FR2758122A1 - Chain and sprocket transmission system with variable gear ratio, e.g. for bicycle - Google Patents

Abstract

The system consists of a driving shaft with a number (N) of toothed sprockets (1, 2, 3, 4, 5) and a driven shaft with a number (n) of toothed sprockets (1', 2', 3', 4', 5'), the two numbers N and n being the same or different by 1. The system has a mechanism for shifting the chain in a given sequence, e.g. between two neighbouring sprockets (2, 3) on the driving shaft and then between two neighbouring sprockets (3', 2') on the driven shaft, giving a total number of gear ratios equivalent to N + n -1. The changing mechanism comprises two gear levers linked by a rocker which allows them to be moved alternately in stages.

Description

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The subject of the present invention is a chain transmission, the ratio of which can be varied by discrete values. In what follows we accept that it is a bicycle transmission, although other applications may be envisaged.

The current trend is to multiply the number of possible reports by installing a plurality of toothed wheels:
- at the crankset (motor axis) the wheels are then called "chainrings"
- at the rear wheel (receiving axle) the wheels are then called "pinions"
The manufacturers then announce as the number of possible ratios the product of the number of chainrings N by the number of pinions n. In a fairly widespread configuration, there will for example be 3 chainrings and 7 pinions, or 21 theoretical reports. The chain moves between the chainrings and the sprockets by two forks whose controls are completely separate.

This device has a certain number of limits and drawbacks:
- The number of significant reports is significantly lower than the product N xn, because one inevitably comes across combinations which have very similar, even identical, reports.

 - Certain combinations are not recommended, namely those for which the chain has a significant obliquity, due to the offset between the plane of the chainring and that of the pinion. There is then, especially in the area where the chain leaves the sprocket and in the area where it enters the chainring, significant lateral friction, which constitutes a loss of energy and a wear factor.

 - The management of reports by the user is very complex. In a certain number of cases, the passage from a report to the closest report requires that one changes simultaneously of plate and pinion, which obliges the user to an intellectual gymnastics hardly compatible with a sporting activity where the attention is occupied elsewhere, and where the reactions must nevertheless be rapid.

The aim of the present invention is to remedy these drawbacks by presenting a device meeting two main objectives:
- the obliquity of the chain is limited, a very low value, in order to limit parasitic friction. This skew limitation is inherent in the design, therefore independent of the user, who cannot go against it.

- Maneuvering by the user is extremely simplified, the most complete form of the invention consisting in having only one handle for controlling the two forks
According to the invention, the numbers N (of chainrings) and n (of sprockets) are either equal or different from 1. Furthermore, a kinematic link is created between the two forks of movement of the chain leading to an alternating actuation of the "chainring" fork and the "pine nuts" fork
If we imagine for example a very simple device with 4 plates numbered from 1 to 4 by increasing diameters, and with 5 pinions numbered from 5 to 1 by decreasing diameters, the possible combinations will be:

 An eight-ratio system is obtained which is described by following the order of the arrows (increasing ratios) or the reverse order (decreasing ratios). We see that we always alternate the horizontal arrows (change of pinion) and oblique arrows (change of chainring). Thus, the chain never finds itself in an exaggerated obliquity position. In particular if the planes of the chainrings and those of the sprockets are staggered, the obliquity of the chain will always correspond to a half notch (a notch here being the distance which separates two chainrings or two consecutive sprockets). Such obliquity can be considered negligible.

 Concerning the numbers N and n, we can clearly see that they cannot be different from more than 1. In the above example, we chose to put one more pinion, which does not correspond to an obligation of principle, but practical convenience (the geometric constraints of the bicycle making it easier to install many sprockets than many chainrings).

Regarding the number of reports available, this is equal to N + n -1, i.e. eight reports for the above example. We could go to ten reports with N = 5 and n = 6 or twelve with N = 6 and n = 7. These figures are obviously much lower than the product N xn, but it should be noted that all the reports are now rationally exploitable, Insofar as:
- they can be staged regularly, without two of them ever being very close.

 - the problem of skewing the chain has disappeared.

 These advantages alone are insufficient to give the invention a decisive interest. This interest will be real only if a simplified command is implemented, allowing the user to describe his reports very clearly, without any particular effort of reflection, by taking advantage of the regular alternation of pinions-chainrings.

 For this order, we can, without departing from the scope of the invention, imagine an almost infinite number of solutions. As part of the application to the bicycle, one can keep for example the actuation of the forks by cables winding on rotary drums.

As it is obviously necessary to create a servo between the two controls, it is advisable to place the two drums side by side on the same axis. From there, we can imagine two families of solutions
- In the first family, each drum is integral with a lever, and can therefore be ordered individually. The enslavement then simply consists in limiting to the equivalent of a gear change notch the angular displacement that the two levers can have between them, this by a simple set of suitable mechanical stops. To change gear, the user is then quite naturally led to act on the lever which is the most behind relative to the direction towards which he wishes to go. At the next change, the positions of the joysticks will have been reversed, and it is therefore the other joystick that will be requested.

The two controllers being side by side, there is no need to look at them, the selection can be done simply by touch.

 Note that if the user actuates the joystick which is in advance, the play of the servo stops will then also lead to driving the other joystick, which will have the effect of simultaneously actuating the two forks, therefore skipping a gear . It is not necessarily a false move, as it may correspond to a real need.

 It should also be noted that this device may or may not be equipped with a mechanical position indexing system.

- In the second family, it is intended to have only one control member, this by placing between the two drums a central element organized to alternately ensure the movement of each drum. The user then acts on this central element only, either by actuating a lever directly integral with this element, or by means of an auxiliary transmission by cable if it is desired, for ergonomic reasons, for the control member to be offset in another sector of the machine (on the handlebars for example)
Such a device accommodates particularly well the presence of mechanical indexing.

 With this type of solution, the action of the user is simplified as much as possible: the device is requested in one direction to increase the ratios and in the other direction to decrease them, without even needing to know if one is acting on the trays or on the gables.

 As an alternative to the above formula, but remaining in the same family of solutions, it is possible to consider switching from a rotary system to a linear device, the drums and the central element being replaced by drawers with rectilinear movement. If you wish to maintain a rotary lever control, you can then attack the central element by any suitable system (rack pinion for example). If a remote control is preferred, the central element will be attacked by a cable connected to this control.

 An interesting fallout of the invention is located at the chain tensioner. With conventional systems, the tensioner must be able to absorb a significant length of chain, corresponding to the difference between the two extreme configurations large chainring / large sprocket and small chainring / small sprocket. With the present invention these configurations no longer exist. There is of course a maximum configuration and a minimum configuration, but the difference between them is much smaller. The role of the tensioner is therefore reduced, which makes it possible to envisage its simplification (for example by providing a single roller instead of two, which gives the chain a less tormented trajectory, thus contributing to the improvement of the yield and to reduced wear).

Other advantages and characteristics will emerge more clearly from the appended drawings, given by way of nonlimiting examples, and in which:
- Figure l schematically shows, in axial section, a set of pinion plates showing the successive positions that can be taken by the chain.

 - Figure 2 is a perspective view of a control device with two slave levers.

 - Figure 3 is a sectional view of a device with a single handle alternately controlling the two drums respectively actuating the two forks.

 FIG. 4 is a sectional view of a device of the same principle with a linear structure, which can also be interpreted as a development of the circular device described in FIG. 3.

 Figure 1 illustrates the configuration described on page 2 which has 4 chainrings (left axis) and 5 pinions (right axis). The broken lines represent the positions that the chain can take, to the exclusion of all others. So we have the eight ratios which we can define the layering as desired (except of course that the numbers of teeth are integers).

 The mechanism which controls the movement of the chain must naturally lead the chain to describe the eight positions while respecting their order of succession. If, for example, you are on the 2-3 'ratio and you want to obtain a higher ratio, you will go to 3-3' (change of plate). If you wish to obtain a lower ratio, you will go to 2-4 '(change of pinion). This selection must be made without special attention from the user.

 Figure 2 presents a solution introducing a slight derogation from this objective, but it can nevertheless be interesting for its simplicity. The cable 11 controls the chain actuation fork on the chainring side. It is wound on the drum 12 integral with the lever 13. On the pinion side the same functions are fulfilled by the cable 21, the drum 22 and the lever 23. The movements of the levers in the direction of the arrow F lead to increasing the ratios , and vice versa. It will be noted that the directions of winding of the cables on the drums respect the conventional rule, namely that the traction of the cable is used to pass from a given toothed wheel to another of larger diameter (whether chainrings or pinions) while the reverse movements are made under the action of the return springs fitted to the forks.

 The particular point of this formula lies in the limitation of the relative angular movement of the levers 13 and 23 to an angle o (corresponding to the passage from one gear to one of the neighboring gears. In the example described, this limitation is obtained by a pin 14 integral with the lever 13 and engaging in a "bean-shaped" slot 24 formed in the lever 23.

 In the position shown, the lever 13 is late with respect to the lever 23 by an angle o (/ 2 (the concept of delay being established according to arrow F).

 Suppose that this position corresponds to the aforementioned example (plate 2 - pinion 3 ') and we want to shift to the higher ratio, i.e. 3 - 3' We then push the lever 13 in the direction of F of the angle o ( corresponding to the movement of the pin 14 in the light 24. At the end of this maneuver, it is the lever 23 which will be late by oX / 2 with respect to the lever 13, and which will therefore have to be requested if one wishes go to the next higher gear (i.e. 3 - 2 ').

 In the reverse maneuvers (reduction of the ratios) we see that from the position of figure 2, it will be necessary to pull the lever 23 of an angle o (in the opposite direction of F. We then pass to the ratio 2 - 4 To continue, we will press lever 13, and so on.

 It is thus clearly seen that the rule to be observed is to act on the lever which is late by (/ 2 with respect to the direction of movement sought. This selection does not require great attention on the part of the user. can be done "to the touch" if the shape of the joysticks is well studied ergonomically. A violation of this rule only results in skipping a gear. For example, if from the position of figure 2, the lever 23 is pushed in the direction of F, the lever 13 will be driven at the same time, and we will go directly from the ratio 2 - 3 'to the ratio 3 - 2'. In some cases, such an operation may even be voluntary.

 Finally, we can note a particularity: if the spacing of the plates is different from that of the pinions, it is possible to achieve this by giving the drum 12 a diameter greater than that of the drum 22. Thus, for the same angular movement o (, the displacement of the cable 11 will be greater than that of the cable 21. One can even possibly respond to the case where the spacings are not regular on the same family of wheels (plates or pinions) by playing on an eccentricity of the corresponding drum relative to the axis of rotation x x '.

 FIG. 3 shows an assembly controlled by a single lever 31 allowing the alternating actuation of the drums 32 and 33, on which it pivots. The drums revolve themselves on the sheath 34, rigidly assembled with the fixed axis 35 which supports the entire mechanism.

 The reciprocating actuation of the drums 32 and 33 is done by the spreader 36, which is articulated on the axis 37 and the ends of which are introduced into notches made in said drums. The axis 37 is rigidly secured to the lever 31.

 The alternating effect is obtained by another part of the spreader bar, barely visible in this figure, shaped like an anchor coming to cooperate with a double toothing belonging to the fixed sheath 34.

 When the lever 31 is requested to rotate about the axis 35, the kinematics of the toothing anchor leads the lifter 36 to describe oscillations around the axis 37. The amplitude of these oscillations is calculated so that each end of the lifter is alternately motionless and mobile at twice the speed of the midpoint. So the two rotating drums are alternately stressed.

FIG. 4 (view in section cc shown in linear development) makes it possible to specify the shapes of the anchor belonging to the spreader 36 and the double toothing belonging to the sheath 34. Let us consider the following points:
A, A '= active vertices of the anchor
S1, S2, S3 S'l S'2 S'3 = vertices of the double toothing
I, I '= centers of the end span of the spreader 36, which engage in the
notches of drums 32 and 33.

The central axis of the spreader, materialized by axis 37, moves along the axis yy '. If it is stressed downwards, the geometry of the anchor and the teeth leads the vertex A to describe the segment S1 S2 and the vertex A 'to describe the segment S'1 S'2. As it can be noted that the perpendicular bisectors of these two segments intersect at point I, it follows that the spreader 36 describes approximately a rotation around point I; the latter therefore practically does not move, and it is the point I 'which moves by a value double that of the displacement of the axis 37. If the movement is continued, it is then the point I' which becomes fixed, and point I which moves in turn
If the axis 37 is biased upwards, the points A and A 'respectively describe the segments S1 S3 and S'1 S'3. There is then a first pivot around 1 ', which will be followed by a pivot around I, and so on.

 We can note that in all logic, the segments s S1 S2 and S'1 S'2 should be arcs of circle centered in I, like the segments s S1 S3 and S'1 S'3 should be arcs of circle centered in I '. You can actually do it that way. However, if the required precision allows, we can stick to straight segments, tolerating a certain play between the anchor and the teeth.

 This figure represents a linear type system, which moreover corresponds to a possible embodiment, in which the rotating parts would be replaced by drawers in translation. If we get closer to Figure 3, Figure 4 should then be considered as the development of a cylindrical structure centered on the axis 35. This results in the fact that the edges such as A, A ', Sol, S2 , S3, S'1, S'2, S'3, as well as the axis 37 are not parallel, but converge towards the axis of rotation common to the entire mechanism. Likewise, the end bearing surfaces of the lifter 36 must be spherical, to take account of the angular clearance existing between the drums 32 and 33.

 In a slightly different embodiment, the vertex A 'could be symmetrical with the vertex A with respect to the axis I I'. We could then be satisfied with a single toothing, but this would have a higher number of teeth.

Claims (6)

<tb> "<SEP> tEt, f,  1 - Transmission by chain presenting a plurality of ratios, between a driving axis comprising a number N of toothed wheels 1, 2, 3, 4 .. and a driven axis comprising a number n of toothed wheels', 2 ', 3', 4 ', 5' ...., the numbers N and n being either equal or different from 1, characterized in that the mechanism ensuring the transfer of the chain, obliges the latter to move alternately in a given direction, between two wheels close to the driving axis, for example from 2 to 3, then between two wheels close to the driven axis, for example from 3 'to 2', the number of reports thus achievable being equal to N + n - I . 2 - Transmission according to claim 1, characterized in that it comprises two levers 13, 23 respectively controlling the transfer of the chain on the driving wheels, for example from 2 to 3, and on the driven wheels, for example from 3 ' at 2 ′, the relative angular movement of these two levers 13, 23 being limited by stops to a value of + o (/ 2, cl being the angle by which any of the two levers must rotate to ensure transfer of the chain between any two neighboring wheels, for example 2 and 3 or 3 'and 2'. 3 - Transmission according to claim 1, characterized in that the alternating movements of the chain at the driving wheels 1, 2, 3, 4 .. and driven wheels 1 ', 2', 3 ', 4', 5 ' . are controlled by a single lever 31. 4 - Transmission according to claim 3, characterized in that the connection mechanism between the lever 31 and the two members 32, 33 for actuating the chain comprises a spreader 36 associated with an anchor cooperating with a fixed toothing 34, the kinematics thus obtained leading the two ends of the spreader 36 to be alternately stationary and in movement of an amplitude corresponding to a notch of transfer of the chain, for example from 2 to 3 or from 3 ′ to 2 ′. 5 - Transmission according to any one of the preceding claims, characterized in that the movements of the various organs, such as 12, 22, 31, 32, 33, are linear movements obeying the same operating principles. 6 - Transmission according to any one of the preceding claims, characterized in that the chain transfer mechanism is organized to be attacked by remote control members acting by successive pulses.