The invention relates to a transmission with a pulling element arranged over a driving wheel and a driven wheel, with both wheels and the pulling element being provided with corresponding teeth and recesses. Such a transmission is known as a part of a bicycle for transmitting a rotary motion of a driving gear, connected with pedals via cranks, to a driven gear coaxial with the rear wheel.

In order to provide several transmission ratios, it is known to make the driven gear, and usually the driving gear as well, of multiple design, with successive, adjacently arranged gears having increasing or decreasing numbers of teeth and the transmission being equipped with means for axial displacement of the pulling element designed as a chain. In view of the available space and the fact that friction and wear increase according as the gears over which the chain rides are further out of alignment, typically a maximum of three driving gears and a maximum of seven driven gears are provided. Further, in practice, driving and driven gears which are located diametrically opposite each other cannot be used in combination. The number of transmissions that are available in practice is therefore smaller than the product of the number of driving and the number of driven gears.

A drawback of this known transmission is that relatively large differences occur between successive, available transmissions. This is the case in particular if a large spread of transmissions has been chosen.

In addition to exhibiting the above-mentioned friction and wear when mutually misaligned gears are used, this known transmission moreover has the drawback that it is complicated and susceptible to failure, and particularly so according as the number of gears is greater.

The object of the invention is to provide a transmission in which the above-mentioned drawbacks have been obviated.

This object is achieved according to the present invention in that in a transmission of the type described in the preamble at least one of the wheels has a variable, adjustable effective diameter and is provided with a string, extending in circumferential direction, carrying the teeth of that wheel/ which string is anchored at at least one point so as to be restrained from rotation about the centreline of the wheel and has a neutral bending line which has substantially the same radius of curvature as the neutral bending line of the pulling element located over that string.

Owing to the provision that at least one of the wheels has a variable effective diameter, it is not necessary to displace the pulling element onto another toothed wheel arranged adjacent thereto in axial direction for varying the transmission ratio. The toothed pulling element can nevertheless remain in engagement with the toothed wheel in that the teeth have been provided on a string extending in circumferential direction. When the effective diameter of the wheel is changed, the part of the string that adjoins the anchored part will move in circumferential direction, so that the pitch of the teeth can be maintained in correspondence with the pitch of the teeth of the pulling element riding over the wheel. The teeth of the pulling element can thus remain in engagement with the teeth of the wheel whose effective diameter is changed.

The effective diameter can be changed in a stepped manner, in such a way that the number of teeth of the wheel increases or decreases by one at a time. Thus, a maximum number of possibilities are available between the highest and the smallest number of teeth that can be ,set.

Eriction and wear due to a pulling element riding over misaligned gears is avoided in that the pulling element always rides over the same teeth.

The pulling element may for instance be a conventional bicycle chain, in which case the pitch of the teeth remains

equal at all times regardless of the effective diameter of the wheel.

However, the pulling element may also be provided with teeth projecting towards the wheel, as in the case of a geared belt, for instance. In that case, the string must be so designed that the pitch of the teeth, in the case of a decreasing effective diameter, decreases in accordance with the effective pitch of a pulling element extending over the wheel. In particular, the neutral line of the string should have substantially the same radius of curvature as the neutral line of a pulling element extending over that string.

The invention is further embodied in a toothed wheel having a variable effective diameter, which is adjusted for use in a transmission according to the invention. Hereinafter, the invention will be further explained and elucidated on the basis of a practical embodiment that is preferred most, with reference to the accompanying drawings. In these drawings:

Fig. 1 shows a side elevation in section, taken on the line I-I in Fig. 2, of a toothed wheel with a part of a pulling element for use in a transmission according to a practical embodiment of the present invention;

Fig. 2 is a rear view in section taken on the line II-II in Fig. 1; and Fig. 3 is a side elevation similar to Fig. 1, of a rear part of a bicycle equipped with a transmission according to the present invention.

Fig. 3 shows an embodiment of the transmission according to the present invention adapted for use on a bicycle, represented in its entirety and in built-in condition.

The transmission comprises a pulling element 1 arranged over a driving wheel 2 and a driven wheel 3, with both wheels and the pulling element provided with corresponding teeth. The pulling element is designed as a chain 1. The chain rides over a bend wheel 4 and a chain tensioning wheel 5.

One of the wheels 2, 3 - the driven wheel 3, in this example - has a variable, adjustable effective diameter and is provided with a string 6 (Figs 1 and 2) , extending in circumferential direction, carrying the teeth of that wheel 3. The wheel 3 is shown in solid lines in a first position and in broken lines in a second position with a smaller effective diameter.

The string 6 is anchored at at least one anchoring point 7 so as to be restrained from rotating about the axis 31 of the wheel 3. The string in turn is designed as a toothed chain 6 having alternate wide links 8 and narrow links 9, which are connected by pivot pins 10. The pivot pins 10 define a neutral bending line 11 of the toothed chain 6, which has the same radius of curvature as the neutral bending line 12 of the drive chain 1.

Inasmuch as the string is designed as a toothed chain 6, the teeth can be made in a simple manner from hard material, it may be of low elongation and is combinable with a conventional drive chain as pulling element. When the effective diameter of the wheel 3 is reduced, for instance, the part of the toothed chain 6 adjoining the anchoring point 7 rotates about the centreline of the wheel away from the anchoring point 7, so that the pitch of the teeth of the toothed chain 6 is not reduced proportionally to the effective diameter of the wheel 3, but remains adjusted to the pitch of the drive chain 1 located over the teeth. When using a drive chain 1 according to the present practical example, the pitch remains the same at all times. Of course, when the effective diameter of the wheel 3 is increased, the reverse effect occurs.

In the transmission according to the practical example shown in Fig. 1, one end 7 of the toothed chain 6 is anchored so as to be restrained from rotating about the centreline of the wheel 3. The toothed chain 6 extends round from that one end 7 to a guide 13 near that one end 7 and further along that guide 13 to a central part 14 of the wheel 3. A part of the

toothed chain 6 that adjoins the other end 15 of the toothed chain 6 is located in the central part 14 of the wheel 3.

When reducing the effective diameter of the wheel 3, a link 10, 11 of the toothed chain is guided along the guide 13 to the central part 14 of the wheel. When increasing the effective diameter of the wheel 3, a link 10, 11 is guided along the guide to the circumference of the wheel 3. Owing to the provision that the toothed chain 6 is guided to a central part 14 of the wheel, it is sufficient to bend the toothed chain 6 about axes parallel to the centreline of the wheel 3 and it is not necessary to bend the toothed chain 6 laterally. This advantage is also obtained if the string is of different design, for instance as a belt with recesses and/or teeth or as a chain without projecting teeth. The other end 15 of the toothed chain 6 is connected to a resilient element 16 which keeps the toothed chain 6 wound up in the central part 14 of the wheel 3. The resilient element 16, which is designed as a spiral spring, maintains the toothed chain 6 in a condition wherein it is pulled taut at all times, while upon increase of the effective diameter of the wheel 3 the resilient element 16 yields so that the required number of links can move towards the periphery of the wheel 3.

To increase the capacity of the central part 14 of the wheel 3 in axial direction for arranging windings of the string 6 side by side in axial direction in the central part without requiring the string 6 to be bent in lateral direction, it can be wound onto a conical core (not shown) . Then, the string 6 need only permit of limited torsion to enable a given axial deflection thereof.

Of corresponding teeth of successive links 8, 9, in each case at least one has a tooth form with an undercut 37, i.e., one of the tooth flanks of that tooth recedes from the outside towards the inside. This provision limits the magnitude of the reaction forces, directed away from the wheel, exerted by the teeth on the drive chain 1. Preferably, the undercut is

located on the outside of a link of the toothed chain, so that the free space between two successive teeth of the toothed chain is not reduced when the chain is being bent.

The wheel 3 having a variable effective diameter comprises a plurality of circumferentially distributed segments 17, 18 movable in radial direction. The toothed chain 6 extends over surfaces of those segments 17, 18 that are remote from the centreline of the wheel.

By moving the segments 17, 18 in radial direction, the effective diameter of the wheel 3 is increased or reduced, while the surfaces of the segments that are remote from the centreline support the toothed chain 6. To that end, the number of segments 17, 18 is preferably so large that in the position wherein the wheel 3 has its largest effective diameter, each link 8, 9 is supported by at least one segment 17, 18.

The segments 17, 18 are provided with grooves 36, collectively forming a groove in circumferential direction, which guides the string 6 and supports it so as to prevent tilting.

The surfaces of the segments 17 remote from the wheel are curved in such a manner that they form a substantially cylinder-shaped surface when the segments 17 are located in the extreme position proximal to the centreline. Thus, a support of the string 6 without sharp transitions is obtained at all times in each position of the wheel 3.

One segment 18 is provided with an opening with the guide 13 along which the string 6 is bent from the effective outer circumference of the wheel to the central part 14, and is also connected with the anchored end 7 of the toothed chain 6. The space between the last link of the toothed chain 6 and the last link of the chain that is still located along the effective outer circumference of the wheel 3 can thus be limited to a minimum. The string 6 is located between two discs 19, 20, each provided with a conical surface 21, 22 facing the string 6. By

moving the discs 19, 20 towards each other, the string 6 is urged outwards, so that the effective diameter of the wheel 3 is increased. Upon displacement of the discs 19, 20 away from each other, the reverse effect occurs. Optionally, only one of the discs may be provided with a conical surface and the other disc may for instance be flat. It is then of importance that steps have been taken to prevent the string 6 from tilting to the flat disc.

For the mutual displacement of the discs 19 and 20, the wheel 3 comprises an adjustment element 23 provided with threads 24, 25 winding in opposite directions, extending around the centreline, and the discs 19 and 20 have threads 26 and 27 cooperating with threads 24, 25. By turning the adjustment element relative to the discs 19, 20, the discs are moved towards each other or moved away from each other. The use of threads offers the advantage that a large force enhancement can be obtained which makes it possible to move the discs 19, 20 towards each other even when a continuous driving force is exerted on the drive chain 1. A further advantage of using threads for adjusting the distance between the discs 19, 20 is that if the pitch is sufficiently small, a self-braking system is obtained, which prevents adjustment by axial forces exerted on the discs.

The discs are connected to a wheel 29 for axial movement and restrained from rotation via pins 28. Of course, a freewheel system may be provided between the discs 19, 20 and the wheel 29.

Instead of being provided with two threads winding in opposite directions, the adjustment element may also be provided with only one thread cooperating with one disc, while the other disc is axially fixed.

For the drive of the adjustment element 23, it is equipped with an adjusting wheel 30, the transmission comprising means 32 for decelerating the adjustment element 23 relative to the discs 19, 20 and means 33 for accelerating the adjustment element 23 relative to the discs 19, 20. The adjusting wheel

30 makes it possible to exert a relatively large moment on the adjustment element 23 and offers an enlarged area of engagement for the above-mentioned means for accelerating or decelerating the adjustment element 23 relative to the discs 19, 20.

The means 33 for accelerating the adjustment element 23 relative to the discs 19, 20 comprise a disc 34, driven by the pulling element 1, which disc can be brought into engagement with the adjusting wheel 30. This makes it possible to take off energy supplied by the pulling element 1 and to use it for energizing the adjustment element 23.

The disc 34 driven by the pulling element 1 is connected with the bend wheel 4 and in operation is moved along by it. By displacing this disc 34 in the direction indicated with an arrow 35, it can be brought into engagement with the adjusting wheel 30.

The means 32 for decelerating the adjustment element 23 relative to the discs 19, 20 are designed as a brake operated by a Bowden cable. Here, too, energy supplied by the pulling element 1 can be used for energizing the adjustment element 23.

To accomplish adjustment of the effective diameter of the wheel 3 in such a manner that a whole number of teeth is added to the outer circumference or is removed from the outer circumference, the pitch of the threads 24, 25 of the adjustment element 23 and the slope of the conical parts 21, 22 of the discs 19, 20 are chosen such that a rotation of the adjustment element 23 through a certain angle relative to the discs 19, 20 results in a change by one tooth of the number of teeth of the string 6 located along the circumference.

Preferably, stops are arranged for rotating the adjustment element 23 through the above-mentioned angle when the means 32 or 33 for rotating the adjustment element 23 relative to the discs 19, 20 are operated once. The string 6 may be in direct abutment with the conical surfaces of the discs. For obtaining easy adjustability of the

wheel 3, however, it is advantageous if, as shown in Figs 1 and 2, the segments 17, 18 can be moved in radial grooves 38 between the discs 19, 20.

For varying the diameter of the wheel 3, in addition to the proposed system, many other systems are conceivable within the framework of the present invention. Thus, for instance, for adjusting the segments in radial direction, a system can be used such as the system described in EP-A-0 084 160 - which is hereby referred to - for adjusting the position of carriers which the pulling element engages.