Abstract:
A variable speed transmission for motor vehicles includes multiple drive gears which are switchable by means of at least one gear changing device, in particular by means of synchronous shifting clutches, in particular multiple forward gears and a reverse gear. The variable speed transmission is subdivided or divided into at least two part-transmissions each with multiple gears, by at least one shifting device in particular least one shifting clutch in such a manner that at least one of the part-transmissions is engaged and at least one of the part-transmissions is disengaged as a function of one of the drive gears as selected by the gear changing device and/or as a function of defined shifting states of the shifting device.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     This application claims the priority of German Patent Application, Serial No. 10 2011 011 170.0, filed Feb. 14, 2011, pursuant to 35 U.S.C. 119(a)-(d). 
     BACKGROUND OF THE INVENTION 
     The invention relates to a variable speed transmission for motor vehicles. 
     For example EP 1 532 383 B1 discloses a variable speed transmission having an input shaft, a layshaft arranged axially parallel to the input shaft and a drive shaft coaxial to the input shaft and in which between a direct gear (highest forward gear) and the other gear wheels a shifting clutch is provided which in the direct gear uncouples the remaining gear wheels, so that the latter are stationary in the corresponding drive mode. This has the advantage that during driving in the direct gear smaller friction and churning losses occur at an improved transmission efficiency. In such a coaxial arrangement of input shaft and driven shaft such a decoupling can be constructed relatively easily; the improved transmission efficiency is only achieved in the highest gear. 
     SUMMARY OF THE INVENTION 
     The object of the invention is to propose a variable speed transmission with multiple forward gears and a reverse gear and preferably with non-coaxial arrangement of input shaft and driven shaft, in which improved efficiencies can be achieved over an extended driving operation with constructively simple means. 
     The solution of this objective is attained by a variable speed transmission for motor vehicles, with multiple pears, in particular multiple forward pears and a reverse gear which are switchable via pear wheel sets by at least one gear changing device, in particular by synchronous clutches, wherein the variable speed transmission is subdivided or divided by at least one shifting device into at least two part-transmissions each including multiple gears, so that at least one of the part-transmissions is engaged and at least one of the part-transmissions is disengaged as a function of one of the drive pears as selected by the pear changing device and/or as a function of defined shifting states of the shifting device. Advantageous refinements of the invention are set forth in the sub claims. 
     According to the invention, a variable speed transmission is proposed with multiple drive gears which are shiftable via gear sets by means of at least one gear changing device in particular by means of synchronous shifting clutches, in particular with multiple forward gears and a reverse gear. According to the invention, the variable speed transmission is divided into at least two part-transmissions by means of at least one shifting device, in particular by means of at least one shifting clutch, such that at least one of the part-transmissions is shifted and at least one of the part-transmissions is disengaged in dependence on a drive gear which is predetermined by means of the at least one variable speed transmission and/or in dependence of defined gear states of the at least one gear device. 
     For reasons of clarity and without intention to narrow the scope of protection defined by the claims, the advantages of the solution according to the invention are explained in the following by way of a concrete embodiment having two part-transmissions, in which the part-transmission which can be disengaged is referred to as part-transmission I and the part-transmission which is shifted active or can be shifted active respectively, when the part-transmission I is disengaged, is referred to as part-transmission II: 
     In particular when the longer transmission ratios are assigned to the part-transmission II and the shorter transmission ratios are assigned to the part-transmission I a driving with more favorable transmission efficiencies is possible over wide ranges of driving conditions, while in ranges of lower speeds of the motor vehicle the friction and churning losses are less severe because of the smaller time fraction and are thus negligible. The proposed solution of disengaging at least one part-transmission in the higher speed range significantly decreases the control and manufacturing effort required therefore. 
     For a variable speed transmission having four or more forward gears it is proposed that at least two highest not necessarily sequential gears are assigned to the part-transmission II. The lower forward gears and the reverse gear are turned off when driving at higher speed. In case of six or more forward gears, the four highest gears can be assigned to the part-transmission II so that the driving range with disengaged part-transmission I is significantly expanded. The proposed transmission layout can even be adjusted beforehand to a driving profile frequently occurring during operation of a motor vehicle. 
     In a structurally advantageous and relatively simple embodiment of the invention, an input shaft of the variable speed transmission can be divided functionally by only one shifting device, preferably by a shifting clutch, into two input shafts, which are assigned to the part-transmissions I and II via corresponding gear sets. According to a preferred refinement, it is proposed that the input shaft of the part-transmission carries the shiftable idler gears of the higher gears whose assigned fixed gears are supported on an driven shaft, while the input shaft of the part-transmission I includes the fixed gears of the lower gears, whose shiftable idler gears are supported on the common driven shaft. As a result, the part-transmission I is completely disengaged with regard to the meshing gear wheels at a minimal additional effort, and does not cause any losses in efficiency. 
     Further, the shifting device can be formed by a twin shifting clutch, which in a neutral position disconnects both input shafts of the part-transmissions, while in one shifting position the shifting clutch couples both input shafts to one another and in the other shifting position forms a reverse gear. For forming the reverse gear it is proposed that an idler gear or in general a reverse gear wheel connects an auxiliary shaft which has reversing gear wheels to a shiftable forward gear wheel which is preferably assigned to a lower gear. The shifting clutch therefore accomplishes in a constructively and functionally simple manner beside the decoupling function also the shifting of the reverse gear of the variable speed transmission. The ratchet of the reverse gear on the input shaft is possible because the auxiliary shaft enables a two-step transmission ratio. 
     In a modified construction of the variable speed transmission, the input shaft which is assigned to the part-transmission I can be a hollow shaft, through which the input shaft of the part-transmission II traverses, wherein the hollow input shaft in reverse drive can be coupled to the input shaft of the part-transmission II via the one shifting clutch. With this, the orientation of the part-transmissions I and II is reversed which may be advantageous for constructive reasons or for reasons of mounting in the motor vehicle. 
     In a further constructive variation, in a variable transmission with multiple forward gears which are assigned to the part-transmission I, these multiple forward gears can be divided again into two sub-transmissions Ia and Ib, between which the part-transmission II is positioned. Particularly preferred is an embodiment in which the hollow input shaft of the part-transmission I is divided into two hollow shaft sections, which optionally can be coupled to the input shaft of the part-transmission II via two twin shifting clutches or which in the other shifting position each connect one idler gear of the part-transmission II to the input shaft. 
     The reverse gear can be configured with an auxiliary shaft which overlaps the part-transmission II and carries two reversing gear wheels. Preferably, the reversing gear wheels mesh on one hand with a fixed gear wheel on the hollow shaft section of the one sub-transmission Ib and a shiftable idler gear on the common driven shaft of the other part-transmission Ia. This combination advantageously makes a separate shifting clutch for shifting of the reverse gear unnecessary. 
     In a further advantageous embodiment of the invention, beside the shifting device which subdivides the input shafts of both part-transmissions I and II, for example a twin shifting clutch, a second shifting device, preferably a twin shifting clutch can be provided on the driven shaft, which optionally connects multiple gear wheels in particular two fixedly interconnected forward gear idler gears and/or a reverse gear idler gear to the driven shaft. Preferably, within this context the twin shifting clutch on the input shaft actively shifts either an idler gear of a forward gear or a double gear wheel which is firmly connected to the input shaft, which double gear wheel meshes with the idler gear of the second forward gear and additionally with a reversing gear wheel of the reverse gear. This allows integrating both shifting clutches in such a way that reverse gear and disengagement of the part-transmissions are combined to allow for a simple control. 
     Further, in an advantageous configuration of the variable speed transmission for a motor vehicle with all-wheel drive and with an interaxle differential connected to the driven shaft, the driven shaft can likewise be two-part, wherein the driven shaft section of the part-transmission I is configured as hollow shaft, which outputs to the differential case of the interaxle differential, whose output shafts effect output of a rear axle differential and in the reverse drive effect output to the second driven shaft section through the hollow shaft to drive a front interaxle differential. 
     For this, the driving connection between the driven shaft section of the part-transmission II and the reverse driving output shaft of the part-transmission I can be released via a shifting clutch which is arranged between the part-transmissions I and II when the part-transmission II is coupled. This simple measure achieves that an all wheel drive is established when driving in the lower forward gears or with part-transmission II respectively, and only one axle, in particular the front axle of the motor vehicle is driven, when driving with the higher forward gears or in the part-transmission II, respectively. 
     The shifting clutch on the driven shaft can preferably again be a twin shifting clutch, by means of which in the neutral position the connection between the driven shaft section and the corresponding output shaft is released, and further in the one shifting position the connection is established and in the other shifting position a reverse gear wheel on the driven shaft section is coupled to a reverse gear-fixed gear on the input shaft of the part-transmission I via the auxiliary shaft with reversing gears. 
     Finally, one of the respective gear wheels of the variable speed transmission which form the reverse gear can each be combined with a gear wheel of a forward gear, which is especially advantageous with regard to the axially required construction space of the transmission as well as with regard to manufacturing costs. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
       In the following, multiple exemplary embodiments of the invention are explained in more detail. The schematic drawing shows in: 
         FIG. 1  a variable speed transmission for motor vehicles with an input shaft, a driven shaft and with six integrated forward gears and a reverse gear, which are divided into two part-transmissions I and II by means of a shifting clutch. 
         FIG. 2  the variable speed transmission according to  FIG. 1  with a different arrangement of the forward gears which are assigned to the part-transmissions I and II. 
         FIG. 3  the variable speed transmission according to  FIG. 1  with an inversed arrangement of the two part-transmissions I and II; 
         FIG. 4  the variable speed transmission according to  FIG. 3  with a further, alternative arrangement or respectively, a division of the one part-transmission I into two sub-transmissions Ia and Ib and with two shifting clutches; 
         FIG. 5  a variable speed transmission modified relative to  FIG. 2  with a shifting clutch integrated in the input shaft and in addition a shifting clutch integrated in the driven shaft; and 
         FIG. 6  a further variable speed transmission configured for an all wheel drive of a motor vehicle with two part-transmissions I and II, an interaxle differential and a drive distribution to either both axles of the motor vehicle or only to the front axle. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       FIG. 1  shows a variable speed transmission  10  or transmission respectively as a block diagram, with an input shaft  14  which is rotatably supported in a transmission housing  12 , a driven shaft  16  and an integrated front axle differential (not shown) which is driven via a spur gear  18  which is connected to the driven shaft  16 . 
     On the input shaft  14  which is driven by a drive machine via a separation clutch (not shown) and on the driven shaft  16  are gear sets for forming forward gears  1  to  6 , which can be shifted by twin synchronous clutches  20  which are common in transmissions and which are only shown as outline. 
     The gear sets of the forward gears  1  to  4  have four fixed gear wheels (uniformly designated  22 ) which are arranged on the input shaft  14 , while their shiftable idler gears (uniformly designated  24 ) are supported on the driven shaft  16 . 
     Further, the fixed gear wheels  22  of the forward gears  5  and  6  are fastened on the driven shaft  16 , while their shiftable idler gears  24  are supported on the input shaft  14 . 
     The input shaft  14  is divided into two input shafts  14   a  and  14   b  by twin shifting clutch  26  to form two part-transmissions I and II, wherein the forward gears  5  and  6  are assigned to the part-transmission II and the forward gears  1  to  4  are assigned to the part-transmission I. Further, as can be seen, the fixed gear wheels  22  of the forward gears  5  and  6  are arranged on the front input shaft  14   a  and the idler gears  24  of the forward gears  1  to  4  are arranged on the rear input shaft  14   b , while the corresponding gear wheels  22 ,  24  are respectively positioned on the common driven shaft  16 . 
     In addition, a shiftable idler gear  28  is rotatably supported on the input shaft  14   a , which idler gear  28  in connection with an axially parallel auxiliary shaft  30  which has two reversing gears  32  and is rotatably supported in the transmission housing  12  forms a reverse gear R, wherein the one reversing gear  32  (indicated by dashed lines) engages with the idler gear  24  for the forward gear  2 . For this, of course, the auxiliary shaft  30  is correspondingly offset out of the drawing plane so that the rotational axes of the shafts  14 ,  16 ,  30  form a triangle when viewed in a cross section. 
     In the drawn neutral position II, the two input shafts  14   a ,  14   b  are drivingly disconnected from one another via the shifting clutch  26  (adjustment in direction of the arrow). In the one shifting position I of the shifting clutch  26  (in the drawing towards the right) the two input shafts  14   a ,  14   b  are coupled to one another, in the opposite shifting position (in the drawing towards the left) the idler gear  28  of the reverse gear is shifted, wherein the force flux flows via the auxiliary shaft  30 , the reversing gears  32  and for example the idler gear  24  of the forward gear  2  to the driven shaft  16 . The synchronous clutch  20  for the forward gear  2  has to be closed in this instant; the disengaged input shaft  14   b  can rotate freely. 
     When driving in the forward gears  1  to  4  (part-transmission I) the shifting clutch  26  is in the right shifting position I and the force flux thus flows via the respective entire input shaft  14  of the transmission  10 , via the respectively shifted forward gears  1  to  4  or their gear sets  22 ,  24  respectively, to the driven shaft  16 . The gear sets of the forward gears  5  and  6  freely co-rotate. 
     When driving in the forward gears  5  and  6  (part-transmission II) the shifting clutch  26  is in its neutral position II so that the input shaft  14   b  of the part-transmission I is disengaged from the fixed gear wheels  22 . The force flux now flows via the respectively shifted forward gear  5  or  6  or respectively its gear set  24 ,  22  to the driven shaft  16 . Because the idler gears  24  on the driven shaft  16  are not shifted, only the corresponding coupling bodies of the synchronous clutches  20  rotate, while the gear wheels  24 ,  22  of the forward gears  1  to  4 , the input shaft  14   b  and the idler gear  28  of the reverse gear R as well as the auxiliary shaft  30  with the reversing gears  32  are stationary or respectively do not cause friction or churning losses in the variable speed transmission  10 . 
       FIG. 2  shows a further variable speed transmission corresponding to the one of  FIG. 1 , which is described only insofar as it significantly differs from the embodiment according to  FIG. 1 . Same parts are designated with same reference signs. This also applies to the additional, further modified transmissions shown in  FIGS. 3 to 6 . 
     In  FIG. 2  the forward gears  1  and  2  as well as the reverse gear R are assigned to part-transmission I and the forward gears  3  to  6  are assigned to the part-transmission II. 
     This means that only the fixed gear wheels  22  of the forward gears  1  and  2  are arranged on the disengageable input shaft  14   b  and mesh with the shiftable idler gears  24  on the driven shaft  16 . 
     The reverse gear R with the idler gear  28 , the auxiliary shaft  30  with the reversing gears  32  and the idler gear  24  of the forward gear  2  are arranged as described before. 
     In the four forward gears  3  to  6 , the fixed gear wheels  22  are positioned on the driven shaft  16  and the shiftable idler gears  24  are positioned on the continuously driven input shaft  14   a.    
     Accordingly, when the motor vehicle drives in the forward gears  3  to  6  or with the part-transmission II respectively, the part-transmission I with the forward gears  1  and  2  and the reverse gear R is disengaged and in this extended driving range does not cause friction and churning losses via the gear sets of the forward gears  1  and  2  and the reverse gear. 
       FIG. 3  shows a variable speed transmission which is also modified relative to  FIG. 1 , in which the part-transmissions I and II, with regard to the forward gears, are arranged inversely or respectively, in which the part-transmission I is arranged in front and the part-transmission II there behind (in the drawing on the right side) and in which again the four forward gears  1  to  4  are assigned to the part-transmission I and the forward gears  5  and  6  to part-transmission II. 
     In distinction to  FIG. 1 , the input shaft  14  is divided into a continuous input shaft  14   a  of the part-transmission II and a hollow input shaft  14   b  coaxially supported in the transmission housing (not shown) relative to the part-transmission I. 
     The hollow input shaft  14   b  of the part-transmission I carries the fixed gear wheels  22  of the forward gears  1  to  4 , whose idler gears  24  are supported on the common driven shaft  16  and are correspondingly shiftable via the synchronous shifting clutches  20 . 
     The idler gear  28  which is shiftable in the one shifting position R is rotatably supported on the continuous input shaft  14   a  and drivingly connected to the idler gear  24  (indicated by the dashed line) of the forward gear  2  via the auxiliary shaft  30  and the reversing gears  32 . 
     In the left shifting position in the drawing, the part-transmission I or respectively the hollow input shaft  14   b  is coupled to the inner input shaft  14   a  and the four forward gears  1  to  4  can be shifted. 
     In the neutral position II of the shifting clutch  26 , the part-transmission I is disengaged and only the forward gears  5  and  6  can be shifted, wherein as described previously, the gear sets  22 ,  24  of the forward gears  1  to  4  and the reverse gear are stationary. 
     In the right shifting position R of the shifting clutch  26 , the reverse gear is shifted, wherein the force flux again flows over the auxiliary shaft  30  and the likewise shifted idler gear  24  of the forward gear  2 . 
       FIG. 4  shows a further modified variable speed transmission  10  in which the part-transmission II is integrated in the part-transmission I which is subdivided into two sub-transmissions Ia and Ib. 
     The gear sets of forward gears  5  and  6  which are assigned to the part-transmission II are supported on the continuous input shaft  14   a  with shiftable idler gears  24  and mesh with the corresponding fixed gear wheels  22  on the common driven shaft  16 . 
     The sub-transmission Ia which is arranged on one side thereto includes the forward gears  3  and  4 , wherein the corresponding fixed gear wheels  22  are positioned on a first hollow shaft section  14   ba  of the input shaft  14  and the corresponding idler gears  24  on the driven shaft  16 . 
     The second sub-transmission Ib is arranged on the other side of the part-transmission II, and has a second hollow shaft section  14  bb, on which the fixed gear wheels  22  of the forward gears  1  and  2  are fastened. The fixed gear wheels  22  mesh with the shiftable idler gears  24  on the driven shaft  16 . 
     The two hollow shaft sections  14   ab ,  14   bb  of the input shaft  14  can alternately be coupled to the input shaft  14   a  via two twin shifting clutches  26 , so that in this shifting position either the forward gears  1  and  2  or the forward gears  3  and  4  are shiftable via the synchronous clutches  20 . 
     From the central neutral position of the shifting clutches  26  towards the right or left respectively, the forward gears  5  and  6  of the central part-transmission II can be shifted alternately, wherein the two sub-transmissions Ia, Ib are then disengaged and their gear sets  22 ,  24  are stationary. 
     The reverse gear R is shifted by simultaneously closing the synchronous clutch  20  of the forward gear  4  of the sub-transmission Ia and the shifting clutch  26  of the sub-transmission Ib, wherein the auxiliary shaft  30  for the reverse gear R overlaps the central part-transmission II and the reversing gears  32  of the auxiliary shaft  30  mesh with the fixed gear wheel  22  (compare dashed drawn in line) of the forward gear  2  on the hollow shaft section  14   bb  and the idler gear  24  of the forward gear  4 . The force flux flows over the fixed gear wheel  22  of the sub-transmission Ib, over the auxiliary shaft  30  and the shifted idler gear  24  of the forward gear  4  onto the driven shaft  16 . 
     Because of the described arrangement of the transmission elements, in spite of the division of the input shaft  14  into three sections  14   a ,  14   ba  and  14   bb , only four shifting clutches  20 ,  26  are required in total as in the previously described embodiments whose functions are merely distributed differently. 
       FIG. 5  shows a further modified variable speed transmission  10  in which the reverse gear R is configured without arrangement of an auxiliary shaft  30 . 
     According to  FIG. 5 , the input shaft  14  of the variable speed transmission  10  is divided into two input shafts  14   a ,  14   b  which are shiftable into the two part-transmissions I and II via the two shifting clutches  26  arranged there between. 
     The part-transmission II again has four forward gears  3  to  6 , whose gear sets are formed by four shiftable idler gears  24  on the input shaft  14   a  and four fixed gear wheels  22  on the driven shaft  16 . The idler gears  24  can be coupled to the input shaft  14   a  via two twin synchronous clutches  20 . 
     The part-transmission I is provided with a hollow shaft  34  which is arranged coaxially on the driven shaft  16  and which carries two fixed gear wheels  22  of the gear sets for the forward gears  1  and  2 , whose shiftable idler gears  24  are supported or arranged respectively, on the input shaft  14   a  (gear  2 ) and on the input shaft  14   b  (gear  1 ). 
     The gearwheel on the input shaft  14   b  which functions as idler gear  24 , is configured with two tracks and at the same time meshes with a reversing gear  36  which is supported in the transmission housing (not shown), and which engages with an idler gear  24  for the reverse gear R, which idler gear  24  is supported on the driven shaft  16 . 
     In the neutral position II, a second shifting clutch  38  on the driven shaft  16  disengages the hollow shaft  34  of the part-transmission I from the driven shaft  16 , which hollow shaft  34  carries the two fixed gear wheels  22 . In the shifting position towards the right the idler gear  24  is connected to the driven shaft  16  for shifting the reverse gear R, and the shifting clutch  26  on the input shaft  14   a  is also shifted toward the right. The force flux for the reverse gear R then flows over the input shafts  14   a ,  14   b  over the idler gear  24 , the reversing gear  24  and the shifted idler gear  24  onto the driven shaft  16 . 
     In the central position of the shifting clutches  26  and  38 , the part-transmission I is disengaged from the part-transmission II and the corresponding gear sets of the gears  1 ,  2  and R do not rotate. 
     In the left shifting position of the shifting clutch  38  and right position of the shifting clutch  26 , the forward gear  1  is shifted; the drive torque is conducted via the idler gear  24  onto the input shaft  14   b  and the fixed gear wheel  22  on the hollow shaft  34  to the shifting clutch  38  and from the latter to the driven shaft  16 . 
     The forward gear  2  is conducted via the idler gear  24  on the input shaft  14   a , the corresponding fixed gear wheel  22  on the hollow shaft  34  and the shifting clutch  38  to the driven shaft  16  when the shifting clutch  38  is still closed and the shifting clutch  26  is shifted. 
     When driving in the geared up forward gears  3  to  6  of the part-transmission II, the two shifting clutches  26 ,  38  are in the neutral position and thus the part-transmission I is disengaged or shut down, respectively. 
       FIG. 6  shows a variable speed transmission  40  which is configured for a partial all wheel drive of a motor vehicle, in which the input shaft  14  is divided in a front input shaft  14   a  and a rear input shaft  14   b  via a first shifting clutch  26 . 
     Likewise, the driven shaft  16  is divided into two sections  16   a  and  16   b , wherein the coaxial driven shaft  16   b  is configured as hollow shaft. 
     The variable speed transmission  40  has seven forward gears  1  to  7  and a reverse gear R, which are again divided into two part-transmissions I and II. The forward gears  1  to  4  and the reverse gear R are assigned to the part-transmission I and the forward gears  5  to  7  to the part-transmission II. 
     The part-transmission I with the forward gears  1  to  4  and the reverse gear R is formed by four fixed gear wheels  22  on the input shaft  14   b , which interact with four idler gears  24  on the hollow shaft  16   b  via synchronous clutches  20 . 
     The hollow driven shaft  16   b  outputs to the differential case  42  of a interaxle differential  44  (for example a bevel gear differential), whose output shaft  48  drives a rear axle differential (not shown) for the drive of the rear wheels of the motor vehicle, while the second output shaft  46  in the reverse drive outputs to the front driven shaft  16   a  through a hollow driven shaft  16   b  and via a second twin shifting clutch  50 , which front driven shaft  16   a  drivingly acts on the front axle differential via the spur gear  18  for driving the front wheels. 
     The shifting clutch  50  between the driven shaft  16   a  and the output shaft  46  of the interaxle differential  44  in the central neutral position II separates the two shafts, while in the right shifting position couples the two shafts  16   a ,  46  to one another. In the left shifting position, it connects the idler gear  24  on the driven shaft  16   a  for the reverse gear R via the auxiliary shaft  30  and the reversing gear  32  of the latter with the fixed gear wheel  22  on the input shaft  14   b  (the tooth engagement between the reversing gear  32  and the fixed gear wheel  22  is indicated again by the drawn in dashed line). 
     The shifting clutch  26  on the input shaft  14  in its neutral position II separates the two input shafts  14   a ,  14   b  while in the right shifting position it couples the latter to one another. In the left shifting position of the shifting clutch  26  the latter connects the corresponding idler gear  24  to the input shaft  14   a  for shifting of the forward drive  7 . 
     When driving in the forward gears  5  and  6  via the transmission II, the two shifting clutches  26  and  50  are in the neutral position. The forward gear  7  of the part-transmission II is shifted in that the shifting clutch  26  in the left shifting position couples the input shaft  14   a  to the idler gear  24 . The output to the driven shaft  16   a  occurs only to the front axle of the motor vehicle, when the gear wheels and shafts of the part-transmission I stand still. 
     When driving in the forward gears  1  to  4  or in the part-transmission I, the two shifting clutches  26 ,  50  are shifted into the right position and connect the two input shafts  14   a ,  14   b  and the driven shaft  16  to the output shaft  46  of the interaxle differential  44 . When the corresponding synchronous clutches  20  are shifted, the force flux flows over the respective gear set  22 ,  24  of the shifted forward gear  1  to  4  onto the hollow driven shaft  16   b  and from this onto the interaxle differential  44 , which distributes the drive torque to the two output shafts  46 ,  48  and finally onto the rear axle differential and via the driven shaft  16   a  and the spur wheel set  18  onto the front axle differential. 
     For shifting the reverse gear R, the shifting clutch  26  remains shifted in the right position, while the shifting clutch  50  is shifted into the left position. The drive torque then flows via the input shaft  14  to the fixed gear wheel  22  of the forward gear  2  and from the latter via the auxiliary shaft  30  with the reversing gear wheels  32  to the corresponding idler gear  24  and via the shifting clutch  50  to the driven shaft  16   a  or the front axle differential of the motor vehicle. 
     Thus, in the forward gears  1  to  4  (at correspondingly high drive torques at the wheels) the motor vehicle is driven with all wheel drive, while in the forward gears  5  to  7  (at lower drive torques) which are assigned to higher speeds, and in the reverse gear R it only outputs to the front axle. Optionally, the arrangement can be modified so that the reverse gear R is also included in the all wheel drive. 
     The shifting clutches  26 ,  38 ,  50  can be known synchronous clutches, as they are usually used in transmissions. 
     Even though by way of the exemplary representations of the  FIGS. 1 to 6  the output in front is previously shown via a spur gear stage to a differential, it is of course also possible, to arrange the output at the rear in a manner known per se. 
     Further, it is possible, to arrange the gear set of the respective embodiment mirror image reversed if this is technically and/or construction space wise possible or useful.