Abstract:
A drive arrangement for a motor vehicle includes two driven axles, wherein at least one of the axles can be connected and disconnected, with a variable speed transmission having at least one input shaft, two driven shafts, each driven shaft driving a respective axle, and several forward gears and optionally one reverse gear which can be switched via gear wheels and synchronous couplings. The drive arrangement can be easily switched between different drive modes by dividing the variable speed transmission into two sub-transmissions, of which one sub-transmission drives exclusively a first axle via a portion of the forward gears and the one driven shaft and the second sub-transmission drives both driven shafts via the remaining portion of the forward gears and optionally the reverse gear.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     This application claims the priority of German Patent Application, Serial No. 10 2011 011 171.9, filed Feb. 14, 2011, pursuant to 35 U.S.C. 119(a)-(d), the content of which is incorporated herein by reference in its entirety as if fully set forth herein. 
     BACKGROUND OF THE INVENTION 
     The invention relates to a drive arrangement for a motor vehicle with two driven axles. 
     Drive arrangements of this type are essentially divided into permanent drive systems with an interposed interaxle differential for preventing stress in drive systems and so-called hang-on systems with a viscous clutch or the like that is integrated in the drive train to one axle of the motor vehicle. In the latter system, the second axle is activated only when wheel slip is detected. 
     An additional clutch for the overdrive for a drivable tool, such as an earth mover, was already proposed in DE 14 55 901 A1, for example for transporting the tool to another site, which automatically disengages the all-wheel drive when the overdrive is engaged and conducts the drive torque to only one axle. The reason for this measure is, more particularly, to disconnect the particularly highly stepped-up transmission part used in normal operation to prevent high rotation speed loads in overdrive. 
     It is an object of the invention to propose a drive arrangement of the generic type, which provides an advantageous gearbox efficiency in motor vehicles with several forward gears (for example with up to eight gears) and an output to both axles and a simple, easily controllable gear structure. 
     SUMMARY OF THE INVENTION 
     This object is attained with the invention by a drive arrangement for a motor vehicle having two driven axles, wherein at least one of the two driven axles is constructed for connection and disconnection. The drive arrangement includes a variable speed transmission having at least one input shaft, two driven shafts, with each driven shaft driving a corresponding axle of the two driven axles of the motor vehicle, and several forward gears and optionally one reverse gear constructed to be switched via gear wheels and synchronous clutches. 
     According to the invention, it is proposed to divide the change speed gearbox into two partial gearboxes, wherein one partial gearbox drives exclusively a first axle via several forward gears and the one driven shaft and the second partial gear drives both driven shafts via the other forward gears and optionally the reverse gear. It is the intent of the invention to drive both axles with higher output torque in the forward gears and to drive only the one axle of the motor vehicle with lower output torque in the forward gears. The aforementioned partial gearboxes may be only functionally implemented through suitable linkage of the gear elements and may therefore not require additional costs. 
     In particular, it is proposed that in a change speed gearbox with several, for example at least five forward gears, at least the lowest forward gear or several low forward gears, for example two or three low forward gears, and optionally the reverse gear are associated with the second partial gearbox driving both driven shafts, whereas the higher stepped-up forward gears, at least one of the highest gears, may only drive one axle, for example the front axle of the motor vehicle. 
     The two partial gearboxes may be housed in a common gearbox housing, wherein the two driven shafts which are coaxially supported in the gearbox housing may be connectable via only a single additionally required switchable clutch. This additional clutch is required, in particular, when the entire axle drive can be stopped by a suitable decoupling device on the other axle or the other axle drive. 
     In an advantageous embodiment of the invention, the switchable clutch may be a dual-synchronous clutch, wherein the two driven shafts are coupled with one another in one switching position and an additional forward gear, in particular a step-up E-gear, can be added in the other switch position, which drives only the one axle of the motor vehicle via the first driven shaft. When an E-gear is already provided in the change speed gearbox, then the switching clutch switching the E-gear may be modified accordingly. 
     In another advantageous design of the gearbox, the second driven shaft of the change speed gearbox may be a hollow shaft driving a downstream interaxle differential having one driven element connected with the disconnectable axle and another driven element which passes through the hollow driven shaft in reverse gear and which can be coupled with the first driven shaft via the switchable clutch and/or the dual-synchronous clutch. This results in a particularly simple arrangement of the interaxle differential, which can be easily adapted downstream to the change speed gearbox and which may be omitted without substantial changes in the gear structure when all-wheel drive has not been requested. 
     In another advantageous embodiment of the invention, the change speed gearbox may be a dual-clutch gearbox with two coaxially supported input shafts which can be alternatingly driven by the engine via the dual clutch and which, when divided into two partial gearboxes, drive only the one axle of the motor vehicle via the first partial gearbox with several forward gears and drive the second driven shaft, which is arranged axis-parallel to the first driven shaft, via the second partial gearbox via several forward gears and optionally a reverse gear, wherein the second driven shaft is additionally coupled with the first driven shaft via a gear wheel set. With suitable design and association of the forward gears with the two input shafts and the two driven shafts, the two drive modes (exclusively one axle or both axles) can be associated with the lower and higher forward gears without an additional clutch. 
     The second driven shaft can also be constructed as a hollow shaft and drive the differential housing of a downstream interaxle differential, which drives the second axle of the motor vehicle via a first output shaft and which is drivingly coupled in reverse gear via a second output shaft passing through the hollow shaft with the first driven shaft via the gear wheel set. 
     In addition, for attaining a simple structure of the gearbox, the switchable idler gear wheels of the forward gears driving both axles and optionally the reverse gear of the second partial gear may be arranged on the second driven shaft constructed as a hollow shaft, thereby establishing the drive connection to the first driven shaft and the second driven shaft and/or the output shaft for driving both axles without an additional switchable clutch. 
     Alternatively, a dual clutch may be arranged in the second driven shaft or in the output shaft of the interaxle differential for driving the one axle of the motor vehicle, wherein the dual clutch couples in one switching position the two partial gearboxes with each other and actively connects the other switching position an additional forward gear with an idler gear wheel on the driven shaft and/or on the output shaft and with a fixed gear wheel on the one input shaft, allowing an additional forward gear, in particular an E-gear, to be switched simultaneously with an additional clutch. 
     Lastly, when a dual-clutch gearbox is employed and the motor vehicle is in a driving condition with only one driven axle and with wheel slip, a forward gear driving both axles may be temporally connected via the second clutch of the dual-clutch device when a clutch is controlled with slip. Such measure can be relatively easily implemented, because such dual clutches (mostly hydraulically controlled multi-plate clutches) are already operated with slip control for providing comfortable gear shifting with uninterrupted power delivery. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
       Several exemplary embodiments of the invention will now be described in more detail. The schematic drawing shows in: 
         FIG. 1  in form of a block diagram, a change speed gearbox designed for front wheel drive and a drive of a rear axle with several forward gears  1  to  6  and a reverse gear R, wherein the gears R,  1  to  4  drive both driven shafts as partial gearbox, whereas the remaining gears drive only the front axle as a second partial gearbox; 
         FIG. 2  the change speed gearbox of  FIG. 1 , with an additional forward gear which can be activated via a dual-synchronous clutch that couples either both driven shafts or connects the forward gear; 
         FIG. 3  a dual clutch gearbox having again two partial gearboxes, wherein the gears R and  1  to  3  drive both axles of the motor vehicle and the gears  4  to  7  drive only the front axle; and 
         FIG. 4  the dual clutch gearbox of  FIG. 3 , however with an additional dual-synchronous clutch, with which either an additional forward gear can be connected or the drive shafts driving both axles can be coupled. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       FIG. 1  shows in form of a block diagram a change speed gearbox  10  for a drive arrangement for motor vehicles, with an integrated front axle differential  12  having a housing  14  in which a gear input shaft  16  and axis-parallel thereto two coaxially arranged driven shafts  18 ,  20  are rotatably supported. 
     The change speed gearbox  10  has forward gears  1  to  6  and a reverse gear R, which are formed from fixed gear wheels and idler gear wheels (without reference symbol) and arranged on the input shaft  16  and the driven shafts  18 ,  20 , via which dual-synchronous clutches (generally indicated with  22 ) and a single synchronous clutch  24  for the reverse gear R can be alternatingly activated. A reversing gear is connected in the gear stage R in a conventional manner. The synchronous clutches  22 ,  24  of the gearboxes are constructed in a conventional manner. 
     The driven shaft  18  of the change speed gearbox  10  drives via an additional gear wheel set  26  the laterally attached differential  12 , which drives the front wheels of the motor vehicle via half shafts  28  extending transversely to the input shaft  16 . 
     The additional driven shaft  20  is constructed as a hollow shaft operating on the differential housing  30  of a schematically indicated interaxle differential  32 . The one output shaft  36  of the interaxle differential  32  is drivingly connected with the rear differential driving the rear wheels of the motor vehicle (not illustrated), whereas the second output shaft  34  passes in reverse gear through the hollow shaft  20  to a switchable interlocking clutch K, with which the driven shaft  18  can be coupled with the output shaft  34 . 
     The clutch K is also constructed as a single synchronous clutch. The switching elements of the change speed gearbox  10  actuating the clutch K and the synchronous clutches  22 ,  24  are not illustrated; however, they are constructed so that the clutch K is automatically engaged when the gears R and  1  to  4  are engaged, whereas the clutch K is disengaged in the gears  5  and  6 . 
     The input shaft  16  driven by the engine of the motor vehicle via a flywheel  38  and a separation clutch  40  drives as a functionally first partial gearbox the driven shaft or a hollow shaft  20  via the forward gears  1  to  4  and the reverse gear R, wherein the driven shaft or a hollow shaft  20  drives the rear wheels of the motor vehicle via the interaxle differential  32  and the output shaft  34 ,  36 , and the driven shaft  18  drives the front wheels of the motor vehicle in all-wheel-drive, when the clutch K is engaged. 
     When the clutch K is disengaged, the input shaft  16  drives in the forward gears  5  and  6  as second partial gearbox only the driven shaft  18  or the front wheels of the motor vehicle via the front differential  12 , while the output shaft  34  and thus functionally the driven shaft  20  with the interaxle differential  32  is uncoupled. 
     The motor vehicle is hence driven in all gears R and  1  to  4  with all-wheels both forward and in reverse when the synchronous clutch  22 ,  24  is suitably switched and the clutch K is automatically engaged, whereas when the forward gears  5  and  6  are engaged and the clutch K is disengaged, only the front wheels of the motor vehicle are driven. 
       FIG. 2  shows another change speed gearbox  10 ′ of the type described in  FIG. 1  wherein, however, an additional stepped-up 7 th  forward gear is provided. This forward gear is composed of a gear wheel set with a fixed gear wheel  42  on the input shaft and an idler gear wheel  44  on the driven shaft  18 . 
     The additional 7 th  forward gear can be connected via the clutch K 2  which is implemented as a conventional dual-synchronous clutch and which couples, starting from a neutral position, either the output shaft  34  of the interaxle differential  32  with the driven shaft  18  or activates in the other switch position the gear wheel stage  42 ,  44  for the 7 th  gear. 
     By modifying the clutch K 2  and with a corresponding design of the switching elements of the change gear  10 ′, the 7 th  gear is especially constructed as a low-revolution overdrive gear and can be connected in addition to a switch from all-wheel-drive (gears R and  1  to  4 ) to front wheel drive (gears  5  to  7 ). In the gears  5  and  6  the clutch K 2  is hereby in a neutral position, whereas the clutch K 2  is in the gears R and  1  to  4  in the switch position coupling both shafts  16 ,  34  with each other or in the opposite switch position activating the 7 th  gear. 
       FIG. 3  shows an alternative embodiment of a change speed gearbox  50  which is from the embodiment in  FIGS. 1 and 2  and is constructed as a dual-clutch gearbox. Functionally identical parts are indicated with identical reference symbols. 
     The change speed gearbox  50  has two coaxial gearbox input shafts  52 ,  54 , with one of the input shaft  54  constructed as a hollow shaft through which the second input shaft  52  extends. 
     The two input shafts  52 ,  54  are driven via a dual clutch  56  which alternatingly transfers the drive torque from the engine via a flywheel (or ZMS)  38  to the input shafts  52 ,  54 . The dual clutch  56  is constructed in a conventional manner from two wet-running, hydraulically controlled multi-disk clutches. 
     The input shafts  52 ,  54  drive two driven shafts  58 ,  60  via the gear wheel sets R and  1  to  7  having respective idler gear wheels and fixed gear wheels (without reference symbols), with the driven shaft  60  again constructed as a hollow shaft. 
     The forward gears  1  to  7  and the reverse gear R can be activated via dual-synchronous clutches  22  by unillustrated switching elements. 
     The hollow driven shaft  60  drives the differential housing  30  of an interaxle differential  32  having an output shaft  36  which drives (not illustrated) a rear axle differential or the rear wheels of the motor vehicle. 
     The second output shaft  34  of the interaxle differential  32  passes in reverse gear through the driven shaft  60  towards the front and carries a gear wheel  62  of a gear wheel drive  64 , with an additional gear wheel  66  on the driven shaft  58  and a gear wheel  68  driving the front axle differential  12 . 
     The driven shafts  58 ,  60  with the output shaft  34  are arranged relative to the input shafts  52 ,  54  of the gearbox  50  with a mutual offset along the periphery such that their driven gear wheels  62 ,  64  are—different from the illustration in FIG.  3 —always in engagement with the gear wheel  68  (indicated by the dashed line). 
     The idler gear wheels of the gear stages  4 ,  6 ,  5 ,  7  which can be switched via the synchronous clutches  22  are arranged on the input shafts  52 ,  54 , whereas their fixed gear wheels are attached on the driven shaft  58  as indicated. 
     In addition, the idler gear wheels of the gear stages R,  1  to  3  are supported on the hollow driven shaft  60  and can be switched via the additional synchronous clutches  22 , whereas the corresponding fixed gear wheels are arranged on the input shafts  52 ,  54 . 
     This arrangement produces functionally two partial gearboxes, with one of the gearboxes driving in the forward gears  4  to  7  only the front axle differential  12 , whereas the output shaft  34 , the interaxle differential  32  and the driven hollow shaft  60  are switched driveless (the hollow driven shaft  60  can rotate freely). 
     In the gear stages R and  1  to  3 , the input shafts  52 ,  54  alternatingly drive the driven shaft  60  and from there via the interaxle differential  32  the output shaft  36  for the rear wheels and the output shaft  34 , which additionally drives the front axle differential  12  or the front wheels of the motor vehicle via the gear wheel set  64 . 
     The forward gear stages  1  to  7  associated with the input shafts  52 ,  54  and the driven shafts  58 ,  60  are designed in a manner known from dual-clutch gearboxes, wherein the respective next higher forward gear (for example from 3 to 4) is associated in the flux of force with a respective other input shaft (for example gear stage  3  via the input shaft  52 , gear stage  4  via the input shaft  54 , etc.) and hence alternatingly activatable via the two separation clutches of the dual clutch  56 . 
     With a corresponding design of the electronic control of the gearbox  50  and by monitoring of, among others, the wheel slip of the driven wheels of the motor vehicle, the 3 rd  forward gear can be connected when in the 4 th  forward gear (only the front axle is driven) and when an impermissible wheel slip on the front wheels is detected, by engaging the corresponding synchronous clutch  22  and through slip-controlled engagement of the second separation clutch, thereby additionally applying drive torque to the rear wheels. It will be understood that this switching operation can only be temporarily performed when wheel slip is large due to the existing rotation speed differences between the two gear stages. 
       FIG. 4  shows another dual-clutch gearbox  50 ′ which is only described to the extent as the dual-clutch gearbox  50 ′ is different from the dual clutch gearbox  50  of  FIG. 3 . Identical components are indicated with identical reference symbols. 
     According to  FIG. 4 , a dual clutch K 2  is connected in the output shaft  34  between the interaxle differential  32  and the output gear wheel  62  of the gear wheel stage  64 . The dual clutch K 2  interconnects, starting from a neutral position, in one switch position the output shaft  34  and couples in the other switch position an idler gear wheel  70  which is rotatably supported on the front section  34   a  of the output shaft  34  with the fixed gear wheel  72  on the input shaft  52 , thereby activating an 8 th  forward gear (E-gear). The flux of force extends hereby from the input shaft  52  to the uncoupled section  34   a  of the output shaft  34  and via the gear wheel set  64  to the front axle differential  12 . 
     The dual clutch K 2  is again preferably constructed as a conventional dual-synchronous clutch. 
     When the interaxle differential  32  is designed in a conventional manner as a limited-slip differential, an all-wheel-drive can temporarily implemented for the gears  4  to  7  in a switching position of the dual clutch K 2  where the section  34   a  is connected with the additional output shaft  34 , wherein the flux of force extends rearward via the driven shaft  58  and the gear wheel set  64  to the output shaft  34  and via the limited-slip function of the interaxle differential  32  from the output shaft  34  to the output shaft  36  and to the rear wheels of the motor vehicle. 
     The output shaft  34  is disconnected and uncoupled from the gear wheel set  64  in the neutral position of the dual clutch K 2 . 
     As a result, the 8 th  forward gear can also only drive the front axle differential  12 , because the output shaft  34  to the interaxle differential  32  is also uncoupled in the corresponding switch position of the dual clutch K 2 . 
     The invention is not limited to the aforedescribed exemplary embodiments. The clutch K and/or the dual clutch K 2  may also be constructed as a hydraulically controls multi-disk clutch. The described partial gearboxes may optionally also be housed in two gearbox housings, with a suitable connection (for example plug-in connections) between the driving shafts and the driven shafts.