Abstract:
A gearbox is provided for a motor vehicle that includes, but is not limited to at least a first input shaft and at least a first output shaft. The first input and output shafts carrying at least a first drive gearwheel and a first driven gearwheel, respectively, for establishing a forward gear ratio, and a second drive gearwheel and a second driven gearwheel, respectively, coupled by a 10 first intermediate gearwheel for establishing a reverse transmission ratio. The second driven gearwheel meshes with a third driven gearwheel on a second output shaft for establishing a further forward gear ratio.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to Great Britain Patent Application No. 0805825.7, filed Mar. 31, 2008, which is incorporated herein by reference in its entirety. 
     TECHNICAL FIELD 
     The present invention relates to a gearbox for a motor vehicle comprising at least a first input shaft and at least a first output shaft. The first input and output shafts carrying at least a first drive gearwheel and a first driven gearwheel, respectively, for establishing a forward transmission ratio, and a second drive gearwheel and a second driven gearwheel, respectively, coupled by a first intermediate gearwheel for establishing a reverse transmission ratio. 
     BACKGROUND 
     In recent times, gearboxes having coaxial input shafts as disclosed (e.g., in DE 199 23 185 A1) have been the subject of intensive development efforts, due to the expectation that in combination with a double clutch for driving the two input shafts they will allow to combine advantages of conventional automatic transmissions, such as easy handling and interruption-free shifting, and of conventional manual transmissions such as fast shifting and high power efficiency. 
     The double clutch required for the operation of a gearbox having coaxial input shafts occupies considerable space, so that a gearbox of this type is difficult to integrate in an engine compartment of a compact motor vehicle. This causes an increasing demand for gearboxes which can accommodate a high number of gears in spite of reduced dimensions, in particular in the longitudinal direction of the shafts. 
     In view of the foregoing, at least one object of the present invention is to provide a gearbox capable of satisfying this demand. In addition, other objects, desirable features, and characteristics will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and this background. 
     SUMMARY 
     The at least object, other objects, desirable features, and characteristics, are achieved by a gearbox for a motor vehicle comprising at least a first input shaft and at least a first output shaft. The first input and output shafts carrying at least a first drive gearwheel and a first driven gearwheel, respectively, for establishing a forward transmission ratio, and a second drive gearwheel and a second driven gearwheel, respectively, coupled by a first intermediate gearwheel for establishing a reverse transmission ratio. The second driven gearwheel meshes with a third driven gearwheel on a second output shaft for establishing a further forward gear ratio. 
     Since the further gear ratio is subject to higher friction loss than a gear ratio established by meshing gearwheels of input and output shafts, it is preferred that the further forward gear ratio is the first gear. Here, the impact of the additional friction loss is smaller than with any higher gear. 
     Preferably, the gearbox comprises a second intermediate gearwheel rigidly coupled to the first intermediate gearwheel, and the first intermediate gearwheel meshes with the second driven gearwheel and the second intermediate gearwheel meshes with the second drive gearwheel. This causes the second drive gearwheel and the second driven gearwheel to be axially offset with respect to each other, and reduces constraints on the radii of these gearwheels. 
     Further, the second intermediate gearwheel should have a larger radius than the first intermediate gearwheel. In a gearbox where the first gear or the reverse gear is established by a pair of meshing gearwheels only, the driven gearwheel must be rather large. If the intermediate gearwheels are used, and if the second intermediate gearwheel is larger than the first, the driven gearwheel may also be made smaller, allowing a compact design of the gearbox. 
     The intermediate gearwheels may be carried by one of the output shafts. In this way the number of components of the gearbox can be kept small, but it may be difficult to satisfy constraints on the radii of the gearwheels which may be imposed (e.g., by a required spacing between the various gear ratios). 
     If the number of output shafts is at least three, the intermediate gearwheels are preferably carried by a third output shaft distinct from the first and second ones. In that case, a synchronizer should be provided for selectively locking the intermediate gearwheels to the third output shaft. 
     In order to reduce constraints on gearwheel radii, it may be desirable to provide a layshaft that carries the intermediate gearwheels. 
     Although the present invention is not restricted to a gearbox in which the first input shaft is hollow and a second input shaft extends coaxially through the first input shaft, this type of gearbox is a preferred field of application of the invention for the reasons given above. 
     In order to enable switching between consecutive gears without interrupting torque flow, drive gearwheels of one of the two input shafts should be used for establishing odd-numbered gears and drive gearwheels of the other input shaft for establishing even-numbered gears. 
     Preferably, a third output shaft is provided. In that case, the axial dimension of the gearbox may be reduced still further if at least one drive gearwheel meshes with driven gearwheels of the first and third output shafts. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will hereinafter be described in conjunction with the following drawing figures, wherein like numerals denote like elements, and 
         FIG. 1  is an axial cross section of a gearbox according to a first embodiment of the invention; 
         FIG. 2  is an axial cross section of a gearbox according to a second embodiment of the invention; 
         FIG. 3  is a radial cross section of a gearbox according to the first or second embodiment; and 
         FIG. 4  is an axial cross section of a gearbox according to a third embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     The following detailed description is merely exemplary in nature and is not intended to limit application and uses. Furthermore, there is no intention to be bound by any theory presented in the preceding background or summary or the following detailed description. 
     The gearbox illustrated in  FIG. 1  has a solid input shaft  1 , output shafts  2 ,  3 ,  4  and a layshaft  5 , rotatably held in bearings of a gearbox casing, not shown. Part of the solid input shaft  1  is surrounded coaxially by a hollow input shaft  6 . The two input shafts are selectively coupled to a combustion engine by a double clutch, not shown. The three output shafts carry output pinions  7 ,  8 ,  9 , all of which mesh with a same ring gear, not shown. 
     Hollow input shaft  6  carries two drive gearwheels  10 ,  11 . Gearwheel  10  meshes with a first intermediate gearwheel  14  of layshaft  5 . A second intermediate gearwheel  15  of layshaft  5  drives a gearwheel  16  rotatably mounted on output shaft  3 . Gearwheel  16 , in turn, drives a gearwheel  23  rotatably  15  mounted on output shaft  2 . Besides, gearwheel  10  meshes with a gearwheel  17  rotatably mounted on output shaft  4 . 
     Gearwheel  11  is engaged with driven gearwheels  18 , rotatably mounted on output shafts  3 ,  4 ,  20  respectively. Solid input shaft  1  carries two drive gearwheels  12 ,  13 . Gearwheel  12  is engaged with driven gearwheels  20 ,  21  rotatably mounted on output shafts  3 ,  4 , respectively. Gearwheel  13  meshes with a driven gearwheel  22  of output shaft  4 . 
     The driven gearwheels  17  to  23  have synchronizers  24  to  28  associated to them for selectively locking one or the other of them to the output shaft carrying it. In the first gear, only synchronizer  28  is engaged, so that torque is transmitted from hollow input shaft  6  via gearwheels  10 ,  14 ,  15 ,  16 ,  23  to output shaft  2 . 
     The second gear is established by engaging synchronizer  27  with gearwheel  22 , so that torque is transmitted from solid input shaft  1  to output shaft  4  via gearwheels  13 ,  22 . 
     Although gearwheel  13  is the smallest of all drive gearwheels, it can be used for the second gear and not for the first, as might be expected, because in the first gear, the speed of rotation is reduced at least two pairs of meshing gearwheels namely  10  and  14 ,  15  and  16 , and might be reduced also between gearwheels  16  and  23 , if desired, and because gearwheel  15  is smaller than gearwheel  14 . 
     Gearwheel  10  is also used for the third gear, by locking gearwheel  17  to output shaft  4  using synchronizer  26 . 
     The fourth gear is established by locking gearwheel  20 , driven by gearwheel  12 , to output shaft  3  using synchronizer  25 . 
     Drive gearwheel  12  is also used for the sixth gear, by locking gearwheel  21  to output shaft  4  using synchronizer  27 . A difference in transmission ratios of fourth and sixth gears is caused by different distances between input shaft  1 , on the one hand, and output shafts  3 ,  4 , on the other. 
     Similarly, drive gearwheel  11  is used in fifth and seventh gears by locking either gearwheel  18  to output shaft  3  using synchronizer  24  or gearwheel  19  to output shaft  4  using synchronizer  26 . Synchronizer  24  is also used for establishing the reverse gear, by locking gearwheel  16  to output shaft  3 . 
     As can be seen in  FIG. 1 , gearwheel  14  extends between gearwheel  16  and its associated synchronizer  24 , so that a locking sleeve of synchronizer  24  must either have a rather small radius in order to pass by gearwheel  14  and engage a toothing of gearwheel  16 , or gearwheel  16  must be solidly connected to a toothing by a sleeve  29  spanning the width of gearwheel  14 , as shown in phantom in  FIG. 1 . This problem is avoided in the embodiment shown in  FIG. 2 . 
     The second embodiment illustrated in  FIG. 2  is identical to the first embodiment in most respects, except for the order of intermediate gearwheels  14 ,  15 . Here, the smaller one of the two intermediate gearwheels is facing synchronizer  24 , so that it can freely accede to gearwheel  16 . Due to the positions of gearwheels being swapped, gearwheel  16  is placed further left than in the embodiment of  FIG. 1 , and in order to provide sufficient space for synchronizer  24 , gearwheel  18  is displaced to the left, too. Since gearwheels  10 ,  17  are displaced to the right, gearwheel  11  must be made broader than in the first embodiment, and gearwheels  18 ,  19  mesh with different zones of the circumference of gearwheel  11 . In fact, gearwheel  11  may be split into two gearwheels  11   a ,  11   b , which might even, if desired, have different radii. 
       FIG. 3  illustrates a schematic cross section of the gearboxes of  FIGS. 1 and 2  perpendicular to the input shafts  1  to  6 . Selected gearwheels are shown and identified by their above-defined reference numerals, not all of which are located in the same plane. Reference numeral  30  refers to the ring gear engaged by pinions  7 ,  8 ,  9 . 
       FIG. 4  is an axial cross section similar to  FIGS. 1 and 2 , of a third embodiment of the invention. In this embodiment, second to seventh gears are established just in the same way as in the embodiments of  FIGS. 1 and 2 , so that a description thereof will not be repeated. 
     The gearbox of  FIG. 4  has no more lay shaft, there are only input shafts  1 ,  6  and output shafts  2 ,  3 ,  4 . The functions of intermediate gearwheel  14  is here fulfilled by gearwheel  17 , which has intermediate gearwheel  15  rigidly coupled to it. Since gearwheel  10  is here placed at the end of hollow shaft  6 , and gearwheel is placed beyond this end, gearwheel  16  can be prevented from interfering with the input shafts in spite of its large diameter. 
     The fact that gearwheel  17  is used for the third gear and for the reverse gear imposes certain constraints on the transmission ratio the reverse gear can have. In order to avoid such constraints for the transmission ratio of the first gear, a further gearwheel is introduced, which is rigidly coupled to gearwheel  16  on output shaft  3 , and which meshes with gearwheel  23  of output shaft  2 . 
     While at least one exemplary embodiment has been presented in the foregoing summary and detailed description, it should be appreciated that a vast number of variations exist. It should also be appreciated that the exemplary embodiment or exemplary embodiments are only examples, and are not intended to limit the scope, applicability, or configuration in any way. Rather, the foregoing summary and detailed description will provide those skilled in the art with a convenient road map for implementing an exemplary embodiment, it being understood that various changes may be made in the function and arrangement of elements described in an exemplary embodiment without departing from the scope as set forth in the appended claims and their legal equivalents.