A CHAIN SPROCKET

Abstract: A sprocket (1) for a drive chain (3) has a plurality of radially retractable teeth (4). As sprocket rotates the teeth pass over a cam (7) that force them in sequence from a retracted position into an extended position where they engage the chain. The teeth are resiliently biased into the retracted position so that after they have engaged the cam they return to the retracted position. The retracting teeth ensure that noise caused by impact of the chain on the sprocket teeth is significantly reduced.

Description: A CHAIN SPROCKET The present invention relates to a chain sprocket, particularly but not exclusively, a sprocket of the type that is used in combination with an endless roller and bush chain. It is well known that a significant proportion of noise generated in chain drives occurs when a chain engages with its drive sprocket and in particular when rollers in the links of the chain impact with teeth defined on the sprocket periphery. It is a object of the present invention to obviate or mitigate the aforesaid disadvantage and provide for a relatively low noise chain drive assembly. According to the present invention there is provided a sprocket for a drive chain, the sprocket comprising a wheel body with at least one chain-engaging tooth that is retractable into the body. The tooth is arranged so that as a chain is presented to the periphery of the sprocket it is in the withdrawn position and as the sprocket rotates the tooth gradually moves towards the extended position so as to engage the chain. The tooth is preferably reversibly retractable from a chain-engaging extended position in which it protrudes from the body to a retracted position. The tooth is preferably radially retractable. The tooth is preferably biased into the retracted position and may be moved to the extended position by means of a cam. The cam may be fixed relative to the sprocket so that the tooth is moved during rotation of the sprocket. In a preferred embodiment the cam is eccentric and engages a radially inward end of the tooth when the latter is moved to the extended position. Preferably the sprocket comprises a plurality of retractable teeth each being moveable in sequence as it passes over the cam. In one embodiment the teeth are supported on a resilient annulus that is engaged by the cam. In this embodiment the cam may rotate about a fixed point offset from a centre of rotation of the sprocket. Specific embodiment of the present invention will now be described, by way - of example only, with reference to the accompanying drawings in which: Figure 1 is a schematic partially sectioned side view of a sprocket according to a first embodiment of the present invention; Figure 2 is an enlarged view of part of the sprocket of figure 1 showing teeth in an extended position; and Figure 3 is a schematic side view illustrating a second embodiment of the present invention Referring now to the drawings, figures 1 and 2 show a rotary drive sprocket 1 of the present invention that has a centre of rotation 2 and is designed to engage with a roller chain 3 only part of the outline of which is represented schematically by parallel lines. The remainder of the chain 3 is removed for clarity but it will be understood that it approaches the sprocket 1 in direction of arrow A, engages approximately 180° of the periphery of the sprocket 1 and emerges in the direction of arrow B. The sprocket 1 has a plurality of equi-angularly spaced teeth 4, each of which is radially moveable within a respective radial slot 5 in the sprocket 1. Each tooth 4 is resiliently biased into a withdrawn position by means of, for example, a spring or hydraulic pressure (not shown). Radially inner ends of the teeth 4 have semi-cylindrical formations 6 that are designed to come into sliding contact with an eccentric cam 7 that is fixed at the centre of rotation 2 of the sprocket 1. In operation the sprocket 1 rotates clockwise to drive the chain 3 in the direction of arrows A and B. As the teeth 4 ride over the eccentric of the cam 7 they are gradually forced radially outwardly to a fully extended position (as represented by tooth 4a in figure 2) and after passing the cam they gradually return to a retracted position. The cam 7 is carefully positioned so that its eccentric portion coincides with the point where the chain 3 fully engages the sprocket 1. At the point where the chain 3 is first presented to sprocket 1, the nearest sprocket tooth (represented by 4b in figure 1) is virtually fully retracted but as the sprocket 1 rotates the tooth 4 gradually protrudes furhter from its radial slot. When fully extended the tooth (as represented by tooth 4b) is in complete engagement with the chain 3. This arrangement significantly reduces noise in a chain drive since the noise caused by the impact of the chain on fully extended teeth is virtually eliminated as the teeth are only extended when required for driving engagement with the chain. In an alternative embodiment shown in figure 3 radially inner ends of the teeth 4 engage an elastic annulus 8 which on its inner circumferential surface is engaged at one point by a rotary circular cam 9. As the sprocket 1 rotates the annulus 8 rides over the cam 9 and is locally deflected so as to cause the adjacent teeth 4 to move radially outwards and into engagement with the chain 3.

Claims: CLAIMS 1. A sprocket for a drive chain, the sprocket comprising a wheel body with at least one chain-engaging tooth that is retractable into the body. 2. A sprocket for a drive chain according to claim 1 , wherein the tooth is preferably reversibly retractable from a chain-engaging extended position in which it protrudes from the body to a retracted position. 3. A sprocket according to claim 2, wherein the tooth is radially retractable. 4. A sprocket according to claim 2 or 3, wherein the tooth is biased into the retracted position and is moved to the extended position by means of a cam. 5. A sprocket according to claim 4, wherein the cam is fixed relative to the sprocket so that the tooth is moved during rotation of the sprocket. 6. A sprocket according to claim 4 or 5, wherein the cam is eccentric and engages a radially inward end of the tooth when the latter is moved to the extended position. 7. A sprocket according to claim 4 or 5, wherein the tooth is supported on a resilient annulus that is engaged by the cam. 8. A sprocket according to claim 7, wherein the cam rotates about a fixed point offset from a centre of rotation of the sprocket. 9.. A sprocket according to any one of claims 4 to 8, wherein the sprocket comprises a plurality of retractable teeth each being moveable in sequence as it passes over the cam. 10. A sprocket substantially as hereinbefore described with reference to figures 1 and 2, and figure 3 of the drawings.

Inventors: BARLOW MICHAEL DAVID, CHRISTMAS MICHAEL CHARLES, BARLOW, MICHAEL, DAVID, CHRISTMAS, MICHAEL, CHARLES
Applicants: BARLOW MICHAEL DAVID, CHRISTMAS MICHAEL CHARLES, RENOLD PLC, BARLOW, MICHAEL, DAVID, CHRISTMAS, MICHAEL, CHARLES
Application year: 1998
Main classification: F16H55
IPC codes: F16H55