EP2666603B1 - Motorised chain saw with a feed pump - Google Patents

Description

The invention relates to a motor chain saw according to the preamble of claim 1, such a motor chain saw is in WO 2011/098097 disclosed.

From the US 6,523,645 B1 is a feed pump in a motor chain saw known, over which a chain driven by a chain saw chain lubricating chain oil is supplied. The pump drive of the feed pump consists of a first drive wheel connected to the feed pump and a second drive wheel connected to the sprocket, which are in drive connection with one another. The second drive wheel is designed as a worm wheel on a drive element which engages over the sprocket with a coupling section in such a way that it is in a form-locking drive connection with the sprocket. The coupling portion has for this purpose on the circumference of a plurality of drivers, which are associated with driving surfaces formed on the sprocket. In this case, all drivers engage between two teeth of the driving sprocket and are in driving connection with this.

If a chainsaw with another saw chain z. B. equipped other division, the sprocket must be replaced according to the type of saw chain used to ensure the shape and pitch of the saw chain adapted drive connection with the sprocket. If the geometry of the sprocket is changed, the drive element to the feed pump must be replaced, since the coupling portion is formed in each case according to the geometry of the sprocket used.

When replacing the Sägekettengarnitur is thus not only the sprocket, but also replace the drive element to the feed pump, which is complicated and cumbersome.

The invention has for its object to simplify the structure of the drive connection between a sprocket and a drive element for driving a feed pump.

The object is achieved according to the features of claim 1.

In the coupling portion of the drive element according to the invention a plurality of first driver are formed, which are associated with a first sprocket first geometry; in the same coupling portion of the same drive element a plurality of second drivers are formed, which are associated with the second sprocket of a second geometry. The first geometry is different from the second geometry. The first geometry differs from the second geometry z. As in the division and / or the number of sprocket teeth and / or the size in diameter. In this case, the drivers are arranged such that the coupling portion rests with the first or second drivers on the respectively associated first or second driving surfaces of the driving first or second sprocket.

By providing a plurality of different sprockets associated driver in the coupling portion, the same drive element for multiple sprockets of different geometry can be used, so that when replacing the Sägekettengarnitur usually the drive element does not need to be replaced. This simplifies the replacement of a Sägekettengarnitur in a motorized chainsaw.

The driver surface is preferably formed on a chain tooth itself, so that no special engagement means on the sprocket itself are necessary. In this case, the driver overlaps the sprocket tooth over a partial width axially, wherein the partial width is smaller than 30% of the total width of the sprocket tooth. This size of the driving surface is sufficient for a pump drive.

In the circumferential direction of the drive element, two first drivers and two second drivers each form a driver pair, wherein a driver pair in the circumferential direction - so in both directions of rotation of the sprocket - each defines a receptacle for an associated sprocket of the first and second sprocket. The training is advantageously provided such that in the circumferential direction of the drive element alternately a first driver pair and a second driver pair successive.

In order to further adapt the drive element for a plurality of sprockets, it is provided that the first receptacles provided in the coupling section relative to the axis of rotation of the drive element are diametrically opposed to one another and the second receptacles each lie diametrically opposite one another. This allows an arrangement of the driver to an axial plane of the drive element carried out such that they are symmetrical to this axial plane.

A first receptacle formed by first drivers is formed in the circumferential direction of the drive element with a smaller extent than the extension of a second receptacle formed by second drivers. Thus, the first receptacle has a smaller extent in the circumferential direction than the second receptacle.

In order to easily consider a different number of teeth and a different pitch of the sprocket, it is provided to provide a larger empty space in the circumferential direction of the drive element between the first and second dog pairs. This empty distance is between the driver pair of first drivers and the driver pair of second drivers and is greater in its extension than the measured between the drivers of a driver pair extension of the recording.

Advantageously, the empty distance between the driver pairs in the circumferential direction of the drive element is at least twice as large as the maximum extent of a receptacle in the circumferential direction.

A sprocket of a driving sprocket is held in the circumferential direction of the drive element in the associated receptacle of a pair of carriers with such tolerance that easy manufacturing and assembly of the drive element is ensured on the sprocket. It may be appropriate to keep the driving sprocket in the receptacle substantially free of play. It is advantageous if adjacent in the direction of rotation of the sprocket over the circumference of the coupling portion adjacent, driving chain teeth on first and second drivers, wherein one of the driving chain teeth is held in each case in the associated receptacle of a driver pair.

The inventive design of the positive drive connection from the pushed onto the existing metal sprocket, consisting of plastic drive element is thus designed so that - regardless of the geometric design of the recorded in the drive connection sprocket - only a subset of the sprockets of the driving sprocket with the drivers be in drive connection. It is advantageous that the drivers lie with radial play to the tooth base between two chain teeth. Due to the different geometry of the sprockets used, the drive element facing connection contour of the sprocket in shape, shape and / or geometry is different; Despite the different connection diagrams of the connection contour of the sprocket can be used according to the invention the same drive element for the pump drive.

Fig. 1

einen Teilschnitt durch ein Arbeitsgerät am Beispiel einer Motorkettensäge längs der Linie A-A in Fig. 3,

Fig. 2

einen Teilschnitt durch das Arbeitsgerät nach Fig. 1 längs der Linie B-B,

Fig. 3

einen Teilschnitt durch das Arbeitsgerät nach Fig. 1 längs der Linie C-C,

Fig. 4

in schematischer Darstellung eine in dem Arbeitsgerät integrierte Förderpumpe,

Fig. 5

in perspektivischer Darstellung ein Antriebselement der Förderpumpe nach Fig. 4,

Fig. 6

eine perspektivische Ansicht auf den Kupplungsabschnitt des Antriebselementes nach Fig. 5,

Fig. 7

eine Stirnansicht auf den Kupplungsabschnitt nach Fig. 6,

Fig. 8

eine schematische Darstellung einer ersten Antriebsverbindung des Antriebselementes mit einem ersten Kettenrad aus sechs Kettenzähnen,

Fig. 9

eine perspektivische Darstellung der ersten Antriebsverbindung nach Fig. 8,

Fig. 10

eine schematische Darstellung einer zweiten Antriebsverbindung des Antriebselementes mit einem zweiten Kettenrad mit acht Kettenzähnen,

Fig. 11

eine perspektivische Darstellung der zweiten Antriebsverbindung nach Fig. 10,

Fig. 12

eine schematische Darstellung einer dritten Antriebsverbindung des Antriebselementes mit einem dritten Kettenrad mit acht Kettenzähnen und geänderter Teilung,

Fig. 13

eine perspektivische Ansicht auf die dritte Antriebsverbindung nach Fig. 12.

Further features of the invention emerge from the further claims, the description and the drawing, in which embodiments of the invention described in detail below are illustrated. Show it:

Fig. 1

a partial section through an implement on the example of a motor chain saw along the line AA in Fig. 3 .

Fig. 2

a partial section through the implement after Fig. 1 along the line BB,

Fig. 3

a partial section through the implement after Fig. 1 along the line CC,

Fig. 4

a schematic representation of an integrated in the working device feed pump,

Fig. 5

in perspective view a drive element of the feed pump to Fig. 4 .

Fig. 6

a perspective view of the coupling portion of the drive element after Fig. 5 .

Fig. 7

an end view of the coupling section after Fig. 6 .

Fig. 8

a schematic representation of a first drive connection of the drive element with a first sprocket of six chain teeth,

Fig. 9

a perspective view of the first drive connection according to Fig. 8 .

Fig. 10

a schematic representation of a second drive connection of the drive element with a second sprocket with eight chain teeth,

Fig. 11

a perspective view of the second drive connection according to Fig. 10 .

Fig. 12

a schematic representation of a third drive connection of the drive element with a third sprocket with eight sprocket teeth and changed pitch,

Fig. 13

a perspective view of the third drive connection according to Fig. 12 ,

In the exemplary embodiment, a portable, hand-guided implement is shown using the example of a motor chain saw 1. The motor chain saw 1 consists essentially of a drive motor 2, which is an internal combustion engine in the illustrated embodiment, in particular a two-stroke internal combustion engine. As a drive motor and electric motors are used.

The drive motor 2 is mounted in a housing 3, which extends in the longitudinal direction 23 (FIG. Fig. 2 ) of the housing 3 extending rear handle 4 with operating elements 5, 6 for the drive motor. A front handle 7 extends transversely to the longitudinal direction 23 of the motor chain saw 1 via the housing 3; in front of this front, provided on the top of the housing 3 handle 7 a hand guard 8 is pivotally mounted, which serves to trigger a safety brake device 9.

As the Figures 2 and 3 show, the safety brake device 9 is received in a removable sprocket 10, into which a clutch drum 11 of a centrifugal clutch 12 protrudes. The driven by the drive motor 2 drive shaft 13, which is formed in the embodiment as a crankshaft of the internal combustion engine, drives the centrifugal clutch 12 with one end 14; above a certain speed, the coupling elements 15 apply to the clutch drum 11 to transmit torque and take this in the direction of rotation of the drive shaft 13 with.

With the clutch drum 11 rotatably connected to a sprocket 16, which in perspective z. In Fig. 9 is shown. The sprocket 16 is - together with the clutch drum 11 - rotatably mounted with a bearing 17 on the drive shaft 13. The end 14 of the drive shaft 13 extends through the bearing 17 and carries at the protruding end 14 the carrier of the coupling elements 15th

The sprocket 16 has circumferentially successive sprockets 18 which engage drivingly in a saw chain 19 and driving them on a guide rail 20 circumferentially. To guide the saw chain 19 is in the peripheral edge of the guide rail 20 has a circumferential groove 21 (FIG. Fig. 2 ) educated. The guide rail 20 is held clamped by the clamping bolt 22 between the housing 3 and the sprocket 10. The guide rail 20 extends in the longitudinal direction 23 of the implement.

For lubricating the circulating on the guide rail 20 saw chain 19 of the circumferential groove 21 chain oil is supplied via a feed pump 30, as in Fig. 3 is shown in partial section. The feed pump 30 is driven by a pump drive 25 which is in drive connection with the sprocket 16.

Like the schematic representation in Fig. 4 shows, the feed pump 30 consists of a delivery cylinder 31, in which a delivery piston 32 is immersed. The delivery piston 32 is driven in rotation, for which purpose on the piston rod 33, a first drive wheel 34 is provided, which is designed as a pinion 35. The free end 36 of the piston rod is supported on a housing-fixed lifting pin 37, which cooperates with a formed on the end of the piston rod 33 lifting cam. By means of a - schematically indicated - spring 38 of the lifting cam is held at the end 36 in contact with the lifting pin 37.

When the drive of the piston rod 33 is rotated, this shifts according to the height H of the lifting cam at the end 36, resulting in a reciprocating movement corresponding to the double arrow 26. In the pump chamber 29 flows in the direction of arrow 28 a fuel, eg. B. chain oil and is in the direction of arrow 27 to the consumer, in the embodiment in the circumferential groove 21 of the guide rail 20, promoted.

If the centrifugal clutch 12 is not engaged, the saw chain 19 does not run with; the saw chain is standing. When standing saw chain 19 promotion of chain oil in the circumferential groove 21 is undesirable. Therefore, the feed pump 30 is driven by the output side of the centrifugal clutch 12, for which purpose a drive element 40 is provided in the embodiment, which in the FIGS. 5 to 7 is shown. The drive member 40 is substantially formed as a cylindrical sleeve with a formed at one end disc-annular collar 42. The outer diameter 43 of the collar 42 is formed larger than the outer diameter 24 of the sprocket 16 (FIG. Fig. 8 ).

As it turned out Fig. 3 results, the feed pump 30 is below the drive shaft 13, wherein the drive shaft 13 extends through the drive member 40. The drive element 40 is rotatably mounted on the drive shaft 13 by means of a bearing 44, preferably a slide bearing. The cylindrical bushing 41 is formed on its outer circumference as a second drive wheel 39, namely as a worm wheel 45 which -. also Fig. 4 - With the pinion 35 of the feed pump 30 meshes.

The provided at one end of the sleeve 41 collar 42 is facing the sprocket 16 with its front side. On the sprocket 16 facing end of the collar 42 is formed as a coupling portion 50. For this purpose, the collar 42 carries on the front side axially projecting drivers 46, 47, 48 and 49. In the embodiment shown, at least two first drivers 46 and 48 and two second drivers 47 and 49 are provided in the circumferential direction 51 of the drive element 40 on the front side. The drivers have in axial front view U-shaped to V-shaped basic shape and are arranged on a common circumferential circle 52. The drivers protrude from the circumferential circle 52 with their tips 53 radially to the axis of rotation 54; in each case two first drivers 46, 48 and two second drivers 47, 49 lie diametrically opposite one another with respect to the axis of rotation 54.

In the circumferential direction 51, a first receptacle 56 is formed between each two first drivers 46 and 48 and a second receptacle 57 is formed between the second drivers 47 and 49. The receptacles 56 and 57 are provided for receiving a sprocket tooth 18 of a arranged on the clutch drum 11 sprocket 16.

In the circumferential direction 51 of the drive element 40, a first driver pair 86 and a second driver pair 87 are alternately provided in the exemplary embodiment, which alternately follow each other. Based on the axis of rotation 54 of the drive element 40, the coupling portion 50 thus has two first receptacles 56 which are diametrically opposed to each other with respect to the axis of rotation 54 and two second receptacles 57 which lie diametrically opposite one another in a corresponding manner relative to the axis of rotation 54. The arrangement is such that the drivers 46, 48 and 47, 49 or the driver pairs 86, 87 are symmetrical to an axial plane 55, 58. The axial plane 55 lies perpendicular to the axial plane 58.

The design of the drive element 40 or its coupling portion 50 is made such that the receptacle 56 formed by the first drivers 46, 48 has a greater extension 60 in the circumferential direction 51 than the extension 61, which is at the same radius R in the second receptacle 57 between the two second drivers 47, 49 results.

The measured in the circumferential direction 51 of the drive member 40 Leerabstand 59 between a first pair of carriers 86 of first drivers 46, 48 and a second driver pair 87 of second drivers 47, 49 is greater than that between the drivers 46, 48; 47, 49 measured extent 60, 61 of the receptacles 56, 57. In the illustrated embodiment, the empty distance 59 is at least twice as large as the maximum extent 60 of a receptacle 56th

The inventive design of the drive element 40 with the trained in a specific way coupling portion 50 allows a drive connection geometrically differently designed sprockets with the drive element 40 of the feed pump 30. The inventively designed coupling portion 50 allows for geometrically different design sprockets (number of sprockets, pitch of the sprocket teeth) each form-fitting drive connection from the sprocket to the drive element 40 and thus to the feed pump 30th

In the FIGS. 8 and 9 a first sprocket 16 is shown, which has six evenly distributed over the circumference of the chain teeth 18. The drive connection in the circumferential direction 51 of the sprocket 16 with the coupling portion 50 of the drive member 40 is achieved in that two teeth 18.1 and 18.4 are received in the first receptacles 56, which are bounded by the first driver 46 and 48. The first sprocket 16 is held with its teeth 18.1 and 18.4 in the associated receptacle 56 of the first cam pair 86 substantially free of play. Due to the in Fig. 7 shown According arrangement according to the invention of the drivers 46, 47, 48 and 49 and their circumferentially provided distances from each other further results that the first sprocket 16 is not only fixed in the first receptacles 56, but also to the following in the circumferential direction 51 on the first receptacle 56 second drivers 47 and 49 is present. The chain tooth 18.2 or 18.6 lying on the first receptacle 56 at an empty spacing 59 is therefore located outside the second receptacle 57 against contact surfaces 67 of the second drivers 47 and 49. In the circumferential direction 51 and thus in both directions of rotation of the sprocket 16 are adjacent, driving sprockets 18.1, 18.2 and 18.6 to first drivers 46 and 48 and second drivers 47 and 49, wherein the interposed sprocket 18.1 held in the associated first receptacle 56 is.

To produce the positive drive connection between the drive element 40 and the first sprocket 16, the drivers 46, 47, 48, 49 engage over the chain teeth 18 axially such that each formed on the chain teeth 18 driving surfaces 70 and 71 at the respectively associated first and second drivers 46th , 48 and 47, 49 are present. As Fig. 9 shows a driver 46, 47, 48 and 49 axially over a partial width T the sprocket tooth 18, wherein the partial width T is preferably less than 30% of the total width G of the sprocket 18.

In the embodiment of the FIGS. 10 to 13 a sprocket 116 or 216 is shown which has eight teeth 118 and 218, respectively. The second sprocket 116 thus differs from the first sprocket 16 by a different number of teeth; the first sprocket has six sprockets 18.1 to 18.6, while the second sprocket has eight teeth 118.1 to 118.8. Despite the other geometric design of the second sprocket 116 this is the same drive element 40 in the same manner in the circumferential direction 51 positively coupled to the sprocket 116, as well as the perspective view in Fig. 11 shows.

While the first sprocket after Fig. 8 a chain tooth 18.1 or 18.4 is held substantially free of play in the first receptacle 56 is at the second sprocket 116 after Fig. 10 a sprocket 118.1 and a diametrically opposite sprocket tooth 118.5 held substantially free of play in the second receptacle 57 between the drivers 47 and 49. As a result, a positive, play-free coupling of the coupling portion 50 of the same drive element 40 is achieved with a second sprocket 116 other geometric shape.

Also in the second sprocket are driving surfaces 70 and 71 of the chain teeth 118.1 to 118.8 on associated drivers; with first cam surfaces 70 is a sprocket 118.1, 118.5 on the drivers 47 and 49 within the second receptacle 57 at; with second cam surfaces 71 are the one received in the receptacle 58 sprocket 118.1 adjacent sprockets 118.8 and 118.2 with cam surfaces 71 on the first drivers 46 and 48 at. Thus, a sprocket 118.1 or 118.5 of the driving second sprocket 116 is held in the circumferential direction 51 of the drive element 40 in the associated second receptacles 57 of a driver pair 87, held substantially free of play. In the circumferential direction 51, that is, in both directions of rotation of the sprocket 116, are located on the circumference of the coupling portion 50 of the sprocket 118.1 adjacent, driving sprockets 118.2 and 118.8 to first dogs 46, 48 at.

In the first receptacle 56 between the drivers 46 and 48 is a following in the circumferential direction 51 chain tooth 118.3 trained in the direction of rotation 51 game.

The sprocket 216 of FIGS. 12 and 13 differs from the sprocket 116 after the FIGS. 10 and 11 by a different pitch with the same number of sprockets 218. This third sprocket is positively held by the coupling portion 50 of the same drive element 40 in the circumferential direction 51, ie in both directions, wherein a same position of the sprocket teeth 218.1 and 218.5 is given in the second receptacle 57 as in Fig. 10 , In that regard, the arrangement is substantially the same; because of the other division, the drivers 46 to 49 are with their tips 53 with greater radial clearance S to the tooth base 72 between two chain teeth 218th

The radial clearance S to the tooth base 72 is in the embodiment according to Fig. 10 lower at the second sprocket 116; at the first sprocket 16, the clearance S to the tooth base is greater. It is essential that the first, second and third sprocket 16, 116, 216 each lie with a minimal clearance S to the tooth base 72 in the coupling portion 50.

All embodiments have in common that with positive drive connection of the pushed onto the sprocket 16, 116, 216 drive element 40 only a subset of the chain teeth 18, 118, 218 of the driving sprocket 16, 116, 216 with the driver 46, 47, 48, 49 in Drive connection is.

In the drive connection between the drive member 40 and one of the sprockets 16, 116, 216, a coupling between the plastic-made drive member 40 and a metal sprocket 16, 116, 216 is provided. The drive element 40 engages over the sprocket 16, 116, 216 axially. In the circumferential direction 51 of the drive element are drivers 46, 47, 48, 49 between the chain teeth 18, 118, 218 of a sprocket 16, 116, 216, whereby in the direction of rotation 51, a driving positive engagement is achieved.

Claims (16)

1. Motorised chainsaw with a drive motor (2), which drives a saw chain (19) via a sprocket (16, 116), wherein chain teeth (18, 118) of the sprocket (16, 116) act on the saw chain (19) and chain saw lubricant is fed via a feed pump (30) to the saw chain (19) for its lubrication, wherein the feed pump (30) is driven via a pump drive (25) consisting of a first drive gear (34) connected to the feed pump (30) and a second drive gear (39) connected to the sprocket (16, 116), which gears are drive-connected to one another, wherein the second drive gear (39) is formed on a drive element (40) provided with a clutch section (50), which establishes a positive drive connection between the drive element (40) and the sprocket (16, 116), for which purpose the clutch section (50) is provided with drivers (46, 47, 48, 49), to which driver faces (70, 71) formed on the sprocket (16, 116) are assigned, and in the circumferential direction of the sprocket (16, 116) several driver faces (70, 71) of the sprocket (16, 116) bear against several drivers (46, 47, 48, 49) of the clutch section (50), characterised in that a first sprocket (16) of a first geometry can be exchanged against a second sprocket (116) of a second geometry, in that several first drivers (46, 48) assigned to the first sprocket (16) are formed in the clutch section (50) of the drive element (40) and several second drivers (47, 49) assigned to the second sprocket (116) are formed in the same clutch section (50) of the same drive element (40), and in that the clutch section (50) bears with the first or second drivers (46, 47, 48, 49) against the associated first or second driver faces (70, 71) of the driving first or second sprocket (16, 116).
2. Motorised chain saw according to claim 1,
   characterised in that the driver faces (70, 71) are formed on a chain tooth (18, 118).
3. Motorised chain saw according to claim 2,
   characterised in that the driver (46, 48; 47, 49) axially overlaps the chain tooth (18, 118) across a part-width (T), the part-width (T) being less than 30% of the total axial width (G) of a chain tooth (18, 118).
4. Motorised chain saw according to any of claims 1 to 3,
   characterised in that, in the circumferential direction (51) of the drive element (40), two first drivers (46, 48) and two second drivers (47, 49) each form a driver pair (86, 87) and each driver pair (86, 87) bounds in the circumferential direction (51) a receptacle (56, 57) for an associated chain tooth (18, 118) of the first and the second sprocket (16, 116).
5. Motorised chain saw according to claim 4,
   characterised in that, in the circumferential direction (51) of the drive element (40), a first driver pair (86) alternates with a second driver pair (87).
6. Motorised chain saw according to claim 4,
   characterised in that, relative to the axis of rotation (54) of the drive element (40), first receptacles (56) are arranged diametrically opposite one another and second receptacles (57) are arranged diametrically opposite one another in the clutch section (50).
7. Motorised chain saw according to claim 5 or 6,
   characterised in that the drivers (46, 48; 47, 49) are arranged symmetrically relative to an axial plane (55, 58) of the drive element (40).
8. Motorised chain saw according to any of claims 4 to 7,
   characterised in that a first receptacle (56) formed by first drivers (46, 48) has a dimension (60) in the circumferential direction (51) of the drive element (40) which is greater than the dimension (61) of a second receptacle (57) formed by second drivers (47, 49).
9. Motorised chain saw according to any of claims 5 to 8,
   characterised in that an empty distance (59) measured in the circumferential direction (51) of the drive element (40) between a driver pair (86) of first drivers (46, 48) and a driver pair (87) of second drivers (47, 49) is greater than the dimension (60, 61) of the receptacle (56, 57) measured between the drivers (46, 48; 47, 49) of a driver pair (86, 87).
10. Motorised chain saw according to claim 9,
    characterised in that the empty distance (59) is at least twice as great as the maximum dimension (60) of a receptacle (56) measured in the direction of rotation (51).
11. Motorised chain saw according to any of claims 4 to 10,
    characterised in that a chain tooth (18.1, 118.1) of a driving sprocket (16, 116) is held substantially without play in the circumferential direction (51) of the drive element (40) in the associated receptacle (56, 57) of a driver pair (86, 87).
12. Motorised chain saw according to any of claims 1 to 11,
    characterised in that driving chain teeth (18.1, 18.2, 18,6; 118.1, 118,2, 118,6) which are adjacent to one another along the circumference of the clutch section (50) in the circumferential direction (51) of the sprocket (16, 116) bear against first and second drivers (46, 48; 47, 49), wherein one of the driving chain teeth is held in an associated receptacle (56, 57) of a driver pair (86, 87).
13. Motorised chain saw according to any of claims 1 to 12,
    characterised in that, if the drive element (40) pushed onto the sprocket (16, 116) is positively drive-connected, only a part-number of the chain teeth (18, 118) of the driving sprocket (16, 116) is drive-connected to the drivers (46, 48; 47, 49).
14. Motorised chain saw according to any of claims 1 to 13,
    characterised in that the drivers (46, 48; 47, 49) lie between two chain teeth (18, 118) with radial play (S) relative to the tooth gullet (72).
15. Motorised chain saw according to any of claims 1 to 14,
    characterised in that a first sprocket (16) of a first geometry has a number of chain teeth (18) which differs from that of a second sprocket (116) of a second geometry.
16. Motorised chain saw according to any of claims 1 to 15,
    characterised in that a first sprocket (16) of a first geometry has a pitch of chain teeth (118) which differs from that of a second sprocket (216) of a second geometry.

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