WO1988000551A1

WO1988000551A1 - Belt drive power transmission

Info

Publication number: WO1988000551A1
Application number: PCT/GB1987/000496
Authority: WO
Inventors: Martin Leslie Tatnall; David Francis Myring
Original assignee: Martin Leslie Tatnall; David Francis Myring
Priority date: 1986-07-18
Filing date: 1987-07-14
Publication date: 1988-01-28
Also published as: GB8617647D0; AU7692587A

Abstract

A belt drive power transmission comprises a tensionable endless flexible belt (26) guided around a driven end pulley (21), at least one intermediate idler (22) pulley and an idler end pulley (23). The pulleys are so mounted on a common transmission housing (18) that in use the outer surface of a length of the belt (26) between the end pulleys (21, 23) contacts an arc of a tyre (10) or other driven element and is held in driving relationship therewith by said intermediate idler pulley (22) or pulleys acting against the inner surface of said contact length of the belt (26) in the central region thereof. The belt (26) acts simultaneously as a drive to the intermediate idler pulley (22) or pulleys and to the driven element (10) on either side of the intermediate idler pulley (22) or pulleys.

Description

1.

BELT DRIVE POWER TRANSMISSION

This invention relates to a belt drive power transmission particularly but not exclusively intended for a pedal cycle, i.e. a bicycle, tricycle or similar wheeled vehicle. 5 It has been previously proposed to motorise bicycles by means of readily affixed or "clip-on" cycle propulsion units. In one class of unit, the motor drives via gears, chains, clutches or pulleys permanently attached to the driven wheel of the ^*10 bicycle but such an arrangement has the disadvantage that operation of the bicycle in the unpowered pedal cycle mode is impeded by the power transmission system. This problem is overcome in another type of transmission system in which a powered friction 15 roller bears down onto the tyre of the driven wheel. Although such a transmission will permit operation of the bicycle in the unpowered pedal cycle mode completely unimpeded by the power transmission system (by raising the drive roller clear of the tyre) there 20 are other problems including the transmission of power from a small diameter roller when high speed motors are used, slip, tyre wear, power loss due to local tyre distortion and contact load distribution, which may limit performance and range. 25 It has also been proposed to propel cycles and other like vehicles by means of a pivotally mounted petrol or like motor through the medium of a pair of pulleys having a belt running round them arranged to act directly on the periphery of the cycle wheel over 30 a lengthened arc. This prior art is acknowledged as such in GB Patent Specification No. 607 558 which claims a driving mechanism in which the drive from the motor is transmitted to the bicycle tyre by means of an endless flexible Vee-belt drive a length of the 0551

2 .

outer surface of which contacts an arc of the wheel or tyre. As with the roller systems, such a driving mechanism permits the bicycle to be used as a normal pedal cycle but lack of adequate motive force between the belt and the tyre, especially in wet conditions, prohibits the development of this mechanism as a practical transmission.

It is an object of the present invention to obviate or mitigate the aforesaid disadvantages of the prior art systems.

According to the present invention there is provided a belt drive power transmission comprising a tensionable endless flexible belt guided around a driven end pulley, at least one intermediate idler pulley and an idler end pulley so mounted in a common transmission housing that in use the outer surface of a length of the belt betwee the end pulleys contacts an arc of a tyre or other driven element and is held in driving relationship therewith by said intermediate pulley or pulleys acting against the inner surface of said contact length of the belt in the central region thereof such that the belt acts simultaneously as a drive to the intermediate pulley or pulleys and to the driven element on either side of the intermediate pulley or pulleys while preferably providing a conformal profiling means for the driven element.

Preferably, both the driven end pulley and idler end pulley are arranged to be clear of the driven element in use. Advantageously, the driven end pulley is mounted on the output shaft of a motor fixed to a flexibly mounted suspension arm upon which the transmission housing is mounted for pivotal movement about an axis passing through the driven pulley so permitting movement, both bodily and pivotally, of the transmission housing into and out /005

3 «

of its operative position in which the belt engages the driven element as aforesaid.

The belt is preferably flat in which case both end pulleys are preferably crowned to stabilise the belt position in use.

Preferably, the idler end pulley is adjustably mounted in the transmission housing in order to vary the belt tension. Advantageously, automatic belt tensioning means is provided for displacing the idler end pulley in the direction away from the driven end pulley during movement of the transmission housing into its operative position so as automatically to tension the belt for use, return movement of the transmission housing causing displacement of the idler end pulley in the opposite direction thereby relaxing the belt.

The invention . will now be further described by way of example only, with reference to the accompanying drawings, in whic :- Fig. 1 is a partly exploded part sectional view of one embodiment of belt drive power transmission in accordance with the invention fitted to a bicycle, with additional component views illustrating the construction and mode of operation of the transmission;

Fig. 2 is a diagram showing the transmission in its operative and inoperative positions; and

Figs. 3 and 4 illustrate the operation of an automatic belt tensioner and a pull-down mechanism. Referring now to Fig. 1, a bicycle (shown only in part) has a frame 1 with a rear fork partly defined by an A-shaped sub-frame 2 with downwardly divergent side members 3, 4 braced by a conventional cross member 5. The rear fork mounts the rear cycle wheel 6 with a central hub 7 connected by spokes 8 to a rim 9 carrying an inflated tyre 10. At the hub end 00551

4.

of the rear fork there is provided at each side of the wheel 6 a conventional fixing 11 for a mudguard stay 12 also providing a fixing for a further similar stay 13. The upper ends of the stays 13 are fixed to pivot, points at the rear ends of respective bell-crank levers 14, 15 the other ends of which are interconnected by an angle section cross piece 16 so as to form a yoke generally indicated at 17 for moving a transmission housing 18. A transmission assembly 19 comprises a driven end pulley 21, an intermediate idler pulley 22 and a idler end pulley 23 rotatably mounted one behind the other in the transmission housing 18 which is of elongate rectangular, box-shape with a bottom opening exposing a flat endless belt 26 of rubber or other elasto eric material trained around the pulleys 21, 22 and 23. The end pulleys 21, 23 are crowned to maintain a stable axial belt position. The idler end pulley 23 is adjustable by means of trunnions slidable in side slots of the transmission housing 18 under the action of an adjusting drive comprising a yoke 30 fixed to a threaded rod 31 extending through an end wall of the transmission housing 18 for engagement with a wing nut 32. Tightening of the nut 32 causes the rod 31 to be pulled rearwardly thereby moving the pulley 23 in the same direction so as to tighten the belt 26.

The transmission housing 18 is accommodated immediately above the tyre 10 in a gap between a front section of the rear mudguard (not shown) and a rear mudguard section 33 hinged to the transmission housing 18.

The driven end pulley 21 is fixedly mounted on the output shaft of a motor generally indicated at 35, the centre line of the output shaft being shown at 36. The diameter of the driven end pulley 21 is 5.

chosen to provide the appropriate belt speed thereby eliminating the need for any reduction gearing. The motor 35 is a lightweight, high speed two-stroke internal combustion engine of around 20σc with a single, vertically reciprocating piston (not shown) within a motor housing 37 driving a crankshaft (not shown) connected to a flywheel (not shown) within a side cover 38 from which the motor output shaft projects for attachment to the driven end pulley 21 in such a way that the entire transmission housing 18 is capable of being pivoted around the axis 36 of the motor output shaft. The motor 35 is of conventional design and need not be described in detail save to say that there is a rearwardly mounted carburettor 40 (almost entirely concealed in the drawing by a cover 41 save for part of a throttle control 42) attached by a linkage 43 of known type to a twist-grip control mounted on the bicycle handlebars (not shown) . In addition, the carburettor includes a manual choke control 44 positioned so as to be within reach of a rider of the bicycle.

The motor casing 37 is fixed by suitable fasteners to a suspension arm 46 which is cantilevered to a flexible torsion mounting 47 comprising an axle 48 fixed to the inboard end of the arm 46 and extending through a cyclindrical tube 50 containing two rubber bushes 51, 52 made of rubber or other elastomeric material fused on the one hand to the tube 50 and on the other hand to the axle 48 such that any twisting action of the axle 48 caused by downward movement of the suspension arm 46 is resisted by the rubber bushes 51, 52, and upward movement is assisted by the torsional spring action of these bushes. The flexible torsion mounting 47 includes a bracket 53 with three- point fastening onto the sub-frame 2. The cylindrical tube 50 is 00551

welded to the front of a mounting plate 54 of the bracket 53, the plate 54 being shaped to fit over the side members 3, 4 of the sub-frame 2 in the region of the cross member 5. The plate 54 has one fixing hole 55 which is traversed by a stud 56 that also serves for the mounting of the conventional brake calipers (not shown) . In addition, the plate 54 has two fixing holes 57, 58 through which bolts 59, 60 pass for engagement with washers and nuts after traversing a clamping member 61, the arrangement being such that the sub-frame 2 is clamped between the plate 54 and the clamping member 61 when the mounting bracket 53 is in position. Additionally, there is provided on each bolt 59, 60 between the plate 54 and the clamping member 61 an eccentric bush 62, 63 by means of which the mounting bracket 53 can be accurately centred with respect to the sub-frame 2.

A petrol tank 65 is pivotally connected to the other end of the axle 48 by means of a mounting arm 66 with a bearing sleeve 67 fitting over the free end of the axle 48 and secured thereto by means of a pin 69. The attachment of the tank 65 to the mounting arm 66 is by way of threaded studs passing through holes in the arm 66 and engaging wing nuts to the inside of the arm. The arm 66 is further attached by way of a spigot 70 to a cup bearing 71 mounted on the bearing housing for the driven end pulley 21. A feedpipe 75 extends from the petrol tank 65 to the carburettor 40. A pull-down mechanism is provided to lower the transmission into engagement with the bicycle wheel. An anchorage 72 mounted on the front of the transmission housing 18 provides a fixing point for one end of a tension spring 73 which (as well as assisting bodily lifting of the transmission housing) provides a torque about the centre line 36 of the 7 .

motor output shaft which causes the transmission housing 18 to rotate away from the wheel into a raised position in which the belt 26 is clear of the tyre 10. Also attached to the anchorage 72 is a Bowden cable linkage 76 connected at one end (not shown) to a pull-grip control with detent catch mounted on the handlebars and at its other end by the inner cable to the cross piece 16 of the yoke 17 which is itself fixed by central portions of the bellcrank levers 14, 15 to a swivel pin 74 traversing the transmission housing 18.

The transmission operates as follows. Assume that the flexible bushes 51, 52 and the tension spring 73 are effective in raising and rotating the transmission housing"18~and hence the drive motor 37, suspension- arm 46, mounting arm 66 and tank 65 into the uppermost position in which the belt 26 is clear of the underlying tyre 10. The bicycle can therefore be pedalled in the usual way through the chain and sprocket drive to the rear wheel 6 completely unimpeded by the transmission of the invention. When the cyclist has reached a reasonable speed and wishes to engage the transmission he actuates the handlebar control to operate the linkage 76 of the pull-down mechanism. This causes the yoke 17 to rotate in a clockwise direction (as seen in the drawings) about the pivot points on the stays 13 and this causes a down load reaction on the pin 74 thereby moving the transmission housing 18 bodily downwards (i.e. with the driven end pulley 21) and pivotally about the centre line 36 of the motor output shaft until driving contact is made with the tyre 10 in the vicinity of the intermediate idler pulley 22 as shown in Fig. 2. The detent catch of the handlebar control is operated to keep the transmission housing 18 in its lowered position. The raised and lowered 3 .

positions of the transmission housing 18 are shown In Fig. 2 with the operation of the pull-down mechanism being illustrated in Figs. 3 and 4, the full line position of the yoke 17 in Fig. 3 being the raised position shown dotted in Fig. 4 in which the full line shows the lowered position.

As best seen in Fig. 2, in the lowered position of the transmission housing 18 the belt 26 is clear of the tyre 10 in the vicinity of the end pulleys 21, 23 but is in contact with the surface of the tyre over a continuous length extending from either side of the idler pulley 22. The motor 35 is started by driving the belt 26 with the wheel 6 whilst the bicycle is being pedalled thereby eliminating the need for an additional starting mechanism. Once the motor has been started the belt 26 is driven by the motor and the drive is transmitted to the tyre 10 by way of the length of the belt that contacts the tyre. No separate clutch mechanism is provided. The transmission may be disengaged from the tyre as required without stopping the engine when the cyclist wishes to come to a halt, the drive being reengaged later as required.

The intermediate idler pulley 22 is set centrally in such a position relative to the end pulleys 21, 23 as to suit the radius^' of curvature of the tyre. The intermediate idler pulley 22 produces significant contact loading of the belt 26 onto the tyre 10 giving rise to the belt contour shown in Fig. 2 when the transmission unit is in operation. The drive onto the tyre 10 is achieved by compressing the belt 26 between the tyre 10 and the intermediate idler pulley 22 which produces sufficient tractive force between the belt and tyre over the range required. The taut belt has an additional beneficial action in reducing tyre distortion in the region of 9.

the intermediate idler pulley 22. Such distortion in the absence of the belt leads to unacceptably high losses as mentioned in relation to prior art roller drives. The belt therefore acts as a conformal device in controlling tyre profile and the belt tension and tyre contact contributes to the tractive force in the regions away from the immediate area of the intermediate idler pulley 22 contact by way of a relatively large area of belt-to-tyre contact. Because both end pulleys 21 and 23 are crowned as previously mentioned the belt maintains a stable position axially of the pulleys and when the belt is brought in contact with the tyre this axial stability is enhanced by the curvature of the tyre surface which effectively acts as a crown. The belt is thus, dynamically stabilised at speed both^' when running free and when powering the wheel. The reaction to the applied belt tension is taken through the transmission housing 18 rather than by the motor bearings.

The illustrated design may advantageously be improved by making the intermediate idler pulley 22 more readily adjustable to suit different wheel diameters. Thus, for example, the intermediate idler pulley 22 could be associated with a pointer indicating on a scale the wheel diameter to which it has been set. A further improvement is to provide for spring loading of the idler end pulley 23 and it is also possible to provide a linkage 80 between the yoke 17 of the pull-down mechanism and the idler end pulley 23 such that during rocking movement of the yoke 17 the pulley 23 is moved rearwardly to tension the belt 26 automatically as the carrier 18 is lowered, the tension being relaxed on movement in the opposite direction (Fig. 3 and bottom right Fig.

1). 00551

10.

An advantage of the described arrangement is that the motor itself is mounted flexibly on the bicycle frame to allow loading symmetry when the drive is pulled down to engage the-* tyre. This symmetry is maintained even in conditions of wheel out-of-roundness. The mounting flexibility additionally reduces the need for accuracy in setting the motor position above the wheel and reduces the level of vibration conducted to the bicycle frame. it will be appreciated that the transmission of the invention may be incorporated in a bicycle during its manufacture or alternatively may be fitted to an ordinary bicycle in the manner shown in the drawings with a section of the rear mudguard being removed to allow the transmission to be mounted correctly.

It will further be appreciated that numerous additional modifications may be made . without departing from the scope of the invention. For example, one or more additional intermediate idler pulleys may be provided in the transmission mechanism and in this case each intermediate idler pulley will be pressed against the tyre with the end pulleys again being so positioned that a length of the belt contacting the tyre in use extends from either side of the intermediate pulleys towards but not up to the end pulleys.

In a further modification, the i.e. engine is replaced by an electric motor with its output shaft connected to the driven end pulley. it will be appreciated that a transmission in accordance with the invention may be utilised with pedal cycles other than a bicycle (for example with tricycles, tandems and similar wheeled vehicles) . In addition, the transmission may also find application as a portable unit for imparting a drive to a rotary component, for example a shaft, particularly if the 05

11.

rotary component has a resilient surface similar to that of the cycle tyre so that conformal cooperation of the belt with the surface to be driven can be achieved, as described above in the case of the bicycle mounted transmission.

Claims

00551- 12 -CLAIMS ;

1. A belt drive power transmission comprising a tensionable endless flexible belt guided around a driven end pulley, at least one intermediate idler pulley and an idler end pulley so mounted on a common transmission housing that in use the outer surface of a length of the belt between the end pulleys contacts an arc of a tyre or other driven element and is held in driving relationship therewith by said intermediate idler pulley or pulleys acting against the inner surface of said contact length of the belt in the central region thereof such that the belt acts simultaneously as a drive to the intermediate idler pulley or pulleys and to the driven element on either side of the intermediate idler pulley or pulleys.

2. A transmission as claimed in claim 1, adapted to provide a conformal profiling means for the driven element when the latter is made of resiliently defor able material.

3. A transmission as claimed in claim 1 or 2, wherein the driven end pulley and idler end pulley are arranged to be clear of the driven element in use.

4. A transmission as claimed in any one of the preceding claims, wherein the driven end pulley is mounted on the output shaft of a motor fixed to a flexibly mounted suspension arm upon which the transmission housing is mounted for pivotal movement about an axis passing through the driven pulley so permitting movement, both bodily and pivotal, of the transmission housing into and out of its operative position in which the belt engages the driven element as aforesaid.

5. A transmission as claimed in claim 4, wherein engagement means is provided for moving the transmission housing from its inoperative position in 13.

which the belt is free of said driven element, into its operative position initially by bodily movement of the transmission housing and motor and subsequently by pivotal movement of the transmission housing about the motor output shaft.

6. A transmission as claimed in any one of the preceding claims, wherein the belt is flat.

7. A transmission as claimed in claim 6, wherein both end pulleys are crowned to stabilise the belt position in use.

8. A transmission as claimed in claim 5, wherein the tensioning end pulley is adjustably mounted in the transmission housing in order to vary the belt tension.

9. A transmission as claimed in claim 8, wherein automatic belt tensioning means is provided for displacing the idler end pulley in the direction away from the driven end pulley during movement of the transmission housing into its operative position so as automatically to tension the belt for use, return movement of the transmission housing causing displacement of the idler end pulley in the opposite direction thereby relaxing the belt.

10. A transmission as claimed in any one of the preceeding claims, wherein one or each intermediate idler pulley is adjustably mounted in the transmission housing so as to suit the radius of the arc over which the belt extends in use.

11. A transmission as claimed in any one of the preceeding claims, wherein the motor is a two-stroke internal combustion engine.

12. A transmission as claimed in claims 4 and 11, wherein a fuel tank is mounted on the axis of the motor output shaft on the opposite side of the carrier from the motor.

13. A bicycle having a frame on which is 0551

- 14 -

mounted a transmission as claimed in any one of the preceeding claims for driving engagement with the rear wheel thereof.

14. A bicycle as claimed in claim 12, wherein the transmission housing fits in a gap in the rear mudguard the back section of which is connected to the rear end of the transmission housing.

15. A belt drive power transmission substantially as herein described with reference to the accompanying drawings.

16. A bicycle fitted with a belt drive power transmission as herein described with reference to the accompanying drawings.