WO2005080824A1 - Multi-gear transmission system - Google Patents

Multi-gear transmission system Download PDF

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Publication number
WO2005080824A1
WO2005080824A1 PCT/FR2005/000355 FR2005000355W WO2005080824A1 WO 2005080824 A1 WO2005080824 A1 WO 2005080824A1 FR 2005000355 W FR2005000355 W FR 2005000355W WO 2005080824 A1 WO2005080824 A1 WO 2005080824A1
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WO
WIPO (PCT)
Prior art keywords
axis
teeth
gear
raceway
pinion
Prior art date
Application number
PCT/FR2005/000355
Other languages
French (fr)
Inventor
Patrick Barthelemy
Original Assignee
Patrick Barthelemy
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Patrick Barthelemy filed Critical Patrick Barthelemy
Publication of WO2005080824A1 publication Critical patent/WO2005080824A1/en

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Classifications

    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F16—ENGINEERING ELEMENTS AND UNITS; GENERAL MEASURES FOR PRODUCING AND MAINTAINING EFFECTIVE FUNCTIONING OF MACHINES OR INSTALLATIONS; THERMAL INSULATION IN GENERAL
    • F16H—GEARING
    • F16H3/00—Toothed gearings for conveying rotary motion with variable gear ratio or for reversing rotary motion
    • F16H3/02—Toothed gearings for conveying rotary motion with variable gear ratio or for reversing rotary motion without gears having orbital motion
    • F16H3/20—Toothed gearings for conveying rotary motion with variable gear ratio or for reversing rotary motion without gears having orbital motion exclusively or essentially using gears that can be moved out of gear
    • F16H3/36—Toothed gearings for conveying rotary motion with variable gear ratio or for reversing rotary motion without gears having orbital motion exclusively or essentially using gears that can be moved out of gear with a single gear meshable with any of a set of coaxial gears of different diameters
    • F16H3/366—Toothed gearings for conveying rotary motion with variable gear ratio or for reversing rotary motion without gears having orbital motion exclusively or essentially using gears that can be moved out of gear with a single gear meshable with any of a set of coaxial gears of different diameters the teeth of the set of coaxial gears being arranged on a generally flat, e.g. disc-type, surface

Definitions

  • the invention relates to a multi-report power transmission system comprising a gear (or disc) having on one of its faces at least two gear rings, separated and concentric with an axis of rotation X, the radially inner ring having a number of teeth equal to Ni and defining a raceway with radius Ri and the radially outer crown having a number of teeth N2 greater than Ni and defining a raceway with radius R 2 greater than Ri, a straight pinion mounted on a shaft having an axis of rotation Y perpendicular to the axis X and capable of meshing with one or the other of said crowns and means for moving said pinion from a path of shift to another to change the transmission ratio.
  • Gearboxes generally have several spur gears, and the heavier and more voluminous they are the higher the number of reports, which complicates the installation of these gearboxes on the devices fitted with them.
  • the transmission is achieved by a chain driven by a bottom bracket and driving a wheel sprocket.
  • the wheel is equipped with a plurality of pinions and the crankset can comprise several plates.
  • the gear change is achieved by devices called derails. Due to their weight, size and cost, it does not appear that gearboxes have ever been mounted on bicycles.
  • WO 02/35117 represents the state of the art closest to the invention and shows in FIG.
  • a flat disc which has, on one of its faces, three concentric crowns of teeth, defining three raceways possible for a straight pinion mounted on a shaft with an axis perpendicular to the axis of rotation of the disc.
  • the crowns are separated from each other by a gap, except in two diametrically opposite zones where two adjacent teeth of each of the crowns are connected to the corresponding teeth of the adjacent crowns in order to form a radial groove between these two sets of teeth in which a tooth of the right pinion slides during its movement from one raceway to another. It is understood that if the disc is stopped, it is possible to slide the pinion on a shaft provided that the groove is positioned parallel to the shaft.
  • the object of the invention is to propose a transmission system as defined in the introduction to this specification, which overcomes the drawbacks mentioned above.
  • the object is achieved according to the invention by the fact that the transfer from one raceway to the other takes place during a time interval equal to a half-rotation of the gear (or disc).
  • the transmission system according to the invention is characterized in that the gear (or disc) has face teeth and comprises at least one third ring of teeth defining a third substantially circular raceway whose radius R3 is equal to (R 2 + R ⁇ ) / 2 and whose center is offset from the X axis by a distance d equal to (R 2 - R ⁇ ) / 2, this third ring having in its radially outer part, relative to the axis X, teeth common with the radially outer ring and in its radially inner part of the teeth common with the radially inner ring.
  • the concentric tooth crowns, the third tooth crown and the pinion have the same module in order to facilitate engagement, which advantageously remains continuous.
  • the gear is flat, or not (gear or disc of flat, concave or convex shape), and the two crowns of facial teeth (crowns of flat, concave or convex shape) each have a even or odd number of teeth, or a crown has an even number of teeth while the other crown has an odd number of teeth.
  • the third ring has between its radially outer part and its radially inner part of the teeth allowing a gradual variation of the speed ratio.
  • the gear comprises a plurality N of concentric rings of teeth defining N raceway spaced apart by a distance equal to R2 - Ri, connected two by two by N - 1 third rings, which are concentric with one another or not ( if there are more than two third crowns, only part of them can be concentric).
  • the pinion is slidably mounted on an axially fixed spline shaft, said pinion being moved by a mechanical speed change device.
  • said gear (or disc) is flat and said pinion is straight. If several gears (or discs) are associated, they are preferably flat and coplanar.
  • FIG. 1 shows the upper face of a flat gear (or disc) according to the invention.
  • FIG. 1 is a section through a transmission system according to a first embodiment, taken along line II II of Figure 1;
  • Figure 3 is identical to Figure 1 and further shows the eccentric used to move back and forth the slider which allows to control the movement of the right pinion on its shaft in order to change gear ratio;
  • Figure 4 shows in developed the grooves formed in the sleeve surrounding the shaft of the spur gear;
  • Figures 5a to 5d show the different positions occupied by the slide rod and the sleeve during a gear change controlled by the rotation of the sleeve;
  • FIG. 1 is a section through a transmission system according to a first embodiment, taken along line II II of Figure 1;
  • Figure 3 is identical to Figure 1 and further shows the eccentric used to move back and forth the slider which allows to control the movement of the right pinion on its shaft in order to change gear ratio;
  • Figure 4 shows in developed the grooves formed in the sleeve surrounding the shaft of the spur gear;
  • Figures 5a to 5d show the different positions occupied
  • FIG. 6 shows a device for transmitting power between a driving shaft and a driven shaft coaxial with the driving shaft by means of the transmission system comprising a single flat gear
  • FIG. 7 shows another device for transmitting power between a driving shaft and a driven shaft parallel to the driving shaft by means of a transmission system comprising two flat gears
  • Figure 8 is a simplified exploded perspective view of a transmission system according to a second embodiment
  • Figure 9 is a perspective view of the transmission system of Figure 8 after assembly
  • Figure 10 is a section through the transmission system, taken along the line X-X in Figure 9
  • Figure 11 is a view similar to that of Figure 10 for an alternative embodiment of the transmission system according to the second embodiment
  • FIGS. 11A and 11B showing other details of this variant
  • FIG. 12 shows the upper face of another flat gear (or disc) which comprises four concentric crowns of teeth of axis X connected by three crowns of non-concentric teeth whose axis is offset from the axis X ' a different distance.
  • Figure 1 shows a flat gear 1, consisting of a disc 2 of axis X, defining the axis of rotation of a shaft passing through the disc 2 at its center O.
  • On the upper face 2a of the disc 2 are provided three crowns concentric teeth, with center O, referenced, Ci for the radially inner crown, Cm for the middle crown and Ce for the radially outer crown.
  • raceways centered in O, the middle circles of which are referenced respectively Chi, Chm and Che for the crowns Ci, Cm and Ce.
  • the middle circles of two adjacent raceways are spaced by a constant distance 2d, equal to R 2 - Ri, R 2 being the radius of the center circle of the largest raceway and Ri being the radius of the center circle of the smallest raceway.
  • the raceways have a width clearly less than 2d, so that on the upper face 2a of the disc 2, flat rings free of teeth separate the raceways.
  • the raceways Chi, Chm and Che are connected to each other on the upper face 2a, by two third circular raceways, referenced Ti and Te, centered at a point Ot distant from the center O by a distance equal to d, c 'is to say equal to (R 2 - R ⁇ ) / 2.
  • the third inner raceway Ti connects the inner raceway Ci to the middle raceway Cm, and its middle circle Cti to a radius equal to (R 2 + R ⁇ ) / 2.
  • the third outer raceway Te connects the middle raceway Cm to the outer raceway Ce, and its middle circle Cte has a radius equal to (R 2 + R ⁇ ) / 2, that is to say equal to half of the sum of the median ribs Chm and Che.
  • These third raceways Ti and Te are also in the form of toothed rings centered in Ot, and the teeth of which are oriented towards the center O.
  • the teeth of the raceways Ci, Cm and Ce are oriented towards the center Ot. These teeth are cut in such a way that they can mesh with the teeth of a spur gear 10 rotating around an axis Y perpendicular to the axis X.
  • the internal raceway Ci has fewer teeth than the raceway median Cm which itself has fewer teeth than the outer raceway Ce. It is thus understood that the speed ratio between the number of revolutions around the axis X of the disc 2 and the number of revolutions of the right pinion 10 around the axis Y varies according to the raceway or track with which the pinion meshes.
  • the width of the third raceways Ti and Te is preferably equal to that of the raceways Ci, Cm and Ce.
  • the third inner raceway Ti has a radially inner part 11, which straddles the inner raceway Ci. In this zone, the teeth cut in the third raceway Ti are in the extension of the corresponding teeth of the raceway inner race Ci.
  • the third inner race Ti has a radially outer part 12 which straddles the middle race Cm. In these regions, the teeth cut in the third internal raceway Ti are in the extension of the corresponding teeth of the median raceway Cm.
  • the third external raceway Te has a general configuration similar to that of the third internal raceway Ti. It also includes a radially inner zone 11 where the teeth are common with those of the median raceway Cm, a radially outer zone 12 where the teeth are common with those of the outer raceway Ce, and an intermediate zone 13 located outside raceways Cm and Ce in which the teeth are cut so as to allow a progressive variation of the speed ratio.
  • the three concentric toothed crowns namely the radially inner crown Ci, the middle crown Cm and the radially outer crown Ce have respectively 29, 40 and 66 teeth: it is therefore understandable that it is possible , according to the present invention to achieve all the speed ratios, between crowns comprising either an even or odd number of teeth Nj.
  • the change of speed or change of reduction between two adjacent crowns is carried out without interruption of movement, while the pinion 10 makes a half-turn of gear 1, that is to say a rotation of 180 °.
  • FIG. 2 shows a gear change device using the flat gear 1 described above, according to a first embodiment.
  • This device comprises a housing casing 20, on which are journalled a driving or driven shaft 21 of axis X secured to the disc 2, and a second driven or driving shaft 22, of axis Y perpendicular to the axis X.
  • the second shaft 22 extends practically as far as shaft 21, and its internal portion 23 located above the disc 2 has grooves, to allow the displacement along the Y axis of the right pinion 10 whose internal bore has ribs cooperating with the splines of the shaft.
  • the right pinion 10 is retained axially on the shaft 22 by means of a fork 24 secured to a sleeve 25 surrounding the internal part of the shaft 22.
  • This sleeve 25 is movable in rotation around the axis Y and in translation on the shaft 22, in order to allow the positioning of the right pinion 10 on one of the tracks Ci, Cm and Ce and its transfer from one track to the other by rolling on one of the third tracks Ti and Te bearing.
  • the disc 2 is equipped with an eccentric which moves in a radial direction relative to the axis X a slide 31 guided by a radial groove 32 formed in the casing 20.
  • the eccentric consists of a disc 33 placed under the disc 2 and off-center from the axis X by a distance d.
  • the axis of the disc 33 is concentric with the third raceways Ti and Te.
  • a connecting rod 34 whose head 36 surrounds the disc 33, drives the slide 31 back and forth in the groove 32 over a distance 2d, which corresponds to the distance between two median circles Chi, Chm and Che of the raceways Ci, Cm and Ce.
  • the groove 32 is arranged outside the geometric cylinder of axis X surrounding the disc 2 and below the shaft 22.
  • An example of configuration of the connecting rod 34 is shown in FIG. 3, as well as the extreme positions of the slide 31 in the groove 32.
  • the slide 31 has a rod 35 directed towards the shaft 22 and the free end of which is housed in a groove among several grooves parallel to the axis Y and formed in the wall of the sleeve 25. As as seen in FIG.
  • the wall of the sleeve 25 has three axial grooves 40a, 40b, 40c, parallel to the Y axis, circumferentially offset around the Y axis and axially in the direction of the Y axis, the ends of two adjacent grooves 40a and 40b, 40b and 40c, being connected by circumferential grooves 41a and 41b whose edges 42 and 43 can cooperate with the end of the rod 35 to axially move the sleeve 25 by a distance 2d in one way or the other so to change the raceway Ci, Cm, Ce of the right pinion 10, this pinion rolling on a portion of a third raceway Ti, Te during this movement.
  • FIGS. 5a to 5d show the way of moving the right pinion 10 from the outer raceway Ce towards the middle raceway Cm by taking the third outer raceway Ti.
  • FIG. 6 shows a transmission system which comprises a single flat gear 1 associated with two diametrically opposite shafts 22 and 22 'and each carrying a spur gear 10, 10', a sleeve 25, 25 '.
  • the rotation of one or the other of the sleeves causes the displacement of the corresponding right pinion by a rod 35, 35 'actuated by a connecting rod 34, 34'.
  • Ce, Cm and Ci 7 speed ratios are obtained, one of these ratios, equal to 1, being obtained by the cooperation of the pinions 10 and 10 'with the same raceway.
  • One of the shafts 22, 22 ' is leading, the other is driven, and the two shafts rotate in opposite directions, with respect to the common axis Y.
  • FIG. 7 shows another transmission system, comprising two gears dishes 1 and the coplanar.
  • the pinions 10 and 10 ' are mounted on the same Y-axis shaft 22, the shafts 21 and 21' of the discs 2 and 2 'rotate in opposite directions. One of these trees is leading and the other is led. Here too we get 7 speed reports.
  • the axis Y of the shaft 22 intersects the axis of rotation X of the disc 2. It is obvious that the axis Y can be offset relative to the X axis in order to constitute an offset or “offset”.
  • the teeth of the tracks Ci, Cm and Ce and the teeth of the tracks Ti and Te are not oriented towards the center O, but are cut so that the teeth which mesh with the pinion 10 are oriented in the direction of the Y axis.
  • an additional disc identical to the disc 2, whose teeth are located on its underside, mesh with the upper teeth of the pinion 10, and which, thanks to a support bearing carried by the frame 20, makes it possible to compensate for the vertical forces exerted on the pinion 10, during the transmission of the torque between the shaft 21 and shaft 22.
  • FIG. 8 to 10 illustrate in a simplified manner a speed change device using the flat gear 1 described above, according to a second embodiment.
  • the same reference signs are used for the same elements as those described above in relation to the first embodiment of Figures 2 to 5.
  • the disc 2 is shown schematically as an annular element forming a crown but it must be considered that it corresponds in particular to the example of FIG. 1.
  • the casing of the housing 20 and the first shaft 21 are not shown: it should be understood that the disc 2 is mounted in rotation about the vertical axis X which is fixed relative to the casing 20.
  • the eccentric 33 which is here surrounded by the head 36 of the connecting rod 34, a part lower diameter of the disc 2 being housed in the opening of the eccentric 33.
  • the head 36 of the connecting rod 34 is provided with a fairly large opening surrounding the eccentric 33 in order to allow a translational movement lon the direction Y of the connecting rod 34.
  • the other end of the connecting rod 34 is secured to a slide secured to a vertical rod 35 parallel to the X axis.
  • the pinion 10 able to mesh with the upper surface of the disc 2 forming the gear (flat in this example), and which is mounted, by means of cooperation by grooves and grooves, at the end of the second shaft 22 extending in the direction Y.
  • the sleeve 25 of the first embodiment is replaced by two forks 126 integral with the shaft 22 and between them by a speed selection plate 125 equipped with a series of grooves similar to the developed grooves visible in FIG. 4, namely three first axial grooves 40a to 40c parallel to the Y axis and two second axial grooves 41a and 41b parallel to a direction orthogonal to the Y axis.
  • the two second axial grooves 41a and 41b are thus orthogonal to the first axial grooves 40a to 40c.
  • the radial groove 32 parallel to the Y axis is formed in a speed change control plate 127 which is arranged under the selection plate 125, so that the radial groove 32 is parallel to the axis Y and to the first three axial grooves 40a to 40c.
  • a speed selection box 128 is used in order to carry out the mounting between the rod 35, the plate 125 secured to the shaft 22 and carrying the pinion 10, and the plate 127.
  • This box 128 retains the rod 35, and the plates 125 and 127 so that the rod 35 remains in the radial groove 32 and in one of the first axial grooves 40a to 40c. More specifically, the housing 128 retains the plate 125 in the vertical direction along the axis X while allowing translation in the direction Y of the plate 125 and the elements which are integral with it (forks 126, shaft 22 and pinion 10).
  • the housing 128 retains the control plate 127 in the vertical direction along the X axis while allowing it to perform a horizontal translational movement in a direction perpendicular to the Y axis in order to control the rod 35, when it is in one of the second axial grooves 41a and 41b, to pass from one of the first axial grooves 40a to 40c to one of its first axial grooves 40a to 40c adjacent.
  • the mode of operation of this speed transmission system according to the second embodiment is therefore quite close to that of the first embodiment, the control not being carried out here by a rotation around the axis Y of the sleeve. 25, but by a horizontal translation of the control plate 127.
  • the disc 2 should in reality not be limited to a gear of the flat type, but it may be in the form of a disc of gears of concave or convex shape but always with facial teeth, in particular with teeth right.
  • FIG. 11 shows an example of an alternative embodiment of the transmission system of FIGS. 8 to 10, in which the flat disc 2 is replaced by a concave disc 202 carrying three concentric toothed rings around the axis X, with teeth facial, in particular with straight teeth, successively forming, from the X axis and in the radial direction towards the outside, the radially inner crown Ci, the middle crown Cm and the radially outer crown Ce.
  • the pinion 10 has straight teeth allowing it to slide from one crown to another.
  • An angle ⁇ is formed between the X axis and the surface of the concave disc carrying the three concentric rings Ce, Cm and Ci, or between the X axis and the Y axis formats the axis of rotation of the pinion 10.
  • this angle ⁇ is around 45 ° but it can vary between a few degrees and almost 180 °.
  • the disc 202 in addition to the fact that it is concave, the disc 202 carries three concentric rings Ce, Cm and Ci which do not have the same difference between them as the difference 2d formed between the three crowns of the disc 2 of FIG. 1. In fact, as can be seen in FIG.
  • the connecting rod 34 is integral with a rod 133, the first end of which forms a slide in a radial groove 132 of the casing 20.
  • the second end of the rod 133 serves as an axis material for a roller 234 housed in a groove 132 'formed on the outer surface of the disk 202. This groove 132' (see FIG.
  • FIG. 11A plays an eccentric role because its position along the axis of rotation X of the disk 202 varies by the distance 2di corresponding to the difference between the radially inner ring Ci and the middle ring Cm.
  • FIG. 11B is visible the speed selection plate 125 adapted to the variant embodiment illustrated in FIG. 11: the three first axial grooves 40a to 40c and the two second axial grooves 41a and 41b have different dimensions.
  • the first axial groove 40a allows the rod 35 to move a distance 2de, while the first axial grooves 40b and 40c allow a displacement of a distance 2di.
  • the grooves 40b and 40c are mutually parallel over a common length greater than the width of the rod 35, namely that they are connected by the second axial groove 41a, located on the left in FIG. 11B, which is wider than the groove 41b and the width of the rod 35: the edge 43 of the groove 41a is 2de distant from the edge 43 of the groove 41b while the edge 42 of the groove 41a is 2di from the edge 42 of groove 41b.
  • the rod 34 always identical (2di), corresponding to the largest difference between the crowns, while the pinion is able to move between the three crowns Ce, Cm and Ci.
  • a set of crowns with face teeth is used forming a gear or disc of flat, concave or convex shape with the same module between them and the same module as the pinion 10 in order to allow the pinion 10 to be permanently driven either on one of the concentric rings corresponding to one of the Chi, Chm or Che raceways of one of the speeds, or on one of the portions of the transition rings forming the third internal raceway Ti and external raceway Te.
  • another flat gear is shown which can be used in the transmission system according to the invention.
  • This flat gear 1 ' consists of a disc 2' of axis X defining the axis of rotation of a shaft (not shown) passing through the disc 2 'at its center O.
  • the four toothed crowns Ci, Cmi, Cme and Ce therefore have alternately an odd and even number of teeth.
  • the center circles of two adjacent raceways are spaced by a distance 2d, equal to R2 - RI, R2 being the radius of the center circle of the largest raceway and RI being the radius of the center circle of the raceway le smaller.
  • this distance 2d is not constant, the smallest 2dm having been chosen between the median circles of the tracks Chmi and Chme, the largest 2de being situated between the median circles of the tracks Chme and Che, and an intermediate value having been chosen for the distance 2di situated between the median circles of the raceways Chi and Chmi.
  • the four raceways Chi, Chmi, Chme and Che are connected to each other on the upper face 2a ', by three third circular raceways, referenced Cti, Ctm and Cte, centered respectively at a point Oti, Otm and Ote distant from center O of a distance respectively equal to di, dm and de.
  • the third circular raceways Cti, Ctm and Cte are therefore not concentric with each other.
  • the third inner raceway Cti is carried by a third radially inner ring Ti
  • the third middle raceway Ctm is carried by a third middle raceway Tm
  • the third outer raceway Cte is carried by a third radially outer race Te.

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Abstract

The invention relates to a multi-gear power transmission system comprising a gear provided with at least two separated and concentric to an axis of rotation (X) crown gears (Ci, Cm, Ce) on one side thereof, wherein a radially internal crown has a number of teeth equal to N1 and defines the raceway a radius R1 and a radially external crown has a number of teeth N2 greater than N1 and defines the raceway of a radius R2 which is greater than that of the R1. The inventive system also comprises a spur pinion (10) which is mounted on the shaft (22) whose axis of rotation (Y) is perpendicular with respect to the axis X and is engaged with one of said crowns and means for moving said pinion (10) from one raceway to the other raceway in order to change the gear transmission ratio. Said transmission system is characterised in that the gear is provided with a third crown gear (Te, Ti) defining a third raceway which is substentially circular and whose radius R3 is equal to (R2+R1)/2 and whose centre is shifted from the axis X at a distance d which is equal to (R2-R1)/2, and said third crown is provided, in the radially external with respect to the axis X part thereof (11), with the teeth which are common with the radially external crown and with the teeth common with the radially internal crown in the radially internal part (12) thereof.

Description

Système de transmission multirapports Multi-report transmission system
L'invention concerne un système de transmission de puissance multirapports comportant un engrenage (ou disque) présentant sur l'une de ses faces au moins deux couronnes de dentures, séparées et concentriques à un axe de rotation X, la couronne radialement intérieure ayant un nombre de dents égal à Ni et définissant un chemin de roulement de rayon Ri et la couronne radialement extérieure ayant un nombre de dents N2 supérieur à Ni et définissant un chemin de roulement de rayon R2 supérieur à Ri, un pignon droit monté sur un arbre ayant un axe de rotation Y perpendiculaire à l'axe X et susceptible d'engrener avec l'une ou l'autre desdites couronnes et des moyens pour déplacer ledit pignon d'un chemin de roulement à l'autre afin de changer le rapport de transmission. Les boîtes de vitesse comportent en général plusieurs engrenages droits, et elles sont d'autant plus lourdes et volumineuses que le nombre de rapports est élevé, ce qui complique l'implantation de ces boîtes sur les appareils qui en sont équipés. Sur les bicyclettes, la transmission est réalisée par une chaîne entraînée par un plateau de pédalier et entraînant un pignon de roue. Pour obtenir plusieurs rapports de transmission, la roue est équipée d'une pluralité de pignons et le pédalier peut comporter plusieurs plateaux. Le changement de rapport de vitesse est réalisé par des dispositifs appelés dérailleurs. Du fait de leur poids, encombrement et coût, il ne semble pas que des boîtes de vitesse à engrenages aient jamais été montées sur des bicyclettes. WO 02/35117 représente l'état de la technique le plus proche de l'invention et montre à la figure 2, un disque plat qui présente, sur l'une de ses faces, trois couronnes concentriques de dentures, définissant trois chemins de roulement possibles pour un pignon droit monté sur un arbre d'axe perpendiculaire à l'axe de rotation du disque. Les couronnes sont séparées l'une de l'autre par un interstice, sauf en deux zones diamétralement opposées où deux dents adjacentes de chacune des couronnes sont raccordées aux dents correspondantes des couronnes adjacentes afin de former entre ces deux ensembles de dents alignées une gorge radiale dans laquelle coulisse une dent du pignon droit lors de son déplacement d'un chemin de roulement à l'autre. On conçoit que si le disque est arrêté, on puisse faire coulisser le pignon sur un arbre à condition de positionner la gorge parallèlement à l'arbre. Par contre, si le dispositif est en fonctionnement, le coulissement du pignon ne peut être réalisé que pendant l'intervalle de temps très bref durant lequel la gorge commune aux trois couronnes est parallèle à l'arbre sinon le pignon droit se met à cheval sur les dentures suivantes de deux couronnes avec les dégâts qui peuvent en résulter du fait, par construction, que les dents suivantes ne sont pas alignées. En outre, le changement de vitesse étant extrêmement rapide, il va se produire des chocs considérables, obligatoirement encaissés par les dents du pignon et des couronnes. Le but de l'invention est de proposer un système de transmission tel que défini dans l'introduction du présent mémoire, qui pallie les inconvénients mentionnés ci-dessus. Le but est atteint selon l'invention par le fait que le transfert d'un chemin de roulement à l'autre se réalise pendant un intervalle de temps égal à une demi-rotation de l'engrenage (ou disque). Plus précisément, le système de transmission selon l'invention est caractérisé par le fait que l'engrenage (ou disque) est à denture faciale et comporte au moins une troisième couronne de dentures définissant un troisième chemin de roulement sensiblement circulaire dont le rayon R3 est égal à (R2 + Rι)/2 et dont le centre est décalé de l'axe X d'une distance d égale à (R2 - Rι)/2, cette troisième couronne ayant dans sa partie radialement extérieure, par rapport à l'axe X, des dents communes avec la couronne radialement extérieure et dans sa partie radialement intérieure des dents communes avec la couronne radialement intérieure. Avantageusement, les couronnes de dentures concentriques, la troisième couronne de dentures et le pignon présentent le même module afin de faciliter l'engrènement, qui reste avantageusement continu. Selon l'invention, l'engrenage (ou disque) est plat, ou non (engrenage ou disque de forme plat, concave ou convexe), et les deux couronnes de dentures faciales (couronnes de forme plate, concave ou convexe) présentent chacune un nombre de dents pair ou impair, ou encore une couronne présente un nombre de dents pair tandis que l'autre couronne présente un nombre de dents impair. Avantageusement, la troisième couronne comporte entre sa partie radialement extérieure et sa partie radialement intérieure des dents permettant une variation progressive du rapport de vitesse. De préférence, l'engrenage comporte une pluralité N de couronnes concentriques de dentures définissant N chemin de roulement espacés d'une distance égale à R2 - Ri, raccordées deux par deux par N - 1 troisièmes couronnes, qui sont concentriques entre elles ou non (s'il y a plus de deux troisièmes couronnes, une partie d'entre elles seulement peuvent être concentriques). En outre, de préférence, le pignon est monté coulissant sur un arbre à cannelures, axialement fixe, ledit pignon étant déplacé par un dispositif mécanique de changement de vitesse. De préférence, ledit engrenage (ou disque) est plat et ledit pignon est droit. Si plusieurs engrenages (ou disques) sont associés, ils sont de préférence plats et coplanaires. On peut aussi prévoir un (ou des) engrenage(s) (ou disques) non plat(s), c'est-à-dire un engrenage (ou disque) de forme concave ou convexe, associé à un pignon à denture droite. De préférence, les axes X et Y sont sécants. On peut aussi prévoir que les axes X et Y ne sont pas sécants, formant entre eux un désaxage ou « offset ». D'autres avantages et caractéristiques de l'invention ressortiront à la lecture de la description suivante fait à titre d'exemple et en référence au dessin dans lequel : la figure 1 montre la face supérieure d'un engrenage (ou disque) plat selon l'invention qui comporte trois couronnes de dentures concentriques d'axe X raccordées par deux couronnes concentriques de dentures dont l'axe est décalé de l'axe X d'une distance égale à la moitié de l'écart entre deux couronnes ; la figure 2 est une coupe d'un système de transmission selon un premier mode de réalisation, prise selon la ligne II II de la figure 1 ; la figure 3 est identique à la figure 1 et montre en outre l'excentrique servant à déplacer en va-et-vient le coulisseau qui permet de commander le déplacement du pignon droit sur son arbre afin de changer de rapport de vitesse ; la figure 4 montre en développé les rainures formées dans le manchon entourant l'arbre du pignon droit ; les figures 5a à 5d montrent les différentes positions occupées par la tige du coulisseau et le manchon lors d'un changement de vitesse commandé par la rotation du manchon ; la figure 6 montre un dispositif pour transmettre une puissance entre un arbre menant et un arbre mené coaxial à l'arbre menant au moyen du système de transmission comportant un seul engrenage plat ; la figure 7 montre un autre dispositif pour transmettre une puissance entre un arbre menant et un arbre mené parallèle à l'arbre menant au moyen d'un système de transmission comportant deux engrenages plats ; la figure 8 est une vue en perspective éclatée simplifiée d'un système de transmission selon un deuxième mode de réalisation ; la figure 9 est une vue en perspective du système de transmission de la figure 8 après montage ; la figure 10 est une coupe du système de transmission, prise selon la ligne X-X de la figure 9 ; la figure 11 est une vue analogue à celle de la figure 10 pour une variante de réalisation du système de transmission selon le deuxième mode de réalisation ; les figures 11A et 11B montrant d'autres détails de cette variante ; et la figure 12 montre la face supérieure d'un autre engrenage (ou disque) plat qui comporte quatre couronnes de dentures concentriques d'axe X raccordées par trois couronnes de dentures non concentriques dont l'axe est décalé de l'axe X d'une distance différente. La figure 1 montre un engrenage plat 1, constitué par un disque 2 d'axe X, définissant l'axe de rotation d'un arbre traversant le disque 2 en son centre O. Sur la face supérieure 2a du disque 2 sont prévues trois couronnes dentées concentriques, de centre O, référencées, Ci pour la couronne radialement intérieure, Cm pour la couronne médiane et Ce pour la couronne radialement extérieure. Ces trois couronnes définissent des chemins de roulement, centrées en O dont les cercles médians sont référencés respectivement Chi, Chm et Che pour les couronnes Ci, Cm et Ce. Les cercles médians de deux chemins de roulement adjacents sont espacés d'une distance constante 2d, égale à R2 - Ri, R2 étant le rayon du cercle médian du chemin de roulement le plus grand et Ri étant le rayon du cercle médian du chemin de roulement le plus petit. Ainsi que cela se voit sur la figure 1, les chemins de roulement ont une largeur nettement inférieure à 2d, afin que sur la face supérieure 2a du disque 2, des anneaux plans exempts de denture séparent les chemins de roulement. Les chemins de roulement Chi, Chm et Che sont raccordés entre eux sur la face supérieure 2a, par deux troisièmes chemins de roulement circulaires, référencés Ti et Te, centrés en un point Ot distant du centre O d'une distance égale à d, c'est-à-dire égale à (R2 - Rι)/2. Le troisième chemin de roulement intérieur Ti raccorde le chemin de roulement intérieur Ci au chemin de roulement médian Cm, et son cercle médian Cti à un rayon égal à (R2 + Rι)/2. Le troisième chemin de roulement extérieur Te raccorde le chemin de roulement médian Cm au chemin de roulement extérieur Ce, et son cercle médian Cte a un rayon égal à (R2 + Rι)/2, c'est-à-dire égal à la moitié de la somme des cerces médians Chm et Che. Ces troisièmes chemins de roulement Ti et Te se présentent également sous la forme de couronnes dentées centrées en Ot, et dont les dents sont orientées vers le centre O. Les dents des chemins de roulement Ci, Cm et Ce sont orientées vers le centre Ot. Ces dents sont taillées de telle manière qu'elles puissent engrener avec les dents d'un pignon droit 10 tournant autour d'un axe Y perpendiculaire à l'axe X. Le chemin de roulement intérieur Ci a moins de dents que le chemin de roulement médian Cm qui a lui-même moins de dents que le chemin de roulement extérieur Ce. On conçoit ainsi que le rapport de vitesse entre le nombre de tours autour de l'axe X du disque 2 et le nombre de tours du pignon droit 10 autour de l'axe Y varie selon le chemin de roulement ou piste avec lequel engrène le pignon droit, lors du fonctionnement du système. La largeur des troisièmes chemins de roulement Ti et Te est de préférence égale à celle des chemins de roulement Ci, Cm et Ce. Le troisième chemin de roulement intérieur Ti présente une partie radialement intérieure 11, qui est à cheval sur le chemin de roulement intérieur Ci. Dans cette zone, les dents taillées dans le troisième chemin de roulement Ti sont dans le prolongement des dents correspondantes du chemin de roulement intérieur Ci. Le troisième chemin de roulement intérieur Ti présente une partie radialement extérieure 12 qui est à cheval sur le chemin de roulement médian Cm. Dans ces régions, les dents taillées dans le troisième chemin de roulement intérieur Ti sont dans le prolongement des dents correspondantes du chemin de roulement médian Cm. Dans la zone 13 du troisième chemin de roulement Ti, reliant la partie radialement intérieure 11 à la partie radialement extérieure 12, les dents de ce troisième chemin de roulement Ti sont dirigées vers le centre O et sont taillées de manière à permettre une variation progressive du rapport de vitesse, lorsque le pignon droit 10 emprunte cette zone 13, entre les rapports obtenus lorsqu'il roule sur le chemin de roulement intérieur Ci et le chemin de roulement médian Cm. Le troisième chemin de roulement extérieur Te a une configuration générale semblable à celle du troisième chemin de roulement intérieur Ti. Il comporte également une zone radialement intérieure 11 où les dents sont communes avec celles du chemin de roulement médian Cm, une zone radialement extérieure 12 où les dents sont communes avec celles du chemin de roulement extérieur Ce, et une zone intermédiaire 13 située en-dehors des chemins de roulement Cm et Ce dans laquelle les dents sont taillées de manière à permettre une variation progressive du rapport de vitesse. Sur l'exemple de la figure 1, les trois couronnes dentées concentriques, à savoir la couronne radialement intérieure Ci, la couronne médiane Cm et la couronne radialement extérieure Ce présentent respectivement 29, 40 et 66 dents : on comprend donc qu'il est possible, selon la présente invention de réaliser tous les rapports de vitesse, entre des couronnes comprenant indifféremment un nombre de dents Nj pair ou impair. De plus, d'après ce qui précède, on comprend que le changement de vitesse ou changement de démultiplication entre deux couronnes adjacentes s'effectue sans rupture de mouvement, alors que le pignon 10 effectue un demi-tour de l'engrenage 1, c'est-à-dire une rotation de 180°. Ce changement de vitesse est possible grâce à la troisième couronne, intermédiaire entre les deux couronnes adjacentes, qui synchronise le mouvement du pignon 10 depuis la première vitesse vers la deuxième vitesse, avec un engrènement permanent. Le disque 2 avec ses dentures peut être réalisé en tout matériau rigide, soit par usinage, soit par moulage sous pression selon les applications. Il peut être notamment réalisé par injection d'une matrice plastique thermodurcissable. Le nombre de chemins de roulement peut évidemment être différent de trois, selon le nombre de rapports de vitesses souhaité : deux chemins de roulement ou plus de trois chemins de roulement et en particulier quatre chemins de roulement. Ce qui compte surtout c'est que la forme des dents des différentes pistes permette un engrènement normal avec le même pignon droit 10. La figure 2 montre un dispositif de changement de vitesse utilisant l'engrenage plat 1 décrit ci-dessus, selon un premier mode de réalisation. Ce dispositif comporte un carter de boîtier 20, sur lequel sont montés tourillonnant un arbre menant ou mené 21 d'axe X solidaire du disque 2, et un deuxième arbre mené ou menant 22, d'axe Y perpendiculaire à l'axe X. Le deuxième arbre 22 s'étend pratiquement jusqu'à l'arbre 21, et sa portion interne 23 située au-dessus du disque 2 comporte des cannelures, pour permettre le déplacement selon l'axe Y du pignon droit 10 dont l'alésage interne présente des nervures coopérant avec les cannelures de l'arbre. Le pignon droit 10 est retenu axialement sur l'arbre 22 au moyen d'une fourchette 24 solidaire d'un manchon 25 entourant la partie interne de l'arbre 22. Ce manchon 25 est mobile en rotation autour de l'axe Y et en translation sur l'arbre 22, afin de permettre le positionnement du pignon droit 10 sur l'un des chemins de roulement Ci, Cm et Ce et son transfert d'une piste à l'autre par roulement sur l'un des troisièmes chemins de roulement Ti et Te. Le disque 2 est équipé d'un excentrique qui déplace dans une direction radiale par rapport à l'axe X un coulisseau 31 guidé par une rainure radiale 32 ménagée dans le carter 20. L'excentrique est constitué par un disque 33 disposé sous le disque 2 et décentré de l'axe X d'une distance d. L'axe du disque 33 est concentrique aux troisièmes chemins de roulement Ti et Te. Une bielle 34 dont la tête 36 entoure le disque 33, entraîne le coulisseau 31 en va-et-vient dans la rainure 32 sur une distance 2d, qui correspond à l'écart entre deux cercles médians Chi, Chm et Che des chemins de roulement Ci, Cm et Ce. La rainure 32 est disposée à l'extérieur du cylindre géométrique d'axe X entourant le disque 2 et en- dessous de l'arbre 22. Un exemple de configuration de la bielle 34 est montré sur la figure 3, ainsi que les positions extrêmes du coulisseau 31 dans la rainure 32. Le coulisseau 31 comporte une tige 35 dirigée vers l'arbre 22 et dont l'extrémité libre loge dans une rainure parmi plusieurs rainures parallèles à l'axe Y et ménagées dans la paroi du manchon 25. Comme on le voit sur la figure 4, la paroi du manchon 25 comporte trois rainures axiales 40a, 40b, 40c, parallèles à l'axe Y, décalées circonférentiellement autour de l'axe Y et axialement dans le sens de l'axe Y, les extrémités de deux rainures adjacentes 40a et 40b, 40b et 40c, étant raccordées par des rainures circonférentielles 41a et 41b dont les bords 42 et 43 peuvent coopérer avec l'extrémité de la tige 35 pour déplacer axialement le manchon 25 d'une distance 2d dans un sens ou dans l'autre afin de changer le chemin de roulement Ci, Cm, Ce du pignon droit 10, ce pignon roulant sur une portion d'un troisième chemin de roulement Ti, Te pendant ce déplacement. Sur la figure 4 et sur la figure 5a, l'extrémité de la tige 35 est positionnée dans la rainure axiale de gauche 40a. La longueur de cette rainure 40a est égale à la distance 2d augmentée du diamètre de la tige 35. Lors du mouvement de va-et-vient du coulisseau 31, par suite de la rotation du disque 2 autour de l'axe X, l'extrémité de la tige 35 coulisse dans la rainure 40a entre son extrémité gauche 42 et le bord droit 43 de la rainure circonferentielle 41a. Dans cette situation le manchon 25 est immobilisé autour de l'arbre 22 d'axe Y, et le pignon droit roule sur le chemin de roulement extérieur Ce. Les figures 5a à 5d montrent la manière de déplacer le pignon droit 10 du chemin de roulement extérieur Ce vers le chemin de roulement médian Cm en empruntant le troisième chemin de roulement extérieur Ti. Si la tige 35 est en butée contre la paroi de droite 43 de la rainure périphérique comme on le voit sur la figure 5a, et si on fait pivoter le manchon 25 autour de l'axe Y dans le sens de la flèche F, l'extrémité de la tige 35 vient se positionner contre la paroi de gauche 42 de la rainure circonferentielle 41a. Le mouvement de retour de la tige 35 vers l'autre position extrême va entraîner le déplacement du manchon 25 vers la gauche et le roulement du pignon droit sur la moitié du parcours du troisième chemin de roulement extérieur Te. Ensuite l'extrémité de la tige 35 se déplace en va-et-vient dans la rainure axiale 40b, et le pignon droit 10 engrène avec le chemin de roulement médian Cm. Si de nouveau on fait pivoter le manchon dans le même sens lorsque la tige 35 est en appui contre la paroi droite 43 de la rainure périphérique 41b, le manchon 25 se déplacera de nouveau vers la gauche d'une distance 2d lors d'un demi-tour de rotation du disque 2, et le pignon droit 10 roulera sur le chemin de roulement intérieur Ci. Pour déplacer le pignon droit d'un chemin de roulement de petit diamètre vers un chemin de roulement de plus grand diamètre, on fait pivoter le manchon 25 dans le sens inverse lorsque l'extrémité de la tige 35 est en butée contre la paroi gauche 42 d'une rainure périphérique 41b ou 41a. La figure 6 montre un système de transmission qui comporte un seul engrenage plat 1 associé à deux arbres 22 et 22' diamétralement opposés et portant chacun un pignon droit 10, 10', un manchon 25, 25'. La rotation de l'un ou l'autre des manchons entraîne le déplacement du pignon droit correspondant par une tige 35, 35' actionnée par une bielle 34, 34'. Avec trois chemins de roulements Ce, Cm et Ci, on obtient 7 rapports de vitesse, l'un de ces rapports, égal à 1, étant obtenu par la coopération des pignons 10 et 10' avec le même chemin de roulement. L'un des arbres 22, 22' est menant, l'autre est mené, et les deux arbres tournent dans des sens opposés, par rapport à l'axe commun Y. La figure 7 montre un autre système de transmission, comportant deux engrenages plats 1 et l' coplanaires. Les pignons 10 et 10' sont montés sur le même arbre d'axe Y 22, les arbres 21 et 21' des disques 2 et 2' tournent dans des sens opposés. L'un de ces arbres est menant et l'autre est mené. Ici aussi on obtient 7 rapports de vitesse. Dans les exemples décrits ci-dessus, l'axe Y de l'arbre 22 coupe l'axe de rotation X du disque 2. Il est évident que l'axe Y peut être déporté par rapport à l'axe X afin de constituer un désaxage ou « offset ». Dans ce cas, les dents des chemins de roulement Ci, Cm et Ce et les dents des chemins de roulement Ti et Te ne sont pas orientées vers le centre O, mais sont taillées de telle manière que les dents qui engrènent avec le pignon 10 soient orientées dans la direction de l'axe Y. En outre, sur l'arbre 21 du disque 2, montré sur les figures 2, 6 et 7, on peut monter fou un disque supplémentaire, identique au disque 2, dont les dents, situées sur sa face inférieure, engrènent avec les dents supérieures du pignon 10, et qui, grâce à un palier de support porté par le bâti 20, permet de compenser les efforts verticaux exercés sur le pignon 10, lors de la transmission du couple entre l'arbre 21 et l'arbre 22. Ce disque supplémentaire tourne autour de l'axe X dans un sens opposé à celui du disque 2, mais il permet d'avoir un meilleur équilibre des forces en présence et il permet ainsi d'augmenter la puissance transmissible. Les figures 8 à 10 illustrent de façon simplifiée un dispositif de changement de vitesse utilisant l'engrenage plat 1 décrit ci-dessus, selon un deuxième mode de réalisation. Ainsi, les mêmes signes de référence sont repris pour les mêmes éléments que ceux décrits précédemment en relation avec le premier mode de réalisation des figures 2 à 5. Sur ces figures 8 à 10, le disque 2 est représenté de façon schématique comme un élément annulaire formant une couronne mais il faut considérer qu'il correspond notamment à l'exemple de la figure 1. Également, sur ces figures 8 à 10, le carter de boîtier 20 et le premier arbre 21 ne sont pas représentés : il faut comprendre que le disque 2 est monté en rotation autour de l'axe vertical X qui est fixe par rapport au carter 20. Dans ce deuxième mode de réalisation, on retrouve l'excentrique 33 qui est ici entouré par la tête 36 de la bielle 34, une partie inférieure de diamètre rétréci du disque 2 étant logée dans l'ouverture de l'excentrique 33. La tête 36 de la bielle 34 est munie d'une ouverture assez grande entourant l'excentrique 33 afin d'autoriser un mouvement de translation selon la direction Y de la bielle 34. Comme dans le cas du premier mode de réalisation, l'autre extrémité de la bielle 34, ne comprenant pas la tête 36, est solidaire d'un coulisseau solidaire d'une tige 35 verticale parallèle à l'axe X. On retrouve le pignon 10, apte à s'engrener avec la surface supérieure du disque 2 formant l'engrenage (plat dans cet exemple), et qui est monté, au moyen d'une coopération par cannelures et rainures, à l'extrémité du deuxième arbre 22 s'étendant selon la direction Y. Selon ce deuxième mode de réalisation, le manchon 25 du premier mode de réalisation est remplacé par deux fourchettes 126 solidaires de l'arbre 22 et entre elles par une plaque de sélection de vitesse 125 équipée d'une série de rainures similaires au développé des rainures visibles sur la figure 4, à savoir trois premières rainures axiales 40a à 40c parallèles à l'axe Y et deux deuxièmes rainures axiales 41a et 41b parallèles à une direction orthogonale à l'axe Y. les deux deuxièmes rainures axiales 41a et 41b sont ainsi orthogonales aux premières rainures axiales 40a à 40c. Dans ce deuxième mode de réalisation, la rainure radiale 32 parallèles à l'axe Y est formée dans une plaque de commande 127 du changement de vitesse qui est disposée sous la plaque 125 de sélection, de sorte que la rainure radiale 32 est parallèle à l'axe Y et aux trois premières rainures axiales 40a à 40c. Afin d'effectuer le montage entre la tige 35, la plaque 125 solidaire de l'arbre 22 et portant le pignon 10, et la plaque 127, on utilise un boîtier de sélection de vitesse 128. Ce boîtier 128 retient la tige 35, et les plaques 125 et 127 de sorte que la tige 35 reste dans la rainure radiale 32 et dans l'une des premières rainures axiales 40a à 40c. Plus précisément, on le boîtier 128 retient la plaque 125 en direction verticale selon l'axe X tout en permettant une translation selon la direction Y de la plaque 125 et des éléments qui lui sont solidaires (fourchettes 126, arbre 22 et pignon 10). En outre, le boîtier 128 retient direction verticale selon l'axe X la plaque de commande 127 tout en lui permettant d'effectuer un mouvement de translation horizontale selon une direction perpendiculaire à l'axe Y afin de commander à la tige 35, lorsqu'elle se trouve dans l'une des deuxièmes rainures axiales 41a et 41b, de passer de l'une des premières rainures axiales 40a à 40c à l'une de ses premières rainures axiales 40a à 40c adjacentes. Le mode de fonctionnement de ce système de transmission de vitesse selon le deuxième mode de réalisation est donc tout à fait proche de celui du premier mode de réalisation, la commande n'étant pas effectuée ici par une rotation autour de l'axe Y du manchon 25, mais par une translation horizontale de la plaque de commande 127. Selon la présente invention, le disque 2 ne doit en réalité pas se limiter à un engrenage du type plat, mais il peut se présenter sous la forme d'un disque à engrenages de forme concave ou convexe mais toujours à denture faciale, notamment à denture droite. Sur la figure 11, est illustré un exemple de variante de réalisation du système de transmission des figures 8 à 10, dans lequel le disque 2 plat est remplacé par un disque 202 concave portant trois couronnes dentées concentriques autour de l'axe X, à denture faciale, notamment à denture droite, formant successivement, depuis l'axe X et en direction radiale vers l'extérieur, la couronne radialement intérieure Ci, la couronne médiane Cm et la couronne radialement extérieure Ce. Dans ce cas, le pignon 10 est à denture droite lui permettant de coulisser d'une couronne à une autre. Un angle α est formé entre l'axe X et la surface du disque concave portant les trois couronnes concentriques Ce, Cm et Ci, ou entre l'axe X et l'axe Y format l'axe de rotation du pignon 10. Sur la figure 11, cet angle α est d'environ 45° mais il peut varier entre quelques degrés et presque 180°. Sur la variante de réalisation illustrée sur la figure 11, outre le fait qu'il est concave, le disque 202 portent trois couronnes concentriques Ce, Cm et Ci qui ne présentent pas entre elles le même écart que l'écart 2d formé entre les trois couronnes du disque 2 de la figure 1. En effet, comme on peut le voir sur la figure 11, l'écart 2di entre la couronne radialement intérieure Ci et la couronne médiane Cm est plus important que l'écart 2de entre la couronne médiane Cm et la couronne radialement extérieure Ce. Dans ce cas, comme on le voit sur la figure 11, la bielle 34 est solidaire d'une tige 133 dont la première extrémité forme un coulisseau dans une rainure radiale 132 du carter 20. La deuxième extrémité de la tige 133 sert d'axe matériel pour un galet 234 logé dans une gorge 132' formée à la surface extérieure du disque 202. Cette gorge 132' (voir figure 11A) joue un rôle d'excentrique car sa position le long de l'axe de rotation X du disque 202 varie de la distance 2di correspondant à l'écart entre la couronne radialement intérieure Ci et la couronne médiane Cm. Sur la figure 11B est visible la plaque de sélection de vitesse 125 adaptée à la variante de réalisation illustrée de la figure 11 : les trois premières rainures axiales 40a à 40c et les deux deuxièmes rainures axiales 41a et 41b présentent des dimensions différentes. La première rainure axiale 40a permet à la tige 35 de se déplacer d'une distance 2de, tandis que les premières rainures axiales 40b et 40c permettent un déplacement d'une distance 2di. En outre, les rainures 40b et 40c sont parallèles entre elles sur une longueur commune plus importante que la largeur de la tige 35, à savoir qu'elles sont reliées par la deuxième rainure axiale 41a, située à gauche sur la figure 11B, qui est plus large que la rainure 41b et que la largeur de la tige 35 : le bord 43 de la rainure 41a est distant de 2de du bord 43 de la rainure 41b tandis que le bord 42 de la rainure 41a est distant de 2di du bord 42 de la rainure 41b. Ainsi, on peut réaliser un déplacement de la biellette 34 toujours identique (2di), correspondant à l'écart le plus important entre les couronnes, tandis que le pignon est apte à se déplacer entre les trois couronnes Ce, Cm et Ci. D'une façon générale, on comprend que pour la présente invention, on utilise un ensemble de couronnes à denture faciale formant un engrenage ou disque de forme plate, concave ou convexe de même module entre elles et de même module que le pignon 10 afin de permettre au pignon 10 d'être entraîné en permanence soit sur l'une des couronnes concentriques correspondant à l'un des chemins de roulement Chi, Chm ou Che de l'une des vitesses, soit sur l'une des portions des couronnes de transition formant les troisième chemin de roulement intérieur Ti et extérieur Te. Ainsi, sur la figure 12, est représenté un autre engrenage plat l' utilisable dans le système de transmission selon l'invention. Cet engrenage plat l' est constitué d'un disque 2' d'axe X définissant l'axe de rotation d'un arbre (non représenté) traversant le disque 2' en son centre O. Sur la face supérieure 2a' du disque 2' sont prévues quatre couronnes dentées concentriques, de centre O, formées, depuis le centre O vers l'extérieur, par : - une couronne radialement intérieure Ci (chemin de roulement Chi) présentant 29 dents, - une couronne médiane intérieure Cmi (chemin de roulement Chmi) présentant 52 dents, - une couronne médiane extérieure Cme (chemin de roulement Chme) présentant 66 dents, et - une couronne radialement extérieure Ce (chemin de roulement Che) présentant 87 dents. Les quatre couronnes dentées Ci, Cmi, Cme et Ce présentent donc alternativement un nombre de dents impair et pair. Les cercles médians de deux chemins de roulement adjacents sont espacés d'une distance 2d, égale à R2 - RI, R2 étant le rayon du cercle médian du chemin de roulement le plus grand et RI étant le rayon du cercle médian du chemin de roulement le plus petit. Dans cet exemple, cette distance 2d n'est pas constante, la plus faible 2dm ayant été choisie entre les cercles médians des chemins de roulement Chmi et Chme, la plus importante 2de étant située entre les cercles médians des chemins de roulement Chme et Che, et une valeur intermédiaire ayant été choisie pour la distance 2di située entre les cercles médians des chemins de roulement Chi et Chmi. Les quatre chemins de roulement Chi, Chmi, Chme et Che sont raccordés entre eux sur la face supérieure 2a', par trois troisièmes chemins de roulement circulaires, référencés Cti, Ctm et Cte, centrés respectivement en un point Oti, Otm et Ote distant du centre O d'une distance respectivement égale à di, dm et de. Les troisièmes chemins de roulement circulaires Cti, Ctm et Cte ne sont donc pas concentriques entre eux. Le troisième chemin de roulement intérieur Cti est porté par une troisième couronne radialement intérieure Ti, le troisième chemin de roulement médian Ctm est porté par une troisième couronne médiane Tm et le troisième chemin de roulement extérieur Cte est porté par une troisième couronne radialement extérieure Te. Le dimensionnement et le fonctionnement de cet engrenage plat l' obéissent aux mêmes règles que celles décrites précédemment en liaison avec l'engrenage 1 de la figure 1. The invention relates to a multi-report power transmission system comprising a gear (or disc) having on one of its faces at least two gear rings, separated and concentric with an axis of rotation X, the radially inner ring having a number of teeth equal to Ni and defining a raceway with radius Ri and the radially outer crown having a number of teeth N2 greater than Ni and defining a raceway with radius R2 greater than Ri, a straight pinion mounted on a shaft having an axis of rotation Y perpendicular to the axis X and capable of meshing with one or the other of said crowns and means for moving said pinion from a path of shift to another to change the transmission ratio. Gearboxes generally have several spur gears, and the heavier and more voluminous they are the higher the number of reports, which complicates the installation of these gearboxes on the devices fitted with them. On bicycles, the transmission is achieved by a chain driven by a bottom bracket and driving a wheel sprocket. To obtain several transmission ratios, the wheel is equipped with a plurality of pinions and the crankset can comprise several plates. The gear change is achieved by devices called derails. Due to their weight, size and cost, it does not appear that gearboxes have ever been mounted on bicycles. WO 02/35117 represents the state of the art closest to the invention and shows in FIG. 2, a flat disc which has, on one of its faces, three concentric crowns of teeth, defining three raceways possible for a straight pinion mounted on a shaft with an axis perpendicular to the axis of rotation of the disc. The crowns are separated from each other by a gap, except in two diametrically opposite zones where two adjacent teeth of each of the crowns are connected to the corresponding teeth of the adjacent crowns in order to form a radial groove between these two sets of teeth in which a tooth of the right pinion slides during its movement from one raceway to another. It is understood that if the disc is stopped, it is possible to slide the pinion on a shaft provided that the groove is positioned parallel to the shaft. On the other hand, if the device is in operation, the sliding of the pinion can only be carried out during the very brief time interval during which the groove common to the three crowns is parallel to the shaft otherwise the right pinion straddles the following teeth of two crowns with the damage which may result from the fact, by construction, that the following teeth are not aligned. In addition, the change of speed being extremely rapid, it will produce considerable shocks, necessarily received by the teeth of the pinion and the crowns. The object of the invention is to propose a transmission system as defined in the introduction to this specification, which overcomes the drawbacks mentioned above. The object is achieved according to the invention by the fact that the transfer from one raceway to the other takes place during a time interval equal to a half-rotation of the gear (or disc). More specifically, the transmission system according to the invention is characterized in that the gear (or disc) has face teeth and comprises at least one third ring of teeth defining a third substantially circular raceway whose radius R3 is equal to (R2 + Rι) / 2 and whose center is offset from the X axis by a distance d equal to (R2 - Rι) / 2, this third ring having in its radially outer part, relative to the axis X, teeth common with the radially outer ring and in its radially inner part of the teeth common with the radially inner ring. Advantageously, the concentric tooth crowns, the third tooth crown and the pinion have the same module in order to facilitate engagement, which advantageously remains continuous. According to the invention, the gear (or disc) is flat, or not (gear or disc of flat, concave or convex shape), and the two crowns of facial teeth (crowns of flat, concave or convex shape) each have a even or odd number of teeth, or a crown has an even number of teeth while the other crown has an odd number of teeth.  Advantageously, the third ring has between its radially outer part and its radially inner part of the teeth allowing a gradual variation of the speed ratio. Preferably, the gear comprises a plurality N of concentric rings of teeth defining N raceway spaced apart by a distance equal to R2 - Ri, connected two by two by N - 1 third rings, which are concentric with one another or not ( if there are more than two third crowns, only part of them can be concentric). In addition, preferably, the pinion is slidably mounted on an axially fixed spline shaft, said pinion being moved by a mechanical speed change device. Preferably, said gear (or disc) is flat and said pinion is straight. If several gears (or discs) are associated, they are preferably flat and coplanar. One can also provide a (or) gear (s) (or discs) non-flat (s), that is to say a gear (or disc) of concave or convex shape, associated with a pinion with straight teeth. Preferably, the axes X and Y are intersecting. It can also be provided that the axes X and Y are not intersecting, forming between them an offset or "offset". Other advantages and characteristics of the invention will emerge on reading the following description given by way of example and with reference to the drawing in which: FIG. 1 shows the upper face of a flat gear (or disc) according to the invention. 'invention which comprises three concentric toothing rings of axis X connected by two concentric crowns of teeth whose axis is offset from the axis X by a distance equal to half the distance between two crowns; Figure 2 is a section through a transmission system according to a first embodiment, taken along line II II of Figure 1; Figure 3 is identical to Figure 1 and further shows the eccentric used to move back and forth the slider which allows to control the movement of the right pinion on its shaft in order to change gear ratio;  Figure 4 shows in developed the grooves formed in the sleeve surrounding the shaft of the spur gear; Figures 5a to 5d show the different positions occupied by the slide rod and the sleeve during a gear change controlled by the rotation of the sleeve; FIG. 6 shows a device for transmitting power between a driving shaft and a driven shaft coaxial with the driving shaft by means of the transmission system comprising a single flat gear; FIG. 7 shows another device for transmitting power between a driving shaft and a driven shaft parallel to the driving shaft by means of a transmission system comprising two flat gears; Figure 8 is a simplified exploded perspective view of a transmission system according to a second embodiment; Figure 9 is a perspective view of the transmission system of Figure 8 after assembly; Figure 10 is a section through the transmission system, taken along the line X-X in Figure 9; Figure 11 is a view similar to that of Figure 10 for an alternative embodiment of the transmission system according to the second embodiment; FIGS. 11A and 11B showing other details of this variant; and FIG. 12 shows the upper face of another flat gear (or disc) which comprises four concentric crowns of teeth of axis X connected by three crowns of non-concentric teeth whose axis is offset from the axis X ' a different distance. Figure 1 shows a flat gear 1, consisting of a disc 2 of axis X, defining the axis of rotation of a shaft passing through the disc 2 at its center O. On the upper face 2a of the disc 2 are provided three crowns concentric teeth, with center O, referenced, Ci for the radially inner crown, Cm for the middle crown and Ce for the radially outer crown. These three crowns define raceways, centered in O, the middle circles of which are referenced respectively Chi, Chm and Che for the crowns Ci, Cm and Ce.  The middle circles of two adjacent raceways are spaced by a constant distance 2d, equal to R2 - Ri, R2 being the radius of the center circle of the largest raceway and Ri being the radius of the center circle of the smallest raceway. As can be seen in FIG. 1, the raceways have a width clearly less than 2d, so that on the upper face 2a of the disc 2, flat rings free of teeth separate the raceways. The raceways Chi, Chm and Che are connected to each other on the upper face 2a, by two third circular raceways, referenced Ti and Te, centered at a point Ot distant from the center O by a distance equal to d, c 'is to say equal to (R2 - Rι) / 2. The third inner raceway Ti connects the inner raceway Ci to the middle raceway Cm, and its middle circle Cti to a radius equal to (R2 + Rι) / 2. The third outer raceway Te connects the middle raceway Cm to the outer raceway Ce, and its middle circle Cte has a radius equal to (R2 + Rι) / 2, that is to say equal to half of the sum of the median ribs Chm and Che. These third raceways Ti and Te are also in the form of toothed rings centered in Ot, and the teeth of which are oriented towards the center O. The teeth of the raceways Ci, Cm and Ce are oriented towards the center Ot. These teeth are cut in such a way that they can mesh with the teeth of a spur gear 10 rotating around an axis Y perpendicular to the axis X. The internal raceway Ci has fewer teeth than the raceway median Cm which itself has fewer teeth than the outer raceway Ce. It is thus understood that the speed ratio between the number of revolutions around the axis X of the disc 2 and the number of revolutions of the right pinion 10 around the axis Y varies according to the raceway or track with which the pinion meshes. right, when operating the system. The width of the third raceways Ti and Te is preferably equal to that of the raceways Ci, Cm and Ce.  The third inner raceway Ti has a radially inner part 11, which straddles the inner raceway Ci. In this zone, the teeth cut in the third raceway Ti are in the extension of the corresponding teeth of the raceway inner race Ci. The third inner race Ti has a radially outer part 12 which straddles the middle race Cm. In these regions, the teeth cut in the third internal raceway Ti are in the extension of the corresponding teeth of the median raceway Cm. In the area 13 of the third raceway Ti, connecting the radially inner part 11 to the radially outer part 12, the teeth of this third raceway Ti are directed towards the center O and are cut so as to allow a gradual variation of the speed ratio, when the right pinion 10 borrows this zone 13, between the ratios obtained when it rolls on the internal raceway Ci and the median raceway Cm. The third external raceway Te has a general configuration similar to that of the third internal raceway Ti. It also includes a radially inner zone 11 where the teeth are common with those of the median raceway Cm, a radially outer zone 12 where the teeth are common with those of the outer raceway Ce, and an intermediate zone 13 located outside raceways Cm and Ce in which the teeth are cut so as to allow a progressive variation of the speed ratio. In the example of FIG. 1, the three concentric toothed crowns, namely the radially inner crown Ci, the middle crown Cm and the radially outer crown Ce have respectively 29, 40 and 66 teeth: it is therefore understandable that it is possible , according to the present invention to achieve all the speed ratios, between crowns comprising either an even or odd number of teeth Nj. In addition, from the above, it is understood that the change of speed or change of reduction between two adjacent crowns is carried out without interruption of movement, while the pinion 10 makes a half-turn of gear 1, that is to say a rotation of 180 °. This change of speed is possible thanks to the third ring, intermediate between the two adjacent rings, which synchronizes the movement of the pinion 10 from the first speed to the second speed, with a permanent engagement. The disc 2 with its teeth can be made of any rigid material, either by machining or by pressure molding depending on the applications. It can in particular be produced by injection of a thermosetting plastic matrix. The number of raceways can obviously be different from three, depending on the number of gear ratios desired: two raceways or more than three raceways and in particular four raceways. What counts above all is that the shape of the teeth of the different tracks allows normal meshing with the same spur gear 10. FIG. 2 shows a gear change device using the flat gear 1 described above, according to a first embodiment. This device comprises a housing casing 20, on which are journalled a driving or driven shaft 21 of axis X secured to the disc 2, and a second driven or driving shaft 22, of axis Y perpendicular to the axis X. The second shaft 22 extends practically as far as shaft 21, and its internal portion 23 located above the disc 2 has grooves, to allow the displacement along the Y axis of the right pinion 10 whose internal bore has ribs cooperating with the splines of the shaft. The right pinion 10 is retained axially on the shaft 22 by means of a fork 24 secured to a sleeve 25 surrounding the internal part of the shaft 22. This sleeve 25 is movable in rotation around the axis Y and in translation on the shaft 22, in order to allow the positioning of the right pinion 10 on one of the tracks Ci, Cm and Ce and its transfer from one track to the other by rolling on one of the third tracks Ti and Te bearing. The disc 2 is equipped with an eccentric which moves in a radial direction relative to the axis X a slide 31 guided by a radial groove 32 formed in the casing 20. The eccentric consists of a disc 33 placed under the disc 2 and off-center from the axis X by a distance d. The axis of the disc 33 is concentric with the third raceways Ti and Te. A connecting rod 34 whose head 36 surrounds the disc 33, drives the slide 31 back and forth in the groove 32 over a distance 2d, which corresponds to the distance between two median circles Chi, Chm and Che of the raceways Ci, Cm and Ce. The groove 32 is arranged outside the geometric cylinder of axis X surrounding the disc 2 and below the shaft 22. An example of configuration of the connecting rod 34 is shown in FIG. 3, as well as the extreme positions of the slide 31 in the groove 32. The slide 31 has a rod 35 directed towards the shaft 22 and the free end of which is housed in a groove among several grooves parallel to the axis Y and formed in the wall of the sleeve 25. As as seen in FIG. 4, the wall of the sleeve 25 has three axial grooves 40a, 40b, 40c, parallel to the Y axis, circumferentially offset around the Y axis and axially in the direction of the Y axis, the ends of two adjacent grooves 40a and 40b, 40b and 40c, being connected by circumferential grooves 41a and 41b whose edges 42 and 43 can cooperate with the end of the rod 35 to axially move the sleeve 25 by a distance 2d in one way or the other so to change the raceway Ci, Cm, Ce of the right pinion 10, this pinion rolling on a portion of a third raceway Ti, Te during this movement. In FIG. 4 and in FIG. 5a, the end of the rod 35 is positioned in the left axial groove 40a. The length of this groove 40a is equal to the distance 2d increased by the diameter of the rod 35. During the back-and-forth movement of the slide 31, as a result of the rotation of the disc 2 around the axis X, the end of the rod 35 slides in the groove 40a between its left end 42 and the right edge 43 of the circumferential groove 41a. In this situation the sleeve 25 is immobilized around the shaft 22 of axis Y, and the right pinion rolls on the external raceway Ce. FIGS. 5a to 5d show the way of moving the right pinion 10 from the outer raceway Ce towards the middle raceway Cm by taking the third outer raceway Ti. If the rod 35 abuts against the right wall 43 of the groove peripheral as seen in FIG. 5a, and if the sleeve 25 is pivoted around the axis Y in the direction of the arrow F, the end of the rod 35 is positioned against the left wall 42 of the circumferential groove 41a. The return movement of the rod 35 to the other extreme position will cause the sleeve 25 to move to the left and the right pinion to roll over half the course of the third external raceway Te. Then the end of the rod 35 moves back and forth in the axial groove 40b, and the right pinion 10 meshes with the central raceway Cm. If the sleeve is again pivoted in the same direction when the rod 35 is in abutment against the right wall 43 of the peripheral groove 41b, the sleeve 25 will again move to the left by a distance 2d during a half -turn of rotation of the disc 2, and the right pinion 10 will roll on the internal raceway Ci. To move the right pinion from a raceway of small diameter towards a raceway of larger diameter, we rotate the sleeve 25 in the opposite direction when the end of the rod 35 abuts against the left wall 42 of a peripheral groove 41b or 41a. FIG. 6 shows a transmission system which comprises a single flat gear 1 associated with two diametrically opposite shafts 22 and 22 'and each carrying a spur gear 10, 10', a sleeve 25, 25 '. The rotation of one or the other of the sleeves causes the displacement of the corresponding right pinion by a rod 35, 35 'actuated by a connecting rod 34, 34'. With three raceways Ce, Cm and Ci, 7 speed ratios are obtained, one of these ratios, equal to 1, being obtained by the cooperation of the pinions 10 and 10 'with the same raceway. One of the shafts 22, 22 'is leading, the other is driven, and the two shafts rotate in opposite directions, with respect to the common axis Y. FIG. 7 shows another transmission system, comprising two gears dishes 1 and the coplanar. The pinions 10 and 10 'are mounted on the same Y-axis shaft 22, the shafts 21 and 21' of the discs 2 and 2 'rotate in opposite directions. One of these trees is leading and the other is led. Here too we get 7 speed reports. In the examples described above, the axis Y of the shaft 22 intersects the axis of rotation X of the disc 2. It is obvious that the axis Y can be offset relative to the X axis in order to constitute an offset or “offset”. In this case, the teeth of the tracks Ci, Cm and Ce and the teeth of the tracks Ti and Te are not oriented towards the center O, but are cut so that the teeth which mesh with the pinion 10 are oriented in the direction of the Y axis. In addition, on the shaft 21 of the disc 2, shown in FIGS. 2, 6 and 7, it is possible to mount an additional disc, identical to the disc 2, whose teeth are located on its underside, mesh with the upper teeth of the pinion 10, and which, thanks to a support bearing carried by the frame 20, makes it possible to compensate for the vertical forces exerted on the pinion 10, during the transmission of the torque between the shaft 21 and shaft 22. This additional disk rotates around the X axis in a direction opposite to that of disk 2, but it allows a better balance of forces present and thus allows to increase the power transferable. Figures 8 to 10 illustrate in a simplified manner a speed change device using the flat gear 1 described above, according to a second embodiment. Thus, the same reference signs are used for the same elements as those described above in relation to the first embodiment of Figures 2 to 5. In these Figures 8 to 10, the disc 2 is shown schematically as an annular element forming a crown but it must be considered that it corresponds in particular to the example of FIG. 1. Also, in these figures 8 to 10, the casing of the housing 20 and the first shaft 21 are not shown: it should be understood that the disc 2 is mounted in rotation about the vertical axis X which is fixed relative to the casing 20. In this second embodiment, we find the eccentric 33 which is here surrounded by the head 36 of the connecting rod 34, a part lower diameter of the disc 2 being housed in the opening of the eccentric 33. The head 36 of the connecting rod 34 is provided with a fairly large opening surrounding the eccentric 33 in order to allow a translational movement lon the direction Y of the connecting rod 34. As in the case of the first embodiment, the other end of the connecting rod 34, not including the head 36, is secured to a slide secured to a vertical rod 35 parallel to the X axis. We find the pinion 10, able to mesh with the upper surface of the disc 2 forming the gear (flat in this example), and which is mounted, by means of cooperation by grooves and grooves, at the end of the second shaft 22 extending in the direction Y. According to this second embodiment, the sleeve 25 of the first embodiment is replaced by two forks 126 integral with the shaft 22 and between them by a speed selection plate 125 equipped with a series of grooves similar to the developed grooves visible in FIG. 4, namely three first axial grooves 40a to 40c parallel to the Y axis and two second axial grooves 41a and 41b parallel to a direction orthogonal to the Y axis. the two second axial grooves 41a and 41b are thus orthogonal to the first axial grooves 40a to 40c. In this second embodiment, the radial groove 32 parallel to the Y axis is formed in a speed change control plate 127 which is arranged under the selection plate 125, so that the radial groove 32 is parallel to the axis Y and to the first three axial grooves 40a to 40c. In order to carry out the mounting between the rod 35, the plate 125 secured to the shaft 22 and carrying the pinion 10, and the plate 127, a speed selection box 128 is used. This box 128 retains the rod 35, and the plates 125 and 127 so that the rod 35 remains in the radial groove 32 and in one of the first axial grooves 40a to 40c. More specifically, the housing 128 retains the plate 125 in the vertical direction along the axis X while allowing translation in the direction Y of the plate 125 and the elements which are integral with it (forks 126, shaft 22 and pinion 10). In addition, the housing 128 retains the control plate 127 in the vertical direction along the X axis while allowing it to perform a horizontal translational movement in a direction perpendicular to the Y axis in order to control the rod 35, when it is in one of the second axial grooves 41a and 41b, to pass from one of the first axial grooves 40a to 40c to one of its first axial grooves 40a to 40c adjacent. The mode of operation of this speed transmission system according to the second embodiment is therefore quite close to that of the first embodiment, the control not being carried out here by a rotation around the axis Y of the sleeve. 25, but by a horizontal translation of the control plate 127.  According to the present invention, the disc 2 should in reality not be limited to a gear of the flat type, but it may be in the form of a disc of gears of concave or convex shape but always with facial teeth, in particular with teeth right. FIG. 11 shows an example of an alternative embodiment of the transmission system of FIGS. 8 to 10, in which the flat disc 2 is replaced by a concave disc 202 carrying three concentric toothed rings around the axis X, with teeth facial, in particular with straight teeth, successively forming, from the X axis and in the radial direction towards the outside, the radially inner crown Ci, the middle crown Cm and the radially outer crown Ce. In this case, the pinion 10 has straight teeth allowing it to slide from one crown to another. An angle α is formed between the X axis and the surface of the concave disc carrying the three concentric rings Ce, Cm and Ci, or between the X axis and the Y axis formats the axis of rotation of the pinion 10. On the Figure 11, this angle α is around 45 ° but it can vary between a few degrees and almost 180 °. In the alternative embodiment illustrated in FIG. 11, in addition to the fact that it is concave, the disc 202 carries three concentric rings Ce, Cm and Ci which do not have the same difference between them as the difference 2d formed between the three crowns of the disc 2 of FIG. 1. In fact, as can be seen in FIG. 11, the difference 2di between the radially inner crown Ci and the middle crown Cm is greater than the difference 2de between the middle crown Cm and the radially outer crown Ce. In this case, as can be seen in FIG. 11, the connecting rod 34 is integral with a rod 133, the first end of which forms a slide in a radial groove 132 of the casing 20. The second end of the rod 133 serves as an axis material for a roller 234 housed in a groove 132 'formed on the outer surface of the disk 202. This groove 132' (see FIG. 11A) plays an eccentric role because its position along the axis of rotation X of the disk 202 varies by the distance 2di corresponding to the difference between the radially inner ring Ci and the middle ring Cm. In FIG. 11B is visible the speed selection plate 125 adapted to the variant embodiment illustrated in FIG. 11: the three first axial grooves 40a to 40c and the two second axial grooves 41a and 41b have different dimensions. The first axial groove 40a allows the rod 35 to move a distance 2de, while the first axial grooves 40b and 40c allow a displacement of a distance 2di. In addition, the grooves 40b and 40c are mutually parallel over a common length greater than the width of the rod 35, namely that they are connected by the second axial groove 41a, located on the left in FIG. 11B, which is wider than the groove 41b and the width of the rod 35: the edge 43 of the groove 41a is 2de distant from the edge 43 of the groove 41b while the edge 42 of the groove 41a is 2di from the edge 42 of groove 41b. Thus, one can achieve a displacement of the rod 34 always identical (2di), corresponding to the largest difference between the crowns, while the pinion is able to move between the three crowns Ce, Cm and Ci. D ' in general, it is understood that for the present invention, a set of crowns with face teeth is used forming a gear or disc of flat, concave or convex shape with the same module between them and the same module as the pinion 10 in order to allow the pinion 10 to be permanently driven either on one of the concentric rings corresponding to one of the Chi, Chm or Che raceways of one of the speeds, or on one of the portions of the transition rings forming the third internal raceway Ti and external raceway Te. Thus, in FIG. 12, another flat gear is shown which can be used in the transmission system according to the invention. This flat gear 1 'consists of a disc 2' of axis X defining the axis of rotation of a shaft (not shown) passing through the disc 2 'at its center O. On the upper face 2a' of the disc 2 'are provided four concentric toothed rings, of center O, formed, from the center O outwards, by: - a radially inner crown Ci (raceway Chi) having 29 teeth, - an inner middle crown Cmi (raceway bearing Chmi) with 52 teeth,  - an outer central crown Cme (raceway Chme) having 66 teeth, and - a radially outer crown Ce (raceway Che) having 87 teeth. The four toothed crowns Ci, Cmi, Cme and Ce therefore have alternately an odd and even number of teeth. The center circles of two adjacent raceways are spaced by a distance 2d, equal to R2 - RI, R2 being the radius of the center circle of the largest raceway and RI being the radius of the center circle of the raceway le smaller. In this example, this distance 2d is not constant, the smallest 2dm having been chosen between the median circles of the tracks Chmi and Chme, the largest 2de being situated between the median circles of the tracks Chme and Che, and an intermediate value having been chosen for the distance 2di situated between the median circles of the raceways Chi and Chmi. The four raceways Chi, Chmi, Chme and Che are connected to each other on the upper face 2a ', by three third circular raceways, referenced Cti, Ctm and Cte, centered respectively at a point Oti, Otm and Ote distant from center O of a distance respectively equal to di, dm and de. The third circular raceways Cti, Ctm and Cte are therefore not concentric with each other. The third inner raceway Cti is carried by a third radially inner ring Ti, the third middle raceway Ctm is carried by a third middle raceway Tm and the third outer raceway Cte is carried by a third radially outer race Te. The dimensioning and the operation of this flat gear obey the same rules as those described previously in connection with the gear 1 of FIG. 1.

Claims

REVENDICATIONS 1. Système de transmission de puissance multirapports comportant un engrenage (1 ; V) présentant sur l'une de ses faces au moins deux couronnes de dentures (Ci, Cm, Ce ; Ci, Cmi, Cme, Ce ), séparées et concentriques à un axe de rotation X, la couronne radialement intérieure ayant un nombre de dents égal à Ni et définissant un chemin de roulement de rayon i et la couronne radialement extérieure ayant un nombre de dents N2 supérieur à Ni et définissant un chemin de roulement de rayon R2 supérieur à Ri, un pignon droit (10) monté sur un arbre (22) ayant un axe de rotation Y perpendiculaire à l'axe X et susceptible d'engrener avec l'une ou l'autre desdites couronnes et des moyens pour déplacer ledit pignon (10) d'un chemin de roulement à l'autre afin de changer le rapport de transmission, caractérisé par le fait que l'engrenage comporte au moins une troisième couronne (Te, Ti ; Te, Tm, Ti) de dentures définissant un troisième chemin de roulement sensiblement circulaire dont le rayon R3 est égal à (R2 + Rι)/2 et dont le centre est décalé de l'axe X d'une distance d (de, dm, di) égale à (R2 - Rι)/2, cette troisième couronne ayant dans sa partie radialement extérieure (11), par rapport à l'axe X, des dents communes avec la couronne radialement extérieure et dans sa partie radialement intérieure (12) des dents communes avec la couronne radialement intérieure. CLAIMS 1. Multi-report power transmission system comprising a gear (1; V) having on one of its faces at least two gear rings (Ci, Cm, Ce; Ci, Cmi, Cme, Ce), separate and concentric to an axis of rotation X, the radially inner crown having a number of teeth equal to Ni and defining a raceway of radius i and the radially outer crown having a number of teeth N 2 greater than Ni and defining a raceway of radius R 2 greater than Ri, a straight pinion (10) mounted on a shaft (22) having an axis of rotation Y perpendicular to the axis X and capable of meshing with one or the other of said crowns and means for moving said pinion (10) from one raceway to another in order to change the transmission ratio, characterized in that the gear comprises at least a third ring gear (Te, Ti; Te, Tm, Ti) of teeth defining a third sen raceway so circularly whose radius R3 is equal to (R 2 + Rι) / 2 and whose center is offset from the axis X by a distance d (from, dm, di) equal to (R 2 - Rι) / 2 , this third crown having in its radially outer part (11), relative to the axis X, teeth common with the radially outer crown and in its radially inner part (12) teeth common with the radially inner crown.
2. Système selon la revendication 1, caractérisée par le fait que la troisième couronne (Ti, Te) comporte entre sa partie radialement externe (11) et sa partie radialement interne (12) des dents permettant une variation progressive du rapport de vitesse. 2. System according to claim 1, characterized in that the third ring (Ti, Te) has between its radially external part (11) and its radially internal part (12) teeth allowing a progressive variation of the speed ratio.
3. Système selon l'une quelconque des revendications 1 ou 2, caractérisé par le fait que l'engrenage comporte une pluralité N de couronnes concentriques de dentures définissant N chemins de roulementespacés d'une distance égale à R2 - Ri, raccordées deux par deux par N - 1 troisièmes couronnes. 3. System according to any one of claims 1 or 2, characterized in that the gear comprises a plurality N of concentric rings of teeth defining N raceways spaced apart by a distance equal to R 2 - Ri, connected two by two by N - 1 third crowns.
4. Système selon la revendication 3, caractérisé par le fait que les N - 1 troisièmes couronnes sont concentriques (Te, Ti). 4. System according to claim 3, characterized in that the N - 1 third rings are concentric (Te, Ti).
5. Système selon l'une quelconque des revendications 1 à 4, caractérisé par le fait que le pignon droit (10) est monté coulissant sur un arbre (22) à cannelures, axialement fixe, ledit pignon étant déplacé par un dispositif mécanique de changement de vitesse. 5. System according to any one of claims 1 to 4, characterized in that the straight pinion (10) is slidably mounted on a shaft (22) with splines, axially fixed, said pinion being moved by a mechanical change device of speed.
6. Système selon la revendication 5, caractérisé par le fait que le dispositif mécanique de changement de vitesse comporte un excentrique (33, 34) solidaire de l'engrenage (1) et décalé radialement par rapport à l'axe X d'une distance égale à (R2 - Rι)/2, un coulisseau (31, 35) guidé par une glissière (32) radiale et entraîné par ledit excentrique (33, 34) sur une distance égale à (R2 - Ri) et un organe de commande du coulissement du pignon droit (10) susceptible d'être actionné par ledit coulisseau (31). 6. System according to claim 5, characterized in that the mechanical speed change device comprises an eccentric (33, 34) integral with the gear (1) and offset radially with respect to the axis X by a distance equal to (R 2 - Rι) / 2, a slide (31, 35) guided by a radial slide (32) and driven by said eccentric (33, 34) over a distance equal to (R 2 - Ri) and a member for controlling the sliding of the right pinion (10) capable of being actuated by said slider (31).
7. Système selon la revendication 6, caractérisé par le fait que l'organe de commande du coulissement du pignon droit comporte une fourchette (24) à cheval sur le pignon droit (10), qui est solidaire d'un manchon (25) entourant l'arbre (22), ledit manchon comportant au moins deux rainures parallèles (40a, 40b, 40c) à l'axe Y dans chacune desquelles peut coulisser l'extrémité d'une tige (35) solidaire du coulisseau (31), lesdites rainures (40a, 40b, 40c) étant décalées circonférentiellement autour de l'axe Y, et axialement d'une distance égale à R2 - Ri, lesdites rainures étant raccordées à leur extrémités voisines par une rainure circonferentielle (41a, 41b) dont les bords (42, 43) peuvent coopérer avec ladite tige (35) pour assurer le déplacement axial dudit manchon (25) d'une distance (R2 - Ri), le changement de vitesse étant commandé par la rotation dudit manchon (25) lorsque ladite tige est positionnée dans une rainure circonferentielle (41a, 41b). 7. System according to claim 6, characterized in that the member for controlling the sliding of the right pinion comprises a fork (24) straddling the right pinion (10), which is integral with a sleeve (25) surrounding the shaft (22), said sleeve having at least two grooves parallel (40a, 40b, 40c) to the Y axis in each of which can slide the end of a rod (35) integral with the slide (31), said grooves (40a, 40b, 40c) being offset circumferentially around the axis Y, and axially by a distance equal to R 2 - Ri, said grooves being connected at their adjacent ends by a circumferential groove (41a, 41b) whose edges (42, 43) can cooperate with said rod (35) to ensure the axial displacement of said sleeve (25) by a distance (R 2 - Ri), the speed change being controlled by the rotation of said sleeve (25) when said rod is positioned in a circumferential groove (41a, 41b).
8. Système de transmission selon la revendication 7, caractérisé par le fait que l'organe de commande du coulissement du pignon droit (10) comporte une plaque de sélection de vitesse (125) solidaire de l'arbre (22), ladite plaque de sélection de vitesse (125) comportant au moins deux premières rainures axiales parallèles (40a, 40b, 40c) à l'axe Y dans chacune desquelles peut coulisser l'extrémité de ladite tige (35), lesdites premières rainures (40a, 40b, 40c) étant décalées axialement par rapport à l'axe X d'une distance égale à R2 - Ri, lesdites premières rainures étant raccordées à leur extrémités voisines par une deuxième rainure axiale (41a, 41b) dont les bords peuvent coopérer avec une tige (35) solidaire du coulisseau pour assurer le déplacement axial de ladite plaque de sélection de vitesse (125) d'une distance (R2 - Ri), le changement de vitesse étant commandé par la translation de ladite plaque de commande (127) lorsque ladite tige (35) est positionnée dans une deuxième rainure axiale (41a, 41b). 8. Transmission system according to claim 7, characterized in that the member for controlling the sliding of the right pinion (10) comprises a speed selection plate (125) integral with the shaft (22), said plate gear selection (125) comprising at least two first axial grooves parallel (40a, 40b, 40c) to the Y axis in each of which the end of said rod (35) can slide, said first grooves (40a, 40b, 40c ) being offset axially with respect to the axis X by a distance equal to R 2 - Ri, said first grooves being connected at their adjacent ends by a second axial groove (41a, 41b) whose edges can cooperate with a rod ( 35) integral with the slide to ensure the axial displacement of said speed selection plate (125) by a distance (R 2 - Ri), the speed change being controlled by the translation of said control plate (127) when said rod (35) is posi actuated in a second axial groove (41a, 41b).
9. Système de transmission selon l'une quelconque des revendications 5 à 8, caractérisé par le fait qu'il comporte deux pignons droits (10, 10') montés chacun de manière coulissante sur un arbre à cannelures (22, 22') et associés chacun à un dispositif de changement de vitesse, l'un des arbres étant menant et le deuxième arbre étant mené. 9. Transmission system according to any one of claims 5 to 8, characterized in that it comprises two straight pinions (10, 10 ') each slidably mounted on a spline shaft (22, 22') and each associated with a speed change device, one of the shafts being driven and the second shaft being driven.
10. Système de transmission selon l'une quelconque des revendications 5 à 9, caractérisé par le fait qu'il comporte deux engrenages (1, l'), et les pignons droits (10, 10') sont montés coulissant sur le même arbre (22) à cannelures, chaque pignon étant associé à un dispositif de changement de vitesse, l'un des engrenages étant menant et l'autre engrenage étant mené. 10. Transmission system according to any one of claims 5 to 9, characterized in that it comprises two gears (1, l '), and the pinions (10, 10') are slidably mounted on the same shaft (22) with splines, each pinion being associated with a speed change device, one of the gears being driving and the other gear being driven.
11. Système de transmission selon l'une quelconque des revendications 1 à 10, caractérisé par le fait que ledit engrenage (1 ; 1 est plat. 11. Transmission system according to any one of claims 1 to 10, characterized in that said gear (1; 1 is flat.
12. Système de transmission selon la revendication 11, caractérisé par le fait que lesdits engrenages sont plats et coplanaires. 12. Transmission system according to claim 11, characterized in that said gears are flat and coplanar.
13. Système de transmission selon l'une quelconque des revendications précédentes, caractérisé par le fait que les axes X et Y sont sécants. 13. Transmission system according to any one of the preceding claims, characterized in that the axes X and Y are intersecting.
PCT/FR2005/000355 2004-02-16 2005-02-16 Multi-gear transmission system WO2005080824A1 (en)

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
FR0401521 2004-02-16
FR0401521A FR2866404B1 (en) 2004-02-16 2004-02-16 MULTIPURPOSE TRANSMISSION SYSTEM USING FLAT GEAR

Publications (1)

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WO2005080824A1 true WO2005080824A1 (en) 2005-09-01

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FR (1) FR2866404B1 (en)
WO (1) WO2005080824A1 (en)

Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
FR2250410A5 (en) * 1973-11-05 1975-05-30 Scao Jean Marie Le Rotational speed control mechanism - spigoted wheel moves between coaxial tracks controlled by track points
FR2640342A1 (en) * 1988-12-14 1990-06-15 Claudeville Xavier Gearing for a constantly engaged speed varier
FR2742203A1 (en) * 1995-12-08 1997-06-13 Denance Raymond Gear box for motor vehicle transmission
WO2002035117A1 (en) * 2000-10-24 2002-05-02 Kim Cheol Soo Transmission using flat gear

Patent Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
FR2250410A5 (en) * 1973-11-05 1975-05-30 Scao Jean Marie Le Rotational speed control mechanism - spigoted wheel moves between coaxial tracks controlled by track points
FR2640342A1 (en) * 1988-12-14 1990-06-15 Claudeville Xavier Gearing for a constantly engaged speed varier
FR2742203A1 (en) * 1995-12-08 1997-06-13 Denance Raymond Gear box for motor vehicle transmission
WO2002035117A1 (en) * 2000-10-24 2002-05-02 Kim Cheol Soo Transmission using flat gear

Also Published As

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FR2866404A1 (en) 2005-08-19
FR2866404B1 (en) 2006-05-19

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