Abstract:
A drive assembly for a motor vehicle driven by a plurality of axles includes a differential unit, an externally controllable hang-on coupling, an externally controllable locking coupling, a first hydraulic actuating unit, and a second hydraulic actuating unit. The differential unit includes an input part that is rotatably drivable around an axis of rotation, and two output parts drivingly connected to the input part. The hang-on coupling drivingly connects the differential unit and a driveshaft. The locking coupling locks the differential movement between the two output parts of the differential assembly. The first hydraulic actuating unit actuates the hang-on coupling and the second hydraulic actuating unit actuates the locking coupling. The first hydraulic actuating unit, the hang-on coupling, the second hydraulic actuating unit and the locking coupling are arranged coaxially relative to the axis of rotation.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a National Phase of International Application No. PCT/EP2009/009097, filed on Dec. 18, 2009, which claims priority to German Application No. 10 2008 063 905.2 filed on Dec. 19, 2008, which applications are hereby incorporated by reference in their entirety. 
     TECHNICAL FIELD 
     The disclosure relates to a coupling assembly for connecting a drive axle in a multi-axle driven motor vehicle. Generally, there are two different drive systems: one where a plurality of drive axles of the motor vehicle is permanently driven and one where a first drive axle is driven permanently, with a second drive axle being connected when required. Such drive systems with an optionally connectable drive axle are also referred to as hang-on or on-demand systems. 
     BACKGROUND 
     From EP 0 466 863 B1 there is known a device for connecting a driveline in a motor vehicle with a transfer case for a plurality of drivelines. One of the drivelines is permanently connected to a drive unit and a further driveline can optionally be connected to the drive unit. For connecting the driveline, there is provided an electronically actuated friction coupling which can be arranged in a transfer case or in a differential drive. 
     DE 10 2008 037 885.2-21 proposes a drive assembly with a hang-on coupling and a differential unit for a multi-axle driven motor vehicle. The hang-on coupling is provided in the form of a friction plate coupling which is arranged coaxially relative to the axis of rotation of the differential carrier of the differential unit. 
     From DE 39 13 487 A1 there is known a four-wheel drive tractor whose front axle differential and rear axle differential are driven permanently during off-road applications. In order to prevent any distortions between the front axle and the rear axle there is provided a coupling between the ring gear of the rear axle differential and the differential carrier. The coupling is ventilated via an actuating cylinder and can thus be controlled under conditions of slip. In the unpressurized condition of the actuating cylinder, the coupling is loaded by a plate spring so that torque is transmitted from the front axle in a rigid through-drive to the rear axle. 
     From DE 42 02 026 A1 there is known a drive assembly for controlling and distributing the driving force for a motor vehicle which comprises two driven axles. The drive assembly comprises a rear axle differential which is controlled via a first valve, a front axle differential lock which is controlled via a second valve, and an intermediate differential lock which is controlled via a third valve. Said valves are electro-hydraulic valves which receive their hydraulic pressure from a pump. 
     SUMMARY 
     A drive assembly for a multi-axle driven motor vehicle is disclosed. The drive assembly comprises a differential unit with an input part which is rotatingly drivable around an axis of rotation A, and two output parts which are drivingly connected to the input part. The two output parts have a differential effect relative to one another. The drive assembly further comprises an externally controllable hang-on coupling for drivingly connecting the differential unit to a driveshaft; an externally controllable locking coupling for locking the differential movement between the two output parts of the differential unit; a first hydraulic actuating unit for actuating the hang-on coupling; and a second hydraulic actuating unit for actuating the locking coupling. The first hydraulic actuating unit, the hang-on coupling, the second hydraulic actuating unit and the locking coupling are arranged coaxially relative to the axis of rotation A. 
     The exemplary drive assembly is advantageous in that it permits two functions, namely additionally connecting a secondary driveline and locking the differential movement of the differential unit, and at the same time, it has a compact design. The actuation of the first hydraulic actuating unit and of the second hydraulic actuating unit can be achieved by one single pump, wherein the level of the hydraulic pressure and thus the degree of locking of the couplings is set by the pump output. The hang-on coupling and the locking coupling are closed by generating a hydraulic pressure. This is advantageous in that no energy is required for keeping the couplings open. If it is required for one of the couplings to be closed, the pump generates a hydraulic pressure which closes the hang-on coupling, respectively the locking coupling. Because the hang-on coupling and the locking coupling as well as the respective hydraulic actuating units are arranged coaxially relative to one another and relative to the axis of rotation of the differential unit, there is achieved a modular design which permits easy integration into existing installation space concepts. A further advantage of the exemplary drive assembly includes that the angle drive, by which torque is introduced from the propeller shaft into the drive assembly, is stationary when the hang-on coupling is open. In this way it is possible to avoid any splashing losses which are caused by the rotation of the ring gear around the axis of rotation, as well as friction losses in the bearings of the ring gear and of the driving pinion, more particularly as a result of pretension. This, in turn, advantageously effects the fuel consumption of the motor vehicle. 
     According to one exemplary embodiment, there is provided a single pump which is hydraulically connected both to the first hydraulic actuating unit and to the second hydraulic actuating unit. In this way, the number of parts of the exemplary drive assembly is reduced, which has an advantageous effect on the installation volume and the weight of the unit. In principle, the pump can have any design; for instance it can be provided in the form of a gerotor pump or a vane type pump. The pump may be driven by an electric motor which, in turn, is controlled by an electronic control unit for controlling the driving dynamics of the motor vehicle. The level of the hydraulic pressure can be variably set by the speed of the electric motor, i.e. the torque to transmitted to the drive axle is determined by the electronic control unit as a function of the vehicle condition, and the electric motor is controlled accordingly in order to actuate the hang-on coupling and the locking coupling in accordance with requirements. Depending on the output of the electric motor, the hang-on coupling and, if necessary, the locking coupling, can be controlled between an open position in which no torque is transmitted from the input part of the coupling to the output part of the coupling, and a closed position in which the coupling is fully closed and in which the input part rotates jointly with the output part. To achieve a high setting accuracy, the hydraulic system may be provided with a pressure sensor which transfers the respective data to the electronic control unit. 
     According to a first exemplary embodiment, the hydraulic assembly is provided with a switching valve which serves to connect the locking coupling via a respective hydraulic actuating unit. The switching valve is arranged in a connecting channel between the pump and the hydraulic actuating unit and can be switched between an open position, in which hydraulic pressure is transmitted to the second hydraulic actuating unit, and a closed position in which the hydraulic pressure supply is interrupted. In the closed condition, hydraulic pressure is transmitted only to the first hydraulic actuating unit for actuating the hang-on coupling, whereas the second hydraulic actuating unit is not loaded, and the locking coupling is open. In this switched condition, the differential unit functions as an open differential, i.e. there occurs no locking effect between the output parts. On the other hand, in the open position of the switching valve, hydraulic pressure is transmitted both to the first actuating unit for the hang-on coupling and to the second actuating unit for the locking coupling, so that torque is transmitted from the primary drive axle to the secondary drive axle and the differential unit is locked. 
     In this exemplary embodiment, the first hydraulic actuating unit and the second hydraulic actuating unit are supplied with the same hydraulic pressure. Thereby, a generatable coupling moment of the hang-on coupling and a generatable coupling moment of the locking coupling comprises a fixed ratio relative to one another. This ratio is substantially influenced by the ratio of the piston faces of the two hydraulic actuating units relative to one another and by the geometry and number of coupling plates of the two couplings. By controlling the pump accordingly, the required torque to be transmitted to the hang-on coupling is controlled as a function of the driving condition of the motor vehicle. If, in addition, there is required a locking function of the differential unit, the switching valve is transferred into the open position. 
     According to a second embodiment, there is provided a proportional valve for connecting the locking coupling by the respective hydraulic actuating unit. The proportional valve which may be arranged in a connecting channel between the pump and the second hydraulic actuating unit can be variably set between an open position in which the hydraulic pressure is transmitted to the second hydraulic actuating unit and a closed position in which the supply of hydraulic pressure is interrupted. 
     Due to this embodiment, the torque transmitted by the hang-on coupling can be variably set, i.e. by controlling the pump accordingly, as well as the torque to be transmitted by the locking coupling can be variably set, i.e. by respectively controlling the proportional valve. As a result, the torque is distributed, as required, to the connectable drive axle and to the individual wheels of the drive axle, so that the driving stability and driving dynamics of the vehicle are always ensured, even under changing operating conditions. If the locking function is required, the proportional valve is set accordingly, so that the locking function is suitably set by the second hydraulic actuating unit. By compressing the locking coupling, the carrier element of the differential unit, also referred to as differential carrier or differential cage, is connected in a friction-locking way to the sideshaft gear. In this way, the balancing effect of the sideshaft gears is suppressed, i.e. the differential unit is locked. 
     According to a third exemplary embodiment, the hydraulic assembly comprises a first proportional valve, by which the first hydraulic actuating unit for the hang-on coupling can be variably controlled, as well as a second proportional valve by which the second hydraulic actuating unit for the locking coupling can be variably controlled. “Variably controlled” in this context means that the respective proportional valve can assume any intermediate position between an open position and a closed position, so that the respective torque to be transmitted can be set accurately according to requirements. 
     This exemplary embodiment permits the torque to be transmitted by the hang-on coupling and also the torque to be transmitted by the locking coupling to be set by accordingly controlling the associated proportional valve. There is no need, in this case, to carry out a control operation by varying the pump output. The hang-on coupling and the locking coupling can be switched completely independently of one another, which permits a high degree of flexibility for controlling the driving dynamics of the motor vehicle. More particularly, this embodiment also permits a method of control in that when the hang-on coupling is open, the locking coupling is closed. In this way, the secondary drive axle is locked, which has a stabilizing effect when the motor vehicle is driven in a straight line. 
     According to an exemplary design which applies to all the above-mentioned embodiments, there is provided a further switching valve which is arranged in a connecting channel between the pump and a pressure sensor and which can be switched between an open position in which hydraulic pressure is transmitted to the first actuating unit, and a closed position in which the supply of hydraulic pressure is interrupted. In an advantageous way, said switching valve permits a reduction in the energy consumption of the pump, more particularly in the case of longer actuation periods. In the closed condition, the hydraulic line is closed, so that the hydraulic pressure in the line system can be maintained even if the pump is not running. The pump can be switched off, which has an advantageous effect on the fuel consumption. In the open position of the switching valve, the pump is again connected to the hydraulic system and generates a hydraulic pressure for loading the first, respectively the second hydraulic activating unit. 
     The hang-on coupling may be effectively arranged between the driveshaft and the input part of the differential unit, and the locking coupling may be effectively arranged between the input part and one of the output parts of the differential unit. To achieve a compact design, in one exemplary arrangement it is advantageous if, with reference to the central plane of the differential unit which extends through the radial axes of the differential gears, the locking coupling and the hang-on coupling are arranged on the same side. Furthermore, from the packaging point of view and to achieve a satisfactory introduction of force, in one exemplary arrangement it is advantageous if the first hydraulic actuating unit and the second hydraulic actuating unit are arranged axially between the locking coupling and the hang-on coupling. The connections for the first and for the second actuating unit are thus positioned closely together, thus achieving short line lengths. 
     The hang-on coupling and the locking coupling are designed in accordance with the required torque to be transmitted, and there is achieved a compact installation space if the locking coupling and the hang-on coupling comprise the same effective friction radius. It is proposed in one exemplary arrangement that the hang-on coupling can be loaded by the first hydraulic actuating unit via a first pressure plate and that the locking coupling can be loaded by the second hydraulic actuating unit via a second pressure plate, with the first and the second pressure plate, when being actuated, being loaded away from one another. The hang-on coupling and the locking coupling may be accommodated in a housing and axially supported relative thereto, which housing can be composed of a plurality of housing parts, thus ensuring that the axial forces are well supported and satisfactorily introduced into the housing when the hang-on coupling and the locking coupling are actuated. 
     In another exemplary embodiment, the first hydraulic actuating unit and the second hydraulic actuating unit are each provided in the form of a piston/cylinder unit. In one exemplary design, in the sense of the concept of having identical components is achieved if the piston/cylinder units are identical in design. The pistons may be provided in the form of annular pistons and arranged coaxially relative to the axis of rotation A. 
     According to another exemplary arrangement, the hang-on coupling is provided in the form of a friction plate coupling, wherein a first plate carrier of the hang-on coupling is at least indirectly connected to the driveshaft in a rotationally fixed way and wherein a second plate carrier of the hang-on coupling is at least indirectly connected to the input part of the differential unit in a rotationally fixed way. Providing the hang-on coupling in the form of a friction plate coupling is advantageous in that the torque to be transmitted can be variably set in accordance with requirements by controlling the first hydraulic actuating unit, and by the friction plate coupling it is possible to achieve any desirable intermediate position between an open position in which no torque is transmitted between the input part of the coupling and the output part of the coupling, and a closed position in which the coupling parts are fully closed and jointly rotate around the axis of rotation. 
     The locking coupling may be provided in the form of a friction plate coupling, wherein a first plate carrier of the locking coupling is at least indirectly connected to the differential carrier of the differential unit in a rotationally fixed way and wherein a second plate carrier of the locking coupling is at least indirectly connected to a sideshaft gear of the differential unit. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Exemplary embodiments will be explained below with a reference to the drawings wherein: 
         FIG. 1  shows an exemplary embodiment of a drive assembly with a hydraulic assembly in half a longitudinal section. 
         FIG. 2  shows a second exemplary embodiment of a drive assembly with a hydraulic assembly in half a longitudinal section. 
         FIG. 3  shows a third exemplary embodiment of a drive assembly with a hydraulic assembly in half a longitudinal section. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows an exemplary drive assembly  2  in a first embodiment. The drive assembly  2  comprises a differential unit  3 , a hang-on coupling  4  which, if required, is able to connect the drive axle arranged behind same in the torque flow, a locking coupling  5  for optionally locking the differential movement of the differential unit  3 , as well as a first actuating unit  6  for actuating the hang-on coupling  4 , and a second actuating unit  21  for actuating the locking coupling  5 . 
     The drive assembly comprises a ring gear  7  via which, by a pinion (not illustrated), torque can be introduced into the drive assembly  2 . The ring gear  7  is firmly connected to a flange of a driveshaft  8  provided in the form of a hollow shaft and supported via bearing mechanisms  9 ,  9 ′ so as to be rotatable around an axis of rotation A in the housing  10  of the drive assembly  2 . At its end facing the differential unit  3 , the hollow shaft  8  comprises splines  11  which engage correspondingly designed splines of a first coupling part  12  of the hang-on coupling  4  for torque transmitting purposes. The coupling part  12  is provided in the form of a coupling cage which is arranged coaxially relative to the axis of rotation A and inside which there is arranged the differential unit  3 . It can be seen that the hang-on coupling  4  is provided in the form of a friction plate coupling which comprises an outer plate carrier to which there are connected outer plates in an axially displaceable and rotationally fixed way, as well as an inner plate carrier to which inner plates are connected in an axially displaceable and rotationally fixed way. The outer plates and inner plates are arranged so as to axially alternate and jointly form the plate package  14  of the hang-on coupling  4 . The inner plate carrier forms the second coupling part  13  of the hang-on coupling  4 , which can also be referred to as coupling output part. 
     The second coupling part  13  is firmly connected to a differential carrier  15  of the differential unit  3  and may be produced so as to be integral therewith. In the differential carrier  15 , differential gears  16  are rotatably supported on journals  17  and jointly rotate with the differential carrier  15  around the axis of rotation A. The differential gears  16  engage sideshaft gears  18 , 18 ′ which are arranged coaxially relative to the axis of rotation A and which, via abutment discs, are axially supported relative to the differential carrier  15 . The sideshaft gears  18 ,  18 ′ each comprise a bore with longitudinal teeth into which an associated sideshaft  19  is inserted for transmitting torque, of which only the righthand sideshaft is shown. The differential carrier  15  comprises a first carrier part  22 , which, via a bearing mechanism  23 , is supported relative to the hollow shaft  8  so as to be rotatable around the axis of rotation A, as well as a second carrier part  24  which is rotatably supported at the opposed end of the sideshaft  19  and comprises a sleeve projection  25 . The differential carrier  15  is connected by said sleeve projection  25  to the input part of the locking coupling  5  in a rotationally fixed way. 
     The first hydraulic actuating unit  6  is provided in the form of a piston/cylinder unit and comprises an annular piston  26  which is axially displaceably arranged in an annular cylinder  27 . The annular piston  26  and the annular cylinder  27  are arranged coaxially relative to the axis of rotation A and, respectively, to the hang-on coupling  4 . The annular piston  26  is sealed relative to the outer wall and the inner wall of the cylinder by annular seals  28 ,  29 . The annular piston  26  delimits an annular hydraulic chamber  30 , i.e. when a hydraulic pressure is generated in the hydraulic chamber, the annular piston  26  is axially loaded towards the hang-on coupling  4 . This leads to the plate package  14  of the hang-on coupling being compressed and, thus, to the secondary drive axle being drivingly connected to the propeller shaft. Via a passage, the hydraulic chamber  30  is connected to a connector  31  which serves to be connected to a conduit  53  of the hydraulic assembly  50 . 
     At its end face facing away from the hydraulic chamber  30 , the annular piston  26  comprises a recess in which an axial bearing  33  is arranged. The annular piston  26  is axially supported via the axial bearing  33  relative to a pressure plate  34 . By axially displacing the annular piston  26 , the pressure plate  34  is also displaced and axially loads the plate package  14 . In this way, torque is transmitted from the first coupling part  12  to the differential carrier  15 . In the fully open condition of the hang-on coupling  4 , no torque at all is transmitted, so that the first coupling part  12  including the hollow shaft  8  and the ring gear  7  are stationary, whereas in the fully closed condition of the hang-on coupling, torque is transmitted from the ring gear  7  to the sideshafts  19 ,  19 ′. For supporting the axial forces which are generated during the actuation of the hang-on coupling  4 , there is provided an axial bearing  37  which is axially arranged between an abutment face of the housing  10  and a supporting face of the first coupling part  12 . Between the pressure plate  34  and the second coupling part  24  there is arranged a returning spring  35  which, in the unactuated condition of the actuating unit  6 , returns the pressure plate  34 , i.e. it axially loads same towards the hydraulic chamber  30 . In the present embodiment, the returning spring is provided in the form of a plate spring, but other types of spring are also conceivable, for instance helical springs. 
     The locking coupling  5  comprises a first coupling part  42  which, via a splined connection  43 , is connected to the differential carrier  15  in a rotational fixed way, as well as a second coupling part  44  which, via a splined connection  46 , is connected to the sideshaft  19  in a rotationally fixed way. It can be seen that the locking coupling  5  is provided in the form of a friction plate coupling, with the first coupling part  42  comprising an inner plate carrier to which inner plates are connected in a rotationally fixed and axially displaceable way. The outer plates and inner plates are arranged so as to axially alternate and, together, form the plate package  45 . The first coupling part  42 , more particularly, comprises two sleeve portions and a flange portion connecting the two sleeve portions. The second coupling part  44 , which is approximately C-shaped in a longitudinal section, is axially supported relative to the housing  10  via an axial bearing  47 . 
     Like the first actuating unit  6 , the second hydraulic actuating unit  21  is provided in the form of a piston/cylinder unit and comprises an annular piston  26 ′ which is axially displaceably arranged in an annular cylinder  27 ′. The annular piston  26 ′ and the cylinder  27 ′ are arranged coaxially relative to the axis rotation A and to the locking coupling  5 . The annular piston  26 ′ is sealed relative to the outer wall and to the inner wall of the cylinder  27 ′ by annular seals  28 ′,  29 ′. The annular piston  26 ′ delimits the hydraulic chamber  30 ′, i.e. when a hydraulic pressure is generated in the hydraulic chamber  30 ′, the annular piston  26 ′ is axially loaded towards the locking coupling  5 . Via a passage, the hydraulic chamber  30 ′ is connected to a connecting device  31 ′ which serves to be connected to a conduit  57  of a hydraulic assembly. 
     At its end face facing away from the hydraulic chamber  30 ′, the annular piston  26 ′ comprises a recess in which there is arranged an axial bearing  33 ′ via which the annular piston  26 ′ is axially supported relative to the pressure plate  34 ′. By axially displacing the annular piston  26 ′, the pressure plate  34 ′ is axially displaced, which pressure plate  34 ′ axially loads the plate package  36 . In this way, torque is transmitted from the differential carrier  15  to the sideshaft  19 . In the fully open condition of the locking coupling  5 , the sideshaft gears  18 ,  18 ′ are able to rotate freely relative to one another; the differential unit functions as a so-called open differential. In the closed condition of the locking coupling  5 , the rotational movement of the sideshaft  19  is adapted to that of the differential carrier, so that the compensating movement between the sideshaft gears  18 ,  18 ′ and the sideshafts  19  connected thereto is reduced. Between the pressure plate  34 ′ and the second coupling part  42  there is arranged a returning spring  35 ′ which, in the unactuated condition of the actuating unit  21 , returns the pressure plate  34 ′, so that the locking coupling  5  is again ventilated. In the present exemplary embodiment, the returning spring is provided in the form of a plate spring, with other types of spring also being conceivable, such as helical springs. 
     The first and the second actuating units  6 ,  21  are axially arranged in the housing  10  between the two couplings  4 ,  5 . An annular housing part  38  forms a separating web between the two hydraulic chambers  30 ,  30 ′. Because the two hydraulic actuating units  6 ,  21  are arranged so as to directly adjoin one another, there may be obtained short line paths for the hydraulic assembly. 
     Below, there will follow a more detailed description of the hydraulic assembly  50  of the drive assembly  2 . The hydraulic assembly  50  comprises a pump  51 , which is able to convey hydraulic fluid from a reservoir  52  and is thus able to generate a hydraulic pressure for actuating the first and the second actuating units  6 ,  21 . 
     In the conduit  53  connecting the pump  51  to the first actuating unit  6  and, respectively, to the second actuating unit  21 , there is provided a pressure sensor  54  which records the pressure in the line  53  and transmits a signal as an input value to the electronic control unit which controls the driving dynamics of the motor vehicle. The pump  51  is preferably controlled by an electric motor  55  which, in turn, is controlled by the electronic control unit. The hydraulic pressure generated by the pump  51  is set by the output of the electric motor  55 , so that the required torque to be transmitted to the optionally connectable secondary drive axle can be set as a function of the driving condition of the motor vehicle. 
     Furthermore, the hydraulic assembly  50  comprises a switching valve  56 , which serves to connect the locking coupling  5  to the respective hydraulic actuating unit  21 . The switching valve  56  arranged in a connecting channel  57  can be switched in an open position in which the hydraulic pressure is transmitted to the hydraulic actuating unit  21  and in a closed position in which the hydraulic pressure supply is interrupted. In the closed position hydraulic pressure is only transmitted from the pump  51  to the first hydraulic actuating unit, whereas the second hydraulic actuating unit  21  is not loaded. In this switched condition, the locking coupling  5  is opened so that the differential unit  3  functions as an open differential, with no locking effect being generated between the sideshaft gears  18 ,  18 ′. In the open condition of the switching valve  56 , hydraulic pressure is transmitted to both the first actuating unit  6  and to the second actuating unit  21 , so that torque is transmitted from the primary driving axle to the secondary driving axle, and addition, the differential unit  3  is locked. 
     Optionally, the hydraulic assembly  50  can comprise a second switching valve  58  which would have to be arranged in the line  53  and which, in the present exemplary illustration, is shown in dashed lines. The switching valve  58  can be switched between an open position in which the hydraulic pressure is transmitted to the first actuating unit  6  as well as to the second actuating unit  21 , and in a closed position in which the hydraulic pressure supply is interrupted. In the closed position, the hydraulic line  53  is closed, so that the hydraulic pressure in the line system is maintained even without an activated pump  51 . The electric motor  55  and thus the pump  51  can be switched off, which has an advantageous effect on the energy consumption. This is advantageous in the exemplary case of longer actuation periods of the hang-on coupling  4  and the locking coupling  5 . In the open position of the switching valve  58 , the pump  51  is again connected to the hydraulic system and generates a hydraulic pressure for actuating the first and the second hydraulic actuating unit  6 ,  21 . 
     In the present exemplary embodiment, the first hydraulic actuating unit  6  and the second hydraulic actuating unit  21  are loaded by the same hydraulic pressure from the pump  51 , with the generatable coupling moment of the hang-on coupling  4  and the generatable coupling moment of the locking coupling being at a fixed ratio relative to one another. By controlling the pump  51  accordingly, the torque to be transmitted by the hang-on coupling  4  is controlled as a function of the driving condition of the motor vehicle in accordance with requirements. If, additionally there is required a locking function of the differential  3 , the switching valve  56  is transferred into the open position. 
       FIG. 2  shows an alternative, exemplary drive assembly  2  in a second embodiment which largely corresponds to that shown in  FIG. 1 . To that extent, as far as common features are concerned, reference is made to the above description, with identical components or components corresponding to one another having the same reference numbers. 
     A special feature of the present exemplary embodiment includes, for actuating the locking coupling  5  by the hydraulic actuating unit  21 , there is provided a proportional valve  59 . The proportional valve  59 , which is arranged in the connecting channel between the pump  51  and the connecting device  31 , is variably settable between an open position, in which the hydraulic pressure is transferred to the second hydraulic actuating unit  21  and a closed position in which the hydraulic pressure supply is interrupted. Between the proportional valve  59  and the reservoir  52  there is provided a return line  60  which is shown in dashed lines. 
     This exemplary embodiment provides that in addition to the torque which has been transmitted by the hang-on coupling  4  and which is variably settable by controlling the pump  51  accordingly, the torque which can be transmitted by the locking coupling  5  is also variably settable by controlling accordingly the proportional valve  59 . This means that the torque can be distributed to the connectable driving axle as required and that the compensating movement of the sideshafts can be reduced as required, thus permitting the driving dynamics of the motor vehicle to be accurately controlled. As in the case of the first exemplary embodiment, there is obtained the advantage of needing only one single pump for actuating the hang-on coupling  4  and the locking coupling  5 . 
       FIG. 3  shows an exemplary drive assembly  2  which largely corresponds to that shown in  FIG. 2 . To that extent, as far as common features are concerned, reference is made to the above description, with identical components or components corresponding to one another having the same reference numbers. 
     The present exemplary embodiment is characterized in that the hydraulic assembly  50  comprises a first proportional valve  39  for controlling the hydraulic pressure transmitted to the first hydraulic actuating unit  6  and a second proportional valve  59  for controlling the hydraulic pressure transmitted to the second hydraulic actuating unit  21 . For this purpose, the proportional valve  39  is arranged in the connecting channel  53  between the pump  51  and the first hydraulic actuating unit  6  and can be individually controlled by the electronic control unit for the driving dynamics. Between the first proportional valve  39  and the reservoir  52  there is provided a return conduit  40  which is shown in dashed lines. The second proportional valve  59  is arranged in a connecting channel  57  between the pump  51  and the second actuating unit  21  and is also individually controlled by the electronic control unit for the driving dynamics. 
     With the help of the present exemplary embodiment, the hang-on coupling and the locking coupling can be actuated completely independently of one another, which permits a maximum degree of flexibility regarding the control of the driving dynamics of a motor vehicle. More particularly, this exemplary embodiment permits a type of control in the case of which the hang-on coupling is opened and the locking coupling is closed. In this way, the secondary driving axle is locked, which has a stabilizing effect on a vehicle driving in a straight line. 
     The above-mentioned exemplary embodiments provide the advantage of a particularly compact design. By arranging the first and the second actuating unit  6 ,  21  in such a way that they directly axially adjoin one another, the conduits of the hydraulic assembly  50  can be particularly short. This, in turn, has a particularly advantageous effect on the reaction behaviour of the vehicle, i.e. the setting accuracy and setting speed of the drive assembly  2 . Furthermore, short conduits also ensure that the hydraulic system  50  has a substantial influence on temperature fluctuations, which, in turn, advantageously influences the setting accuracy. A further substantial advantage includes that the drive assembly  2  permits the operations of locking the differential unit  3  and of optionally connecting the secondary driving axle to take place at one driveline, with only one pump  51  being required. By arranging the hang-on coupling  4  and the locking coupling  5  including the associated actuating units at, and coaxially relative to, the differential unit  3 , the disconnection of the angle drive for increasing energy efficiency and the locking function can be combined in a compact and cost-effective form.