Abstract:
In a twin-clutch transmission, in a first forward gear, the drive torque is transferred by an interconnection of a first transmission part having one countershaft and a second part transmission having another countershaft by means of an intermediate stage whose gearwheels can be used multi-functionally in connection with additional forward gears of the transmission thereby providing for a relatively small transmission with a relatively large number of transmission ratios.

Description:
[0001]     This is a Continuation-In-Part Application of International Application PCT/EP2004/005920 filed 06/06/04 and claiming the priority of German application 103 25 647.4 filed 06/06/03.  
       BACKGROUND OF THE INVENTION  
       [0002]     The invention relates to a twin-clutch transmission with a number of a load-shiftable forward gears and at least one reverse gear and two co-axial shafts connected to the twin clutches.  
         [0003]     A generic twin-clutch transmission is known from the publication DE 198 60 251 C1. Further twin-clutch transmissions are known, for example, from the publications WO 00/39484, U.S. Pat. No. 6,250,171, DE199 39 334, DE 198 21 164, DE 10108881, DE 10102028, U.S. Pat. No. 6,427,547, DE 10015336 and the publication by Tenberge, P.: “Doppelkupplungsgetriebe in Windungsanordnung” [“Twin-clutch transmissions in winding arrangement”], VDI Seminar No. 31 03 01 “Stufenlose Fahrzeuggetriebe” [“Continuously variable vehicle transmissions”], Stuttgart, 2001.  
         [0004]     It is the object of the present invention to provide a compact twin-clutch transmission.  
       SUMMARY OF THE INVENTION  
       [0005]     In a twin-clutch transmission, in a first forward gear, the drive torque is transferred by an interconnection of a first transmission part having one countershaft and a second part transmission having another countershaft by means of an intermediate stage whose gearwheels can be used multi-functionally in connection with additional forward gears of the transmission thereby providing for a relatively small transmission with a relatively large number of transmission ratios.  
         [0006]     A twin-clutch transmission is equipped with N (in particular, sequentially) load-shiftable forward gears and at least one reverse gear and has two intermediate shafts which are arranged coaxially with respect to one another and to a transmission axis. By a twin clutch, the intermediate shafts can be brought into drive connection in each case with an input shaft, one intermediate shaft being designed as a hollow shaft. As compared with a form of construction with intermediate shafts not arranged coaxially with respect to one another, for example according to the publication U.S. Pat. No. 6,427,547 B1, the invention has the advantage of a small radial overall size, particularly in the region of the clutches, that is to say in the front part region of the transmission. This may be advantageous, for example in terms of integrating the transmission into a vehicle tunnel or for achieving a necessary ground clearance of the motor vehicle.  
         [0007]     Furthermore, the twin-clutch transmission according to the invention has two countershafts. These are arranged parallel to one another and parallel to the transmission axis. In comparison with a design having two countershafts which are arranged coaxially with respect to one another and one of which is designed as a hollow shaft, cf., for example, WO 00/39484, the design according to the invention results in an axially relatively short construction.  
         [0008]     An input shaft of the twin-clutch transmission and a transmission output shaft are in each case arranged coaxially with respect to the transmission axis. An axial offset can thereby be avoided.  
         [0009]     For at least one forward gear A, four step-up stages are interposed between the input shaft and the transmission output shaft. The overall step-up to the transmission output shaft is therefore obtained from the product of the four individual step-up stages. For at least one further forward gear stage B, only two step-up stages are interposed between the input shaft and the transmission output shaft. For at least one forward gear A and one forward gear B, the force flux takes place via (at least) one identical gearwheel. According to the invention, therefore, one gearwheel is multi-functional, with the result that construction space requirements and/or the number of necessary components are reduced. Alternatively or additionally, improved possibilities with regard to the staging of the forward gears are afforded. In particular, a version with relatively short transmission shafts is possible, so that, under load, lower shaft flexions occur or the shaft cross sections have a smaller dimensioning, as compared with other transmission arrangements. The load on the rolling bearings supporting the transmission shafts is likewise reduced as a result of a shortening of the lever arms for support.  
         [0010]     The at least one forward gear A is a first forward gear (or the first forward gears) for which four step-up stages are interposed between the input shaft and a transmission output shaft. According to the invention, via the additional two step-up stages for implementing the first forward gear which are interposed in this way in the first gear stage, an additional step-down can be implemented. As a result, the first input stage of the transmission, that is to say a first step-up stage between intermediate shaft and counter-shaft, can be designed to be relatively “long”. This has, in particular, the following advantages: 
        The torque introduced into the countershaft by means of the first input stage is lower than if a relatively short input step-up were used. This has advantages in terms of the dimensioning of the components involved.     Where an odd number of forward gears is concerned, the highest forward gear is assigned to the same input stage as the first forward gear. With the first input stage being designed to be long, the transmission stage for implementing the highest gear can be designed to be relatively “short”, or else a particularly long overall step-up can be achieved in the highest gear.        
 
         [0013]     According to a preferred embodiment of the twin-clutch transmission, for the at least one forward gear A with four interposed step-up stages, two gearwheels, which are connected (or connectable) to one another fixedly in terms of rotation, are interposed into the force flux between the input shaft and transmission in output shaft. The gearwheels rotate about the (main) transmission axis. In a forward gear A, one of the abovementioned gearwheels assumes the drive torque from one countershaft, while the other gearwheel transfers the drive torque to the other countershaft. Thus, in particular, in the forward gear A, a transfer of the drive torque from an intermediate shaft to one countershaft, to the gearwheels, to the other countershaft and to the output shaft takes place, in each case with a transmission stage being interposed. Thus, according to the invention, the two part transmissions formed by the respective counter-shafts are introduced in succession into the force flux.  
         [0014]     In at least one further forward gear B, at least one of the abovementioned gearwheels transfers the drive torque directly, that is to say with a transmission stage formed by this gearwheel being interposed, from an intermediate shaft to a countershaft, from which the drive torque is transferred to the transmission output shaft via a further transmission stage. The gearwheels connected to one another thereby form a multiply usable intermediate stage. A particularly advantageous embodiment arises when, in a further forward gear B, the other of the abovementioned gearwheels transfers the drive torque directly, that is to say with a transmission stage formed by this gearwheel being interposed, from the same intermediate shaft to the same or the other countershaft.  
         [0015]     According to a further embodiment of the invention, the twin-clutch transmission has a forward gear which is designed as a direct gear. This is, in particular, the prepenultimate, the penultimate or the last forward gear. Improvements in efficiency can thereby be achieved.  
         [0016]     In a preferred twin-clutch transmission, the input step-up assigned to the lowest forward gear is designed to be longer than the other input step-up. This has advantages when the highest gear stage is an odd gear, for example a seventh gear. In this case, the transmission stage, following the input step-up, for implementing the seventh gear may be designed to be longer. In particular, the input step-up designed to be longer is arranged nearer to the twin clutch than the other input step-up. Because of the relatively large diameter of the gearwheel which corresponds to the input step-up, the intermediate shaft assigned to the other gearwheel may advantageously be relatively far under the gearing for the input step-up gears.  
         [0017]     According to the invention, the twin-clutch transmission has, in particular, at least seven gears which are shiftable via four shift elements. This results, as compared with the prior art, in a reduction in the number of shift elements, this being accompanied by a reduction in the weight, in the components required, in the construction space and/or in the costs.  
         [0018]     Preferably, one of the abovementioned gearwheels is helically toothed with an angle α. The other gearwheel is helically toothed with an angle β. The angles α and β have identical signs and are dimensioned such that the axial forces acting on the two gearwheels in the first forward gear approximately cancel one another. Necessary bearing forces for the gearwheels or a hollow shaft supporting the gearwheels can thereby be reduced, which, for example, leads to a smaller dimensioning and/or increased running time.  
         [0019]     In a particular embodiment, the twin-clutch transmission has a shift element which has its neutral position in an outer shift position, in a first shift position connects an intermediate shaft to the transmission output shaft, so that a direct gear is formed, in a second shift position connects the abovementioned gearwheels to the transmission output shaft, and, in a third shift position, makes no connection between assigned transmission elements, the second shift position lying between the first shift position and the third shift position. An outer shift position of the shift element thus forms a “neutral position”. Preferably, the abovementioned gearwheels are connected fixedly in terms of rotation to the assigned intermediate shaft via the same shift element in the first shift position. A simple power shift from the direct gear to an adjacent gear thereby becomes possible.  
         [0020]     Preferably, the two countershafts and the transmission axis are arranged triangularly in cross section. This affords an especially compact type of construction, particularly a small extent of the twin-clutch transmission transversely with respect to the transmission axis.  
         [0021]     The invention will become more readily apparent from the following description of the invention on the basis of the accompanying drawings, wherein preferred exemplary embodiments of the twin-clutch transmission according to the invention are explained in more detail with reference to the drawing: 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0022]      FIG. 1  shows a gear arrangement of a twin-clutch transmission according to a first embodiment,  
         [0023]      FIG. 2  shows a table of the shift states of the clutches and shift elements for the twin-clutch transmission according to  FIG. 1 ,  
         [0024]      FIG. 3  shows a gear plan of the twin-clutch transmission illustrated in  FIG. 1 , in a first forward gear,  
         [0025]      FIG. 4  shows a gear plan of the twin-clutch transmission according to  FIG. 1 , in a sixth forward gear,  
         [0026]      FIG. 5  shows a gear plan of a twin-clutch transmission according to the invention in a second embodiment of the invention,  
         [0027]      FIG. 6  shows a table of the shift states of the clutches and shift elements for the twin-clutch transmission illustrated in  FIG. 5 ,  
         [0028]      FIG. 7  shows a gear plan of a twin-clutch transmission according to a third embodiment of the invention,  
         [0029]      FIG. 8  shows a table of the shift states of the clutches and shift elements of the twin-clutch transmission according to  FIG. 7 ,  
         [0030]      FIG. 9  shows a gear plan of a twin-clutch transmission according to a fourth embodiment of the invention, and  
         [0031]      FIG. 10  shows a table of the shift states of the clutches and shift elements of the twin-clutch transmission according to  FIG. 9 . 
     
    
     DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0032]     A twin-clutch transmission  10  according to the invention is used in a drive train of a motor vehicle. In this case, the twin-clutch transmission  10  is interposed between a drive assembly and an output shaft, for example a propeller shaft or cardan shaft. The drive train is preferably a standard drive train with a drive assembly, which is frontmounted lengthways in a vehicle, and drives the rear wheels of the vehicle.  
         [0033]     The twin-clutch transmission has an input shaft  11 , in particular a crankshaft or a shaft rotating with the latter, and a transmission output shaft  12 . The input shaft  11  and the transmission output shaft  12  are arranged coaxially with respect to transmission axis X-X. The input shaft  11  is connected to a drive assembly, if appropriate with at least one further transmission stage being interposed. The transmission output shaft  12  is connected to vehicle wheels, if appropriate with at least one further transmission stage being interposed.  
         [0034]     The input shaft  11  is drive-connected to a twin clutch  13 , designed here as a structural unit. The twin clutch  13  has a clutch K 1  and a clutch K 2  which, via suitable devices, not illustrated, and ensuring a suitable overlap control, transfer the drive torque of the input shaft  11  to the intermediate shaft  14 , with a clutch K 2  closed, and to the intermediate shaft  15 , with a clutch K 1  closed. The clutch K 1  is arranged, in the direction of the transmission axis X-X (hereafter “in the axial direction”), on that side of the clutch K 2  which faces away from the drive assembly. The intermediate shaft  15  is a hollow shaft and the solid intermediate shaft  14  extends through the hollow intermediate shaft  15 . In that end region of the intermediate shaft  15  which faces away from the twin clutch  13 , the drive torque is transmitted from the intermediate shaft  15  to a countershaft  16  via a gear structure  17  which forms an input step-up and has a driving gearwheel  18  connected fixedly for rotation with the intermediate shaft  15  and also a driven gearwheel  19  connected firmly for rotation with the first countershaft  16 .  
         [0035]     A driving gear  20  of a gear stage VII is connected fixedly for rotation with the first countershaft  16 . Via a shift element  21 , the countershaft  16 , in a shift position S 3 , is connected for rotation with a driving gear  22  of a gear stage V and, in a shift position S 4 , is connected to a driving gearwheel  23  of a gear stage III. In a neutral position between the shift positions S 3  and S 4  the first countershaft  10  independent of the driving gearwheels  20 ,  22 .  
         [0036]     In an end region, projecting out of the hollow intermediate shaft  15 , the inner intermediate shaft  14  is connected fixedly in terms of rotation to a driving gear  24  of a gear structure  25  which forms the input step-up between the inner intermediate shaft  14  and a second countershaft  26 . The driven gear  27  of the gear structure  25  and a driving gear  28  of the gear stages I, II are connected for rotation with the second countershaft  26 . Via a shift element  29 , a driving gear  30  of a gear stage IV is connected to the second countershaft  26  in a shift position S 5  and a driving gear  31  of a gear stage RI, RII is connected to the countershaft  26  in a shift position S 6 , while, in a neutral position lying between the shift positions S 5  and S 6 , the driving gearwheels  30 ,  31  are rotatable relative to the second countershaft  26 .  
         [0037]     A shift element  32  is disposed between the inner intermediate shaft  14 , the transmission output shaft  12  and a hollow shaft  33  receiving the transmission output shaft  12 . In a shift position S 1 , the shift element  32  rotationally interconnects the intermediate shaft  14 , the transmission output shaft  12  and the hollow shaft  33 . In a middle position, the shift element  32  rotationally interconnects the hollow shaft  33  and the transmission output shaft  12 . In a shift position S 2 , there is no drive connection between the intermediate shaft  14 , the transmission output shaft  12  and hollow shaft  33 . The hollow shaft  33  carries the gearwheel  34  which is assigned to the gear stage V and, offset in the circumferential direction, to the gear stage IV and which meshes with the driving gearwheels  22  and  30 , and also the gearwheel  35  which is assigned to the gear stage III and which meshes with the driving gearwheel  23 .  
         [0038]     A shift element  36  is disposed between the transmission output shaft  12  and the gearwheels  37 ,  38 . In a shift position S 7 , the shift element  36  connects the transmission output shaft  12  to the gearwheel  37 . In a shift position S 8 , the shift element  36  connects the transmission output shaft to the gearwheel  38 . In a neutral position lying between the shift positions S 7  and S 8 , the gearwheels  37 ,  38  and the transmission output shaft  12  have no drive connection via the shift element  36 . The gearwheel  38  forms with the driving gearwheel  28  the gear stage I, II. The gearwheel  37 , on the one hand, forms with the driving gearwheel  20  the gear stage VII. Furthermore, offset in the circumferential direction with respect to the driving gearwheel  20 , the gearwheel  37  meshes with a reverse-gearwheel  39  which, in turn, is drive-connected to the driving gearwheel  31  so as to form the gear stage RI, RII.  
         [0039]     In transmission planes which are oriented transversely (hereinafter “radially”) with respect to the transmission axis X-X, are arranged axially one behind the other in the following order: 
        the twin clutch  13 ,     the gear structure  17  with driving gearwheel  18  and driven gearwheel  19 ,     the gear structure  25  with driving gearwheel  24  and driven gearwheel  27 ,     shift element  32 ,     gear stage V with driving gearwheel  22  and gearwheel  34  and gear stage IV with driving gearwheel  30  and gearwheel  34 ,     shift elements  29 ,  21 ,     gear stage III with driving gearwheel  23  and gearwheel  35  and gear stage RI, RII with driving gearwheel  31 , reverse-gearwheel  39  and gearwheel  37 ,     shift element  36 , and     gear stage I, II with driving gearwheel  28  and gearwheel  38 .        
 
         [0049]     In a second reverse gear, with the clutch K 1  open, the clutch K 2  is closed. Shift element  32  is in the middle position, shift element  21  is in the shift position S 3 , shift element  29  is in shift position S 6  and shift element  36  is in the neutral position. The force flux runs via the input shaft  11 , clutch K 2 , intermediate shaft  14 , driving gearwheel  24 , driven gearwheel  27 , countershaft  26 , shift element  29 , driving gearwheel  31 , reverse-gearwheel  39 , gearwheel  37 , driving gearwheel  20 , countershaft  16 , shift element  21 , driving gearwheel  22 , gearwheel  34  and shift element  32  to the transmission output shaft  12 .  
         [0050]     In a first reverse gear, the clutch K 2  is closed, shift element  32  is in the middle position, shift element  21  is in shift position S 4 , shift element  29  is in shift position S 6  and shift element  36  is in the neutral position. In this case, the force flux runs via the input shaft  11 , clutch K 2 , intermediate shaft  14 , driving gearwheel  24 , driven gearwheel  27 , countershaft  26 , shift element  29 , driving gearwheel  31 , reverse-gearwheel  39 , gearwheel  37 , driving gearwheel  20 , countershaft  16 , shift element  21 , driving gearwheel  23 , gearwheel  35  and shift element  32  to the transmission output shaft  12 .  
         [0051]     In a first forward gear, the force flux extends from an input shaft  11  via the clutch K 1 , the intermediate shaft  15 , the driving gear  18 , the driven gear  19 , the countershaft  16 , the shift element  21 , the driving gear  23 , the gear  35 , the hollow shaft  33 , the gear  34 , the driving gear  30 , the shift element  29 , the countershaft  26 , the driving gear  28 , the gear  38  and the shift element  36  to the transmission output shaft  12 . The shift element  32  is in shift position S 2 , the shift element  21  is in shift position S 4 , the shift element  29  is in shift position S 5  and the shift element  36  is in shift position S 8  (cf.  FIG. 3 ).  
         [0052]     In a second forward gear, the clutch K 2  is closed. Shift element  32  is in shift position S 2 , shift element  21  is in shift position S 4 , shift element  29  is in the neutral position and shift element  36  is in shift position S 8 . In the second forward gear, the force flux runs from the input shaft via the clutch K 2 , intermediate shaft  14 , driving gearwheel  24 , driven gearwheel  27 , countershaft  26 , driving gearwheel  28 , gearwheel  38  and shift element  36  to the transmission output shaft  12 , cf.  FIG. 4 .  
         [0053]     In a third forward gear, the clutch K 1  is closed. Shift element  32  is in the middle position, shift element  21  is in shift position S 4 , shift element  29  is in the neutral position and shift element  36  is in the neutral position. In the third forward gear, the force flux runs from the input shaft  11  via the clutch K 1 , intermediate shaft  15 , driving gearwheel  18 , driven gearwheel  19 , countershaft  16 , shift element  21 , driving gearwheel  23 , gearwheel  35 , hollow shaft  33  and shift element  32  to the transmission output shaft  12 .  
         [0054]     In a fourth forward gear, the clutch K 2  is closed. Shift element  32  is in the middle position. Shift elements  21  and  36  are in the neutral position, while shift element  29  is shifted into shift position S 5 . The force flux takes place from the input shaft  11  via the clutch K 2 , intermediate shaft  14 , driving gearwheel  24 , driven gearwheel  27 , countershaft  26 , shift element  29 , driving gearwheel  30 , gearwheel  34  and shift element  32  to the transmission output shaft  12 .  
         [0055]     In a fifth forward gear, the clutch K 1  is closed. Shift element  32  is in the middle position. The shift elements  29  and  36  are in the neutral position, while shift element  21  is shifted into shift position S 3 . The force flux takes place from the input shaft  11  via the clutch K 1 , intermediate shaft  15 , driving gearwheel  18 , driven gearwheel  19 , countershaft  16 , shift element  21 , driving gearwheel  22 , gearwheel  34  and shift element  32  to the transmission output shaft  12 .  
         [0056]     In a sixth forward gear, the clutch K 2  is closed. Shift element  32  is in shift position S 1 , while the shift elements  21 ,  29  and  36  are shifted into the neutral position. The sixth forward gear is a direct gear for which a force flux takes place from the input shaft via the clutch K 2  and intermediate shaft  14  to the transmission output shaft  12  by means of the shift element  32 .  
         [0057]     In a seventh forward gear, the clutch K 1  is closed. Shift element  32  is in shift position S 1 . The shift elements  21 ,  29  are in the neutral position, while shift element  36  is shifted into shift position S 7 . The force flux takes place from the input shaft  11  via the clutch K 1 , intermediate shaft  15 , driving gearwheel  18 , driven gearwheel  19 , countershaft  16 , driving gearwheel  20 , gearwheel  37  and shift element  36  to the transmission output shaft  12 .  
         [0058]     For the second exemplary embodiment according to  FIG. 5 , with a design otherwise corresponding to the exemplary embodiment according to  FIG. 2  and with corresponding shift positions of the clutch  13  and of the shift elements  21 ,  29 ,  32 ,  36  according to  FIG. 2 , the reverse-gearwheel  39  is omitted. For this exemplary embodiment, a single reverse gear is implemented, in that a driven gearwheel  51  and a driving gearwheel  52  are mounted fixedly in terms of rotation with respect to an intermediate shaft  50  which extends parallel to the transmission axis X-X and to the countershafts  26 ,  16 . The driven gearwheel  51  meshes with the driving gearwheel  31 , while the driving gearwheel  52  meshes with the gearwheel  35 .  
         [0059]     For reverse gear, according to the table in  FIG. 6 , the clutch K 2  is closed. Shift element  32  is in the middle position. The shift elements  21 ,  36  are in the neutral position, while shift element  29  is shifted into shift position S 6 . In reverse gear, the force flux takes place from the input shaft  11  via the clutch K 2 , intermediate shaft  14 , driving gearwheel  24 , driven gearwheel  27 , countershaft  26 , shift element  29 , driving gearwheel  31 , driven gearwheel  51 , intermediate shaft  50 , driving gearwheel  52 , gearwheel  35 , hollow shaft  33  and shift element  32  to the output shaft  12 . With a design otherwise corresponding to the first exemplary embodiment according to  FIG. 1 , for the third exemplary embodiment according to  FIG. 7 , a reverse gear is implemented solely via gearwheels which are arranged coaxially with respect to the transmission axis X-X or coaxially with respect to the axes defined by the countershafts  16 ,  26 . For this purpose, the reverse-gearwheel  39  is omitted. The driving gearwheel  31  is offset (with enlarged diameter) out of the transmission plane for the transmission staging VII and RI, RII into a new transmission plane which lies between the transmission plane having the gear stage III and the transmission plane having the gear stage VII and RI, RII. In this transmission plane, the driving gearwheel  31  meshes with a gearwheel  60  which is connected fixedly in terms of rotation to the countershaft  16 . The transmission axis X-X and the axes defined by the countershafts  16 ,  26  are arranged triangularly in cross section (in contrast to the illustration in  FIG. 7 ).  
         [0060]     In a second reverse gear, the clutch K 2  is closed. The shift element  32  is in the middle position. The shift element  39  is in a neutral position, while shift element  39  is shifted into shift position S 6  and shift element  21  into shift position S 3 . The force flux runs, in the second reverse gear, from an input shaft  11  via the clutch K 2 , intermediate shaft  14 , driving gearwheel  24 , driven gearwheel  27 , countershaft  26 , shift element  29 , driving gearwheel  31 , gearwheel  60 , countershaft  16 , shift element  21 , driving gearwheel  22 , gearwheel  34  and shift element  32  to the transmission output shaft  12 .  
         [0061]     In a first reverse gear, the clutch K 2  is closed. The shift element  32  is in the middle position. Shift element  36  is shifted into the neutral position, while shift element  29  is shifted into shift position S 6  and shift element  21  into shift position S 4 . The force flux runs, in the first reverse gear, from the input shaft  11  via the clutch K 2 , intermediate shaft  14 , driving gearwheel  24 , driven gearwheel  27 , countershaft  26 , shift element  29 , driving gearwheel  31 , gearwheel  60 , countershaft  16 , shift element  21 , driving gearwheel  23 , gearwheel  35 , hollow shaft  33  and shift element  32  to the transmission output shaft  12 .  
         [0062]     Insofar as the description or the illustration in  FIG. 9  is not to the contrary, the exemplary embodiment illustrated in  FIG. 9  corresponds essentially to the first exemplary embodiment according to  FIG. 1  and, in terms of the design of the single reverse gear, to the exemplary embodiment illustrated in  FIG. 5  and described. In contrast to this, according to  FIG. 9 , a shift element  70  is provided instead of the shift element  32 . The shift element  70  acts between the intermediate shaft  14  or the driving gearwheel  24 , the transmission output shaft  12  and the hollow shaft  33 . In a shift position S 1 , the shift element  70  connects the intermediate shaft  14  directly to the transmission output shaft  12  (direct gear). In the middle neutral position of the shift element  70 , the latter is inactive. In the outer shift position S 2 , the shift element  70  connects the hollow shaft  33  to the transmission output shaft  12 . Whereas, according to the exemplary embodiment illustrated in  FIG. 1 , the gearwheel  34  meshes both with the driving gearwheel  30  and with the driving gearwheel  32 , according to the exemplary embodiment illustrated in  FIG. 9  the gearwheel  34  meshes solely with the driving gearwheel  30 . The driving gearwheel  22  meshes with the driving gearwheel  24  in a transmission plane in common with the driven gearwheel  27 . The following transmission planes are thus formed in the following axial order: 
        twin clutch  13 ,     constant  17  with gearwheels  18 ,  19 ,     constant  25  and gear stage VII with the gearwheels  22 ,  24 ,  27 ,     shift element  70 ,     gear stage IV with gearwheels  30 ,  34 ,     shift elements  29 ,  21 ,     gear stage III with gearwheels  23 ,  25  and the gearwheel  52  assigned to reverse gear,     gear stage V with gearwheels  20 ,  37  and gearwheels  31 ,  51  assigned to the reverse gear,     shift element  36 ,     gear stage I, II with gearwheels  28 ,  38 .        
 
         [0073]     According to  FIG. 10 , in the single reverse gear, the clutch K 1  is closed. Shift element  70  is in the neutral position, while shift element  21  is shifted into shift position S 4 , shift element  29  into shift position S 6  and shift element  36  into shift position S 8 . The force flux takes place from the input shaft  11  via the clutch K 1 , intermediate shaft  15 , driving gearwheel  18 , driven gearwheel  19 , countershaft  16 , shift elements  21 , driving gearwheel  23 , gearwheel  35 , driving gearwheel  52 , intermediate shaft  50 , driven gearwheel  51 , driving gearwheel  31 , shift element  29 , countershaft  26 , driving gearwheel  28 , gearwheel  38  and shift element  36  to the transmission output shaft  12 .  
         [0074]     In a first forward gear, the clutch K 1  is closed. The shift element  70  is in the neutral position. Shift element  21  is shifted into shift position S 4 , shift element  29  into shift position S 5  and shift element  36  into shift position S 8 . The force flux takes place from the input shaft  11  via the clutch K 1 , intermediate shaft  15 , driving gearwheel  18 , driven gearwheel  19 , countershaft  16 , shift element  21 , driving gearwheel  23 , gearwheel  35 , hollow shaft  33 , gearwheel  34 , driving gearwheel  30 , shift element  29 , counter-shaft  26 , driving gearwheel  28 , gearwheel  38  and shift element  36  to the transmission output shaft  12 .  
         [0075]     In a second forward gear, the clutch K 2  is closed. The shift elements  70  and  29  are in the neutral position, shift element  29  is shifted into shift position S 4  and shift element  36  into shift position S 8 . A force flux takes place from the input shaft  11  via the clutch K 2 , intermediate shaft  14 , driving gearwheel  24 , driven gearwheel  27 , countershaft  26 , driving gearwheel  28 , gearwheel  38  and shift element  36  to the transmission output shaft  12 .  
         [0076]     In a third forward gear, the clutch K 1  is closed. The shift elements  29 ,  36  are in the neutral position, while shift element  70  is shifted into shift position S 2  and shift element  21  into shift position S 4 . The force flux takes place from the input shaft  11  via the clutch K 1 , intermediate shaft  15 , driving gearwheel  18 , driven gearwheel  19 , countershaft  16 , shift element  21 , driving gearwheel  23 , gearwheel  35 , hollow shaft  33  and shift element  70  to the transmission output shaft  12 .  
         [0077]     In a fourth forward gear, the clutch K 2  is closed. The shift elements  21 ,  36  are in the neutral position, while shift element  70  is shifted into shift position S 2  and shift element  29  into shift position S 5 . The force flux takes place from the input shaft  11  via the clutch K 2 , intermediate shaft  14 , driving gearwheel  24 , driven gearwheel  27 , countershaft  26 , shift element  29 , driving gearwheel  30 , gearwheel  34 , hollow shaft  33  and shift element  70  to the transmission output shaft  12 .  
         [0078]     In a fifth forward gear, the clutch K 1  is closed. The shift elements  21 ,  29  are in the neutral position, while shift element  70  is shifted into shift position S 2  and shift element  36  into shift position S 7 . The force flux takes place from the input shaft  11  via the clutch K 1 , intermediate shaft  15 , driving gearwheel  18 , driven gearwheel  19 , countershaft  16 , driving gearwheel  20 , gearwheel  37  and shift element  36  to the transmission output shaft  12 .  
         [0079]     In a sixth forward gear designed as a direct gear, the clutch K 2  is closed. The shift elements  21 ,  29 ,  36  are in the neutral position, while shift element  70  is shifted into shift position S 1 . The force flux takes place, here, from the input shaft via the clutch K 2 , intermediate shaft  14  and shift element  70  to the transmission output shaft  12 .  
         [0080]     In a seventh forward gear, the clutch K 1  is closed. Shift elements  29  and  36  are in the neutral position. Shift element  70  is shifted selectively into the neutral position or into shift position S 1 . Shift element  21  is shifted into shift position S 3 . The force flux takes place from the input shaft via the clutch K 1 , intermediate shaft  15 , driving gearwheel  18 , driven gearwheel  19 , countershaft  16 , shift element  21 , driving gearwheel  22 , driving gearwheel  24  and shift element  70  to the transmission output shaft  12 .  
         [0081]     According to  FIG. 9 , the single reverse gear is designed, similarly to the first gear, as a winding gear, using both part transmissions. A change from the first forward gear to the reverse gear, or vice versa, is made possible only by the actuation of the shift element  29 . Contrary to the other embodiments of the invention, according to  FIG. 9 , only conventional shift elements with a middle neutral position are used. According to the exemplary embodiment illustrated in  FIG. 9 , all forward gears are power-shiftable sequentially, whereas the transition from the first forward gear to the reverse gear (and vice versa) is not power-shiftable. The stage jumps  1 → 2  and  3 → 4  are identical, irrespective of the profile displacement and of a selected axial distance.  
         [0082]     The tables according to  FIG. 2 ,  FIG. 6 ,  FIG. 8  and  FIG. 10  indicate by squares shift states which are not necessarily required in order to implement the gear specified in each case. However, possibly needless shift movements may be avoided by a selection of the shift state indicated in the tables. Shift positions deviating from the shift positions illustrated are likewise possible.  
         [0083]     For all the exemplary embodiments illustrated, diameter and step-up ratios of the twin-clutch transmission may be gathered from the wheel plans in the drawing, in particular 
        the exact diameter ratios,     whether a step-down or a step-up takes place or     whether a step-up of one transmission stage is designed to be higher or lower than the step-up of another transmission stage.        
 
         [0087]     For the embodiments illustrated, as an additional intermediate stage in the first forward gear, the gear stages of the third and the fourth forward gear (the latter in reverse direction) are used, which together result in an additional step-down. For this reason, the selected input step-up (constant  17 ) may be relatively long.  
         [0088]     The number s of the (minimum) required shift units for a compact design according to the invention is calculated from a number N of forward gears on the following principle:  
         [0000]     if N is odd, then 
 
 s= ( N+ 1)/2; 
 
 if N is even, then 
 
 s= ( N+ 2)/2. 
 
         [0089]     The shaft axes illustrated may be arranged in one plane or else in a spatial arrangement, in particular in a triangular arrangement.