GB2433571A

GB2433571A - Chain tensioning device

Info

Publication number: GB2433571A
Application number: GB0525919A
Authority: GB
Inventors: Israel Garcia Sanchez
Original assignee: Nissan Motor Manufacturing UK Ltd; Nissan Technical Centre Europe Ltd
Current assignee: Nissan Motor Manufacturing UK Ltd
Priority date: 2005-12-21
Filing date: 2005-12-21
Publication date: 2007-06-27
Anticipated expiration: 2025-12-21
Also published as: GB0525919D0; GB2433571B; WO2007071535A1

Abstract

A chain tensioning member 58 for tensioning a chain 56 which passes around a sprocket 52, said chain thereby having an incoming span 60 and an outgoing span 62, wherein a span of the chain is tensioned using force derived from the other span. The tensioning member preferably has first and second end portions 68,70 disposed in engagement with the respective spans, and an intermediate arcuate portion which is mounted on the radially outer periphery of the chain where the chain passes around the sprocket. The tensioning member may be C-shaped, or M shaped, and able to rotate about the axis of the sprocket. The tension member may be resilient while its ends portions may comprise pulleys. It maintains tension in the chain regardless of the direction of rotation of the sprocket/chain.

Description

CHAIN DRiVE APPARATUS AND

CHAIN TENSIONING DEVICE

The invention relates to chain drive apparatus and particularly, but not exclusively, to tensioning chain drive apparatus for use in automobiles. The invention also relates to a chain tensioning device for use in a chain drive apparatus, and to a method of tensioning a chain drive.

Figures 1 and 2 illustrate the behaviour of a chain in a chain drive during forward and reverse rotation of the chain drive. The chain drive comprises a drive sprocket 10 connected with a drive, or input, shaft (not shown), a driven, or output, sprocket 12 connected with a driven, or output, shaft 12 (not shown) and an endless chain 14 mounted on the two sprockets. When a drive force is applied to the drive shaft, the drive sprocket 10 is rotated and that rotation is transmitted to the driven sprocket 12 by the chain 14. The chain 14 has a first (upper) chain region or span 16 extending between the two sprockets 10, 12 and a second (lower) chain region or span 18 that extends between the two sprockets and is disposed generally opposite the first chain span 16.

As viewed in Figure 1, when the drive sprocket 10 is driven clockwise, the upper span 16 of the chain 14 is put in tension as it is pulled onto the drive sprocket. In this condition the upper span 16 is the drive side and transmits the rotation of the drive sprocket 10 to the driven sprocket 12. The lower span 18 of the chain 14 is free of load and adopts an uncontrolled shape. Figure 2 shows the state of the chain 14 if the direction of rotation of the drive sprocket 10 is reversed. In this condition, the lower span of the chain is the drive side and straightens as it transmits the drive force to the driven sprocket 12 and the upper span 16 becomes the free side and adopts an uncontrolled shape.

Uncontrolled movement of the chain spans is undesirable and so it is usual to tension the chain to avoid such movement. One way of tensioning a chain is to make one of the shafts movable relative to the other so that the chain can be stretched by moving the movable shaft away from the fixed shaft. This fonn of chain tensioning is used on many bicycles, where the movable shaft is fitted to the rear wheel/driven sprocket.

However, in many engineering applications, such movement cannot be provided. For example, where the camshaft in an automobile engine is driven off the crankshaft by a chain drive, it is not possible to have one shaft movable relative to the other. In applications such as this, where there is no relative movement between the shafts/sprockets, a chain tensioning device is usually provided.

Figures 3 and 4 illustrate the way in which a known chain tensioning device 20 acts on the chain drive shown in Figures 1 and 2. As shown in Figure 3, the chain tensioning device 20 is designed to act as a guide for the lower span 18 of the chain 14, which is the free side when the drive sprocket 10 is driven clockwise. In this condition of the chain drive, the shape of the chain 14 is controlled, with the upper span 16 being tensioned by the applied drive force and the lower span 18 being guided by the chain tensioning device 20. However, as shown in Figure 4, if the direction of rotation of the drive sprocket 10 is reversed, the upper span 16 becomes the free side of the chain 14 and its shape is not controlled, while the lower span 18 tries to straighten, so stressing and wearing the chain tensioning device 20. This is undesirable in chain drives that are required to be operable in both directions or, at least, are subject to frequent transient reverse rotation.

One method of providing chain tensioning for reverse rotation is to have a chain tensioning device in the form of a third sprocket mounted on a pivot ann. This is an arrangement sometimes used on bicycles and an example of such a chain tensioning device is shown in JP 1110 5774. This chain tensioning device is relatively complex and may not be suitable for applications where the space around the chain is limited and/or mounting positions for the pivot arm are limited or unavailable.

Another method of providing chain tensioning that can adjust to reverse rotation is disclosed by US 6 117 034, which discloses a floating chain tensioning device mounted on the drive chain and having two sprockets. One of those sprockets engages the uppermost face of the upper span of the chain and the other sprocket engages the underside of the lower span of the chain such that the two sprockets are disposed one above the other with the two spans between them. The respective axles on which the two sprockets of the chain tensioning device are mounted are connected by elastomer bands so that the vertical distance between the sprockets can vary according to the forces transmitted by the chain. This chain tensioning device is relatively complex and may not be suitable where the space around the chain is relatively limited.

It is an object of the present invention to avoid or alleviate the above mentioned problems and/or provide an alternative to existing chain tensioning devices.

According to a first aspect of the invention, there is provided a chain tensioning device comprising a tensioning member having an arcuate portion having a first end portion and a second end portion spaced from the first end portion. The tensioning member is arranged such that, in use, the arcuate portion can be mounted on a radially outer periphery of an endless chain of a chain drive apparatus where the chain passes around a first sprocket of the chain drive apparatus to transmit drive to a second sprocket of the chain drive apparatus, with the first and second end portions disposed in engagement with respective spans of the chain so that force from one of the chain spans is at least partially transferred to the other of the chain spans so as to tension said other of the chain spans.

In a preferred embodiment, the tensioning member is arranged such that, in operation of the chain drive apparatus, force from one of the chain spans is at least partially transferred to the other of the chain spans so as to tension said other of the chain spans by sliding movement of the arcuate portion on the outer periphery of the endless chain.

It will be appreciated that in order to permit drive to be transmitted from the first sprocket of the chain drive apparatus to the second sprocket of the chain drive apparatus, the force from the chain span that is loaded is only partially transferred to the other chain span.

In a preferred embodiment, force from one of the chain spans is partially transferred to the other of the chain spans so as to tension said other of the chain spans by sliding movement of the arcuate portion on the outer periphery of the endless chain.

In a preferred embodiment, the arcuate portion can slide on the outer periphery of the chain so that force from one of the chain spans is partially transferred to the other of the chain spans so as to tension said other of the chain spans by sliding movement of the arcuate portion on the outer periphery of the endless chain.

In a preferred embodiment, the tensioning member is a resilient arcuate member.

The tensioning member is preferably generally C-shaped. Alternatively, the arcuate portion of the tensioning member may comprise a chain engaging portion disposed intermediate the first and second end portions such that the tensioning member is generally M-shaped and only engages the chain by the chain engaging portion and the first and second end portions.

Preferably, the first and second end portions subtend an angle less than 180 degrees.

Preferably, the tensioning member further comprises respective curved free end portions extending from the first and second end portions of the arcuate portion, the free end portions extending outwardly of the arcuate portion. The outward curve of the free end portions avoids the danger of the ends of the tensioning member digging into the chain when there is relative movement between the two parts.

According to a second aspect of the present invention, there is provided a chain drive apparatus comprising a first sprocket, a second sprocket and an endless chain for transmitting rotation from the first sprocket to the second sprocket and a chain tensioning device. The endless chain has a first span extending between the first and second sprockets and a second span extending between the first and second sprockets and generally opposite the first span. A chain tensioning device is provided having a first part for bearing against the first span and a second part for bearing against the second span such that, in use, force generated in the one of the first and second spans that is loaded is at least partially transferred to the other of the spans so as to tension the other of the spans.

Force generated in the one of the first and second spans that is loaded is partially transmitted to the other of the spans so as to tension the other of the spans, whilst permitting drive torque to be imparted to the second sprocket.

The chain tensioning device is, in one embodiment, rotatable about an axis of rotation in response to forces generated in the first and second spans.

In this ease, the axis of rotation for the chain tensioning device is preferably the axis of rotation of the first sprocket.

In a further preferred embodiment, the chain tensioning device is slidable on the outer periphery of the chain, in use, so that force from one of the chain spans is partially transferred to the other of the chain spans for tensioning said other of the chain spans by sliding movement of the chain tensioning device on the outer periphery of the chain.

Advantageously, the chain tensioning device is mounted on the outer periphery of the endless chain about the first sprocket, so that no additional supporting structure is required.

Preferably, the chain tensioning device is an arcuate member having an angular extent about the axis of rotation that is greater than 180 degrees and less than 360 degrees.

The arcuate member may have a generally circular cross-section, which has the advantage of limiting the contact between the arcuate member and the chain to what is substantially a line contact.

Preferably, the first and second parts of the chain tensioning device are defined by respective end portions of the arcuate member.

Preferably, each end portion of the arcuate member has a free end that projects outwardly from the axis of rotation. By this means, the ends of the arcuate member will not dig into the chain when there is relative movement between the two parts.

In a preferred embodiment, the chain tensioning apparatus has application in an automobile, and in particular wherein the chain drive is for driving a compressor of an air conditioning system.

According to a third aspect of the invention, there is provided a method of tensioning an endless chain of a chain drive apparatus comprising a first sprocket, a second sprocket and the endless chain mounted on the sprockets for transmitting drive from the first sprocket to the second sprocket, the endless chain having a first unsupported chain span extending between the first sprocket and the second sprocket and a second unsupported chain span extending between the first sprocket and the second sprocket and disposed generally opposite the first chain span. The method comprises transferring force from a loaded one of the first and second chain spans to an unloaded one of the first and second chain spans by means of a chain tensioning device, preferably freely rotatable, such that the transferred force tensions said unloaded one of the first and second chain spans.

Preferably, the chain tensioning device is rotatable about the axis of rotation of the first sprocket.

Preferably, the chain tensioning device comprises a resilient arcuate member and the method further comprises snap fitting the chain tensioning device onto an outer periphery of the endless chain where the chain passes around the first sprocket.

According to a fourth aspect of the invention, there is provided a chain drive apparatus comprising a first sprocket, a second sprocket, an endless chain for transmitting rotation from the first sprocket to the second sprocket and a chain tensioning device, the chain tensioning device engaging respective spans of the chain that extend from the first sprocket to the second sprocket and being mounted for rotation about the axis of rotation of the first sprocket so as to transmit force from the chain span that is loaded to the chain span that is substantially free of load, thereby to tension the chain span that is substantially free of load.

It will be appreciated that preferred and/or optional features of the first aspect of the invention may be incorporated within the second, third or fourth aspects of the invention alone or in appropriate combination, and vice versa.

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 arrangement of a prior art chain drive showing the behaviour of the chain when the drive sprocket is rotated clockwise; Figure 2 shows the behaviour of the chain of the chain drive of Figure 1 when the drive sprocket is rotated anti-clockwise; Figure 3 shows the chain drive of Figure 1 provided with a prior art chain tensioning device and the behaviour of the chain when the drive sprocket is rotated clockwise; Figure 4 shows the behaviour of the chain of the chain drive of Figure 3 when the drive sprocket is rotated anti-clockwise; Figure 5 is a schematic representation of a chain drive according to a first embodiment of the invention showing the behaviour of the chain when the drive sprocket is rotated clockwise; Figure 6 shows the behaviour of the chain of the chain drive of Figure 5 when the drive sprocket is rotated anti-clockwise; Figure 7 is a plan view of a portion of the chain of the chain drive of Figures 5 and 6 showing a portion of a chain tensioning device mounted on the chain; Figure 8 is a schematic representation of a chain drive according to a second embodiment of the invention; and Figure 9 is partial top plan view of the embodiment of Figure 8 showing a bearing part of a chain tensioning device engaging the upper side of the upper span of the chain of the chain drive.

Referring to Figure 5, a chain drive apparatus comprises a drive sprocket (input sprocket) 52, a driven sprocket (output sprocket) 54, an endless chain 56 and a chain tensioning device or member 58. The drive sprocket 52 is fitted on a drive shaft (not shown) so that when the drive shaft rotates, the drive sprocket is caused to rotate with it.

The driven sprocket 54 is fitted on a driven shaft (not shown) such that when the driven sprocket is rotated, it causes rotation of the driven shaft. The chain 56 extends partially around the circumference of each of the sprockets 52, 54 and between them in a conventionaj manner so that when the drive sprocket 52 is rotated, that rotation is transmitted to the driven sprocket 54. In the same way as the chain drives illustrated in Figures 1 to 4, the chain 56 has a first (upper) chain span 60 extending between the two sprockets 52, 54 and a second (lower) chain span 62 extending between the two sprockets and disposed generally opposite the first span 60.

The respective shafts on which the sprockets 52, 54 are fitted are fixed relative to one another and so any slack in the chain 56 cannot be taken up by moving one shaft/sprocket relative to the other. In order to tension the chain 56, the chain tensioning device 58 is fitted on the chain 56 where it passes around the drive sprocket 52.

The chain tensioning device 58 is an arcuate, generally C-shaped, member fitted on the radially outer periphery of the chain 56 where it extends around the drive sprocket 52.

The chain tensioning device 58 is made of a resilient material so that it can clip onto the chain/sprocket in the maimer of a spring clip. As shown in Figure 7, the chain 56 is conventional, being made up of two rows of interleaved links 64 that are disposed in parallel spaced apart relation and interconnected by pins 66. The chain tensioning device 58 seats on the pins 66 between the two rows of links 64 and is supported by the drive sprocket 52 via the chain 56, which is sandwiched between the chain tensioning device 58 and the drive sprocket 52.

The chain tensioning device 58 has a central axis 67 that coincides with the axis of rotation of the drive sprocket 52 and extends about that axis by an angle greater than degrees and less than 360 degrees so that respective end portions 68, 70 of the chain tensioning device 58 can engage with the first chain span 60 and the second chain span 62. The free ends 72, 74 of the chain tensioning device 58 curve away from (i.e. outwardly from) the central axis 67 of the device so that they will not dig into the chain 56.

The chain tensioning device 58 clips onto the chain 56 with sufficient force to ensure that it is retained on the chain, while at the same time allowing relative rotation of the chain 56 and the chain tensioning device 58 when the drive sprocket 52 is rotated.

Operation of the chain tensioning device 58 will now be described with reference to Figures 5 and 6.

Initially, as the drive sprocket 52 rotates clockwise, the upper chain span 60 begins to straighten and the device 58 rotates clockwise together with that portion of the chain 56 carried on the drive sprocket 52. As the upper chain span 60 straightens further under tension, its outer periphery (upper side) pushes against the end portion 68 of the chain tensioning device 58. During the phase for which the upper chain span 60 starts to straighten, the end portion 70 of the device 58 starts to push against the outer periphery (underside) of the lower chain span 62. Once the upper chain span 60 is fully tensioned, as shown in Figure 5, there is a contact force between the pins 66 of the upper chain span 60 and the end portion 68 of the device 58. Due to the contact force between the pins 66 and the end portion 68 of the device 58, the device 58 is driven to rotate in a clockwise direction, until a point is reached at which the contact force between the pins 66 on the lower chain span 62 and the end portion 70 of the device 58 has increased sufficiently to prevent further rotation. At this point the device 58 therefore starts to slide relative to the chain 56. The contact force between the pins 66 on the lower chain span 62 and the end portion 70 of the device 58 provides the tensioning force for the lower chain span 62. Thereafter, the chain tensioning device 58 will be maintained in position with the end portion 70 of the device 58 pressing into the lower chain span 62.

In summary, when the drive sprocket 52 is rotated clockwise, the lower chain span 62 becomes the free side. However, the uncontrolled movement shown in Figure 1 is not permitted because the end portion 70 of the chain tensioning device 58 presses into the lower chain span 62, thereby pushing it inwardly, towards the upper chain span 60, to tension the lower chain span and so maintain a desired shape and tension in the chain.

Referring to Figure 6, if the direction of rotation of the drive sprocket 52 is reversed so that it rotates anti-clockwise, the upper chain span 60 becomes the free side and the lower chain span 62 tends to straighten under tension. The straightening of the lower chain span 62 causes its outer periphery to press against the end portion 70 of the chain tensioning device 58. The lower chain span 62 begins to straighten and the device 58 rotates anti-clockwise, together with that portion of the chain 56 carried on the drive sprocket 52. As the lower chain span 62 straightens further under tension its outer periphery (underside) pushes against the end portion 70 of the chain tensioning device 58. During the phase for which the lower chain span 62 starts to straighten, the end portion 70 of the device 58 starts to push against the outer periphery (upper side) of the upper chain span 66.

Once the lower chain span 62 is fully tensioned, as shown in Figure 6, there is a contact force between the pins 66 of the lower chain span 62 and the end portion 70 of the device 58. Due to the contact force between the pins 66 and the end portion 70, the device 58 is driven to rotate in an anti-clockwise direction, until a point is reached at which the contact force between the pins 66 on the upper chain span 66 and the end portion 68 of the device 58 has increased sufficiently to prevent further rotation. The contact force between the pins 66 on the upper chain span 66 and the end portion 68 of the device 58 provides the tensioning force for the upper chain span 66. Thereafter, the chain tensioning device 58 will be maintained in position with the end portion 70 of the device 58 pressing into the upper chain span 66. This prevents the shape of the upper chain span 60 becoming uncontrolled in the way shown in Figure 2. At the same time as controlling the shape of the upper chain span 60, the chain tensioning device 58 allows the lower chain span 62 to straighten.

It will be appreciated that the chain tensioning device 58 floats' on the outer periphery of the chain 56 around the drive sprocket 52 and will assume an equilibrium position, according to the direction of rotation of the chain, which results in the control of the shape of the free side of the chain. At the same time, the chain tensioning device 58 is arranged so as permit the side that is under tension (load) to straighten, without stressing or wearing the chain tensioning device or the chain. It will also be appreciated that the chain tensioning device is self-adjusting and will automatically move to the correct position according to the direction of rotation of the drive sprocket in response to forces generated in the chain.

The wrap angle of the chain tensioning device 58 about the central axis 67, which is a measure of the extent to which the device 58 causes the chain to be wrapped' around the drive sprocket 52, may be varied between an angle greater than 180 degrees and less than 360 degrees. In other words, the angle subtended by the end portions 68, 70 of the chain tensioning device 58 must be less than 180 degrees. The wrap angle must be greater than 180 degrees, at least to the extent necessary for the end portions 68, 70 to maintain contact with the chain so that the device 58 does not fall off the sprocket 57.

The chosen wrap angle should also take account of the amount of slack that will be in the chain 56 and the desired tensioning effect. Thus, for example, if there is only relatively little slack in the chain the wrap angle does not need to be as great as if there is relatively high slack. The resilience of the material from which the chain tensioning device 58 is made will also be a factor in determining the wrap angle around the central axis 67.

The optimum wrap angle of the chain tensioning device 58 about the central axis 67, and so the gap between the end portions 68, 70, can be determined by experiment. For example, it may be preferable that the angle is selected such that the gap between the end portions 68, 70 approximates to the distance between the centre points of the upper and lower chain spans 60, 62 when pushed towards one another to the extent necessary to just remove the slack in the chain.

The chain tensioning device 58 must be made of a material that is sufficiently resilient to permit it to be clipped onto the chain 56 around the drive sprocket 52 and be retained on the chain. Suitable materials include spring steel, piano wire, fluorocarbon elastomers (FKM), and polyamides (PA). It is believed desirable that the strength and resilience of the chain tensioning device should be similar to that of an automotive piston ring and so the materials from which such piston rings are made should in general be suitable. At present, it is believed that it may be preferable to make the chain tensioning device 58 from the same material as the pins 66 of the chain 56.

In the embodiment described with reference to Figures 5 and 6, the tensioning device 58 is configured such that before torque is applied to the drive sprocket 52, the chain spans 60, 62 are not under tension. However, in another embodiment the tensioning device 58 may be configured so that, even when there is no torque applied to the drive sprocket 52, both the upper and lower chain spans 60, 62 are tensioned due to the inherent pre-loading of the device 58. Consider operation of this embodiment when the drive sprocket 52 is rotated clockwise. Because the device 58 is pre-loaded, there is an additional contact force between the device 58 and the pins 66 of the chain 56.

Therefore, when sliding of the device 58 relative to the chain 56 commences, an additional driving torque is applied to the device 58. Since the end portion 70 prevents the device 58 from rotating relative to the chain 56, the contact force between the end portion 70 and the lower chain span 62 increases, so increasing the tension in the lower chain span 62. In other respects this embodiment operates in a similar manner to that described previously.

In cross-section, the chain tensioning device 58 may be circular or rectangular (oblong or square). A circular cross-section may be more desirable, as this will reduce the surface contact between the chain tensioning device and the chain.

In the aforementioned embodiments, the chain tensioning device 58 is a simple C-shape between the end portions 68, 70. However, it may be desirable to reduce the amount of contact between the chain tensioning device 58 and the chain 56. For this purpose, the chain tensioning device could be provided with a chain engaging portion in the form of a projection midway between the two end portions 68, 70, so as to be generally M-shaped. In this case, the chain tensioning device would only contact the chain via the projection and the two end portions.

It is believed that mounting the chain tensioning device directly on the chain is desirable, since then no additional supporting structure is required. This is advantageous where there is limited space around the chain drive or no suitable point for locating a supporting structure. This feature makes the chain tensioning device particularly suitable for retrofitting. However, this is not an essential feature of the invention. By way of example, Figures 8 and 9 illustrate an embodiment in which the chain tensioning device is provided with a supporting structure and not mounted directly on the chain.

Referring to Figures 8 and 9, a chain drive 98 comprises a first sprocket 100, a second sprocket 102, a chain 104 and a chain tensioning device 106. The chain tensioning device 106 is mounted on the drive shaft 108 on which the first sprocket 100 is fitted.

The chain tensioning device 106 comprises a tubular bush 110 that is mounted on the drive shaft 108 such that the drive shaft and bush can rotate relative to one another, while at the same time support the chain tensioning device. A first arm 112 projects radially outwardly from the bush 110 and has a circular section bearing part 114 provided at its free end. As shown in Figure 9, the bearing part 114 extends perpendicular to the first arm 112 such that in use, it can engage the outer periphery of the upper span 116 of the chain 104. A second arm 118 projects radially outwardly from the bush 110 and has a circular section bearing part 120 projecting from its free end. The bearing part 120 extends perpendicular to the second arm 118 so that it can engage the underside of the lower span 122 of the chain 104. The chain tensioning device 106 functions inthe same way as the chain tensioning device 58 (the bearing parts 114, 120 taking the place of the end portions 68, 70) and so its operation will not be described in detail.

It will be appreciated that as an alternative to mounting the bush 110 on the drive shaft 108, the bush could be mounted for relative rotation on a spigot provided on the first sprocket 100.

It will be understood that in both embodiments, the force generated in the drive side of the chain is transmitted to the free side of the chain to tension the free side and that this is achieved by simple rotation of the chain tensioning device in response to the forces generated in the drive side of the chain. In each case, the chain tensioning device is a simple device that rotates freely in response the forces generated in the drive side to assume an equilibrium position in which it provides the desired tensioning of the free side of the chain, the equilibrium position being achieved when there is a force balance between the forces generated in the two sides of the chain.

The configuration of the chain tensioning devices and, in particular, the spacing between the two chain engaging portions can readily be determined by experimentation and so the performance of the device can readily be matched to the characteristics of a particular drive. This, in combination with the simplicity of the structure of the chain tensioning device, makes the device suitable for retrofitting to existing chain drives. It will be appreciated that the chain tensioning device of the first embodiment is particularly suitable for retrofitting, since it simply clips onto the chain and does not require a dedicated support structure.

It will be understood that a further advantage of the chain tensioning device of the first embodiment is that it simply snap fits onto the periphery of the chain and so no fitting tools are required.

Claims

CLAIMS

1. A chain tensioning device comprising a tensioning member (58) having an arcuate portion with a first end portion (68) and a second end portion (70) that is spaced from the first end portion, the tensioning member being arranged such that, in use, the arcuate portion can be mounted on a radially outer periphery of an endless chain (56) of a chain drive apparatus where the chain passes around a sprocket (52) of the chain drive apparatus with the first and second end portions (68, 70) disposed in engagement with respective spans (60, 62) of the chain (56), whereby, in operation of the chain drive apparatus, force from one of the chain spans is transferred to the other of the chain spans for tensioning said other of the chain spans.

2. A chain tensioning device as claimed in claim 1, whereby the arcuate portion of the tensioning member (58) is slidable on the outer periphery of the chain, in use, so that force from one of the chain spans is transferred to the other of the chain spans for tensioning said other of the chain spans by sliding movement of the arcuate portion on the outer periphery of the chain.

3. A chain tensioning device as claimed in claim 1 or claim 2, wherein the tensioning member is generally C-shaped.

4. A chain tensioning device as claimed in claim 1 or claim 2, wherein the arcuate portion comprises a chain engaging portion disposed intermediate the first and second end portions (68, 70) such that the member is generally M-shaped and only engages the chain by the chain engaging portion and the first and second end portions.

5. A chain tensioning device as claimed in any one of claims 1 to 4, wherein the first and second end portions subtend an angle less than 180 degrees.

6. A chain tensioning device as claimed in any one of claims 1 to 5, wherein the tensioning member further comprises respective curved free end portions (72, 74) extending from the first and second end portions (68, 70), wherein the free end portions extend outwardly from the arcuate portion.

7. A chain tensioning device as claimed in any one of claims 1 to 6, wherein the tensioning member is a planar member.

8. A chain drive apparatus comprising: a first sprocket (52: 100), a second sprocket (54;102), an endless chain (56; 104) for transmitting rotation from the first sprocket to the second sprocket, wherein the chain has a first chain span (60; 116) extending between the first and second sprockets and a second chain span (62; 122) extending between the first and second sprockets and disposed generally opposite the first chain span, and a chain tensioning device (58; 106) having a first part (68; 114) for bearing against the first chain span (60; 116) and a second part (70; 120) for bearing against the second chain span (62; 122) such that, in use, force generated in the one of the first and second chain spans that is loaded is transferred to the other of the first and second chain spans for tensioning the other of the first and second chain spans.

9. A chain drive apparatus as claimed in claim 8, wherein the chain tensioning device (58) is rotatable about an axis of rotation in response to forces generated in the first and second chain spans (60, 62).

10. A chain drive apparatus as claimed in claim 9, wherein the axis of rotation is the axis of rotation of the first sprocket.

11. A chain tensioning device as claimed in any one of claims 8 to 10, whereby the chain tensioning device (58) is slidable on the outer periphery of the chain, in use, so that force from one of the chain spans (60, 62) is transferred to the other of the chain spans for tensioning said other of the chain spans (60, 62) by sliding movement of the chain tensioning device (58) on the outer periphery of the chain.

12. A chain drive apparatus as claimed in any one of claims 8 to 11, wherein the chain tensioning device is mounted on the outer periphery of the chain (56) about the first sprocket (52).

13. A chain drive apparatus as claimed in claim 12, wherein the chain tensioning device (58) is an arcuate member having an angular extent about the axis of rotation of the first sprocket that is greater than 180 degrees and less than 360 degrees.

14. A chain drive apparatus as claimed in claim 13, wherein the arcuate member has a generally circular cross-section.

15. A chain drive apparatus as claimed in claim 13 or claim 14, wherein respective end portions (68; 70) of the arcuate member are the first and second parts of the chain tensioning device.

16. A chain tensioning device as claimed in claim 15, wherein each end portion (68; 70) of the arcuate member has a free end (72; 74) that projects outwardly of the axis of rotation of the first sprocket.

17. A chain drive apparatus comprising: a first sprocket (52), a second sprocket (54), an endless chain (56) for transmitting rotation from the first sprocket to the second sprocket and having a first chain span (60) extending between the first and second sprockets and a second chain span (62) extending between the first and second sprockets and disposed generally opposite the first chain span, and a chain tensioning device as claimed in any one of claims 1 to 7.

18. An automobile provided with a chain drive apparatus as claimed in any one of claims8tol7.

19. An automobile as claimed in claim 18, wherein the chain drive apparatus is for driving a compressor of an air conditioning system.

20. A method of tensioning an endless chain (56) of a chain drive apparatus comprising a first sprocket (52), a second sprocket (54) and the endless chain which is mounted on the sprockets for transmitting drive from the first sprocket to the second sprocket, the endless chain having a first unsupported chain span extending between the first sprocket and the second sprocket and a second unsupported span extending between the first sprocket and the second sprocket and disposed generally opposite the first span, the method comprising transferring force from a loaded one of the first and second chain spans to an unloaded one of the first and second spans by means of a chain tensioning device such that the transferred force tensions the unloaded one of the first and second spans.

21. A method as claimed in claim 20, wherein the chain tensioning device is rotatable about the axis of rotation of the first sprocket.

22. A method as claimed in claim 20 or claim 21, wherein the chain tensioning device comprises a resilient arcuate member, the method further comprising snap fitting the chain tensioning device onto an outer periphery of the chain where the chain passes around the first sprocket.

Amendments to the claims have been filed as follows 6. A chain tensioning device as claimed in any one of claims 1 to 5, wherein the tensioning member further comprises respective curved free end portions (72, 74) extending from the first and second end portions (68, 70), wherein the free end portions extend outwardly from the arcuate portion.

7. A chain tensioning device as claimed in any one of claims 1 to 6, wherein the tensioning member is a planar member.

8. A chain drive apparatus comprising: a first sprocket (52: 100), a second sprocket (54; 102), an endless chain (56; 104) for transmitting rotation from the first sprocket to the:* second sprocket, wherein the chain has a first chain span (60; 116) extending between the first and second sprockets and a second chain span (62; 122) extending between the first and second sprockets and disposed generally opposite the first chain span, and a chain tensioning device (58; 106) mounted on the outer periphery of the chain *::: :* (56) about the first sprocket (52), the chain tensioning device (58; 106) having a first::: : part (68; 114) for bearing against the first chain span (60; 116) and a second part (70; 120) for bearing against the second chain span (62;

122), such that in use, force generated in the one of the first and second chain spans that is loaded is transferred to the other of the first and second chain spans for tensioning the other of the first and second chain spans.

9. A chain drive apparatus as claimed in claim 8, wherein the chain tensioning device (58) is rotatable about an axis of rotation in response to forces generated in the first and second chain spans (60, 62).

10. A chain drive apparatus as claimed in claim 9, wherein the axis of rotation is the axis of rotation of the first sprocket.

11. A chain tensioning device as claimed in any one of claims 8 to 10, whereby the chain tensioning device (58) is slidable on the outer periphery of the chain, in use, so that force from one of the chain spans (60, 62) is transferred to the other of the chain spans for tensioning said other of the chain spans (60, 62) by sliding movement of the chain tensioning device (58) on the outer periphery of the chain.

12. A chain drive apparatus as claimed in any one of claims 8 to 11, wherein the chain tensioning device (58) is an arcuate member having an angular extent about the axis of rotation of the first sprocket that is greater than 180 degrees and less than 360 degrees. * * * . . I. **

13. A chain drive apparatus as claimed in claim 12, wherein the arcuate member has:* a generally circular cross-section. *...* * .

S.....

14. A chain drive apparatus as claimed in claim 12 or claim 13, wherein respective end portions (68; 70) of the arcuate member are the first and second parts of the chain * S...

tensioning device. *::: : 15. A chain tensioning device as claimed in claim 14, wherein each end portion (68; 70) of the arcuate member has a free end (72; 74) that projects outwardly of the axis of rotation of the first sprocket.

16. A chain drive apparatus comprising: a first sprocket (52), a second sprocket (54), an endless chain (56) for transmitting rotation from the first sprocket to the second sprocket and having a first chain span (60) extending between the first and second sprockets and a second chain span (62) extending between the first and second sprockets and disposed generally opposite the first chain span, and a chain tensioning device as claimed in any one of claims 1 to 7.

17. An automobile provided with a chain drive apparatus as claimed in any one of claims 8to 16.

18. A method of tensioning an endless chain (56) of a chain drive apparatus comprising a first sprocket (52), a second sprocket (54) and the endless chain which is mounted on the outer periphery of the chain about the first sprocket for transmitting drive from the first sprocket to the second sprocket, the endless chain having a first unsupported chain span extending between the first sprocket and the second sprocket and a second unsupported span extending between the first sprocket and the second sprocket and disposed generally opposite the first span, the method comprising: *. *.

transferring force from a loaded one of the first and second chain spans to an unloaded one of the first and second spans by means of a chain tensioning device such S....

that the transferred force tensions the unloaded one of the first and second spans.

*SS...

19. A method as claimed in claim 18, wherein the chain tensioning device is. "S. S...

rotatable about the axis of rotation of the first sprocket.

20. A method as claimed in claim 18 or claim 19, wherein the chain tensioning device comprises a resilient arcuate member, the method further comprising snap fitting the chain tensioning device onto an outer periphery of the chain where the chain passes around the first sprocket.