In using mechanical conveyors there is a need for separate, spaced articles on the conveyor to be allowed to accumulate in line in a predetermined zone, for example for subsequent division of the line or for other processing. Although the line of articles can readily be brought to a halt by the use, for example, of a mechanical stop, it is important to preclude the conveyor rollers from continuing to rotate relative to the bottoms of the stationary articles. Stopping the rollers by cutting the power to the conveyor is not a practicable course of action, largely because the conveyor needs to be kept moving so as to drive rollers (usually by chain and sprocket means) in other parts of the line.

Our British Patent No. 2 290 358 discloses a torque limitation device, suitable for use with a conveyor, comprising a driving member, e. g. a chain sprocket having a driving surface ; a driven member, e. g. a reaction plate fixed to a roller shaft, having a driven surface ; and disposed between and in frictional contact with said driving and driven surfaces, a resilient force- transmitting means, e. g. elastomeric balls in apertures in a cage.

This torque limitation device has proved most successful in practical use. In particular, the torque limits it provides are adjustable within wide limits, and the device is thereby effective for conveyed articles of very different weights. In that torque limitation device the torque limits are normally adjusted by selecting a predetermined number of the resilient elements to be present in the cage.

The present invention now provides an improved torque limitation device which, while being adjustable so as to effect a wide selection of torque limits, does not require the device to be dismantled in order to change from one torque limit to another.

Accordingly, in a first aspect the present invention provides a torque limitation device which comprises : a driving member having a driving surface ; a driven member having a driven surface, said driving surface and said driven surface being so disposed as to define a gap between them ; and, disposed in said gap and in frictional contact with the driving surface and the driven surface, a resilient force-transmitting means capable in a first mode of transmitting force from said driving member to said driven member and, in a second mode, of ceasing to transmit the force and to rotate while in contact with said driving surface and said driven surface ;

characterised in that said device includes adjustment means for mechanically adjusting the amount of compression applied to said resilient elements when they are in said frictional contact with said driving surface and said driven surface.

In a second aspect the invention provides a roller conveyor in which one or more of the rollers are provided with a torque limitation device according to the first aspect of the invention.

In a third aspect the invention provides an adjustment means for use with, or as part of, the torque limitation device of the invention.

The invention is concerned, in particular, with a torque limitation device for roller conveyors driven by chain and sprocket wheel means, and it is with particular reference to such conveyors that the following description relates.

In a preferred form of the invention the force-transmitting member comprises one or more resilient rotatable elements accommodated in a cage or other carrier, the carrier being disposed between the driving and the driven surfaces for rotation about an axis of the carrier. As the rotatable elements there can be used, for example, balls (which can be, for example, of spherical or spheroidal shape) or rollers (for example of cylindrical or frusto-conical shape). Any convenient number of rotatable elements can be used, for example any number in the

range 3 to 10 (for example 5, 6, 7 or 8), the number depending to some extent on the size of the carrier and on the torque limit required for the device. In general, the torque limit is greater the greater the number of balls or other rotatable elements.

In order to provide for flexibility in use, it is convenient to use a carrier having apertures to accommodate the rotatable elements in a number greater than the minimum number required. For example, the carrier can comprise a rotatable disc having formed therein, for example, 6 to 10 apertures in which are disposed, normally equi-angularly, the balls or other rotatable elements, which can be, for example, 3 to 10, for instance 5, 6 7 or 8, in number. If desired every one of the apertures can contain a rotatable element.

The rotatable elements are preferably disposed equi- angularly with respect to each other ; they are conveniently disposed at a common radial distance from the axis of rotation of the carrier.

The rotatable elements normally are ones having a substantial degree of resilience. The resilience of the ball or other rotatable member is preferably such that it is in a compressed, distorted state in its location between the driving surface and the driven surface. Thus, where the rotatable elements are spherical balls in their normal, relaxed state, they will assume a spheroidal shape when in a compressed state between

the driving and driven surfaces.

The ball or other rotatable element is conveniently of a polyurethane elastomer having an appropriate resilience. Suitable materials are polyurethane elastomers having a Shore A hardness in the range from 80 to 100, for example in the range from 90 to 98.

A preferred form of carrier is a cylindrical plate having formed therein a plurality of apertures each to accommodate an element rotatable within its respective aperture or recess. The apertures are preferably disposed equi-angularly in the plate and preferably disposed at a common radial distance from the axis of rotation of the plate.

The driven surface can be one having formed therein a plurality of recesses each sequentially to receive the rotatable elements as they move relative to the driven surface.

In normal operation the balls or other resilient members have two modes of operation. In the first, torque-transmitting,

mode the resilient members are in a compressed state between the driving and driven members and do not move with respect to either. In the second mode the resilient members, still in a compressed state, roll gainst an adjacent surface of the driving and/or driven member. This rolling motion is normally without slipage.

Reference is now made to the adjustment means of the torque limitation device of the invention.

In one form the adjustment means comprises means to mechanically adjust, by incremental amounts, the width of the gap between the driving surface of the driving member and the driven surface of the driven member, and thereby adjust the degree of compression of said resilient elements.

In another form, for example in combination with said first form, the adjustment means comprises a first member for transmitting a lateral force to the driven member so as to urge it against the resilient elements in the cage, a second member adapted for rotational movement co-axial with that of the driven member, and means operatively connecting said first and second members whereby said rotational movement of the second member results in lateral force being applied to the first member. In one embodiment of said another form said first member is carried by, or is comprised by, the reaction plate and said second member is a plate mounted co-axially with the reaction plate. The means operatively connecting the first and second members is

conveniently a ramp device comprising co-operating ramps disposed for relative motion while in face-to-face contact.

In a preferred embodiment the adjustment means includes an adjustment plate mounted co-axially and substantially parallel to the reaction plate for relative rotational movement and relative axial movement. The opposed faces of the reaction plate and the adjustment plate each carries co-operating arcuate ramps disposed for face-to-face sliding contact, whereby relative movement between them results in the reaction plate and adjustment plate being moved apart, thereby urging the reaction plate against the resilient balls or other resilient elements.

The present invention provides torque limitation devices suitable for use with conveyors having rotatable axles and torque limitation devices suitable for use with conveyors having static axles. Normally, in a rotatable axle conveyor the axles rotate in the conveyor frame and the rollers are fixed on the axles, whereas in a static axle conveyor the axles are fixed in the conveyor frame and roller rotate on the axles. Normally, in chain and sprocket conveyors, if the torque limitation device is for the use with a rotatable axle conveyor the device is mounted outboard of the chain-driven sprocket ; if it is for use with a static axle conveyor it is normally mounted inboard of the chain- driven sprocket.

A further advantage provided by the present invention is in starting up the conveyor, in which circumstance the torque limitation device acts as a clutch and smooths the transmission of power from the driven sprocket wheel to the driven member.

The balls or other rotatable elements can be of any suitable material that provides a suitable resilience and/or coefficient of friction. Suitable materials, mentioned by way of example, are thermoplastic polyurethane elastomers, for instance those sold by BASF under the trade mark"Elastollan", especially those of the C series based on polyester, for example C90A and C74D.

Physical properties of members of the Elastollan C series are as follows :

Units DIN C78A C80A C85A Hardness shore A 53505 80 82 87 Hardness shore D 53050 36 Density g/cm'53479 1. 18 1. 19 1. 19 Tensile strengthl N/mm2 53504 45 45 50 Elongation at break¹ % 53504 600 600 600 20% Tensile modulusl N/mm2 53504 2 2. 5 3 100% Tensile modulusl N/mm2 53504 3. 5 4. 5 5. 5 300% Tensile modulusl N/mm2 53504 7. 5 8. 5 9. 5 Modulus of elasticity - tensile test (Test specimen 3mm) N/mm2 53457 Tear strength N/mm 53515 60 65 75 Abrasion loss mm3 53516 30 30 30 Compression set at room temperature % 53517 25 25 25 Compression set at 70°C % 53517 35 35 35 Tensile strength after storage in water at 80°C for 21 days N/mm2 53504 35 35 38 Elongation at break after storage in water at 80°C for 21 days % 53504 650 650 650 KEY 1 measured with test specimen S2 with a strain rate of 100 mm/min.

The test samples were punched out of 2mm or 6mm injection moulded sheets and tempered at 100°C for 20 hours

C90A C59D C60D C64D C74D Hardness shore A 93 Hardness shore D 41 57 60 63 75 Density 1. 20 1. 23 1. 23 1. 24 1. 25 Tensile strengthl 50 55 55 55 55 Elongation at breakl 550 400 400 400 350 20% Tensile modulusl 7 12 16 17 28 100% Tensile modulusl 9 17 20 24 30 300% Tensile modulusl 15 30 35 35 35 Modulus of elasticity - tensile test (Test specimen 3mm) 250 330 390 730 Tear strength 100 170 190 210 250 Abrasion loss 3G 35 35 35 35 Compression set at room temperature 25 30 40 40 40 Compression set at 70°C 40 50 50 55 60 Tensile strength after storage in water at 80°C for 21 days 40 48 48 48 50 Elongation at break after storage in water at 80°C for 21 days 550 480 450 420 380 KEY 1 measured with test specimen S2 with a strain rate of 100 mm/min.

The test samples were punched out of 2mm or 6mm injection moulded sheets and tempered at 100°C for 20 hours

The driven surface can, if desired, have formed therein a plurality of recesses to provide an increase in torque. Thus, for example, these recesses can be recesses bounded by a spherical or conical surface or can be in the form of grooves radially disposed in the driven surface. The balls or other rotatable elements, when in engagement with the respective recesses, can be either in a relaxed, uncompressed state or in a distorted, compressed state. In either event, when the balls move from a first recess to the next adjacent recess they assume a distorted, compressed state between the first recess and said adjacent recess. In one form, given by way of example, the recesses can be shallow conical recesses disposed equi-angularly and at a common radial distance from the centre of a driven plate.

The number and disposition of the recesses is preferably such that there is some misalignment of the recesses with the resilient balls so as to avoid all of the balls being located in recesses at the same time and all moving out of the recesses at the same time. This latter arrangement, though included in the invention is not preferred because it results in sudden changes in the torque transmitted by the resilient balls. Instead, it is preferred that balls move into, and out of, the recesses in a consecutive manner. For example, where the balls are disposed equi-angularly in their carrier and the recesses are disposed equi-angularly in the driven surface, the number of balls is unequal to the number of recesses (either greater than or less than), thereby providing that some but not all of the balls are located in recesses at any one time.

There are now described, by way of example and with reference to the accompanying drawings, preferred embodiments of torque limitation devices of the present invention and of conveyors equipped with the devices. FIG. 1 to FIG. 8C relate to a conveyor having rotatable axles and FIG. 9 and FIG. 10 relate to a conveyor having static axles.

In the drawings : FIG. 1 is a median cross-section of an accumulator roller assembly including a torque limitation device of the present invention ; FIG. 2 is an enlargement of the torque limitation device portion of the roller assembly of FIG. 1 ; FIG. 3 is an enlargement of a portion of FIG. 2 showing, in greater detail, a hub and a bush, both components of the torque limitation device ; FIG. 4A is an elevation of the hub shown in FIG. 3 ; FIG. 4B and FIG. 4C are opposed end views of the hub of FIG. 4A ; FIG. 4D is a median cross-section of the hub of FIG.

4A ; FIG. 5A and FIG. 5B are opposed end views of the bush

shown in FIG. 3 ; FIG. 5C is a cross-section on the line A-A of FIG. 5A ; FIG. 6A and FIG, 6B are opposed face views ofa pressure plate of a pressure adjustment device ; FIG. 6C is a cross-section on the line A-A of FIG. 6B ; FIG. 7A and FIG. 7B are opposed face views of an adjustment ring of the pressure adjustment device ; FIG. 7C is a side elevation of the adjustment ring of FIG. 7A and 7B ; FIG. 7D is a cross-section on the line A-A of FIG. 7A ; FIG. 8A and FIG. 8B are opposed face views of an index disc of the pressure adjustment device ; FIG. 8C is a cross-section on the line A-A of FIG. 8A ; FIG. 9 shows in its lower portion an elevation and in its upper portion a section of an accumulator roller assembly including a torque limitation device of the present invention, the roller assembly having a static axle ; and

FIG. 10 is an enlargement of the torque limitation device portion of the roller assembly of FIG. 9.

The conveyor includes a plurality of parallel cylindrical rollers (10), respective rollers being fixedly secured to hexagonal axles (11) whose end portions are journalled in bearings (12). The conveyor is driven by a motor (not shown) through a drive sprocket wheel (not shown) and a chain (13) which engages sprocket wheels (14), one of which is shown in FIGS. 1, 2 and 3. The sprockets (14) have formed therein hexagonal bores and are a sliding fit on the respective axles (11).

The mechanism is enclosed in housing (29) having secured thereto a polyethylene wear strip guard (13. 1).

With particular reference to FIGS. 1 and 2, the end portion of axle (11) carries, as a sliding fit, a hub (15) comprising an annular portion (15. 1) having a bore of hexagonal cross-section and a periphery of circular cross-section, and a flange portion (15. 2). Thus, the hub (15) can be regarded, in practice as part of the axle. The hub is held in position on the axle by means of a washer (16) and a retaining bolt (17) engaging a screw- threaded recess (18), and at the other end by a spacer (19) which spaces it apart from bearing (12). An annular bush (20) on which sprocket (14) can rotate, is carried on the hub.

As shown in particular in Figures 4A to 4D, the annular portion (15. 1) of the hub (15) has formed therein three like,

parallel, longitudinally extending, grooves (15. 3) spaced apart from each other by 120°, the purpose of which is described below.

The hub annular portion also has a circumferential groove (15. 4) to accommodate circlet clip (21), the purpose of which is described below.

On the annular portion of hub (15) and adjacent to sprocket (14) is mounted an annular cage (22) ; the latter contains, in a plurality of spaced apertures (23), a plurality of resilient balls (24). In the embodiment illustrated in the drawings the cage is in the form of a circular, annular disc rotatably mounted on hub (15) and having a plurality of circular apertures (23) disposed equi-angularly and at a common radial distance from the centre of the disc, each aperture containing a like torque- transmitting polyurethane rubber ball (24) of high resilience.

The balls have a common diameter which is greater than the thickness of disc and thus the balls protrude from each face of the disc.

An annular reaction (pressure) plate (25) is fixedly mounted on hub (15). The cage (22) is disposed between the reaction plate (25) and sprocket (14) so that the resilient balls are in contact both with driving surface (14. 1) of the sprocket and with the driven surface (25. 1) of the reaction plate.

The reaction plate (25) (shown in greater details in FIGS.

6A, 6B and 6C), has an inner periphery of circular cross-section to engage the annular portion of hub (15). However, it is

prevented from relative rotation on hub (15) by means of like equi-angularly spaced lugs (25. 1) which engage respectively the parallel grooves (15. 3) in hub (15). As shown in FIGS. 6A to 6C, the reaction plate is of a generally annular shape. One face (FIG. 6A) is substantially planar ; the other face (FIG. 6B) has three, like, inclined portions or ramps (25. 2) each of arcuate shape and rising in an anti-clockwise direction from the"-" sign to the " + "signv The ramps can be contiguous, the upper end of one ramp being adjacent the lower end of the next adjacent ramp. The angle of inclination of the ramps is, for example, a uniform angle, for example in the range 0. 5° to 5°, for instance in the range 2° to 4°. In the embodiment shown in the drawings the angle of the ramps was a uniform angle of 2. 2°.

Adjacent the upper end of each ramp is a stop (25. 3). Each ramp is preferably accompanied by a scale of values, for example 0 to 9 as shown in FIG. 6B, to facilitate pre-determined setting of the device.

FIGS. 7A, 7B, 7C and 7D show an adjustment ring (26) disposed adjacent, and in contact with, the reaction plate (25) but, unlike the latter, mounted for rotation on hub (15).

Adjustment ring (26) is an annular plate, conveniently of the same or similar dimensions to the reaction plate (25). On a first face (FIG. 7A) it includes inclined portions or ramps (26. 2) for co-operation with the ramps (25. 2) of the reaction plate, and stops (26. 5). The shape and dimensions of the ramps of the adjustment ring and/or stops (26. 5) are conveniently the same as, or similar to, those of the reaction plate. On its

other (second) face (FIG. 7B) the adjustment ring (26) has calibration means, for example, a plurality of like protrusions (26. 3), depressions or other engagement means for co-operation with means carried by the index disc described below.

The adjustment ring preferably has slots (26. 4) or equivalent by means of which the ring can be engaged by a spanner or other tool to adjust the angular position of the ring with respect to the reaction plate (25). The slots or other means can be conveniently disposed equi-angularly of each other in the periphery of the adjustment plate. Said means can be so dimensioned and positioned as to provide an observation aperture to observe respective aligned index numbers of the reaction plate.

FIGS. 8A, 8B and 8C illustrate an index means which is, in this embodiment, a disc (27) fixedly mounted on hub (15), being prevented from rotation thereon by means of lugs (27. 1) which engage, respectively, the parallel grooves (15. 3) in the hub.

The index disc has means for said co-operation with the calibration means of the adjustment plate, for example a plurality of depressions (27. 2) (or protrusions) to engage a plurality of protrusions (26. 3) (or depressions) of the adjustment plate.

The index plate is captured on the axle (11) by means of circlet clip (21) which engages circumferential groove (15. 4) of hub (15).

In the embodiment illustrated in the drawings the diameter of index disc (27) is sufficiently smaller than that of adjustment plate that it does not obscure the respective index numbers of pressure plate (25) that are visible in windows (26. 4) of the adjustment ring.

The balls used in the preferred embodiment described above were of a polyurethane elastomer having the following a Shore A Hardness of 98.

MODE OF OPERATION OF THE FIRST EMBODIMENT A general mode or method of operation of the torque limiting device described above with reference to FIGS. 1 to 8C of the accompanying drawings can, for example, be as follows : When the conveyor is in its normal operating mode, that is, being driven by the motor through the drive sprocket wheel, chain (13) and driven sprocket wheel (14), torque is transmitted from the sprocket wheel (14) through the balls (24) to the reaction plate (25), which causes the axle (11) and roller (10) to rotate at the same angular velocity as the sprocket wheel, the balls (24), in a compressed state, being stationary with respect to the driving surface (14. 1) of sprocket wheel (14) and the driven surface (25. 1) of reaction plate (25). In this mode a line of articles carried by the conveyor moves forward at a constant

linear velocity. If that flow of articles is interrupted, for example by means of a mechanical stop, the articles accumulate on the conveyor and a torque is set up in the rollers supporting the articles, this torque being transmitted through axle (11) to reaction plate (25). When the torque limit of the torque limitation device is reached the balls begin to rotate while remaining compressed and in contact both with the driving surface (14. 1) and with the driven surface (25. 1) ; and the drive between sprocket wheel (14), and reaction plate (25) is thereby disengaged allowing the rollers (10) supporting the articles to come to rest.

When the mechanical stop or other braking means is removed frictional force between the balls (24) and the driving and driven surfaces is re-established, the balls come to rest between the surfaces and transmission of power from the sprocket wheels (14) to reaction plate (25) is resumed.

A cylindrical oil shield (28), secured to sprocket (14) by welding, is disposed coaxially with respect to the cage (22) to prevent accidental ingress of oil.

The dimensions and disposition of the various component parts are such that, in use, the balls are held in compression between sprocket and reaction member. The degree of compression is such that the diameter of the balls is, for example, about one and a half mm. less in the fully compressed state than in their uncompressed state.

The balls used in the embodiment described above were spherical balls and composed of Elastollan C90A (or alternatively Elastollan C74D).

The size and number of balls used in the device described above with reference to the drawings were as follows : Number of balls : 10 Diameter of balls : 8 mm As stated above, the degree of compression of the balls can be one and a half mm. The term"degree of compression"in this context indicates the difference between the diameter of the balls in their uncompressed state and the distance between the driving surface (for example the sprockets shown in the drawings) and the driven surface (for example the reaction plates shown in the drawings). Degrees of compression other than one and a half mm can, if desired be used ; the greater the degree of compression the greater is the torque limit that can be obtained.

Suitable alternative degrees of compression are in the range 0. 5mm to 4mm, for example, 1,2 or 2. 5mm.

The degree of compression can be adjusted, for example, by choosing balls or other resilient rotatable elements of a particular size ; an increase in the diameter of resilient balls, for instance, results in a greater degree of compression and, in consequence, an increase in the torque limit of the device.

The adjustment means feature of the torque limitation device can be used as follows. By way of example, it is assumed that the lowest value of torque limit is required. Accordingly, the adjustment ring (26) is turned, as necessary, by means of a spanner (or other suitable tool) so that the index digit"0"of the pressure plate/reaction plate (25) appears in each of the slots/windows (26. 4) thereby indicating that the co-operating ramps (25. 2) of the reaction plate (25) and the (26. 2) of the adjustment ring overlap to a minimum extent (if any). In this position the face (25. 1) of pressure plate (25) in contact with balls (24) exerts a minimum pressure on them. Thus, a low torque limit of the torque limitation device is effected. The adjustment ring (26) is held in position with respect to reaction plate (25) by the engagement of the protrusion calibration means (26. 3) of the adjustment ring with the depression calibration means (27. 2) of the index disc (27).

If, by way of example, it is then wished to use the highest value of torque limit, the adjustment ring (26) is turned until the index digit"9"of reaction plate (25) appears in each of the slot/windows (26, 4), thereby indicating that the co-operating ramps (25. 2) of reaction plate and (26. 2) of the adjustment ring overlap, in face-to-face contact, to a maximum, or substantially maximum, extent. In that position the reaction plate is displaced in a lateral direction from adjustment ring, exerts a substantially maximum pressure on balls (24) and thereby compresses them to a substantial extent. Thus, a high torque limit of the torque limitation device is effected. As before,

the adjustment ring (26) is held in position with respect to reaction plate (25) by the engagement, in a new position, of the protrusion calibration means (26. 3) of the adjustment ring with the depression calibration means (27. 2) of the index disc (27).

A second embodiment of the invention, given by way of example, is now described with reference to FIGS. 9 and 10. In this embodiment the axles of the conveyor are static, that is static with respect to the conveyor frame, and each carries a roller rotatably mounted thereon. Thus, the conveyor includes a plurality of parallel cylindrical rollers (110) each mounted, by means of bearings (110. 1), on a static axle (111) held in the conveyor frame and driven by a motor through a drive sprocket wheel (not shown) and a chain (not shown) which engages sprocket wheels one of which (112) is shown in FIGS. 9 and 10.

The roller comprises a hollow cylindrical member (110. 2) secured by crimping to an end piece (110. 3). The end piece (110. 3) comprises a cylindrical portion (110. 4) and a flange portion (110. 5). The outer surface of the end piece has secured thereto an annular wear plate (110. 6) which provides a driven surface of the torque limitation device. In the angle of the end piece there is secured by welding an annular spacer (110. 8) whose purpose is to hold the bearing (110. 1) apart from the flange (110. 5).

Axle (111) has secured to it first and second stops (111. 1 and 111. 2). In between the first stop (111. 1) and adjacent

bearing the axle carries a compression spring (111. 3) against which the axle can be moved axially and thereby compress the spring in locating the axle in the conveyor frame.

Axle (111) carries a hub (113) which has a hexagonal bore, by means of which it is a sliding fit on the axle and has cylindrical periphery. The hub is held in position on the axle by said second stop (111. 2) at one end and by the conveyor frame (not shown) at the other end. Sprocket (112) is mounted on the hub by means of bearing (114) which is prevented from outward movement by circlip (115) located in a circumferential groove in the hub. Sprocket bearing (114) is protected from the ingress of dust or other contamination by annular dust cap (114. 1) whose annulus is a push fit on the end of hub (113). Sprocket (112) includes an inwardly directed annular flange or extension portion (116) carrying three equally-angularly spaced lugs one of which (117) is shown in FIGS. 9 and 10.

On the hub (113) is rotatably mounted a carrier or cage (118) in the form of a circular plate and having a plurality of spaced apertures containing a plurality of resilient balls (119).

The carrier and/or balls can be, for example, similar to those described above with reference to FIGS. 1 to 8C. The resilient balls are disposed between, on one side, a pressure plate or other pressure member (120) and on the other side the wear plate (110. 6) of the roller end piece (110. 3). The pressure member (120) is conveniently in the form of a circular plate and is so disposed that it rotates synchronously with the sprocket (112).

This synchronous operation can be effected, for example, by slots formed in the pressure plate to engage respectively the lugs (117) carried by the sprocket extension (116). The pressure plate includes a wear plate for contact with the resilient balls (119).

Thus, in terms of the general concept of the invention, the pressure plate (120) comprises the driving member and the pressure plate wear strip (120. 1) comprises the driving surface ; and the roller end piece (110. 3) comprises the driven member and the roller end piece wear plate (110. 6) comprises the driven surface.

The outer face (that is the face nearer sprocket 112) of the pressure plate (120) has formed therein three, like, inclined portions or ramps each of arcuate shape. These ramps can, for example, be identical with, or similar to, the ramps described above with reference to FIGS. 6A, 6B and 6C.

In between the outer face of the pressure member and the opposed inner face of the sprocket extension (116) there is disposed an adjustment ring (121) rotatably mounted on hub (113) and in contact with sprocket extension (116), whose purpose is similar to the adjustment ring described above with reference to FIGS. 7A to 7D, being formed on the face directly opposed to the outer face of pressure member (120) with inclined portions or ramps for co-operation with the ramps of the pressure member.

The shape and dimensions of the ramps of the adjustment ring

and/or stops are conveniently the same as, or similar to, those of the pressure member.

Adjustment ring (121) has slots or their equivalent, by means of which the ring can be engaged by a spanner or other tool to adjust the angular position of the ring. The ring preferably has a display of comparative values of torque. On its other face the adjustment ring has calibration means, for example a plurality of protrusions, depressions or engagement means for co- operation with corresponding means carried by the opposed face of the sprocket extension (116).

Roller end piece (110. 3) carries an outwardly extending circular flange (122) as an oil shield to prevent the ingress of oil onto the carrier (118).

In FIG. 9 the values of"15. 5 or 11. 5 mm"for the dimension "X" are given by way of example.

MODE OF OPERATION OF THE SECOND EMBODIMENT The mode or method of operation of the embodiment described above with reference to FIGS. 9 and 10 can, for example, be similar to that described above with reference to FIGS. 1 to 8C but taking into account the fact that in the FIGS. 9 and 10 embodiment the torque limitation device lies inboard of the sprocket whereas in the FIGS. 1 to 8C embodiment the devices lies

outboard of the sprocket. In general terms, in the second embodiment the transmission of power from sprocket (112) to the roller (110) is provided, in succession by pressure member (120) as the driving member, resilient balls (119) and roller end piece (110. 3) as the driven member. The adjustment ring or other adjustment means (121) is comprised by the ramp means of the pressure plate and the ramp means of the adjustment ring and (optionally) by the calibration means.

The resilient balls used in the embodiments of the invention described with reference to the drawings were either Elastollan C 90 A 55 000 or Elastollan C 98A 10 000. These elastomers have the following properties : Elastollan C 90 A 55 000 Property Value Unit Test Method MFR g/10 min Density 1. 20 0. 01 g/cm3 DIN 53479 Hardness shore A 93 + 2 -- DIN 53505 Hardness shore D -- Tensile strength 45 N/mm2 DIN 53504 Elongation at break 450 W DIN 53504 Tear strength 80 N/mm DIN 53515 Abrasion loss 35 mm3 DIN 53516

Elastollan C 98 A 10 000 Property Value Unit Test Method MFR g/10 min Density 1. 22 0. 01 g/cm3 DIN 53479 Hardness shore A 98 _ 2 -- DIN 53505 Hardness shore D 52 + 3 -- Tensile strength 35 N/mm2 DIN 53504 Elongation at break 400 % DIN 53504 Tear strength 110 N/mm DIN 53515 Abrasion loss 45 mm3 DIN 53516