TITLE: Chain or belt drive system

DESCRIPTION

The present invention relates to a chain or belt drive system.

One applicauon of the drive system is for a bicycle but the system is not limited to such an application.

In the application to a bicycle the drive system of the present invention can replace the conventional gearing, eg. Sturmey- Archer or derailleur type. The aim of the present invention is to provide a stepless drive system within a prescribed adjustment range which will avoid the need for manual gear changing.

According to the present invention, there is provided a chain or belt drive system comprising a driving wheel which is rotatable about a fixed axis of rotation, a driven wheel having a driving connection with a chain or belt for driving same, means providing a driving connection between the driving wheel and the driven wheel whereby rotation of the driving wheel generates rotation of the driven wheel, and wherein the axis about which the driven wheel rotates is movable relative to the axis of rotation of the driving wheel to automatically alter the effective gear ratio according to load requirements, said movement being opposed by resilient biasing means and wherein the means providing the driving connection between the driving wheel and the driven wheel comprises a plurality of driving connection elements carried slidably by the driving wheel and cooperable sequentially with the driven wheel, wherein said sliding connection allows relative movement of the driven wheel

with respect to the driving wheel whilst maintaining the driving connection.

The sliding movement of the carrier serves to adjust the effective gearing ratio since the effective radius of the driving connection with the chain or belt is altered. The adjustment is continuously variable within the adjustment range. The resilient biasing force is preferably a constant force or presents a slightly increasing resistance. The resilient biasing force may be adjustable to suit a particular user. The resilient biasing force which is applied to the resist movement of the driven wheel determines the force which can be exerted on the pedals before sliding movement occurs. Whenever increasing load is demanded, the gear ratio is effectively lowered so that the driving force applied to the driving wheel (ie. appUed at the pedals) cannot exceed the design maximum except when the device is at the end position corresponding to the lowest gear ratio.

The driven wheel is conveniently mounted for rotation on a carrier and the carrier is mounted for linear sliding movement. In the case of an application to a bicycle, the carrier is slidable with respect to a back plate secured to the bicycle frame. Drive to the driving wheel is provided by a pair of cranks and respective pedals.

More particularly, the driving wheel comprises a spider having a plurality of radial slots each receiving a slidable ratchet assembly comprising a slider block received captively in the radial slot and to which is secured a respective driving connection element comprising one or more teeth and hereinafter referred to as a ratchet element as in the preferred embodiment it is constructed to transmit drive to a ratchet wheel in only one direction of rotation. The slots are preferably straight slots

and may be radial or at some other angle. For example tangential to a notional circle centred on the axis of rotation. In other embodiments said ratchet assembly may be required to transmit drive in both directions. The ratchet wheel is fixedly secured with respect to the driven wheel which incoφorates a chain wheel with which the belt or chain cooperates. The carrier for the chain wheel incoφorates a cam track. Each slider block has associated with it cam follower means which may comprise a pin which acts as a cam follower and runs in the cam track. The cam track follows an eccentric (non-circular) path relative to the axis of rotation of the driven wheel. The cam track serves to move or otherwise cause each carrier and hence the ratchet element to move into and out of engagement with the ratchet wheel in the course of revolution of the driving wheel. Thus, the ratchet teeth of each ratchet assembly are brought into cooperation with the ratchet wheel sequentially. The arc over which a ratchet element is brought into engagement with the ratchet wheel is sufficient to ensure that there is always at least one ratchet element in contact with the ratchet wheel at any one time. The radial slots in the spider are of a length to accommodate the movement range of the driven wheel (60mm in one embodiment) whilst also allowing movement of the ratchet teeth into and out of engagement with the ratchet wheel at the opposite extremes of the movement range.

The present invention will now be described further hereinafter, by way of example only, with reference to the accompanying drawings; in which :-

Figure 1 is a side view of a transmission mechanism according to the present invention applied to a cycle.

Figure 2 is a sectional view taken on line 1 -1 of Figure 1 ,

Figures 3 and 4 show the mechanism of Figure 1 schematically, and illustrating alternative end positions,

Figure 5 is a side view of an alternative embodiment of transmission mechanism according to the present invention,

Figure 6 is a staggered section on line II-II of Figure 5,

Figure 7 is view looking in the dirction of arrow A of Figure 5 showing more detail of the sliding connection, but ommitting other detail of the drive for clarity.

Figure 8 is a side view showing the driven wheel in further detail,

Figure 9 and 10 are side views of the drive mechanism in alternative positions and showing part of a cycle frame and details of the reslient biasing system,

Figures 11 and 12 illustrate a side and end view of a main body portion of a driving connection element as used in the embodiment of Figure 5,

Figures 13 to 17 illustrate various views of a slider element which mates with the body of Figures 11 and 13, and

Figure 18 shows an alternative configuration of driving wheel.

The mechanism of the present invention is described by way of example only in relation to one possible applicauon, namely a bicycle transmission. Other applications will be apparent to the reader and are briefly discussed herein. Referring to the drawings, two tubes of a cycle frame are shown at 1 and 3 and join at a bottom bracket 5. An axle 7 is journalled for rotation in the bottom bracket 5 and opposite ends of the axle carry respective crank arms 9 which extend in opposite directions from the axle as is conventional for a cycle crank. A pedal 13 is attached to the outer end of each crank arm in a conventional manner. The axle 7 has a fixed axis of

rotation.

In place of the conventional chain wheel, the axle 7 has fixedly attached to it a spider 15 (which acts as the driving wheel) has a plurality of radial guide slots 19, preferably an odd number and in the illustrated embodiment employs 9 such slots. Each radial guide slot 19 receives captively and slidably therealong a respecuve slider block 21 of a ratchet assembly. Each slider block has attached to it a ratchet block 23 which may have one or more ratchet teeth 25 facing radially inwardly and designed to make contact with the corresponding teeth 26 of a ratchet wheel 27 during a predetermined arc of rotation of the spider. The root and tip of the ratchet wheel

teeth 26 are denoted by dotted lines 27a, 27b. For ease of illustration only two ratchet assemblies are illustrated. In the illustrated embodiment, contact between the ratchet element 23 and the ratchet wheel 27 is over an arc of approximately ±40° either side of the centre line XX.

The ratchet wheel 27 is mounted fixedly with respect to a toothed chain wheel 29 and both the ratchet wheel and the toothed chain wheel are journalled for rotation about a bearing carrier 31. A sealed bearing 33 is provided for this purpose. Dotted line 33' represents the pitch circle diameter of the bearing 33. The bearing carrier is preferably tubular and is fixedly secured to a carrier 35 which is mounted for sliding movement with respect to a back plate 37. The latter is secured to the cycle frame. Conveniently linear bearings 39, 41 are provided to guide the carrier slidably in a direction normal to the axis of axle 7. The bearing carrier surrounds the axle 7 and is hollowed out to accommodate the aforesaid sliding movement of the bearing carrier.

The tubular bearing carrier has a cam surface in the form of a groove denoted

by chain dotted line 43 which receives a pin 45 of each slider block 21. The groove defines a cam track whose path is non-concentric relative to the axis of rotation of the toothed chain wheel and the ratchet wheel 27. For convenience intersection of the respective pins 45 with the groove 43 are marked on the drawings. Other details of the slider assembly are only shown for two such assemblies. Cooperation between the pin 45 and the eccentric groove 43 results in a sliding movement of the slider block 21 as the spider 15 is rotated relative to the tubular bearing carrier 31 which is fixed against rotation. The path of the eccentric groove 43 is such as to bring each successive ratchet element 23 into contact with the ratchet wheel 27 over a prescribed arc it moves with the spider. In the illustrated embodiment contact is from a position

50 degrees after top dead centre, and contact over an arc of 80 degrees has been found satisfactory, ie. +40° either side of the centre line XX. The arc of contact is denoted by arrows RR and the number of slider blocks are chosen to ensure that there is always one ratchet element in contact with the ratchet wheel at any one time.

Usually one ratchet element takes up engagement with the ratchet wheel just before another disengages. The plurality of ratchet teeth and their cooperation with the ratchet wheel over the prescribed arc serve to transmit a driving force from the crank arms to the chain wheel.

Pin 45 is received in a slot in the slider block 21 and is urged to one end of the slot by a spring 49. This serves two purposes, firstly as the ratchet tooth is brought by the cam track 43 to a posiuon where it will be expected to mate with the ratchet wheel 27, the spring allows for any initial mis-engagement. Once contact has been established the profile of the teeth is such as to draw the at least one ratchet

tooth (but usuallay a series of teeth) into cooperating engagement with the ratchet

wheel during continued movement over the contact range. The second function of the spring is to function as a free wheel arrangement, ie. the tooth or teeth of the ratchet element can override the ratchet wheel teeth where the ratchet wheel is travelling relatively faster than the spider 15. This mechanism means that it is not necessary to

provide a free wheel in the rear wheel in a conventional manner. In an alternative embodiment the spring 49 could be applied to the ratchet block 23 so that it is biased witii respect to the slider block 21. In such a case the pin 49 would be connected directly to the slider block 21. It will be seen that the ratchet element comprises a block which has shoulders 23a, 23b which serve to locate it laterally with respect to the ratchet wheel teeth. This prevents any lateral displacement during engagement.

A resilient biasing force is applied to the carrier 35. This is denoted schematically by arrows F. Preferably a substantially constant force spring is provided for this purpose and it can take any convenient form. Alternatively, a gas strut may be utilised for this purpose. In the at rest position, the spring force biases the carrier to the right as shown in Figure 3. This corresponds to the highest effective gear ratio. Although not illustrated, the force exerted by the biasing force may be adjustable. In fact it may be preferable that the force exerted by the resilient biasing force increase to a small extent as the carrier moves from its rest position to its other end position which is illustrated in Figure 4.

In use, a driving force will be applied to the pedals and hence the driving wheel causing it to rotate. Engagement of successive ratchet elements with the ratchet wheel will cause the chain wheel to rotate and consequently a driving force will be

generated in the chain or belt (c) mated with the chain wheel This in turn is connected via a drive gear to the rear wheel of the cycle (not illustrated) which gear may be a free wheel. At any instant a certain dπving force will be required to rotate the cycle wheel and propel it forwards. For example, attempting to move uphill will demand more load than moving downhill. When the force exerted on the pedals generates a load in the chain which exceeds the force of the resilient beaπng force, the earner will move to the left m the illustrated embodiment. This will effectively reduce the effective radius of the chain wheel and hence the force input into the chain will be increased for the same mput force. The earner will continue to move to the left until the force generated in the cham is sufficient to overcome the resistance to forward movement. Figure 4 shows the carrier moved to the leftmost end position, corresponding to the lowest gear ratio. Means, not illustrated, may be provided to take up slack in die cham or belt which arises as a result of such movement.

Thereafter if the effort required to move the cycle decreases, the earner will move towards the nght, as viewed m the drawings, balancing the forces The mechanism is designed so that the effort exerted by the rider will be substantially constant whilst operating within the adjustment range, and as will be apparent from the foregoing descnption, the maximum possible effort which can be exerted by the πder is limited to a finite amount by the vaπable drive aπangement, except when the dπve is in the end posiuon conesponding to the lowermost gear ratio, ie with the earner in the extreme left position, as viewed in the drawings.

The dπve system can be used in other applications, for example where a motor, for example an electric motor, provides the dnve to the dnvmg wheel The

effect of the drive system is to automatically lower the gearing when there is an

increased load requirement. The drive system can be used in many applications which conventionally require gearing to adapt the drive to different load conditions.

For example, one application for the system is for a conveyor belt drive on start up.

Another application is in a vehicle drive system.

It will be apparent from the above that the drive system is designed to operate with a substantially constant power input. However, in certain circumstances it may be desirable to be able to input a higher power without lowering the effective gear ratio. This can be achieved by arranging for the resilient biasing force to be increased. This may be done automatically when the power input is increased. This can be applied to any motive power source which is under operator or computer control.

Reference is now made to the embodiment of the invention illustrated in Figures 5 to 17. The mechanism is essentially the same as the embodiment described above and like reference numerals have been used to denote coπesponding parts to which the foregoing description applies. The construction differs in respect of the detail design of the bearing carrier, the slide block and the ratchet block and the description in that respect is detailed below. The bearing carrier 31 has inner and outer cam surfaces 100, 102 which are part circular and cooperate with inner and outer follower elements 104, 106 of a ratchet block 123. When the follower elements are in contact with the cam surface teeth 125 of the ratchet blocks are held out of driving engagement with die teeth 26 of the driven wheel 27 (as viewed at X in Figure 8). Where the outer follower separates from the outer cam track, the

respective teeth 125, 26 make contact to provide a driving connection (as viewed at Y in Figure 8).

As with the previously described embodiment, there is always at least one ratchet block in driving engagement with the driven wheel. A spring 108 engages with the inner followers 106 to draw the ratchet teeth into initial engagement with one another. The angle of the teeth ensures continued engagement under load.

The ratchet block 123 is illustrated in further detail in Figures 1 1 and 12. Teeth 125 are carried by a seperate block 110 which is received in slot 112 and is secured in place with pins or rivets received in holes 114. The ratchet block has a bore 116 which is dimensioned to receive a boss 118 of a slider block 121. A rib with side edges 120 of the slider block is dimensioned to be received slidably in the slot 19 of the driving wheel. Each slot has an associated slider block and ratchet block as previously. Figures 16 and 17 illustrate a retention washer which is secured to the end of the slider block to hold it captive in the slot 19.

The mechanism operates in essentially the same manner as die first described embodiment. Figures 9 and 10 show how the movement of the carrier plate 35 is resisted by a resiliant biasing force which in the embodiment of Figures 9 and 10 is provided by a gas strut (122). Figure 9 shows the starting position and Figure 10 the position under substantial maximum load. A cranked lever 124 transmits movement between the carrier plate and the gas strut. A series of holes 126 in die crank provide a simple means of adjusting the effective lever ratio of the gas strut and thereby adjusting the resilient biasing force which is applied to the mechanism and thereby determining how much effort has to be expended to generate a change to die gear

ratio.

Finally, Figure 18 illustrates an alternative embodiment of driving wheel 15 in which the angle of the slots 19 is changed away from a radial disposition to provide for improved driving contact between die ratchet teeth when under load. This version of die driving wheel may be used as an alternative to tiiose described above.