre of a roller seating curve <NUM> with radius rs sits at each vertex of the regular polygon forming the bases of the teeth. The radius of the arc may be compared with the radius of the roller and given as a ratio ρ. In addition, the steepness of the working curve relative to the centreline of the tooth at the contact point of the load bearing roller <NUM> may be denoted by Θ. In embodiments of the invention, the ratio ρ was found to be <NUM> regardless of the number of teeth on the sprocket <NUM>.

Θ was found to vary depending on the number of teeth forming the sprocket.

A representative, but non-exhaustive list of values for Θ is set out below:.

Thus, it can be seen that in a transmission system according to an embodiment of the invention, the teeth <NUM> of the sprocket <NUM> will have a profile that hardly varies depending on the number of teeth forming the sprocket.

By means of the embodiments of the invention therefore a standard roller chain, for example a roller chain meeting the ISO <NUM> standard, is able to engage a sprocket such that dual engagement is achieved.

In embodiments of the invention where the sprocket <NUM> has n teeth, the roller seating arc has a fixed radius rs for all n, and this radius is slightly larger than the radius of each roller <NUM>.

Referring initially to <FIG> a transmission system according to an embodiment of the invention is designated generally by the reference numeral <NUM>. The transmission system comprises a sprocket <NUM> and a drive member comprising a roller chain <NUM>.

As shown in <FIG> particularly, in use of the transmission system <NUM>, every other engagement pocket <NUM> will engage with a respective tooth <NUM> whilst the remaining every other engagement pocket <NUM> will not engage a tooth. This is because, due to the dimensions of the sprocket, and particularly the profile of the tooth, relative to the dimensions of the rollers <NUM>, when the roller chain <NUM> is engaged with the sprocket <NUM> there will be two rollers <NUM> positioned between adjacent teeth. This in turn means that every other engagement pocket <NUM> will engage with a tooth <NUM>, with every other engagement pocket effectively engaging with spaces between adjacent teeth <NUM> of the sprocket.

The working curve <NUM> is convex, and the convex arc forming the working curve <NUM> curves towards the tooth centreline R. The surface of working curve <NUM> makes contact with the load bearing roller <NUM>, enabling torque transfer between the roller chain <NUM> and the sprocket <NUM>. As the chain pitch elongates due to internal wear, this surface also accommodates the climbing of the load-bearing roller.

As mentioned above, the rollers <NUM> of the roller chain <NUM> are able to articulate relative to one another via the links connecting adjacent rollers to one another.

In <FIG> two articulations angles are shown, a<NUM> and a<NUM> and these will now be explained further.

First roller <NUM> and second roller <NUM> are shown forming a first engagement pocket <NUM> which meshes with a first tooth <NUM>. A third roller <NUM> is in contact with a second tooth <NUM> and is positioned to one side of the first roller <NUM>. The third roller <NUM> and the first roller <NUM> together form a second engagement pocket <NUM>.

A fourth roller <NUM> is positioned adjacent to second roller <NUM> and is in contact with a third tooth <NUM>. The second and fourth rollers <NUM>, <NUM> together form a third engagement pocket <NUM>.

In this embodiment of the invention, the first roller <NUM> is a load bearing roller, and the second roller <NUM> is a support roller. Every other roller starting with the load bearing roller <NUM> will also be a load bearing roller. With reference to <FIG> therefore, the fourth roller <NUM> is also a load bearing roller. This pattern will repeat itself around the sprocket <NUM>.

At the point that first roller <NUM> makes contact with first tooth <NUM>, and third roller <NUM> is also in contact with a second tooth <NUM>, first roller <NUM> and third roller <NUM> are positioned on their respective engagement surfaces, and the second roller <NUM> is in position, a first articulation angle a<NUM> is formed at an articulation point <NUM>, which in this embodiment coincides with the axis of the first roller <NUM>.

Considering now the second roller <NUM> and fourth roller <NUM>, the second articulation angle a<NUM> is formed at the second roller <NUM> when the second roller <NUM> and the fourth roller <NUM> are in contact with a respective tooth <NUM>, and the first roller <NUM> is in contact with tooth <NUM>.

The magnitude of the first articulation angle a<NUM> at the point defined above, is in this example greater than the second articulation angle a<NUM> at the point defined above.

Similarly, every other roller starting with the second roller <NUM> is a support roller. In this embodiment therefore the third roller <NUM> is also a support roller and this pattern will repeat itself around the sprocket <NUM>.

In this embodiment, every other articulation angle will be the same. This means that the articulation angle a<NUM> will be at every load bearing roller, and the articulation angle a<NUM> will be at every support roller.

Adjacent rollers are connected to one another by a link which provides a rigid connection between adjacent rollers.

In this embodiment, first roller <NUM> is connected to second roller <NUM> by link <NUM>. Third roller <NUM> is connected to first roller <NUM> by link <NUM>, and second roller <NUM> is connected to fourth roller <NUM> by link <NUM>.

It is the links <NUM>, <NUM>, <NUM> which articulate relative to one another as shown by the articulation angles.

Because the articulation angle at each load bearing roller <NUM>, <NUM> is larger in this embodiment that the articulation angle a<NUM> at every support roller <NUM>, <NUM>, each load bearing roller <NUM> will articulate for a longer duration than is the case with each support roller <NUM>. This can improve the efficiency of the transmission system.

By means of the present invention therefore it is possible to achieve selective articulation by setting the articulation angle at each tooth to be different, or to follow a regular pattern as is the case in this embodiment.

This is desirable from the perspective of both power transmission efficiency and chain wear. Articulation under load causes inevitable friction between adjacent chain links. This leads to both energy loss and component wear. The size of these losses is roughly proportional to the size of the articulation angle.

The losses associated with each articulation alternates with the alternating inner and outer chain links of a standard power transmission roller chain. The articulation of the outer link is more efficient than the inner, while the articulation of the inner link leads to less chain elongation than the outer. By using selective articulation, the magnitude of the beneficial or deleterious effects of a given articulation can be manipulated to improve the drive trains overall performance.

As shown particularly in <FIG> and <FIG>, the articulation points <NUM> in a transmission system according to embodiments of the invention define a n sided irregular polygon <NUM>.

In embodiments of the invention where the first articulation angle is a<NUM> and the second is a<NUM>, the pattern is repeated for every pair of links around the sprocket circumference.

Thus, the relationship between these new articulation angles and the original exterior, angle of a polygon of n sides, a is, a<NUM>+a<NUM>=2a as shown in <FIG>.

To achieve this n sided irregular polygon, a sprocket of n/<NUM> teeth is used, where a tooth sits between the vertices of every other side of the polygon. This is shown more clearly in <FIG>.

Turning now to <FIG>, a sprocket <NUM> according to another embodiment of the invention is illustrated schematically. The sprocket <NUM> forms part of a transmission system <NUM> comprising the sprocket <NUM> and a roller chain <NUM>.

Parts of the transmission system <NUM> that are equivalent to the transmission system <NUM> described above will be given corresponding reference numerals for ease of reference.

As shown particularly in <FIG>, the roller chain <NUM> comprises a plurality of rollers <NUM>. The rollers <NUM> are connected to adjacent rollers by means of inner links <NUM> and outer links <NUM>.

The inner links <NUM> serve to connect two rollers <NUM> together to form a roller pair <NUM>. The outer links serve to connect roller pairs <NUM> together to form the roller chain <NUM>. The distance between inner surfaces <NUM> of inner links <NUM> is indicated by the reference numeral d<NUM> in <FIG>. The distance between inner surfaces <NUM> of facing outer links <NUM> is indicated by the reference numeral d<NUM>. As shown in <FIG>, d<NUM> is greater than d<NUM>.

Turning now to <FIG> and <FIG>, the sprocket <NUM> is described in more detail.

The sprocket comprises a plurality of teeth <NUM> spaced apart around the sprocket. Each tooth has a first width <NUM> that is equal to or slightly less than the distance between inner surfaces of facing inner links <NUM> (d<NUM>).

Each tooth <NUM> also has a second width <NUM> which is equal to or slightly less than the distance between the inner surfaces <NUM> of outer links <NUM> (d<NUM>).

In this embodiment of the invention each tooth comprises a middle tooth portion <NUM> and outer tooth portions <NUM>, <NUM> which together define the second width.

When the sprocket <NUM> engages with the roller chain <NUM>, the teeth will be positioned between two outer links as shown in <FIG>. The width of the outer tooth portions <NUM>, <NUM> together with the width of the middle portion <NUM> results in an overall tooth width that is the same as or slightly less than the distance (d<NUM>) between inner surfaces of facing outer links, and greater than the distance (d<NUM>) between the inner surfaces of facing inner links. This means that the fit between the tooth <NUM> and the chain <NUM> is such that there is little clearance between the tooth and the chain. Furthermore, the presence of the outer tooth portions <NUM>, <NUM> prevents the teeth from engaging between inner links, and thus the alignment of the chain is substantially maintained during use of the drive transmission system.

Claim 1:
A drive sprocket (<NUM>) comprising a plurality of teeth (<NUM>) for meshing with a drive member (<NUM>) to transmit rotary motion, the drive member including a plurality of engagement pockets (<NUM>) engaging the teeth of the drive sprocket, wherein the teeth are formed integrally with the drive sprocket and each tooth of the plurality of teeth has a tooth profile defined by a first side (<NUM>) comprising a first engagement surface (<NUM>) and an opposite second side (<NUM>) comprising a second engagement surface (<NUM>), and characterized in that, said engagement surfaces are configured such that when driven, a tooth meshes to one of the engagement pockets at a first contact location (<NUM>) on the first engagement surface and also at a second contact location (<NUM>) on the second engagement surface, wherein the first contact location is radially offset from the second contact location.