Abstract:
A hydrostatically power-splitting transmission ( 10 ), particularly for agricultural and construction equipment, comprises at least two hydrostats (H 2 ), which are hydraulically connected to each other and operate as pumps or as motors, wherein at least one of the hydrostats (H 2 ) can be adjusted or pivoted by means of a controller ( 16, 20, 21 ; SK 1 , . . . SK 4 ), mechanical coupling means (K 1 , K 2 ; Z 1 , . . . , Z 12 ), which couple the hydrostats (H 1 , H 2 ) to an inner drive shaft (W 1 ) and an inner driven shaft (W 7 ), a housing ( 14, 31 ) comprising a cover ( 14 ) and a housing bottom part ( 31 ), wherein the hydrostats (H 1 , H 2 ), the inner drive and driven shafts (W 1 , W 7 ), and the mechanical coupling means (Z 7 , Z 9 ) are disposed and attached on the bottom of the cover ( 14 ), and in the lower housing part an outer drive shaft accessible from the outside and a driven shaft are supported, which are operatively connected to the inner drive shaft or driven shaft when the housing is assembled. In such a transmission, a compact design, while simultaneously providing easy accessibility and high flexibility in the adaptation to different vehicles, is achieved in that the controller ( 16, 20, 21 ; SK 1 , . . . SK 4 ) for adjustment or pivoting of the at least one hydrostat (H 2 ) is disposed on the top of the cover ( 14 ) and acts upon the at least one hydrostat (H 2 ) through the cover ( 14 ).

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to the field of transmission technology and a (continuously variable) hydrostatically power-splitting transmission. 
     2. Discussion of Related Art 
     Power-splitting transmissions, particularly for employment in vehicles of agricultural or building use, such as, for example, tractors, have been known for a long time. In such power-splitting transmissions, the power prevailing at an input shaft or drive shaft and usually output by an internal combustion engine is apportioned to a first mechanical power branch with a fixed step-up ratio and a second power branch having a continuously variable step-up ratio and is subsequently combined again in order to be available at an output shaft or take-off shaft. The second power branch is mostly designed as a hydrostatic branch in which two hydrostatic axial piston engines (hydrostats) of the oblique axis or swashplate type, which are connected to one another hydraulically, operate selectively as a pump or as a motor. The step-up ratio can in this case be varied by a variation of the pivot angle of the cylinder block or the swashplate. The apportionment of the power to the two power branches and the combining of the split powers usually take place by means of an epicyclic transmission. Power-splitting transmissions of the type described are disclosed in various configurations in DE-A1 27 57 300, in DE-C2-29 04 572, in DE-A1-29 50 619, in DE-A1-37 07 382, in DE-A1-37-26 080, in DE-A1-39 12 369, in DE-A1-39 12 386, in DE-A1-43 43 401, in DE-A1-43 43 402, in EP-B1-0 249 001 and in EP-A2-1 273 828. 
     So that a power-splitting transmission can be successfully employed in practice, it should, in general, be distinguished by the following properties:
         The transmission should have high efficiency over the entire speed range. This should be the case particularly at the high driving speeds which are adopted in road traffic for a relatively long period of time.   The transmission should have a compact construction in order to allow installation in the most diverse possible vehicles, as far as possible without structural restrictions.   The transmission should allow the transfer of high powers.   The transmission should have as simple a construction as possible in order to limit the power losses and increase the operating reliability.   The transmission should allow fully comprehensive electronic control in conjunction with the engine management and should make available sufficient emergency running programs even in the event of a failure of specific control elements.       

     The initially mentioned DE-A1-43 43 402 has already described a power-splitting transmission, designated as a CHP transmission (Continuously variable Hydrostatic Power-splitting transmission), which is distinguished by two hydraulically coupled identical hydrostats of the oblique axis type of construction, which can be coupled in different ways to an epicyclic differential transmission via pairs of clutches or change-shift elements K 1 /K 2  or K 3 /K 4 . The known CHP transmission has been employed and tested under the type designation SHL-Z in city buses. The two hydrostats employed have a pivoting range of only 0-25°. For forward travel, in this case, three driving steps or driving ranges are obtained: in the first driving range, at the starting point the hydrostatic fraction of the transferred power is 100% and then moves linearly with the speed toward zero. In the second driving range, it moves from zero to a maximum of about 27% and then back again to zero. In the third driving range, it moves from zero to a maximum value of 13% at the highest forward speed. 
     The hydrostatic power transfer branch of such a transmission usually comprises two hydrostatic axial piston engines which are connected hydraulically to one another and of which in each case one operates as a pump and the other as a motor. Depending on the driving step, in this case, the two engines can interchange their roles. 
     The hydrostatic axial piston engines constitute an essential component of the hydrostatic power-splitting transmission and decisively affect the properties of the transmission, such as, for example, the efficiency, overall size, complexity, speed range covered, type and number of driving steps, and the like. Examples of hydrostatic axial piston engines of this type are disclosed in DE-A1-198 33 711 or in DE-A1-100 44 784 or in US-A1-2004/0173089. The functioning and theory of hydrostatic axial piston engines and of a power-splitting tractor transmission equipped with them are described in a publication of TU Munich from the year 2000 by H. Bork et al., “Modellbildung, Simulation and Analyse eines stufenlosen leistungsverzweigten Traktorgetriebes [Modelling, Simulation and Analysis of a continuously variable power-splitting tractor transmission]”. 
     In the known hydrostatic transmissions, the parts in the transmission (hydrostats, clutches, shafts, epicyclic drives, gearwheels, etc.) are installed in a housing which is oriented specially with respect to the transmission and consists of a multiplicity of housing segments. If, then, such a transmission is to be installed in a corresponding agricultural or building vehicle, either the vehicle has to be coordinated in its design with the already prefabricated transmission or the transmission has to be coordinated with the given conditions of an already existing vehicle and therefore redesigned. In both instances, a considerable extra outlay arises due to the special adaptation of the vehicle or entire transmission. 
     In the publication DE-A1-26 33 718, it has already been proposed to construct a simple hydrostatic transmission without power splitting so that it forms a structural unit with the cover of the transmission housing. In the transmission housing itself, only the drive and take-off shafts accessible from outside are mounted, and come into engagement via internal gearwheels with the corresponding inputs and outputs of the transmission when the cover together with the transmission is placed on the transmission housing. 
     What is achieved thereby is that the housing together with the drive and take-off shafts can be installed in the vehicle at an early stage, while a decision can be made later, by a cover together with a corresponding transmission unit being put in place, as to whether a mechanical or a hydrostatic transmission is to be used. Correspondingly, transmissions can be exchanged in a simple way in the already finished vehicle. 
     The transmission concept (structural unit of transmission and cover) known from DE-A1-26 33 718 may be useful for the simple case of a transmission without power splitting, when neither clutches nor summing members are required and only one of the hydrostats is adjusted. It is sufficient here to arrange the adjusting mechanism for the one hydrostat directly on the hydrostat inside the housing. 
     For the substantially more demanding concept of a continuously variable hydrostatic power-splitting transmission, however, other ways must be found not only to accommodate the markedly more complicated control, but also to place it suitably in terms of assembly and of maintenance. 
     SUMMARY OF THE INVENTION 
     One object of the invention, therefore, is to provide a hydrostatically power-splitting transmission which, while maintaining the flexible concept of the separation of transmission and housing, is distinguished by an improved arrangement of the components and, in particular, is suitable for implementing a complex continuously variable hydrostatic power-splitting transmission. An object of the invention, furthermore, is to specify a transmission concept which is especially suitable for this purpose. 
     The one object is achieved by means of the whole of the features of claim  1 . A characterizing feature of the novel transmission is that the control for adjusting or pivoting the at least one hydrostat is arranged on the top side of the cover and acts through the cover upon the at least one hydrostat. By the control being shifted onto the top side of the cover, there is not only space for the transmission components arranged in the housing, but also access to the control from outside for assembly or maintenance purposes is made considerably easier, while close spatial proximity to the transmission components to be controlled on the underside of the cover is maintained. Thus, testing and setting work can be carried out on the transmission, without the transmission housing having to be opened. Moreover, if required, electrical and electronic and also hydraulic control components (electrically actuated hydraulic valves, measurement and control electronics, etc.) can be combined on the top side of the cover into a structural unit which does not subject the rest of the housing to any restrictions, is not exposed to the rough ambient conditions inside the housing and nevertheless is located near the transmission. 
     A preferred refinement of the transmission according to the invention is characterized in that the two hydrostats can be adjusted or pivoted by means of the control through the cover, in that a plurality of clutches are provided for controlling the power split, and in that a multistep epicyclic drive is provided for summing the split powers. 
     Another refinement of the invention is distinguished in that the two hydrostats are in each case pivotable about a pivot axis through at least about +/−45° for controlling the hydraulic power, in that the cover lies essentially in one plane, in that the pivot axes of the hydrostats are arranged perpendicularly to the plane of the cover, in that the control comprises hydraulically actuated lifting pistons which pivot the hydrostats about their pivot axis via a lever mechanism, and in that control hydraulics are provided for controlling the lifting pistons inside the control and are controlled by means of an electric control motor. 
     Preferably, the hydrostats are arranged with their axes of rotation parallel next to one another and parallel to the plane of the cover, and the inner drive and take-off shafts and the outer drive and take-off shafts have a common axis which is oriented parallel to the axes of rotation of the hydrostats and which is arranged between the axes of rotation of the hydrostats. 
     One development is characterized in that, overall, two or four clutches are provided, which are assigned in pairs to the hydrostats and are arranged in the axis of rotation of the assigned hydrostat, and in that the multistep epicyclic drive is arranged in the common axis of the inner and outer drive and take-off shafts. 
     Another development is distinguished in that the hydrostats are mounted pivotably between the cover and a bearing bottom parallel to the cover, which bearing bottom is fastened to the cover via lateral posts standing vertically on the cover, and in that bearing walls which stand vertically on the underside of the cover and are screwed to the bearing bottom are provided for mounting the shafts of the transmission. 
     According to another refinement of the invention, a lower-lying pan is formed on the housing lower part, and a hydraulic pump is arranged and fastened on the underside of the cover and, when the transmission is in the assembled state, penetrates with an intake connection piece into the pan. 
     Furthermore, it is conceivable and advantageous that control electronics are provided for the transmission, and that the control electronics are arranged on the top side of the cover. 
     The other object is achieved by means of the whole of the features of claim  13 . It is essential, in this case, that, to achieve a wide continuous adjustment range, the two hydrostats are in each case pivotable about a pivot axis at least in a range of between −45° and +45° for controlling the hydraulic power. 
     Especially advantageously, this transmission may be provided for a hybrid drive and be coupled to an electric motor. 
     According to one refinement, in this case, the electric motor is coupled to the inner drive shaft via a transmission. 
     According to another refinement, the electric motor is arranged directly on the inner drive shaft. In particular, a disk-shaped three-phase machine known per se is suitable for this purpose. 
     Preferably, the electric motor is connected via control electronics to a battery, from which it obtains energy or into which it can feed energy for storage. 
     In addition, the electric motor may be capable of being used as a generator and/or starter and/or retarder. 
     Particularly in the case of vehicles of agricultural use, a second electric motor may be provided, which drives a power take-off shaft, the second electric motor being connected to a battery via second control electronics. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be explained in more detail below, by means of exemplary embodiments, in conjunction with the drawing in which: 
         FIG. 1  shows a diagrammatic illustration of the basic set-up of a continuously variable hydraulic power-splitting transmission with four clutches overall, which is especially suitable for implementing the invention; 
       FIG.  1 ′ shows an illustration, comparable to  FIG. 1 , of a comparable continuously variable hydraulic power-splitting transmission with only two clutches, which is especially suitable for implementing the invention; 
         FIG. 2  shows the various driving steps of the power-splitting transmission according to  FIG. 1  with a first forward driving step ( FIG. 2(   a   1 ) to  2 ( a   3 )), with a second forward driving step ( FIG. 2(   b   1 ) to  2 ( b   3 )) and with a reverse driving step ( FIG. 2(   c   1 ) to  2 ( c   3 )); corresponding driving steps with the same pivoting movements of the hydrostats H 1  and H 2  and with the same positions of the clutches K 1  and K 2  also apply to the transmission according to FIG.  1 ′; 
         FIG. 3  shows the pivot angles SW 1 ,  2  of the two hydrostats and the hydraulic power fraction HL against the speed v in the two forward driving steps for the transmission according to  FIGS. 1 and 2 ; 
         FIG. 4  shows a perspective illustration (seen obliquely from above) of a transmission according to the principle illustrated in  FIG. 1 , according to a preferred exemplary embodiment of the invention, only the cover with the transmission arranged below it and with the control arranged above it being shown; 
         FIG. 5  shows a perspective illustration (seen obliquely from below) of a transmission according to  FIG. 4 ; 
         FIG. 6  shows a bottom view of the transmission from  FIG. 4 ; 
         FIG. 7  shows a rear view of the transmission from  FIG. 4 ; 
         FIG. 8  shows a front view of the transmission from  FIG. 4 ; 
         FIG. 9-11  show two side views of a housing lower part fitting with the transmission according to  FIG. 4 ; 
         FIG. 12  shows a view from above into the housing lower part according to  FIG. 9-11 ; 
         FIG. 13  shows a transmission according to  FIG. 1  with additional electric motors for a hybrid drive or an electrically driven power take-off shaft; and 
         FIG. 14  shows a transmission according to  FIG. 1  with an additional electric motor, seated directly on the drive shaft, for a hybrid drive. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  illustrates a diagrammatic illustration of the basic set-up of a continuously variable hydraulic power-splitting transmission which is especially suitable for implementing the invention. The transmission  10  transfers the power of an internal combustion engine  11  which is symbolized in  FIG. 1  by a piston seated on a crankshaft. The transmission  10  is connected to the internal combustion engine  11  by means of an input shaft (drive shaft) W 1 . Said transmission discharges the transferred power via an output shaft (take-off shaft) W 7 . If required, a power take-off shaft W 8  extends through the transmission  10  and is a direct continuation of the input shaft W 1 . 
     The core of the transmission  10  is formed by a multistep epicyclic drive  12  with a large sun wheel Z 1  and a small sun wheel Z 1 ′, with the double planet wheels Z 2 , Z 2 ′, with the ring wheel Z 3  and with the planet web  13  connected fixedly in terms of rotation to a gearwheel Z 8 , and with two hydrostatic axial piston engines or hydrostats H 1 , H 2 , the take-off shafts of which, W 6  and W 12  respectively, can in each case be coupled differently via a pair of clutches K 3 , K 4  and K 1 , K 2 , respectively, to the input shaft W 1 , to the output shaft W 7  and to the multistep epicyclic drive  12 . The hydrostats H 1  and H 2 , which operate selectively as a pump and as a motor, are connected to one another hydraulically via high-pressure lines, not illustrated. The first hydrostat H 1  can be coupled with its take-off shaft W 6  to the ring wheel Z 3  by means of the clutch K 3  via a counter gear consisting of the gear wheel Z 5  and of a gear wheel Z 4  connected fixedly in terms of rotation to the ring wheel Z 3 . However, it can also be coupled to the input shaft W 1  by means of the clutch K 4  via the gearwheel Z 11 , the intermediate wheel Z 12  and the gearwheel Z 10  arranged fixedly in terms of rotation on the input shaft shaft W 1 . 
     The second hydrostat H 2  can be coupled with its take-off shaft W 12 , on the one hand, to the planet web  13  and consequently to the output shaft W 7  by means of the clutch K 1  via the hollow shaft W 11  and the gearwheel Z 9  which is arranged fixedly in terms of rotation on the latter and which meshes with the gearwheel Z 8 . It can, on the other hand, be coupled to the smaller sun wheel Z 1 ′ of the multistep epicyclic drive  12  by means of the clutch K 2  via the pair of gearwheels Z 7 , Z 6  and the hollow shaft W 2 . 
     The power prevailing at the input shaft W 1  is apportioned in the transmission  10 , by the multistep epicyclic drive  12 , to two power branches, to be precise to a mechanical power branch and a hydraulic power branch, and is combined again later at the output shaft W 7 . The mechanical power branch runs from the input shaft W 1  via the larger sun wheel Z 1  which is connected fixedly in terms of rotation to the input shaft W 1 , the double planet wheels Z 2 , the planet web  13  and the gearwheel Z 8 . The hydraulic power branch runs via the two hydraulically connected hydrostats H 1  and H 2  and is designed differently, depending on the shifting of the clutches K 1 , . . . , K 4 . As indicated in  FIG. 1  of the drawing, the two hydrostats H 1  and H 2  can in each case be pivoted through +/−45°. 
     The shifting of the clutches K 1 , . . . , K 4  and the pivoting position of the hydrostats H 1 , H 2  for the various operating states of the transmission  10  are illustrated in  FIG. 2 ,  FIG. 2(   a   1 ) to  2 ( a   3 ) showing the first forward driving step,  FIG. 2(   b   1 ) to  2 ( b   3 ) the second forward driving step and  FIG. 2(   c   1 ) to  2 ( c   3 ) reverse drive. During starting ( FIG. 2(   a   1 )), as in the entire first forward driving step, the clutches K 3  and K 1  are actuated (marked in  FIG. 2  by the short arrows), so that the first hydrostat H 1  is coupled to the ring wheel Z 3  of the multistep epicyclic drive  12  and the second hydrostat H 2  is coupled to the planet web  13  or to the gearwheel Z 8  or to the output shaft W 7 . The first hydrostat H 1 , which operates as a pump in the first forward driving step, is first unpivoted (pivot angle 0°), whereas the second hydrostat H 2  operating as a motor is pivoted out fully (maximum pivot angle 45°). On account of the zero position of the first hydrostat H 1 , no pressure medium is pumped to the second hydrostat H 2 , and therefore also no power is transferred hydraulically. The starting operation is initiated in that the first hydrostat H 1  is pivoted gradually, the volume increasingly being pumped to the second hydrostat H 2 , and the second hydrostat beginning to rotate with high torque and an increasing speed. When the first hydrostat H 1  is pivoted out fully ( FIG. 2(   a   2 )), the first phase of the first driving step is concluded. In the second phase, with the first hydrostat H 1  pivoted out fully, the second hydrostat H 2  is gradually moved back from the maximum pivot angle to the pivot angle 0° ( FIG. 2(   a   3 )), the rotational speed being increased ever further with a decreasing torque. At the end of the first driving step, the second hydrostat H 2  no longer absorbs any torque and the rotational speed of the first hydrostat H 1  approaches zero. The hydrostatically transferred power approaches zero, and the entire power is transferred mechanically (this corresponds to about 33% of the maximum driving speed in  FIG. 3) . 
     For the transition from the first driving step to the second driving step ( FIG. 2(   a   3 )→ FIG. 2(   b   1 )), the clutch K 1  is opened and the clutch K 2  is closed. Since the second hydrostat H 2  does not absorb any torque at the pivot angle 0°, the changeover takes place virtually without any shift moment. The second hydrostat H 2  is then coupled to the smaller sun wheel Z 1 ′ of the multistep epicyclic drive  12 . As a result of the full pivoting of the hydrostats H 1 , H 2 , the directions of flow between the hydrostats are automatically reversed. In the second driving step, the first hydrostat H 1  operates as a motor and the second hydrostat H 2  as a pump. As in the first driving step, the hydrostat operating as a pump (now the second hydrostat H 2 ), in a first phase, is pivoted out gradually from the pivot angle 0° onto the other side to the maximum pivot angle ( FIG. 2(   b   2 )), while the hydrostat operating as a motor (now the first hydrostat H 1 ) remains pivoted out fully on the same side. In a subsequent second phase ( FIG. 2(   b   2 )→ FIG. 2(   b   3 )), the first hydrostat H 1  is then pivoted back into the zero position. At the end of the second driving step, the hydraulically transferred power once again approaches zero; the entire power is transferred via the mechanical power branch. 
     The graph, obtained for a power-splitting transmission according to  FIG. 1-8  in a tractor, of the pivot angles SW 1 ,  2  of the two hydrostats and of the percentage of the hydrostatically transferred power HL as a function of the vehicle speed v, is reproduced in  FIG. 3 . On account of the 45°-hydrostats employed in the transmission  10 , the entire driving range extending from 0 to the final speed can be subdivided into only two driving steps, the first driving step extending from 0 to about 33% and the second driving step from 33% to 100%. In the first driving step, the fraction of hydrostatically transferred power decreases from an initial 100% linearly to 0. In the second driving step, the fraction of hydrostatically transferred power rises from 0 to a maximum of approximately 30% at about 50% of the maximum driving speed and then falls to 0% again. The result of this is that the efficiency at the end of the second driving step does not fall again. This results, for high driving speeds which are maintained over a lengthy period of time during driving across country, in an especially high efficiency of the transmission which leads to markedly lowered operating costs. 
     In reverse drive ( FIG. 2(   c   1 ) to  2 ( c   3 )), starting from the situation in  FIG. 2(   a    1 ), there is a changeover from the clutch K 3  to the clutch K 4  (in the configuration of FIG.  1 ′ operating without the clutches K 3  and K 4 , the changeover is to a power-split reverse drive). The first hydrostat H 1  operating as a pump is then driven directly by the input shaft W 1  and is pivoted out from 0° gradually onto the other side. The fully pivoted-out second hydrostat H 2  is pivoted back ( FIG. 2(   c   3 )) and thus picks up rotational speed further. 
     In the transmission configuration illustrated in FIG.  1 ′, the clutches K 3  and K 4  and the associated shafts W 3 , W 5  and gearwheels Z 10 , Z 11  and Z 12  are absent. The driving steps of this transmission  10 ′ operating with only two clutches K 1  and K 2  have the same division as shown in  FIG. 2 . The hydrostats H 1  and H 2  execute the same pivoting movement and the clutches K 1  and K 2  are changed over between the driving steps in the same way. 
     In a transmission of the type illustrated in  FIG. 1  or  1 ′, then, according to the invention installation in a housing consisting of a cover and of a housing lower part is carried out that the actual transmission with the hydrostats, the shafts, the clutches, the gearwheels and the multistep epicyclic drive is arranged on the underside of the cover and with the cover forms a structural unit, while the electrical, electronic, mechanical and hydraulic control is arranged on the top side of the cover and likewise with the cover forms a structural unit. This affords a compact form of the transmission, high flexibility in adapting the housing lower part to the respective vehicle and excellent accessibility to the control with its various components. 
     A power-splitting transmission implemented according to the transmission diagram from  FIG. 1 , according to a preferred exemplary embodiment of the invention, is reproduced in  FIG. 4 to 8 , as seen from various viewing angles, only the cover with the transmission arranged below it and with the control arranged above it being shown. The associated housing lower part may be configured differently, depending on requirements. 
     A (non-restricting) example of such a housing lower part is illustrated in  FIG. 9 to 12 , as seen from various viewing angles. 
     The transmission  10  of  FIG. 4 to 8  comprises as a carrying part an essentially rectangular cover  14  which is bordered by a continuous flange  15  lying in one plane and provided with bores for screwing to the housing lower part  31  from  FIG. 9 to 12 . The transmission components (hydrostats, clutches, gearwheels and shafts), illustrated diagrammatically in  FIG. 1 , are arranged and mounted on the underside of the cover  14  in the actual transmission core  17  in three mutually parallel axes forming an equilateral triangle. The first hydrostat H 1  with the shafts W 3 , W 5  and W 6 , with the gearwheels Z 5  and Z 11  and with the clutches K 3  and K 4  is located in one axis. The second hydrostat H 2  with the shafts W 9 , W 11  and W 12 , with the gearwheels Z 7  and Z 9  and with the clutches K 1  and K 2  is located in the second axis. The third, middle axis comprises W 2 , W 7  and W 10 , the multistep epicyclic drive  12  and the gearwheels Z 4 , Z 6 , Z 8  and Z 10 . 
     Essential components for mounting and holding the transmission core  17  on the underside of the cover  14  are a bearing bottom  27  oriented parallel to the cover  14 , two lateral posts  26 ,  26 ′ emanating vertically downward from the cover  14  and two bearing walls  28 ,  28 ′ likewise emanating vertically downward from the cover  14 . The bearing bottom  27  delimits the transmission core  17  on the underside. Said bearing bottom is screwed to the posts  26 ,  26 ′ and to the bearing walls  28 ,  28 ′. The lower pivot bearings  24 ,  25  for the housings, in each case pivotable about a vertical axis, of the hydrostats H 1  and H 2  are arranged in the bearing bottom  27 . The upper pivot bearings are accommodated in the cover  14  itself, but this cannot be seen. The mutually parallel bearing walls  28 ,  28 ′ standing perpendicularly to the three axes of the transmission core  17  serve for mounting the shafts belonging to the axes. 
     In particular, the shafts W 9  and W 3  coming from the clutches K 1 /K 2  and K 3 /K 4  are mounted in the front bearing wall  28 . The associated bearings are in each case designed as a structural unit with control hydraulics  29  and  30  which are connected to the control on the top side of the cover and actuate the clutches K 1 , . . . , K 4  via axial bores inside the shafts W 3  and W 9 . The oil pressure required for the control hydraulics is generated by a hydraulic pump  22  which sucks in oil, via a downwardly directed intake connection piece  23 , out of the oil sump formed in a pan  32  of the housing lower part  31  ( FIG. 9-11 ) and which conducts it further on to the control via ducts integrated in the bearing wall  28 . 
     The input shaft or inner drive shaft W 1 , which is provided with a serration and via which the power from the engine is fed into the transmission by means of an outer drive shaft ( 40  in  FIG. 12 ) mounted in the housing lower part  31 , projects out of the front bearing wall  28  in the third, middle axis ( FIG. 5 ,  8 ). The inner take-off shaft W 7 , which is likewise provided with a serration and via which the power from the transmission can be discharged outward by means of an outer take-off shaft ( 39  in  FIG. 12 ) mounted in the housing lower part  31 , is accessible through the rear bearing wall  28 ′. The two outer shafts  39 ,  40  are coaxial to the third, middle axis of the transmission core  17 . They are in each connected to a clutch  34  and  35 , respectively, which is located outside the housing lower part  31  and via which the transmission  10  can be installed in the drive train of the associated vehicle. 
     The transmission control necessary for operating the transmission core  17  is accommodated on the top side of the cover  14  so that action upon the transmission induced by the transmission control  16  takes place directly through the cover  14 : one type of action is the control of the hydrostats H 1  and H 2 , which, on the one hand, requires a pivoting of the pivot housings through a maximum of +/−45° and, on the other hand, influences the hydraulic connection between the two hydrostats. For this purpose, control hydraulics  20  in the form of control blocks are provided on the cover top side directly above the two hydrostats H 1 , H 2 . Each of the two hydrostats H 1 , H 2  is assigned two opposite, hydraulically actuated lifting pistons SK 1 , SK 2  and SK 3 , SK 4 , respectively, which pivot the associated hydrostat H 2  or H 1  via a lever mechanism located in the control block  20 . The hydraulic control of the lifting pistons SK 1 , SK 4  and of the hydraulic connection between the hydrostats H 1 , H 2  is controlled by a rotatable control piston in the control block  20 , said control piston being driven by an electric control motor  21 . The direct connection between the control block  20  and the hydrostats H 1 , H 2  lying below it achieves an extremely compact set-up which allows easy access to the individual components of the control from above and at the same time permits high adaptability to the vehicle surroundings on the housing lower part  31 . 
     A compact set-up, good accessibility and short travels also arise due to the arrangement of the control electronics  18  in a box directly on the cover  14 . The control electronics  18  evaluate physical measurement variables from the transmission and also commands from the engine control and the operating elements of the vehicle and outputs control commands to the control motor  21  and to hydraulic valves which are arranged around the control electronics  18  on the cover  14  and with the aid of which the clutches K 1 , . . . , K 4  are actuated. For this purpose, the necessary microprocessors and power outputs are accommodated in the control electronics  18 . Likewise located on the cover  14  is a closable filling orifice  19  for the oil which is required in the transmission for the hydraulic tasks. 
     The compact transmission block illustrated in  FIG. 4 to 8  and consisting of a cover  14 , transmission core  17  (below the cover) and transmission control  16  (above the cover) contains everything which is required for the functioning of the hydrostatic power-splitting transmission. As reproduced in  FIG. 9 to 12 , the housing lower part  31  has correspondingly only the functions of protecting the transmission core  17 , of holding the oil for the transmission and of feeding the power into the transmission and out of the transmission again. As shown in the example of  FIG. 9 to 12 , the infeed and outfeed may in this case take place by means of simple coaxial shafts  39 ,  40  which are mounted rotatably in the housing lower part  31 . However, deflection and/or conversion transmissions may also be provided, which change the position and orientation of the axes. Thus, by means of the same transmission block, a multiplicity of drive solutions can be implemented in different vehicles by the housing lower part  31  simply being adapted to the vehicle. 
     An appropriate flange  36  is formed on the housing lower part  31  for oil-tight connection to the cover  14 . The shafts  39  and  40  are mounted rotatably in the end walls of the housing lower part  31  by means of corresponding bearings  37 ,  38 . Formed in the bottom of the housing lower part  31  is a recessed pan  32  which extends in the longitudinal direction and in which a sump of the hydraulic oil can collect and be sucked in on the transmission core  17  by the hydraulic pump  22 . Access orifices  33  which are closable by means of covers and through which access can be had to the inside of the transmission when the latter is closed can be arranged in the side walls of the housing lower part  31 . 
     The transmission according to the invention is distinguished, overall, by the following properties and advantages:
         The multistep epicyclic drive acts as a power-splitting and summing transmission and is used as an optimal solution for the basic set-up.   The hydrostatic power range is implemented by the +/−45° large-angle technique with major advantages in terms of efficiency and spread in this transmission.   If, therefore, the basic mechanical set-up is taken and combined with the large-angle technique, and if it is supplemented, as required, with axial offset, power take-off shaft and all-wheel drive, an optimal transmission concept is obtained, which can satisfy all vehicle requirements and allows both the axially offset and an inline variant.   The transmission is constructed according to a modular principle.   It is power-split hydrostatically.   It has a multistep epicyclic drive with splitting and summing.   There are 2 forward driving ranges without a traction interruption.   2 large-angle hydrostats with a +/−45° pivot angle are used.   The transfer of force is continuous in the entire operating range.   The transmission has high overall efficiency without dips.   Full hydrostat power is required only during starting.   Full traction during starting is always available.   A driving clutch is unnecessary since the function is already present.   Speeds higher than 65 km/h are possible.   Low speeds with reduced engine rotational speed are possible.   The initial rotational speed can be regulated continuously between 0 and 3000 rev/min without a traction interruption.   The torque spread from input to output amounts to approximately 7.8.   Various driving strategies are possible.   Control takes place via an actuating unit.   The electronics have a modular set-up.   Further operation or emergency drive is possible even in the event of a fault of the electrics or electronics.       

     It will appreciated that the transmission  10  and  10 ′ set up according to  FIG. 1  and/or FIG.  1 ′ can be used advantageously not only within the framework of the present compact cover/housing concept, but also in another connection or with another housing configuration. 
     In particular, the continuous regulation of the initial rotational speed, without shift operations and without traction interruption, as is afforded in the transmission concept of  FIG. 1 , makes this concept especially suitable, irrespective of the actual design and installation configuration of the transmission, for hybrid drives in the sector of buses and agricultural and building vehicles, in which the drive takes place selectively via an internal combustion engine and/or an electric motor and, in the case of regenerative braking, kinetic energy can be recovered via the electric motor acting as a generator and be stored in the battery. Although it is already known from the prior art (DE-A1-38 42 632) to provide hydrostatic/mechanical power-splitting transmissions in a hybrid drive, nevertheless this known solution results in highly complicated and cumbersome control and regulation due to the use of a flywheel and of a change-shift clutch with a neutral shift position. 
     If, by contrast, a hybrid drive with a continuously variable hydrostatic power-splitting transmission according to  FIG. 1  is implemented, the control of the electric drive part can be simplified considerably on account of the uniform operation of the transmission. A first exemplary embodiment of such a hybrid drive is reproduced in  FIG. 13  in a greatly simplified diagram: a first electric motor E 1  acting as a driving motor is coupled fixedly to the gearwheel Z 11  and consequently to the input shaft W 1  via a gearwheel Z 13 . The first electric motor E 1  is supplied with the necessary electrical energy from a suitable battery  42  via first control electronics  41 . The first control electronics  41  cooperate with the motor and transmission control (not illustrated in  FIG. 13 ). In certain instances, the first electric motor E 1  may in this case drive the vehicle alone (for example, a bus in city traffic). It may, however, also assist the internal combustion engine  11 . In particular, it is advantageous if the first electric motor E 1  is used as an electrodynamic retarder or if the electric motor E 1 , particularly within the framework of regenerative braking, operates as a generator and feeds energy back into the battery  42  for storage (see the double arrows between the first control electronics  41  and the first electric motor E 1  and also the battery  42 ). By means of an appropriate control of the transmission  10 , the electric motor/generator E 1  can in this case always be operated in the optimal range. A lithium ion battery, which combines a high storage capacity with a high performance, is preferably used as a battery  42 . 
     The hybrid drive by means of the battery  42  and by the first electric motor E 1  affords the possibility of driving and controlling a power take-off shaft W 8  according to  FIG. 13  by means of a second electric motor E 2  independently of the other operating conditions of the vehicle drive. For this purpose, second control electronics  43  are provided between the battery  42  and the second electric motor E 2 . The second control electronics  43  can operate largely independently of the motor and transmission control, but must take into account at least the instantaneous loading and the charging state of the battery  42 . 
     In the coupling of the first electric motor E 1  to the input shaft W 1  via a gearwheel mechanism Z 11 , Z 12 , Z 13 , the type of electric motor E 1  can be chosen largely freely, because the electric motor E 1  can, for example, be arranged laterally on the transmission, where the overall length pays only a minor role. 
     However, it is also conceivable, according to the exemplary embodiment shown in  FIG. 14 , to arrange the rotor of an electric motor E 3  fixedly in terms of rotation on the input shaft W 1  directly. For reasons of space, what is known as a disk-shaped three-phase machine, such as is described, for example, in the publication DE-A1-10 2006 019 837, is especially suitable for this purpose. This can take over not only the function of a drive motor, but at the same time the functions of a starter and dynamo and also a retarder and can be flanged directly to the transmission in a space-saving way. As compared with the normal hybrid drive which in any case saves energy, the continuously variable power-splitting transmission affords a considerable additional energy saving. When a disk-shaped three-phase machine is used as an electric motor/generator, an especially compact and efficient drive train is implemented to great advantage.