Abstract:
An axle drive block for a motor vehicle, comprising a first and a second differential in a driven housing. Both of the differentials are coaxially aligned planetary spur gears, the sun wheels of which are drivably connected to the semiaxes of the first driven axle. The planet wheels of both differentials mesh the joint ring gear of the sun wheels. In order to lock the interaxle differential, the housing is provided with a first striking surface interacting with a second striking surface which is pressed thereto by ball ramps.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The invention involves an axle-drive block for motor vehicles having a first and a second driven axle which contains a first and a second differential in a housing driven by an engine/transmission block, the first differential distributing the torque fed to it between a first half axle of the first driven axle and the second differential, and the second differential further distributing the torque fed to it between a second half axle of the first driven axle and a power take-off for the second driven axle, the two differentials being spur-gear-type planetary gears with parallel axes, the sun wheels of which are in each case connected in terms of drive to the half axles of the first driven axle, and planet wheels of the two differentials meshing with the common ring gear of said planetary gears, one planet carrier being connected in a rotationally fixed manner to the housing accommodating the two differentials and the other planet carrier being connected in terms of drive to the power take-off for the second driven axle.  
         [0002]     An axle-drive block of this type is disclosed in DE 44 18 891 C2. In the latter, owing to the particular design and arrangement of the two differentials, optimum adaptation of the moment distribution ratio is achieved with a minimum outlay on construction. Provided between the driven housing, which contains the two differentials, and the power take-off for the second driven axle is a fluid friction clutch as a longitudinal differential lock. The latter is not only complicated and bulky (for which reason it has to be driven via a hollow shaft and arranged outside the housing of the two differentials), it also has the disadvantage of not being suitable for interaction with brake-force and slip-limiting systems (ABS, ESP). The fluid friction clutch is a lock sensing the rotational speed.  
         [0003]     EP 94 870 A1 discloses an axle-drive block, in which two bevel gear differentials are connected in series and, in the first differential, the moment for just one side of the first driven axle is branched off. To totally lock the longitudinal differential, the cages of the two bevel gear differentials can be connected by means of a claw clutch. With this arrangement, neither the desired moment distribution nor a symmetrically acting locking can be obtained. In particular, with only a partial locking by means of friction, an asymmetrical distribution of torque with respect to the longitudinal axis of the vehicle would be produced, which is not allowed.  
         [0004]     For compatibility with ABS or ESP and for other reasons concerned with driving dynamics, a torque-sensing lock which carries out its task without an external control action is desired. A lock of this type is specified, for example, also in a torsen differential of particular design, but has an unfavorable behavior in terms of wear.  
         [0005]     It is therefore the object of the invention to provide, with a minimum outlay on construction, an automatic locking of the longitudinal differential, with it being possible for the automatic locking which can be obtained to be structurally influenced in such a manner that it is adapted to the requirements in terms of driving dynamics.  
       SUMMARY OF THE INVENTION  
       [0006]     According to the invention, the object is achieved by providing a friction clutch formed between a planet carrier of a second differential and a housing, wherein the friction clutch comprising an axially normal first frictional surface on an inner wall of the housing and a ring which is operatively connected to the planet carrier of the second differential and has a second frictional surface, wherein the ring is rotatable with respect to the second planet carrier and, during rotation, the distance between the first and second frictional surfaces changes.  
         [0007]     The locking thus takes place by means of friction, gradually. Owing to e), only the central differential is lockable (i.e. the locking action between the first and second driven axle) without the moment distribution between the two wheels of the first driven axle becoming asymmetrical. Because it requires little space, the friction clutch can be accommodated within the housing and can be connected into the power flux over the shortest possible distance (without the interconnection of shafts whose twistability may result in oscillations). The operative connection between the planet carrier and the ring, via which the torque is conducted to the second driven axle, renders the lock torque-sensing, as a result of which it is self-controlling without external intervention and is ABS- and ESP-compatible. The desired locking behavior can be represented within wide limits by means of the structural design of the operative connection.  
         [0008]     In one preferred embodiment, the second planet carrier and/or the ring has an end surface with ramps rising in the axial direction. The desired locking behavior is produced by means of the shape and inclination of the ramps. In particular, the ramps may rise from a neutral position at different inclinations in both circumferential directions (i.e. in a different direction of rotation). This enables the locking behavior to be designed differently in the traction-mode than in the overrun mode. It may also be said that, in one direction of rotation, a dropping ramp is followed by a rising ramp.  
         [0009]     There are various possibilities for the structural design of the ramps. For reasons of symmetry, at least two ramps distributed over the circumference are provided in every case.  
         [0010]     In one possible embodiment, the second planet carrier and the ring has an end surface with ramps rising in the axial direction. During mutual rotation, the ramps of the two parts slide on each other and thus determine the axial position of the ring. The mechanical friction between the two ramps means that the action responds only upon a certain “sensed” torque.  
         [0011]     In another possible embodiment, the second planet carrier and/or the ring has a planar end surface with depressions which form ramps and accommodate rolling bodies. The action therefore occurs virtually without any friction and in a more sensitive manner. In addition, the space required and the outlay on manufacturing are thus smaller.  
         [0012]     In a development of the invention, in addition there are first and second clutch disks between the first and the second friction surface, of which the first clutch disks are connected in a rotationally fixed and axially displaceable manner to the inner wall of the housing and the second clutch disks are connected in a rotationally fixed and axially displaceable manner to the ring. This enables relatively high locking moments to be obtained and to manage with lower locking forces. Another possibility of increasing the locking action with the minimal additional amount of space being required involves providing at least one of the frictional surfaces with a friction lining. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]     The invention will be described and explained below with reference to figures, in which:  
         [0014]      FIG. 1  shows a diagrammatic view of the entire drive train of a vehicle together with the axle-drive block according to the invention,  
         [0015]      FIG. 2  shows an axial section through the double differential of  FIG. 1 ,  
         [0016]      FIG. 3  shows detail III in  FIG. 2  in a first embodiment,  
         [0017]      FIG. 4  shows a circumferential section according to IV-IV in  FIG. 3 ,  
         [0018]      FIG. 5  shows detail III in  FIG. 2  in a second embodiment,  
         [0019]      FIG. 6  shows detail III in  FIG. 2  in a third embodiment,  
         [0020]      FIG. 7  shows a circumferential section according to VII-VII in  FIG. 6 . 
     
    
     DETAILED DESCRIPTION  
       [0021]     In the all-wheel-drive motor vehicle illustrated in  FIG. 1 , the engine is referred to by  1 , the clutch by  2 , and the manual transmission by  3 . The transmission  3  ends in a driven gearwheel  4  which meshes with a large driving gearwheel  5 . The driving gearwheel  5  is already part of the axle-drive block  6 . The latter is adjoined by a power take-off  7  for the rear axle drive and by a right and a left half axle  8 ,  9  for the drive of the front wheels. Located in the interior of the power take-off  7  is a pair of bevel gears  10 ,  11 , and the torque for the rear axle is fed via a propeller shaft  12  to a, for example, conventional differential transmission  13 , in which the half axles  16 ,  17  of the rear wheels are driven in a known manner via a pair of bevel gears  14 ,  15 . A first and a second planetary gear  18 ,  19  are located in the interior of the axle-drive block  6  and will be described in greater detail below.  
         [0022]     The rotating part of the axle-drive block illustrated in  FIG. 2  is, starting from the driving gearwheel  5 , a driven housing  22  which contains the two planetary gears  18 ,  19 . The housing  22  comprises two housing parts  23 ,  24  which are jointly clamped together here to the driving gearwheel  5  by means of threaded bolts  25 . The axial position of the joint between the two housing parts can be established in accordance with external requirements; either the two housing parts can have the same depth or one of the housing parts is a flat cover and the other is a deep bell.  
         [0023]     The first housing part  23  is at the same time the planet carrier of the first differential transmission  18  and, for example, is connected fixedly to the bell-shaped housing part  24  by means of the threaded bolts  25 . The two together thus form a rigid part which is mounted rotatably in the housing  20 ,  21  by means of bearings  26 ,  27 . This first planetary gear  18  also includes planet wheels  31  which can rotate about axes  30 , and a sun wheel  32 , which is connected by means of a wedge-shaped toothing  33  to the left output shaft  34  to which the half axle  9  ( FIG. 1 ) is connected.  
         [0024]     A ring gear  35  surrounds the planet wheels  31  of the first planetary gear  18  and is at the same time also the ring gear of the second planetary gear  19 . It meshes with the outer planet wheels  36  of the second planetary gear  19 , which planet wheels are mounted on spindles  37  which, for their part, are fastened in the second planet carrier  40 . The first planet wheels  36  also mesh with second planet wheels  38  which are likewise mounted on the planet carrier  40  on spindles  39 . The second planet wheels  38  mesh with a sun wheel  43  which is connected via a wedge-shaped toothing  44  to the right output shaft  45 . The latter leads via the right axle-drive shaft  8  ( FIG. 1 ) to the right front wheel. Reference is made in this respect to DE 44 18 891.  
         [0025]     The power flux runs as follows: the torque acting on the housing  24  is firstly divided in the first planetary gear  18  between the sun wheel  32 , and hence the left, front axle drive shaft  9 , and the ring gear  35  which produces the connection between the first and second planetary gear. In the second planetary gear  19 , the torque is distributed via the planet wheels  36 ,  38  to, on the one hand, their planet carriers  40 , and hence to the power take-off  7  for the rear wheels, and, on the other hand, to the sun wheel  43  and hence to the right half-axle  8  of the front wheel drive.  
         [0026]     According to the invention, a friction clutch  50  is now provided between the planet carrier  40  of the second differential  19  and the housing  24  ( FIG. 1 ), the friction clutch acting between an axially normal first frictional surface  51  and a second frictional surface  53  formed on a ring  52 . The ring  52  is operatively connected to the planet carrier  40  and at the same time produces the driving connection between the latter ( 40 ) and the hollow shaft  42  leading to the drive of the second axle. The planet carrier  40  is connected via a wedge-shaped toothing  41  to a hollow shaft  42  which leads into the power take-off  7  for the rear axle ( FIG. 1 ).  
         [0027]      FIG. 3  shows the operative connection in more detail. The ring  52  has a hub  54  sitting on the hollow shaft  42  and, on the side facing away from the frictional surface  51 ,  53 , at least two depressions  55  distributed uniformly on the circumference. In the exemplary embodiment depicted, the disk  57  of the planet carrier  40  has depressions  56  just like this. Balls  58  are situated between the depressions  55 ,  66 . There may be, but does not have to be, a friction lining  70 .  
         [0028]     It can be seen in  FIG. 4  that the depressions  55 ,  56  in each case form a first ramp ( 59 ,  59 ′) and a second ramp ( 60 ,  60 ′)  59 ,  60 . Since their lengths  61 ,  62  differ, the inclination of the two ramps also differs. During a relative movement of the ring  52  and planet carrier corresponding to the traction mode and forward travel and arrows  63 ,  63 ″, the ball  58  will exert a force  64  on the ring with which it is pressed with its second frictional surface  53  against the first frictional surface  51  ( FIG. 3 ) and thus retards the relative movement  63 ,  63 ′. During a relative movement in the opposite direction during overrun mode forward or during traction mode rearward, the steeper ramps  60 ,  60 ′ come into effect and the torque transmitted via the ball  58  brings about a smaller press-on force between the frictional surfaces  51 ,  53 .  
         [0029]     In  FIG. 5 , first and second clutch disks  165 ,  166  are provided between the first frictional surface  151  and the second frictional surface  153  in order to increase the friction with the same press-on pressure. The former ( 165 ) are connected to the housing  24  in a rotationally fixed and axially displaceable manner by means of teeth  167  therein; the latter ( 166 ) are connected to the ring  58  via teeth  168 . Springs which are optionally present cannot be seen.  
         [0030]      FIG. 6  differs from  FIG. 3  by the fact that, instead of the depressions and rolling bodies, connecting links which slide on each other are provided, as can better be seen in the circumferential section in  FIG. 7 . The disk  256  of the planet carrier  40  has two or more hump-shaped ramps  265  which are distributed over the circumference and come into contact along their generatrices with the ramps  259 ,  260 , which may again be shaped differently. The ramps  265 ,  259 ,  260  protrude in the axial direction from the end surfaces  268 ,  269  of the ring  252  and disk  256 .