GB2385569A

GB2385569A - Drive wheel for loop driven manually powered machine

Info

Publication number: GB2385569A
Application number: GB0204388A
Authority: GB
Inventors: Philip Henry Evans
Original assignee: Individual
Current assignee: Individual
Priority date: 2002-02-26
Filing date: 2002-02-26
Publication date: 2003-08-27
Also published as: GB0204388D0

Abstract

The drive wheel (20), which in use is rotated about a centre of rotation (A), has a periphery with each part of which, as the wheel (20) rotates, the loop (12) consecutively becomes engaged, and wherein the periphery includes four peripheral sections (21, 22, 23, 24), configured so that as the wheel (20) rotates, at a point where the loop (12) engages the wheel (20), for the first and third peripheral sections (21, 23) the distance (d) of the loop (12) from the centre of rotation (A) continuously increases and for the second and fourth peripheral sections (22, 24), the distance (d) of the loop (12) from the centre of rotation (A) continuously decreases. Each section can be of part circular form with its centre of curvature displaced from the axis of rotation.

Description

Title: Improvements in or Relating to Drive Wheels for Loop Drives Description of Invention This invention relates to a drive wheel for a loop drive and more particularly but not exclusively to a sprocket for use as a front sprocket of a cycle chain drive.

Conventionally such front sprockets are circular, and the chain drive includes a pair of pedal arms which are operated by a cyclist, to rotate the front sprocket and thus impart drive to a chain entrained about a part of the sprocket.

Such circular sprockets are inefficient at delivering drive to the chain for the reasons explained in more detail below, largely as a result of the configuration of cyclist's'legs, which are able to deliver maximum power to the pedals when straightening, to push the pedals down, but are unable to deliver full power when bending, and as the pedals move upwardly.

Notwithstanding that a pair of pedal arms and pedals are provided, there are still positions during the sprocket rotation when neither leg is in an ideal position to deliver power through the pedals to the chain.

On some cycles, toe clips are provided on the pedals on the pedal arms, so that a cyclist can impart drive to the chain both not only during downward movements of the pedals as the cyclist's legs straighten, but during at least some upward pedal movements too. Even so, circular sprockets are not wholly efficient.

It has been proposed to utilise a non-circular sprocket, and in particular an oval shaped sprocket. This is intended to compensate for drive being unavailable during at least some of the sprocket rotation. However such sprockets present substantial problems because the symmetry of the sprocket leads to severe chain oscillations particularly as a cyclist rotates the sprocket at a constant frequency. Although some compensation can be provided using a

Deraileur arrangement, the oscillations resulting from the use of an oval sprocket can be disconcerting to a cyclist, and particularly at high sprocket rotational speeds, can result in cycle imbalance and instability.

According to a first aspect of the invention I provide a drive wheel for a loop driven manually powered machine, which wheel in use is rotated about a centre of rotation, the wheel having a periphery with each part of which in use, as the wheel rotates, the loop consecutively becomes engaged, and wherein the periphery includes four peripheral sections, the peripheral sections being configured so that as the wheel rotates, at a point where the loop engages the wheel, for the first and third peripheral sections the distance of the loop from the centre of rotation continuously increases and for the second and fourth peripheral sections, the distance of the loop from the centre of rotation decreases.

Particularly where such a wheel is a sprocket of a cycle chain drive, the sprocket presents many advantages over conventional circular and oval sprockets in that increasing power can be transferred during pedalling to the chain as the power becomes available as the cyclist's legs are straightened, and when the cyclist's legs are bending and power cannot usefully be delivered to the chain, the distance of the chain from the centre of rotation of the sprocket is reduced as the pedal rises in preparation for the next downward pedal movement. Moreover, although the sprocket is not circular, because there are four peripheral sections, the sprocket does not cause severe chain or other loop oscillations as is the case with an oval sprocket, and thus a generally smooth drive output through a chain or other loop can be achieved which is substantially free from vibration and unwanted oscillations.

In a preferred embodiment, at least the first and third peripheral sections are part-circular with a centre displaced from the centre of the rotation of the wheel, and from the centre of the other part-circular section.

Preferably, each of the peripheral sections is curved and where the second and fourth peripheral sections are part-circular, their centres too may be displaced from the centre of rotation of the wheel, and from the centres of the other part-circular sections.

Preferably, to ensure a smooth continuous drive transfer to the chain or other loop, the radii of the four part-circular peripheral sections are generally equal.

The fourth and first peripheral sections will meet at a first peripheral position, and the first and second peripheral sections will meet at a second peripheral position, and the second and third peripheral sections will meet at a third peripheral position and the third and fourth peripheral sections will meet at a fourth peripheral position, and preferably the peripheral sections at their respective meeting positions, meld so as to provide a substantially smooth transition for the loop from each of the peripheral sections to the next peripheral section.

Particularly but not exclusively where the drive wheel is a sprocket for a chain drive of a bicycle or other cycle, which usually is driven by a cyclist's leg movements, in order that the drive wheel is of an appropriate configuration to accommodate a cyclist's leg configuration, the first and third peripheral sections may be greater in extent than the second and fourth peripheral sections. This assists in preventing unwanted oscillations in the chain drive and moreover, at a point where the chain engages the sprocket, for the first and third peripheral sections the distance of the chain from the centre of rotation continuously increases for the whole extent of the respective peripheral section where the cyclist's leg is able to deliver maximum power.

The drive wheel is preferably a sprocket for a loop being a drive chain, but may in another example be a pully wheel for a drive band type loop.

Where the drive wheel is a sprocket, although the invention encompasses a sprocket in which the ratio of the number of teeth provided on

the first to second and/or third to fourth peripheral parts is greater than 1: 1 and less than 22: 2, in a practical arrangement, preferably the ratio of the number of teeth provided on the first to second and/or third to fourth peripheral parts is greater than 7: 5 and less than 19: 5. Within this range, a smooth transition for the chain from one peripheral section to the next can be achieved.

More particularly the ratio of the number of teeth provided on the first to second and/or third to fourth peripheral parts is about 17: 7 or 18: 7.

The invention is particularly applicable where the sprocket is a front sprocket for a cycle chain drive which includes first and second pedal arms the first and second pedal arms being secured relative to the sprocket, and wherein the first pedal arm extends along a radius from the centre of rotation of the sprocket through a peripheral position within the fourth peripheral section and the second pedal arm extends along a radius from the centre of rotation of the sprocket through a peripheral position within the second peripheral section.

In this way, whilst a cyclist's leg is bending, as the pedals rise, where the chain engages the sprocket, for the second and fourth peripheral sections, the distance of the chain from the centre of rotation continuously decreases.

Preferably the peripheral position through which the first pedal arm radius extends, is generally mid way along the fourth peripheral section and the peripheral position through which the second pedal arm radius extends is generally mid way along the second peripheral section, opposite to the first pedal arm.

The second pedal arm may be connected to the sprocket e. g. through a drive shaft or crank, and to the first pedal arm for rotation with the sprocket and first pedal arm about an axis which passes through the centre of rotation of the sprocket, the second pedal arm extending radially from the axis in a direction opposite to the first pedal arm.

According to a second aspect of the invention I provide a cycle having a chain drive including a sprocket being a drive wheel according to the first aspect of the invention.

The cycle may have a plurality of sprockets in accordance with the invention with differing numbers of teeth, arranged as a sprocket set for rotation about a common axis, with there being a change mechanism to enable the chain to be changed from engagement with one sprocket to engagement with another sprocket. Preferably the sprockets are arranged sequentially with a sprocket having the most teeth at one end of the sprocket set and the sprocket with the least number of teeth arranged at an opposite end of the sprocket set.

Typically, drive is imparted from the chain to a gear sprocket of the rear wheel of the cycle. By providing a drive sprocket set, and if desired a plurality of rear wheel gear sprockets, with an appropriate gear changing mechanism, different drive sprocket to gear sprocket teeth ratios may be selected as appropriate to the ability of the cyclist and the terrain over which the cycle may be travelling.

The invention will now be described with reference to the accompanying drawings in which :- FIGURE 1 is an illustrative view of a conventional prior art chain drive of a cycle, in a first condition; FIGURE 2 is a view similar to Figure 1 but showing the drive in an alternative condition; FIGURE 3 is a schematic view of a sprocket of the drive of Figure 2 in a first rotational condition; FIGURE 4 is a view similar to Figure 3 but showing the sprocket in another rotational condition; FIGURE 5 is a view similar to Figures 4 and 5 showing the sprocket in a further rotational condition.

Referring to Figure 1 there is shown illustratively, part of a conventional chain wheel drive 10 including a circular front drive sprocket 11 and a chain 12. The front drive sprocket 11 has secured thereto a first pedal arm 14 which has a pedal 15 arranged to be operated by a cyclist's foot 16. The sprocket 11 is arranged to be rotated about an axis A of rotation, and as the front sprocket 11 is rotated, drive is imparted to the chain 12 which is always partially in engagement with teeth 25 of a part of the periphery of the sprocket 11. The first pedal arm 14 extends generally radially from the axis of rotation A beyond the periphery of the sprocket 11.

The chain drive 10 most efficiently transmits drive to the sprocket 11 and hence to the chain 12, as a cyclist's leg 17 straightens, which occurs as the pedal arm 14 moves from a position on the sprocket periphery indicated at PI in Figure 1, at which the pedal arm 14 is rotated clockwise as seen in the drawings, beyond an upright vertical orientation to about 45 , to a position P2 indicated in the Figure at which the pedal arm 14 is at rotated clockwise beyond the vertical to about 135 .

The chain drive 10 includes a second pedal arm 18 with a pedal 19, the second pedal arm 18 extending radially outwardly from the axis of rotation A oppositely to the first pedal arm 14. The second pedal arm 18 is secured to the sprocket 11 and first pedal arm 14 via a drive shaft S or the like and is rotatable about the axis A of rotation of the sprocket 11.

As the second pedal arm 18 moves from position PI to point P2, the cyclist's other leg (not shown) will be straightening and thus imparting the maximum power from the sprocket 11 to the chain 12. Notwithstanding a pair of pedals 15,19 are provided, when one of the pedal arms 14,18 is at position P2, a minimal amount of power can be transferred from the cyclist's leg 17 to the sprocket 11 and hence to the chain 12, until the other pedal arm moves to position PI when again drive may usefully be imparted from the other cyclist's leg to the sprocket 11. This is because as one leg reaches a position in which

the leg is substantially fully straightened, or at least is as straight as is allowed by the cycle geometry, the cyclist's other leg will not yet be in a position where it can begin to straighten.

Hence the chain drive 10 lacks power as the sprocket 11 rotates in the regions Rl, R2 indicated in Figure 1.

Referring now to Figures 2 to 5 there is shown a sprocket 20 in accordance with the present invention.

The sprocket 20 has a centre of rotation A about which drive shaft S in use rotates. However the sprocket 20 is non-circular.

The sprocket 20 periphery, includes four part-circular peripheral sections, 21,22, 23,24, but each part-circular peripheral section 21,22, 23,24 has a respective centre 21a, 22a, 23 a, 24a which is displaced from the centre of rotation A. Moreover, each respective centre 21a, 22a, 23a, 24a is displaced from the centres of the other peripheral sections. However the periphery of the sprocket 20, where the sprocket teeth 25 are provided, is substantially continuous, the part-circular peripheral sections 21,22, 23,24 in this example each have the same radius from their respective centres 2 la, 22a, 23a, 24a and meld into one another at respective meeting positions 26,27, 28,29 to provide a smooth transition for the chain 12 from each of the respective part-circular peripheral sections 21, 22, 23, 24 to the next section as the sprocket 20 rotates.

At a point P where the chain 12 first engages the sprocket 20 as the sprocket rotates, it will be appreciated that from a first meeting position 26 where the fourth and first peripheral sections 24,21 meet; to a second meeting position 27 where the first and second peripheral sections 21,22 meet, the distance d from the centre of rotation A of the sprocket 20 to the chain 12 at point P will continuously increase. From the second meeting position 27 where the first and second peripheral sections 21,22 meet, to a third meeting position 28 where the second and third peripheral sections 22,23 meet, the distance d from the centre of rotation A to the chain 12 at position P will continuously

decrease. From the third meeting position 28 to the fourth meeting position 29, the distance d will again continuously increase, and from the fourth meeting position 29 to the first meeting position 26, the distance d will again decrease as the sprocket 20 rotates.

The first pedal arm 14 is arranged to extend along a radius from the centre of rotation A, to pass the periphery of the sprocket 20 approximately half way between the first 26 and fourth 29 meeting positions, along the fourth peripheral section 24, and the second pedal arm 18 passes the sprocket periphery approximately half way between the second 27 and third 28 meeting positions, along the second peripheral section 22.

It can be seen that the first and third 21,23 part-circular peripheral sections extend over greater arc than the second and fourth part-circular peripheral sections 22,24.

As the sprocket 20 is non-circular, the ratio of the number of teeth 25 provided on the first 21 to second 22, or the third 23 to fourth 24 peripheral sections, must be greater than 1: 1. Preferably, so that the sprocket 20 can provide advantage over the conventional circular sprocket 11 indicated in figure 1 the gear teeth ratio of the first to second peripheral sections 21,22 or third to fourth peripheral sections 23,24, is greater than 7: 5.

A sprocket 20 which is too eccentric, e. g. has a ratio of the number of teeth 25 provided on the first 21 to second 22, or the third 23 to fourth 24 peripheral parts of for example 22: 1, may suffer from the problem of the chain 12 disengaging the sprocket. Preferably therefore a sprocket 20 in accordance with the invention has a ratio of the number of teeth provided on the first 21 to second 22, or the third 23 to fourth 24 peripheral sections of less than 22: 1, and more preferably in the order of less than 19: 5.

For a typical forty eight teeth 25 sprocket 20 shown in Figure 2, a preferred ratio of the number of teeth provided on the first 21 to second 22, or the third 23 to fourth 24 peripheral sections has been found to be 17: 7 although

if greater eccentricity is required, a ratio of 18: 7 may be provided, on a sprocket having fifty chain engaging teeth 25. This may involve moving the centres 21a, 22a, 23a, 24a, of the part-circular peripheral sections 21,22, 23,24 from the positions shown in figure 2.

In Figure 3, the first pedal arm 14 is shown in a position PI which is rotated clockwise about 450 beyond the upright position, where a cyclist's leg 17 driving the first pedal arm 14 is just beginning to straighten and develop maximum power. The first meeting position 26 is at point P where the first peripheral section 21 begins to receive the chain 12 as the sprocket 20 continues to rotate clockwise as seen in the drawings. Thus the power imparted to the chain 12 through the sprocket 20 from the cyclist's leg 16 will continuously increase as the pedal 15 moves downwards, to the position shown at P2 in Figure 4, as the distance d of the chain 12 at point P, to the centre of rotation A increases.

Of course, when the second pedal arm 18 is in the position shown occupied by the first pedal arm 14 in Figure 3, the power imparted to the chain 12 as the second pedal arm 18 is rotated will also continuously increase as the distance d at point P, increases from the third meeting position 28 to the fourth meeting position 29.

Conversely, when the first pedal arm 14 is moved from the position indicated in Figure 4 towards the end of the pedal arm 15 downward movement and beyond, to the position indicated in Figure 5 at P3 when the third meeting point 28 is adjacent point P, the cyclist's leg 16 will deliver little power to the sprocket 20. The distance d of the chain 12 at point P from the centre of rotation A will decrease continuously, as the first pedal arm 14 is moved from position P2 to P3, and similarly, as the second pedal arm 18 is rotated from the P2 to P3 positions, the distance d will continuously decrease.

Thus using a sprocket 20 in accordance with the invention, maximum power can be delivered to the sprocket 20 and hence the chain 12, whilst the

cyclist's legs are straightening, and power is not wasted when that power cannot usefully be delivered to the sprocket 20, as the cyclist's legs are bending, or at least are insufficiently straight for useful power to be delivered to the sprocket 20.

In the particular example described, the sprocket 20 has a teeth 25 pitch of 12. 7mm to match the pitch of the links of the chain 12.

Where it is desired to provide for varying gear ratios between the front 20 and rear gear sprocket provided on a rear wheel of the cycle, this may be achieved conventionally by providing a plurality of rear gear sprockets arranged as a sprocket set, each gear sprocket having a different number of teeth, and e. g. a Deraileur type gear changing arrangement to enable the chain 12 to be moved between the rear gear sprockets of the set to a desired sprocket.

Alternatively of additionally, a plurality of front sprockets 20 with differing numbers of teeth 25 may be provided, at least one of which is in accordance with the present invention, with an appropriate e. g. conventional chain changing mechanism for moving the chain 12 between the alternative front sprockets 20. Preferably all the front sprockets 20 would be in accordance with the invention with the sprocket 20 having the greatest number of teeth 25 at one end of the sprocket set, and a sprocket with the least number of teeth 25 arranged at the opposite end of the sprocket set, with any intermediate sprockets 20 arranged between the two outermost sprockets.

However it has been found that by providing a cycle with a sprocket 20 in accordance with the present invention, pedalling, even at slow speeds with a high front-to-rear gear ratio selected, is easier than with a conventional arrangement, and accordingly there is less requirement for a large number of alternative gear ratios to be selectable.

However the use of a Deraileur type arrangement for the rear gear sprocket is useful, because this helps ensure that point P where the chain 12 first engages the sprocket 20, is generally 180 from a point where the chain 12

last engages the sprocket 20, as the sprocket 20 rotates. Also the Deraileur provides some spring damping.

Using a sprocket 20 in accordance with the present invention, it is also much easier for a cyclist to start from standing, even using a high front to rear gear ratio, and acceleration is exceptional.

Because efficiency of pedalling is improved by the invention, cycling slowly is easier to achieve, and cycling up gradients is considerably easier than with a conventional arrangement.

Because pedalling presents less strain to the cyclist, the cyclist's posture and balance are also improved, as there is less need for body and head movements.

The invention is particularly applicable to a bicycle, but could be adapted for other cycles such as tricycles and four wheeled bicycles or even man-powered aircraft. A sprocket 20 in accordance with the invention could be provided for each cyclist of a tandem or other multi-cyclist cycle.

Although the invention has particularly been developed for a chain type drive, the invention may be applied to other drives, such as pulley and drive belt drives, where manual effort is exerted, e. g. by the legs or arms to rotate the drive wheel to perform work, and where drive is not evenly imparted to the drive wheel during its entire rotation.

In another embodiment, the peripheral sections 21,22, 23,24 need not be exactly circular but may otherwise be configured so that for the first and third peripheral sections 21, 23, the curvature is such that the distance at point P, where the chain 12 or other loop first engages the sprocket 20 or other wheel to the centre of rotation A continuously increases. In an extreme embodiment, the second and fourth peripheral sections 22,24, when of sufficiently small extent, may even be straight, but even so the distance d from point P to the centre of the rotation A continuously decreases as the sprocket 20 rotates.

The features disclosed in the foregoing description, or the following claims, or the accompanying drawings, expressed in their specific forms or in terms of a means for performing the disclosed function, or a method or process for attaining the disclosed result, as appropriate, may, separately, or in any combination of such features, be utilised for realising the invention in diverse forms thereof.

Claims

CLAIMS 1. A drive wheel for a loop driven manually powered machine, which wheel in use is rotated about a centre of rotation, the drive wheel having a periphery with each part of which in use, as the drive wheel rotates, the loop consecutively becomes engaged, and wherein the periphery includes four peripheral sections, the peripheral sections being configured so that as the wheel rotates, at a point where the loop engages the wheel, for the first and third peripheral sections the distance of the loop from the centre of rotation continuously increases and for the second and fourth peripheral sections, the distance of the loop from the centre of rotation decreases.
2. A wheel accordingly to claim I wherein at least the first and third peripheral sections are part-circular with a centre displaced from the centre of rotation of the wheel, and from the centre of the other part-circular peripheral section.
3. A wheel according to claim 1 or claim 2 wherein each of the peripheral sections is curved.
4. A wheel according to claim 3 where appendant to claim 2 wherein the second and fourth peripheral sections are part-circular with their centres being displaced from the centre of rotation and from the centres of the other partcircularperipheral sections.
5. A drive wheel according to claim 4 wherein the radii of the four partcircular peripheral sections are generally equal.

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6. A drive wheel according to any one of the preceding claims wherein the fourth and first peripheral sections meet at a first peripheral position, and the first and second peripheral sections meet at a second peripheral position, and the second and third peripheral sections meet at a third peripheral position and the third and fourth peripheral sections meet at a fourth peripheral position, the peripheral sections at their respective meeting positions, melding so as to provide a substantially smooth transition for the loop from each of the peripheral sections to the next peripheral section.
7. A drive wheel according to any one of the preceding claims wherein the first and third peripheral sections extend for greater extent than the second and fourth peripheral sections.
8. A drive wheel according to any one of the preceding claims wherein the drive wheel is a sprocket of a chain drive, each peripheral section having teeth for engagement with a drive chain.
9. A drive wheel according to claim 8 wherein the ratio of the number of teeth provided on the first to second and/or third to fourth peripheral parts is greater than 1 : 1 and less than 22 : 2.
10. A drive wheel according to claim 9 wherein the ratio of the number of teeth provided on the-first to second and/or third to fourth peripheral parts is greater than 7: 5 and less than 19: 5.
11. A drive wheel according to claim 10 wherein the ratio of the number of teeth provided on the first to second and/or third to fourth peripheral parts is about 17 or 18: 7.

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12. A drive wheel according to any one of the preceding claims which a front sprocket for a cycle chain drive which includes first and second pedal arms the first and second pedal arms being secured relative to the sprocket, and wherein the first pedal arm extends along a radius from the centre of rotation of the sprocket through a peripheral position within the fourth peripheral section and the second pedal arm extending along a radius from the centre of rotation of the sprocket through a peripheral position within the second peripheral section.
13. A drive wheel according to claim 12 wherein the peripheral position through which the first pedal arm radius extends, is generally mid way along the fourth peripheral section and the peripheral position through which the second pedal arm radius extends is generally mid way along the second peripheral section.
14. A drive wheel according to claim 12 or claim 13 wherein the second pedal arm is connected to the sprocket and to the first pedal arm for rotation with the sprocket and first pedal arm about an axis which passes through the centre of rotation of the sprocket, the second pedal arm extending radially from the axis in a direction opposite to the first pedal arm.
15. A drive wheel substantially as hereinbefore described with reference to and as shown in Figures 2 to 5 of the accompanying drawings
16. A cycle having a chain drive including a sprocket being a drive wheel according to any one of the preceding claims.

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17. A cycle substantially as hereinbefore described with reference to the accompanying drawings.
18. Any novel feature or novel combination of features described herein and/or as shown in the accompanying drawings.