Abstract:
An automated transmission device with torque transfer having an input shaft ( 18 ) that supports drive pinions ( 20 ), each of which is engaged with a driven pinion ( 22 ) that is mounted freely rotating on a output shaft ( 12 ); controlled unidirectional locking mechanism ( 24 ) which are mounted between the shaft ( 12 ) and the driven pinions ( 22 ); and mechanical device for activating the locking mechanisms ( 24 ) in a positive manner including at least one cam ( 26 ) which translationally moves inside the output shaft ( 12 ).

Description:
BACKGROUND OF THE INVENTION 
   The present invention concerns an automatic transmission device with torque transfer, in particular for a motor vehicle, allowing a change in transmission ratio without interruption of a rotation torque between a driving shaft and a driven shaft. 
   DESCRIPTION OF RELATED ART 
   Transmissions with torque transfer are already known, comprising an input shaft which is connected to a driving shaft by a clutch and which carries driving pinions, a driven shaft which carries driven pinions, each driving pinion being constantly in engagement with a driven pinion and defining a transmission ratio, the driving pinions being rotationally fixed to the input shaft and the driven pinions being free to rotate on the driven shaft and being associated with synchronisation and clutching means which make it possible to selectively connect a driven pinion and the driven shaft, for engaging a given transmission ratio. 
   One of the driving pinions, for example that of the last transmission ratio, is connected to the input shaft by a clutch, the corresponding driven pinion being rotationally fixed to the driven shaft. By controlling the clutch of the driving pinion of the last transmission ratio, it is possible to control the speed of rotation of the input shaft of the transmission during a change in transmission ratio and to make it adopt a given value allowing the engagement of a new transmission ratio. 
   The clamping of the clutch of the driving pinion of the last transmission ratio during a change in ratio also makes it possible to maintain a driving torque on the driven shaft during the change in ratio. 
   In the known torque-transfer transmission devices, controlling the synchronisation and clutching means is relatively tricky, the declutching having to take place when the driving torque of the driven pinion is substantially zero and the clutching having to take place when the speed of rotation of the driven pinion is equal to that of the driven shaft, so that the means controlling these synchronisation and clutching means must be relatively very precise and are relatively complex. 
   An automatic transmission with torque transfer is already known in which the usual synchronisation and clutching means are replaced by unidirectional locking mechanisms of the toothed wheel and pawl type, which are controlled by magnetic means comprising permanent magnets carried by a movable rod and cooperating by magnetic attraction or repulsion with permanent magnets carried by the toothed wheels and pawls of the unidirectional locking mechanisms. This “contactless” magnetic control has the drawback of not guaranteeing the movement of the pawls between their idle and service positions, the control rod carrying the permanent magnets being freely movable with respect to the pawls, even if the latter do not move as expected. 
   SUMMARY OF THE INVENTION 
   The aim of the invention is in particular to avoid these drawbacks of the prior art, whilst simplifying the control and actuation means used in an automatic transmission with torque transfer for a change in transmission ratio. 
   It proposes for this purpose a device of the aforementioned type, intended to connect a driving shaft and a driven shaft and comprising an input shaft, driving pinions mounted on the input shaft, driven pinions mounted on the driven shaft, each driving pinion being constantly in engagement with a driven pinion and defining a transmission ratio, one of the pinions of each ratio being rotationally fixed to its shaft and the other being free to rotate on its shaft, at least one clutch connecting the driving pinion of the last transmission ratio to the input shaft, and means of selective connection of each free pinion and its shaft for the engagement of the corresponding transmission ratio, these means comprising, for at least one free pinion, two controlled unidirectional locking mechanisms, mounted in opposite directions between the free pinion and its shaft in order to prevent a speed of rotation of the free pinion which is lower or respectively higher than the speed of rotation of its shaft and each being able to occupy two positions, one idle and the other service, characterised in that these locking mechanisms are associated with mechanical actuation means comprising at least one cam able to be moved in translation and/or rotation which acts positively on the said mechanisms in order to bring them into the service position, means of controlling the actuation means and the said clutch being provided for controlling the speed of rotation of the input shaft according to the change in transmission ratio to be effected, the change in ratio comprising a step of passing from an initial state in which a ratio is engaged, to an intermediate state in which the free pinion of the engaged ratio and the free pinion of the ratio to be engaged each have a mechanism for locking in the service position and a mechanism for locking in the idle position, a step of passing from this intermediate state to a final state in which the mechanisms for locking the free pinion of the ratio to be engaged are in the service position and those of the free pinion of the previously engaged ratio are in the idle position, and a step of controlling the clutch which takes place between the initial state and the final state. 
   The actuation by cam of the aforementioned unidirectional locking mechanisms guarantees the movement of these mechanisms into their service position by the movement of the cam which is itself controlled positively by mechanical means. The result is great security in functioning, allowing the automatic control of the transmission. In addition, the sequence of operations executed for a change in ratio is rapid and precise by virtue of the clutch control which makes it possible to control the speed of rotation of the input shaft in the required fashion for actuating the locking mechanisms. 
   According to another characteristic of the invention, the changes in transmission ratio which do not concern the last ratio comprise the same sequence of operations of actuating the unidirectional locking mechanisms for the increase and decrease in the ratios, this sequence being executed in a given order for the increase in the ratios and in the reverse order for the decrease in the ratios. 
   Because of this, the means of actuating the aforementioned unidirectional locking mechanisms can comprise a single actuator, which acts in one direction for an increase in the ratios and in the other direction for a decrease in the ratios. The result is great simplification of the control of the transmission. 
   In a first embodiment of the invention, the free pinions are disposed in the order of the transmission ratios and the device comprises a single cam able to be moved in translation inside the shaft carrying the free pinions. 
   In another embodiment, the free pinions are disposed on their shaft in the order  1 ,  3 ,  5 , . . . ,  2 ,  4 ,  6 , . . . and the device comprises two cams able to be moved simultaneously in translation inside the said shaft, one for the actuation of the locking mechanisms for the free pinions for the ratios  1 ,  3 ,  5 , the other for the actuation of the locking mechanisms for the free pinions for the ratios  2 ,  4 ,  6 , . . . . 
   Advantageously, the two cams are carried by the same support and the travel of each cam is reduced by half compared with the embodiment comprising only one actuating cam for the aforementioned mechanisms. 
   In another embodiment of the invention, the free pinions are disposed on their shaft in the order  1 ,  4 ,  2 ,  5 ,  3 ,  6 , the device then comprising three cams able to be moved simultaneously in translation inside the shaft carrying the free pinions, one for the actuation of the locking mechanisms for the free pinions for ratios  1  and  4 , the second for the actuation of the locking mechanisms for the free pinions for ratios  2  and  5 , and the third for the actuation of the locking mechanisms for the free pinion for ratio  3 . 
   The invention thus makes it possible to vary the relative arrangement of the transmission ratios in a transmission device with torque transfer and therefore to choose a configuration according to the constraints imposed by other factors. 
   In a preferred embodiment of the invention, the cams are able to be moved in translation by a nut and screw system, where the nut carries the cams and the screw is a threaded rod driven in rotation by an actuator such as an electric motor. 
   Advantageously, the nut is guided in translation and immobilised with respect to rotation on a fixed axial rod on which it is engaged at one end, and screwed at its other end on the said threaded rod. 
   The cams can be fixed in rotation, or turn in synchronism with the shaft in which they are mounted. 
   In a particular embodiment of the invention, the cam or cams are mounted by screwing on a threaded rod fixed in translation and connected to a drive member by an epicyclic gear, preferably a double epicyclic gear. 
   In this case, the cam turns the driven shaft and remains immobile in translation with respect to the driven shaft whilst the drive member does not make the threaded rod turn in one direction or the other. 
   According to another characteristic of the invention, the transmission device comprises another clutch connecting the drive shaft to the input shaft, the pinion driving the first transmission ratio being rotationally fixed to the input shaft and the associated driven pinion being able to be rotationally connected with the driven shaft by two unidirectional locking mechanisms of the aforementioned type. 
   In a variant, the said other clutch connects the input shaft to the driving pinion of the first transmission ratio, whose driven pinion is rotationally fixed to the driven shaft. 
   This other clutch serves mainly for starting the vehicle. 
   Advantageously, filtering the vibrations are provided on the driving shaft and/or on the input shaft of the device according to the invention. 
   In general terms, the invention combines the advantages of automatic transmissions with hydrokinetic couplers and planetary gear trains and those of manual gear boxes, whilst avoiding their respective drawbacks. 
   The invention will be better understood and other characteristics, details and advantages thereof will emerge more clearly from a reading of the following description, given by way of example with reference to the accompanying drawings, in which: 

   
     BRIEF DESCRIPTION OF DRAWINGS 
       FIG. 1  is a schematic representation of a first embodiment of a transmission device according to the invention; 
       FIG. 2  is a partial schematic view of a variant embodiment of the invention; 
       FIG. 3  is an axial schematic view of a unidirectional locking mechanism according to the invention; 
       FIG. 4  is a graph showing diagrammatically the function of the unidirectional locking mechanisms for several transmission ratios; 
       FIG. 5  is a graph illustrating the four possible cases of change in transmission ratio according to the rotation torque applied to the driven shaft and the increase and decrease in the ratios; 
       FIGS. 6 and 7  are tables showing the states of the unidirectional locking mechanisms in two possible cases of change in transmission ratio; 
       FIGS. 8 and 9  are graphs illustrating the clutch commands during changes in transmission ratio corresponding to the tables in  FIGS. 6 and 7  respectively; 
       FIG. 10  is a schematic representation of a variant embodiment of the device according to the invention; 
       FIG. 11  is a partial schematic representation in axial section of the part of the device comprising the driven shaft; 
       FIG. 12  is a partial schematic view illustrating a different arrangement of the transmission ratios in a device according to the invention; 
       FIG. 13  is a schematic representation of another variant embodiment of the invention; 
       FIG. 14  is a schematic view in axial section of this variant; and 
       FIG. 15  is a schematic representation of another variant embodiment of the invention. 
   

   DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
   Reference is made first of all to  FIG. 1 , which shows schematically a first embodiment of an automatic transmission device with torque transfer according to the invention, intended to connect the output shaft  10  of a thermal engine M, such as in particular an internal combustion engine for a motor vehicle, to a driven shaft  12  which is the output shaft of the device according to the invention and which, in the case of a motor vehicle, is intended to drive driving wheels  14  in rotation by means of a differential  16 . 
   The device according to the invention comprises an input shaft  18  which is connected to the output shaft  10  of the motor M by a clutch E 1 . The input shaft  18  is parallel to the output shaft  12  and carries driving pinions  20 , each of which is in constant engagement with a driven pinion  22  mounted on the output shaft or driven shaft  12 , the pairs of pinions in engagement  20 ,  22  defining transmission ratios which are numbered from  1  to  6  and which are progressively increasing, the pinions of the various transmission ratios being mounted in the order of these ratios on the shafts  18  and  12 . 
   In this example embodiment, the driving pinions  20  for ratios  1  to  5  are rotationally fixed to the input shaft  18  and the driven pinions of these ratios are free to rotate on the driven shaft  12  and can be rotationally fixed thereto by means of controlled unidirectional locking means  24 , each free pinion  22  being associated with two controlled unidirectional locking mechanisms  24  which are mounted in opposite directions between the pinion  22  and the shaft  12 , one of these mechanisms being intended to prevent the free pinion  22  from turning more quickly than the driven shaft  12 , the other mechanism being intended to prevent the free pinion  22  from turning less quickly than the shaft  12 . 
   The driving pinion  20  of the sixth transmission ratio is free to rotate on the input shaft  18  and is rotationally fixed thereto by means of a clutch E 2 , which is for example mounted on the end of the input shaft  18  on the side opposite to the input clutch E 1 . 
   The driven pinion  22  of the sixth transmission ratio is rotationally fixed to the driven shaft  12  and does not have any controlled unidirectional locking mechanism  24 . 
   The actuation means of the mechanisms  24  are advantageously housed within the driven shaft  12  and comprise at least one cam  26  mounted on an axial rod  28 , one end of which is connected to an actuator  30 , outside the driven shaft  12 . A system  32  for managing and controlling the device according to the invention is connected to the clutches E 1  and E 2  and to the actuator  30  for controlling the changes in transmission ratio and is also connected to means of controlling the motor M for exchanges of information or actions. 
   The cam  26  can be mounted so as to be fixed in rotation within the driven shaft  12  and is then moved in translation inside the shaft  12 , between the unidirectional locking mechanisms  24  for the free pinions  22  of the first five transmission ratios by the axial rod  28  which is itself driven by the actuator  30  in translation inside the shaft  12  or in rotation, the axial rod  28  then being a threaded rod fixed in translation, on which the cam  26  is mounted by screwing. 
   A particular embodiment of the unidirectional locking mechanisms and their actuation means is depicted in detail in  FIGS. 2 and 3 . 
   In this example embodiment, an internal set of teeth  36  is formed in a large face of each free pinion  22  in order to cooperate with at least two pawls  38  rotationally fixed to the shaft  12  and each of which is able to be moved by a pusher  40  between an idle position in which it is away from the set of teeth  36  on the pinion  22  and a service position in which it is in engagement with the set of teeth, preventing the rotation of the pinion  22  about the shaft  12  in a given direction, which is indicated by an arrow close to each pawl  38 . 
   The pushers  40  are guided in radial holes in the cylindrical wall of the shaft  12  and are able to be moved radially outwards by the cam  26 . The pushers  40  act on the pawls  38  by means of elastically deformable means  42  such as compression springs, which are mounted between the pushers  40  and the pawls  38 . 
   Advantageously, the pushers  40  are tubular and the springs  42  are partly housed inside the pushers  40  and bear on the bottom of these pushers. 
   Other elastic members  44  such as spring blades push the pawls  38  in the direction tending to move them away from the internal set of teeth  36  on the pinion  22 . 
   The pawls  38  and the spring blades  44  are mounted on bracing washers  46  which are rotationally fixed at the shaft  12  and which are arranged between the free pinions  22 , as depicted in  FIG. 2 . 
   The cam  26  is mounted on the rod  28  and is rotationally fixed to the shaft  12 , by means of a longitudinal rib  48  engaged in a corresponding groove on the internal surface of the shaft  12 . 
   The rod  28  is fixed in translation to the inside of the shaft  12  and is connected to the actuator  30  by an epicyclic gear and preferably a double gear such as the one shown in  FIG. 2 . 
   In this embodiment, the output shaft of the actuator  30 , which is for example a small electric motor, is connected to the sun pinion  50  of a first epicyclic gear, the planet carrier  52  of which is fixed to one end of the threaded axial rod  28  and the crown wheel  54  of which is fixed to that of a second epicyclic gear. The sun pinion  56  of this second gear is fixed with respect to rotation and its planet carrier  58  is rotationally fixed to the shaft  12 . The pinion ratios are the same in the two epicyclic gears, the first of which forms a differential and the second a reducing gear. Under these circumstances, the cam  26 , which is driven in rotation by the shaft  12 , is fixed in translation to the axial rod  28  whilst the output shaft of the actuator  30  does not turn. The rotation of this output shaft in one direction or the other makes it possible to move the cam  26  in translation in one direction or the other inside the shaft  12 . 
   As the double epicyclic gear which connects the actuator  30  to the shaft  12  and to the threaded rod  28  transmits practically no force, it can be produced conveniently from plastics material, at a relatively very low cost. 
   In the example embodiment in  FIGS. 2 and 3 , the two controlled unidirectional locking mechanisms  24  which are associated with a free pinion  20  are on the same flank of the pinion and are diametrically opposed with respect to the axis of rotation. 
   In a variant embodiment, these two mechanisms could be one on one flank of the pinion and the other on the other flank of the pinion, each mechanism being able to have a single pawl or several. 
   The cam  26  is a substantially cylindrical tubular piece whose external surface is formed so as to comprise abutment surfaces making it possible to move the pushers  40  radially towards the outside when the corresponding locking mechanisms have to be in the service position. As will be seen below, one and the same cam can successively control all the locking mechanisms for the free pinions for the various transmission ratios, when it is moved in one direction, for an increase in the transmission ratios and, when it is moved in the other direction, for a decrease in the ratios. 
   Reference is now made to  FIGS. 4 to 9  in order to describe the functioning of the device according to the invention. 
   In the following, it is assumed that the speed of rotation of the shaft  12  is constant and it is in this case that the way in which the changes in transmission ratio are controlled and effected will be described. 
   Use will be made in this description of the following conventions, which are illustrated by  FIGS. 4 and 5 . 
     FIG. 4  is a graph depicting the staging of the rotation speeds of the driving shaft corresponding to the first four transmission ratios for a given constant speed of the output shaft  12 . The references A 1 , B 1 , A 2 , B 2 , A 3 , B 3  and A 4 , B 4  designate respectively the unidirectional locking mechanisms  24  associated with the driven pinions  22  of the first four transmission ratios and their actions on the pinions, according to the rotation torque transmitted to the output shaft  12 , which can be a positive driving torque designated by Cm&gt;0 exerting a traction on the vehicle or a negative driving torque designated by Cm&lt;0 exerting a braking on the vehicle, and a direction of variation in the speed of rotation of the driving shaft  10 , this speed being indicated by increasing values on the vertical axis in  FIG. 4 . 
   In this figure, the references A are allocated to the unidirectional locking mechanisms which prevent the free pinions  22  from turning more quickly than the output shaft  12  and the references B are allocated to the mechanisms  24  which prevent them from turning less quickly than the shaft  12 . 
     FIG. 5  shows schematically the four possible cases of change in transmission ratio according to the driving torque and the increase or decrease in the transmission ratios. 
   Cases I and II are those for which the rotation torque applied to the shaft  12  is positive, case I corresponding to the increase (change from ratio N to ratio N+1) and case II to that of decrease (change from ratio N to ratio N−1). 
   Cases III and IV are those where the driving torque applied to the shaft  12  is a negative torque (Cm&lt;0), case III being that of the decrease from the ratio N to the ratio N−1 and case IV that of the increase from the ratio N to the ratio N+1. 
   The table in  FIG. 6  shows the states of the unidirectional locking mechanisms for the free pinions of ratios N and N+1 in the case of the change from the ratio N to the ratio N+1 with a positive driving torque. The commands for clutches E 1  and E 2  are shown schematically in  FIG. 8  as a function of time. 
   Initially, the ratio N being engaged, the locking mechanisms associated with the driven pinion  22  of this ratio are in the service position, which corresponds to A N =B N =1, the clutch E 1  connecting the driving shaft  10  to the input shaft  18  being clamped or engaged, the clutch E 2  associated with the driving pinion of the sixth transmission ratio being released or declutched (as a general rule, the state at rest of a clutch is its engaged or clamped state, and its activated state is the declutched state). 
   Under these circumstances, the pawl or pawls of the mechanism A N  which prevent the driven pinion  22  of the ratio N from turning more quickly than the shaft  12  are in abutment on the internal set of teeth  36  on the driven pinion, because of the fact that the driving torque transmitted to the shaft  12  is positive. When a command to change transmission ratio is given in order to change to the higher ratio N+1, the pawl or pawls of the mechanism B N  which prevent the pinion  22  from turning less quickly than the shaft  12  are not in abutment on the internal set of teeth  36  on the driven pinion, and can therefore be disengaged from this set of teeth. As the driven pinion of the ratio N+1 is driven by its driving pinion at a speed of rotation which is greater than that of the shaft  12 , the mechanism B N+1  of this pinion can be brought into the service position. 
   A movement of the cam  26  inside the shaft  12  simultaneously controls the bringing to rest of the mechanism B N  and the bringing into service of the mechanism B N+1 . This then gives the state depicted in the second column of the table in  FIG. 6 . 
   Next, the clutch E 2  is actuated in order to make it pass from the partially engaged state, which will make it possible to transmit a torque through the driving pinion  20  of the sixth transmission ratio to the input shaft  18  and therefore slow down the speed of rotation of this input shaft, since the output shaft  12  which rotates at a constant speed is then connected to the input shaft  18  by the sixth transmission ratio ( FIG. 8 ). 
   The partial or progressive damping of the clutch E 2  therefore causes a reduction in the speed of rotation of the input shaft  18  to a value equal to the speed of synchronisation with the output shaft  12  by means of the ratio N+1. Under these circumstances, it is possible to bring the mechanism A N  to the idle state, since its pawl or pawls are no longer in abutment on the set of internal teeth on the associated pinion, and simultaneously to bring the mechanism A N+1  into the service position. 
   This then gives a state corresponding to the third column in the table in  FIG. 6 , illustrating the engagement of the ratio N+1. 
   The states A N =0 and A N+1 =1 are obtained simultaneously, by movement of the cam  26  in the same direction as before. 
   The clutch E 2  is next unclamped or declutched for complete engagement of the ratio N+1, with a resumption of abutment on A N+1 . 
   During the change from ratio N to N+1, a positive driving torque is transmitted continuously to the shaft  12 , first of all by the mechanism A N  of the ratio N as far as at least partial clamping of the clutch E 2 , and then by the sixth transmission ratio for a brief moment, and finally by the mechanism A N+1  of the ratio N+1, the total duration of the change in transmission ratio being around one second or less. 
   When the change in transmission ratio takes place under conditions corresponding to case II (positive driving torque applied to the output shaft  12  and change from the ratio N to the ratio N−1), the following procedure is followed, summarised in the table in  FIG. 7  and depicted schematically in  FIG. 9 , with a succession of operations controlling the locking mechanisms which is identical to that described for case I but which is performed in the opposite direction. 
   The starting point is the state corresponding to the first column in the table in  FIG. 8 , with A N =B N =1, the ratio N being engaged, and A N−1 =B N−1 =0. 
   When a command to change to the ratio N−1 is given, the clutch E 2  of the driving pinion of the sixth transmission ratio is partially clamped in order to transmit a torque at least equal to the driving torque and to reduce the speed of rotation of the input shaft  18 , as already indicated above, and the cam  26  is moved towards the pinion  22  of the ratio N−1 in order to bring the locking mechanism A N  to rest and the mechanism A N-1  into service, which corresponds to the second column in the table in  FIG. 7 . Next, the clutch E 2  is progressively unclamped, which transmits less torque, which has the effect of making it possible to increase the speed of rotation of the input shaft  18  and to come into abutment on A N−1 . Then the rapid unclamping of the clutch E 2  is finished and the cam  26  is moved once again, in the same direction, in order to bring the locking mechanism B N  to rest and the mechanism B N−1  into service, which corresponds to the third column in the table in  FIG. 7 . 
   By comparing the tables in  FIGS. 6 and 7 , it is noted that the sequences of operations controlling the locking mechanisms which are executed in order to change from the transmission ratio with a positive driving torque are opposite to each other for an increase in the transmission ratios and for their decrease. This assumes of course that the locking mechanisms are arranged in the same way on all the driven pinions  22  of the transmission ratios  1  to  5 . 
   When the driving torque applied to the output shaft is negative (the vehicle is under motor braking), cases I and II, which have just been described, are returned to, in order to change transmission ratio, by means of a temporary control of the motor M in order to give a positive value to the driving torque applied to the output shaft, this positive value preferably being fairly low. 
   The same procedure is followed when the driving torque applied to the shaft  12  has a value close to zero. This modification, of very short duration (less than one second for example), is not perceptible to the driver of the vehicle but makes it possible to use a single control law for all the cases of change in transmission ratio. 
   Consequently the same actuator controlled by the management system  32  makes it possible to effect all possible cases of change in transmission ratio. 
   When it is necessary to change from the fifth to the sixth transmission ratio with a positive driving torque, the procedure as described with reference to  FIGS. 6 and 8  is followed, but the clutch E 2  is left engaged in order to keep the sixth transmission ratio engaged. 
   In the opposite direction, when it is necessary to pass from the sixth to the fifth ratio, the procedure is followed as depicted in  FIG. 9 , without having to first of all actuate the clamping of the clutch E 2 . 
   In all cases, the clutch E 1  remains in the clamped state, this clutch being used solely for the starting of the vehicle (passage from neutral to the first transmission ratio). 
   This clutch also serves for passing from neutral to reverse. 
   In the variant embodiment depicted schematically in  FIG. 10 , the arrangement of the various pinions on the input and output shafts is identical to that described with reference to  FIG. 1 , but the driving pinion  20  of the first transmission ratio is free to rotate on the input shaft  8  and can be fixed with respect to rotation with the latter by means of the clutch E 1 . The driven pinion  22  of the first transmission ratio is fixed rotationally to the output shaft  12  and is not equipped with any aforementioned locking mechanisms  24 . 
   For the rest, this transmission device is identical to that of  FIG. 1 . 
   The functioning for the changes in transmission ratio is identical to that which has just been described, except with regard to the first transmission ratio, which is controlled directly by the clutch E 1 . The passage from the first to the second transmission ratio takes place by releasing the clutch E 1  and clamping the clutch E 2 , and then proceeding as depicted in  FIG. 8 . 
   The change from the second to the first transmission ratio takes place by first of all clamping the clutch E 2  and then, after setting A 2  and B 2  to idle, by clamping the clutch E 1  and simultaneously releasing the clutch E 2 . 
   In the variant embodiment depicted in  FIG. 11 , the output shaft  12  is supported at its ends by bearings mounted in a casing  62  and comprises an output pinion  64  at one of its ends. 
   Two cams  26  are mounted so as to rotate on a tubular support  66 , one end of which forms a nut screwed onto the end of a threaded axial rod  28 , the other end of which comprises a pulley  68 , a pinion or the like, connected by a synchronous transmission  70  to the actuator  30 , which is offset. 
   The other end of the tubular element  66  is guided in translation on a fixed axial rod  72  mounted on a wall of the casing  62  and comprising a longitudinal groove in which there is engaged a toe carried by the tubular element  66 , for immobilising this element with respect to rotation. 
   The axial rods  28  and  72  are aligned, almost end to end, inside the output shaft  12 . 
   In this device, the driven pinions  22  of the various transmission ratios are mounted in alternation on the output shaft  12 , in the order  1 ,  3 ,  5 ,  2 ,  4 ,  6 , the corresponding driving pinions being mounted in the same order on the input shaft. One of the cams  26  controls the locking mechanisms for the driven pinions of ratios  1 ,  3  and  5 , the other cam  26  controlling the locking mechanisms for the driven pinions of ratios  2  and  4 . 
   It can be seen in  FIG. 11  that the cams  26  which act on the diametrically opposed locking mechanisms of each free pinion  22  have symmetrical profiles with respect to a median point, situated on the rotation axis, for the actuation of the mechanisms in accordance with the sequences depicted in  FIGS. 6 and 7 . 
   In  FIG. 11 , the positions of the cams corresponding to the actuation of the locking mechanisms of a driven pinion are indicated by lines marked with figures surrounded by a circle. The lines marked by figures not surrounded by a circle indicate the positions adopted by a cam when it is the other cam which acts on locking mechanisms of a driven pinion. For example, the  FIG. 2  situated between  FIGS. 1 and 3  entered in circles indicates the position of the left-hand cam when the right-hand cam is in the position of actuation of the locking mechanisms of the driven pinion of the second ratio, this position being indicated by the  FIG. 2  surrounded by a circle. The small intermediate lines between the positions marked by figures corresponds to intermediate positions occupied by the cams during a change in transmission ratio. 
   This design has a reduced axial dimension, by virtue of the driving of the cams by a nut and screw system of reduced length, nevertheless allowing good axial guidance of the cams and rotational immobilisation of their support. 
   In the variant embodiment depicted schematically in  FIG. 12 , the driven pinions (and the driving pinions) of the various transmission ratios are disposed in the order  1 ,  2 ,  5 ,  3 ,  6  and the means of actuating the locking mechanisms associated with these pinions comprise three cams  26  able to be moved simultaneously in translation inside the shaft  12  by the axial rod  28 , each cam being able to occupy four positions which are represented by transverse broken lines. The left-hand cam  26  can actuate the locking mechanisms for the driven pinions of ratios  1  and  4 , the cam  26  in the middle can actuate the locking mechanisms for the driven pinions of ratios  2  and  5 , and the right-hand cam  26  can actuate the locking mechanisms for the driven pinion of the third ratio. 
   Various modifications can be made to the devices which have just been described, without departing from the scope of the invention. For example, the free pinions can be carried by the input shaft  18  rather than by the input shaft  12 . As a variant, some free pinions can have been mounted on the input shaft  18  whilst the others are on the output shaft  12 . It is advantageous for all the free pinions to be associated with controlled unidirectional locking mechanisms of the type described, but some of them could be equipped with usual synchronisation and clutching means, if necessary. 
   Naturally, the transmission device according to the invention comprises a reverse running ratio, of which a driving pinion is mounted on the input shaft  18 , of which an intermediate pinion for reversing the direction of rotation is mounted on an intermediate shaft parallel to the shafts  12  and  18  and of which a driven pinion is mounted on the output shaft  12  and is equipped either with usual synchronisation and clutching means, or controlled unidirectional locking mechanisms of the type indicated above. 
   In the embodiment depicted in  FIGS. 13 and 14 , the driving pinion of the reverse running ratio is the pinion  20  of the third transmission ratio, which is in engagement with a pinion  74  mounted fixed on the intermediate shaft  76 . An intermediate pinion  78  for reversing the direction of rotation is free in rotation on the intermediate shaft  76  and is associated with conventional synchronisation and clutching means  80  mounted on the intermediate shaft  76 . The intermediate pinion  76  is in engagement with a pinion  82  which is mounted fixedly on the output shaft  12  and which is therefore fixed rotationally thereto. The pinion  20  of the sixth transmission ratio is mounted fixedly on the input shaft  18  and its driven pinion  22  is connected to the output shaft  12  by controlled unidirectional locking mechanisms  24  of the type described above. 
   When the driven pinions  22  have a relatively small diameter, as is the case with the driven pinions of the fifth and sixth transmission ratios, it is not possible to use the method of mounting the locking mechanisms which is depicted in  FIGS. 2 and 3  and therefore an annular piece or a hub which comprises the controlled unidirectional locking mechanisms and which is juxtaposed axially with the corresponding driven pinion is therefore associated with these pinions. This is what is depicted in  FIGS. 13 and 14 , when the reference  84  designates the annular pieces or hubs comprising the unidirectional locking mechanisms for the driven pinions of the fifth and sixth transmission ratios. 
   The functioning of the device depicted in  FIGS. 13 and 14  is identical to the functioning of the other devices depicted in the previous figures. In these other embodiments, the clutch E 2  associated with the driving pinion for the sixth transmission ratio made it possible to have the lowest speed of the input shaft  18 . In the embodiment in  FIGS. 13 and 14 , it is the synchronisation means associated with the intermediate reverse pinion  78  which make it possible to give the input shaft  18  the lowest rotation speed: it is in fact possible, for this purpose, to brake the rotation of the input shaft  18  or to tend to drive it in the opposite direction. In the latter case, which corresponds to the embodiment in  FIGS. 13 and 14 , use is made solely of the synchronisation means of the reverse pinion  78 , but without using the clutching means. 
   In the variant embodiment depicted in  FIG. 15 , the rotation of the input shaft  18  is slowed down by means of a brake  86  carried by the casing of the transmission device and which acts on the input shaft  18 , for example at the end of this shaft situated on the side of the driving pinion  28  of the sixth transmission ratio. 
   For the rest, the same arrangement as in  FIGS. 13 and 14  is found, the driving pinion  20  of the sixth transmission ratio being fixed to the input shaft  18 . 
   Another variant embodiment consists of lowering the rotation speed of the input shaft  18  using motor control means, but this results in a slower reduction in the rotation speed of the input shaft  18 .