Drive sprocket

Abstract

A drive sprocket can include a plurality of teeth for meshing with a drive member to transmit rotary motion. The drive member can include a plurality of engagement pockets engaging the teeth of the drive sprocket, where 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.

Claims

1. 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.

2. A drive sprocket as claimed in claim 1, wherein the first face arc of each tooth comprises a base portion of the first side of the said tooth, and the second face arc of each tooth forms a base portion of the second side of the respective tooth, wherein the first and second face arcs each comprise a roller seating curve.

3. A drive sprocket as claimed in claim 1, wherein 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.

4. A drive sprocket as claimed in claim 1, further comprising a supporting curve extending from the roller seating curve of a first tooth towards the roller seating curve of an adjacent tooth.

5. A transmission system comprising a drive sprocket according to claim 1, the transmission system further comprising a drive member, which drive member is adapted to mesh with the drive sprocket.

6. A transmission system as claimed in claim 5, wherein the drive member comprises 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.

7. A transmission system as claimed in claim 6, wherein the drive member comprises a roller chain, and wherein the engagement pockets are defined between adjacent rollers forming the roller chain.

8. A transmission system as claimed in claim 7, wherein 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.

9. A transmission system as claimed in claim 7, wherein 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.

10. A drive member forming part of a transmission system as claimed in claim 5.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention will now be further described by way of example only with reference to the accompanying drawings in which:

(2) FIG. 1 is a schematic representation of a roller chain engaged with a sprocket according to the first aspect of the invention and forming a transmission system according to an embodiment of the second aspect of the invention;

(3) FIG. 2 is a schematic representation of the transmission system of FIG. 1 with some links removed to show more clearly how the rollers of the chain engage with the sprocket;

(4) FIG. 3 is a perspective view of part of the sprocket of FIG. 1 showing the tooth profile of the teeth of the sprocket;

(5) FIG. 4 is a schematic representation of the transmission system of FIG. 1 showing how the rollers of the roller chain engage with the teeth of the sprocket;

(6) FIG. 5 is a schematic representation of the transmission system of FIG. 1 showing where the engagement pockets lie during use of the transmission system;

(7) FIG. 6 is a schematic representation showing the dimensions of the rollers of the roller chain shown in FIG. 1;

(8) FIG. 7 is a schematic representation of the sprocket arc construction of the transmission system shown in FIG. 1; and

(9) FIG. 8 is a schematic representation showing movement of the rollers caused by wear of the drive member shown in FIG. 1.

DETAILED DESCRIPTION

(10) Referring initially to FIGS. 1 and 2 a transmission system according to an embodiment of the invention is designated generally by the reference numeral 2. The transmission system comprises a sprocket 4 and a drive member comprising a roller chain 6.

(11) In this embodiment of the invention the roller chain 6 is a standard roller chain comprising a plurality of rollers 8 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 10 in a known manner. The roller chain 6 is able to articulate between adjacent rollers 8. An engagement pocket 40 is defined between adjacent rollers 8. Each engagement pocket 40 is adapted to engage with a tooth 12 as will be described in more detail below.

(12) By means of the present invention, however, only every other engagement pocket 40 will engage with a tooth during use of the transmission system 2. The remaining every other engagement pockets 40 will effectively engage with the space between adjacent teeth 12.

(13) Turning now to FIG. 3, the sprocket 4 is shown in more detail.

(14) The sprocket 4 comprises a plurality of teeth 12 which are all shaped substantially identically to one another. Each tooth has a tooth face or profile 14 which is symmetrical about a radial axis R of the sprocket 4.

(15) The tooth profile 14 is defined by a first side 16 comprising a first engagement surface 18, and a second side 20 defining a second engagement surface 22. Each of the first and second sides 16,20 comprises a base portion 24 which forms a roller seating curve 25. Each side further comprises a portion 26 extending from the roller seating curve towards a tip 28 of the tooth. The portion 26 is convex and defines a working curve 29.

(16) The sprocket 4 comprises a further curve 30 forming a supporting curve 31 which extends between adjacent teeth.

(17) As shown in FIGS. 4 and 5 particularly, in use of the transmission system 2, every other engagement pocket 40 will engage with a respective tooth 12 whilst the remaining every other engagement pocket 40 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 8, when the roller chain 6 is engaged with the sprocket 4 there will be two rollers 8 positioned between adjacent teeth. This in turn means that every other engagement pocket 40 will engage with a tooth 12, with every other engagement pocket effectively engaging with spaces between adjacent teeth 12 of the sprocket.

(18) Referring to FIG. 4 the manner in which the rollers 8 engage with the sprocket 4 during use of the transmission system 2 is shown schematically.

(19) When considering a pair of rollers 8 positioned on either side of a tooth 12, one roller 32 will be a load bearing roller, and the second roller 8 will be a supporting roller 34.

(20) The roller seating curve 25 provides an initial seating position for the engaged rollers 8 of the roller chain 6. 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 25 enables rollers to easily transition between supporting and load bearing positions if the drive direction is ever reversed.

(21) The load bearing roller 32 will engage with the tooth 12 on a first engagement surface 36, and the support roller 34 will engage with the tooth at a second engagement surface 38.

(22) The first and second engagement surfaces 36,38 are radially offset from one another. This enables the pair of rollers 8 engaging the tooth 12 to engage with dual engagement, since the roller chain makes contact with the sprocket teeth 12 at two contact points 37, 39 on engagement surfaces 36, 38 in each tooth of the sprocket.

(23) The two contact points 37, 39 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.

(24) The combination of these features leads to a secure engagement of the drive sprocket tooth by the roller chain 6 and ensures that the rollers 8 do not become wedged on the tooth. In addition, there is little to no relative movement between the tooth and the rollers 8 whilst in contact.

(25) The first contact point 37 is load bearing and transfers the load between the roller chain 6 and the tooth 12. The second contact point 39 is supporting and thus stabilises the roller chain 6 on the sprocket 4 and increases the load distribution over the sprocket teeth 12.

(26) As shown in FIGS. 3 and 4, each tooth 12 further comprises a working curve 26 that extends from the roller seating curve towards the tip 28 of the tooth.

(27) The working curve 26 is convex, and the convex arc forming the working curve 26 curves towards the tooth centreline R. The surface of working curve 26 makes contact with the load bearing roller 32, enabling torque transfer between the roller chain 6 and the sprocket 4. As the chain pitch elongates due to internal wear, this surface also accommodates the climbing of the load-bearing roller as shown in FIG. 8.

(28) The tip 28 of each tooth does not need to have a pointed profile. This is because when an engagement pocket 40 is at the point of engagement with the tooth 12 it has a single degree of freedom only which is the articulation of the engagement pocket about the centre of the roller.

(29) 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 32, 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.

(30) Turning again to FIG. 3, the sprocket further comprises a supporting curve 50 which extends between the roller seating curves of adjacent teeth.

(31) The supporting curve is designed to accommodate the supporting roller 34. The supporting curve may also accommodate some movement of the supporting roller 34 over the lifetime of the roller chain 6, as the worn chain adopts an altered position on the sprocket.

(32) Referring specifically to FIGS. 4 and 5, the engaging pockets 40 are represented by lines 42.

(33) The line 42 of each engagement pocket sits with endpoints situated on a circle 44 known as the pitch circle. The pitch circle defines the centre point of all the roller seating curves 25.

(34) 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 40 sitting marginally off the pitch circle 44. 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.

(35) Referring to FIG. 6, the dimensions of the roller chain 6 are shown in more detail.

(36) As can be seen from FIG. 6, 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 d.sub.r.

(37) Referring now to FIG. 7, a schematic representation of part of the transmission system 2 of FIG. 1 is shown. The circle radius r.sub.p represents the pitch circle. This is the circle which passes through all of vertices of a regular polygon of n sides, for a sprocket 4 which has n/2 teeth. Each side of the regular polygon has a length ρ. FIG. 7 shows three of the sides of the regular polygon showing the length as ρ.

(38) The radius of the arc forming the first face arc and the second face arc may be represented by r.sub.s. The centre of a roller seating curve 25 with radius r.sub.s 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 32 may be denoted by Θ. In embodiments of the invention, the ratio ρ was found to be 1.01 regardless of the number of teeth on the sprocket 12.

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

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

(41) TABLE-US-00001 Number of teeth in the sprocket Theta (degrees) 6 0.3 8 2.3 10 2.7 25 5.4 30 4.5 32 4.2

(42) Thus, it can be seen that in a transmission system according to an embodiment of the invention, the teeth 12 of the sprocket 4 will have a profile that hardly varies depending on the number of teeth forming the sprocket.

(43) By means of the embodiments of the invention therefore a standard roller chain, for example a roller chain meeting the ISO 606 standard, is able to engage a sprocket such that dual engagement is achieved.

(44) In embodiments of the invention where the sprocket 4 has n teeth, the roller seating arc has a fixed radius r.sub.s for all n, and this radius is slightly larger than the radius of each roller 8.