Abstract:
The invention relates to a hydrostatic axial piston machine having a cylinder block which can be rotated about a first axis, the cylinder block being provided with a plurality of cylinder bores which extend in the axial direction and which are arranged on a partial circle which is concentric in relation to the first axis, in addition to a plane which can be rotated about a second axis, whereon a number of pistons associated with the bores, can be articulated in a pivotable manner on a second partial circle which is concentric in relation to the second axis and form a ring. Also provided is a device for synchronizing the rotation of the cylinder block about the first axis and the drive shaft about the second axis. The cylinder block and the drive shaft can be adjusted in a continuous manner using both axes, between a first position, wherein both axes are parallel, and a second position, wherein both axes together form a maximum pivoting angle which is different from zero.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to the field of axial piston machines. It refers to a hydrostatic axial piston machine according to the preamble of claim  1  and to the use of said machine. 
   2. Description of Related Art 
   Continuously variable hydrostatic power-branched transmissions, particularly in vehicles used for construction or agricultural purposes, have long been known from the prior art (see, for example, the publications DE-AS-1 113 621, DE-C2-29 04 572, DE-A1-37 07 382, DE-A1-43 43 401 and EP-A1-1 195 542). In these transmissions, the transferred power is apportioned to a mechanical branch and a hydrostatic branch of the transmission as a function of the driving speed, transferred and subsequently combined again. 
   The hydrostatic power transfer branch of the transmission comprises conventionally two hydrostatic axial piston machines which are connected hydraulically to one another and one of which operates in each case as a pump and the other as a motor. The two machines can in this case interchange their roles, depending on the driving step. 
   The hydrostatic axial piston machines constitute an essential component of the hydrostatic power-branched transmission and decisively influence the properties of the transmission, such as, for example, the efficiency, the overall size, the complexity, the speed range covered, the type and number of driving steps and the like. Examples of hydrostatic axial piston machines of this type are disclosed in DE-A1-198 33 711 or DE-A1-100 44 784. The functioning and theory of the hydrostatic axial piston machines and a power-branched tractor transmission equipped with them are described in a publication of TU Munich from the year 2000 by H. Bork et al., “Modellbildung, Simulation und Analyse eines stufenlosen leistungsverzweigten Traktor-getriebes” [“Modeling, simulation and analysis of a continuously variable power-branched tractor transmission”]. 
   In the hydrostatic axial piston machines, the cylinder block, into which the axial pistons penetrate, can be pivoted with respect to the driving flange, on which the axial pistons are mounted pivotably, over a pivot angle out of the axially parallel basic configuration. Depending on the pivot angle, in the case of a constant rotational speed, an axial piston machine operating as a pump conveys more or less volume per unit time. In an axial piston machine operating as a motor, the pivot angle influences the torque output and the rotational speed. Owing to the cooperation of two axial piston machines operating as a pump and as a motor in a power-branched transmission, the driving speed can be set independently of the engine rotational speed of the driving internal combustion engine, in that the pivot angles of the pump and motor are suitably varied. Thus, in a tractor, it is possible, for example, in spite of a changing driving speed, to keep the rotational speed of the diesel engine constant and to operate the engine at the most beneficial operating point, or to adapt the rotational speed of the take-off shaft optimally to the work task of the accessory driven by means of the take-off shaft. 
   The maximum possible pivot angle of the axial piston machine determines the operating range of the axial piston machine and therefore also the properties of the transmission. In axial piston machines known hitherto, the maximum pivot angle is restricted to values equal to or lower than 45°. This leads to a restriction in power and in the range of variation. The result of this, too, is that the power-branched transmissions in which the axial piston machines are used are restricted in efficiency, cover a restricted speed range per driving step and entail a comparatively high outlay in terms of construction and space requirement. 
   The object of the invention, therefore, is to provide a hydrostatic axial piston machine which is distinguished by markedly improved properties, as compared with the known axial piston machines, and, when used in a power-branched transmission, leads to corresponding improvements in the transmission properties. 
   SUMMARY OF THE INVENTION 
   The object is achieved by means of the whole of the features of claim  1 . The essence of the invention is, in the axial piston machine, to provide a maximum pivot angle of greater than 45°, in particular equal to or greater than 50°. By the maximum pivot angle being increased, the efficiency of the machine is improved. At the same time, the spread is increased, that is to say, in the case of a constant rotational speed, a broadened operating range is obtained. Furthermore, with the overall size being the same, the power is increased or, with the power being the same, a reduced overall size becomes possible. These improvements in the individual axial piston machine also lead to corresponding improvements in a power-branched transmission equipped in such machines. 
   A preferred refinement of the hydrostatic axial piston machine according to the invention is distinguished in that the rotatable plane is arranged at one end of a driven shaft rotating about the second axis, in that the synchronizing means comprise a central synchronizing shaft which is arranged within the belt of pistons and which is in rotationally fixed engagement at one end via a first joint with the driven shaft and at the other end by a second joint with the cylinder block, and in that in order to achieve a pivot angle different from zero, the synchronizing shaft is pivotable on all sides in a central funnel-shaped orifice of the driven shaft, said orifice being arranged within the belt of pistons, in such a way that the maximum pivot angle of the machine is decisively determined by the size and configuration of the funnel-shaped orifice and of the cross section of the synchronizing shaft, the first and the second joint being designed in each case as a tripod joint, the synchronizing shaft being mounted axially displaceably in the drive shaft in order to compensate distance changes in the event of changes in the pivot angle, and means being provided in the drive shaft which prestress the synchronizing shaft resiliently in the direction of the cylinder block, and the prestressing means comprising an axial compression spring which exerts pressure via a pressure piston and a first pressure pin connected pivotably to one end of the synchronizing shaft, and the synchronizing shaft being supported at the other end, via a second pressure pin connected pivotably to it, on the cylinder block. 
   In a hydrostatic axial piston machine designed in this way, to increase the maximum pivot angle, preferably the funnel-shaped orifice is widened locally in each case between adjacent pistons by means of bulges, and the cross-sectional contour of the synchronizing shaft is adapted to the bulges. The bulges may basically have various forms, insofar as, in conjunction with the adapted cross-sectional contour of the synchronizing shaft, they lead to the synchronizing shaft being capable of being pivoted outward to a greater extent. It is particularly beneficial if, according to a preferred further development, the edge contour of the funnel-shaped orifice is a polygon with a number of corners corresponding to the number of pistons, and if the corners of the polygon are arranged in each case between adjacent pistons and form a bulge. 
   In particular, a number of pistons divisible by 3 may be provided, and the cross-sectional contour of a synchronizing shaft may have a rotational symmetry which merges into itself as a result of rotation through 120°, the 120° rotational symmetry of the cross-sectional contour of the synchronizing shaft being generated by three groove-shaped bulges in the synchronizing shaft which run in the axial direction and are arranged in each case so as to be rotated through 120°. Preferably, in this case, 9 pistons are provided. 
   A further refinement of the hydrostatic axial piston machine according to the invention is characterized in that the pistons are arranged in each case at one end of a piston shank, in that the piston shank tapers toward the other end and at the other end is mounted pivotably with a spherical head in a spherical bearing, in that the spherical bearings are fastened on the second reference circle in a flange formed on the driven shaft and forming the plane rotatable about the second axis, and in that the pistons, the piston shanks and the spherical heads are in each case parts of a one-piece element. 
   According to the invention, the hydrostatic axial piston machine according to the invention is used in a power-branched transmission of a vehicle, in particular a tractor, driven by an internal combustion engine, part of the drive power transferred by the power-branched transmission being transferred hydraulically, and, for the hydraulic power transfer, at least two hydrostatic axial piston machines connected hydraulically to one another being used, which operate alternately as a pump and as a motor. 
   Further embodiments may be gathered from the dependent claims. 

   
     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 longitudinal section through a hydrostatic axial piston machine with an unpivoted cylinder block according to a preferred exemplary embodiment of the invention; 
       FIG. 2  shows a detail of the hydrostatic axial piston machine from  FIG. 1  with the cylinder block pivoted through +50°; 
       FIG. 3  shows a detail of the hydrostatic axial piston machine from  FIG. 1  with the cylinder block pivoted through −50°; 
       FIG. 4  shows a longitudinal section through an enlarged illustration of the head part of the driven shaft of the machine from  FIG. 1  with the clearances for one tripod joint and the synchronizing shaft; 
       FIG. 5  shows a top view of the head part of the driven shaft from  FIG. 4  in the axial direction; 
       FIG. 6  shows, in several part figures a) to e), various views of the synchronizing shaft from  FIG. 1  with the recesses on the shank and the tenons, formed at both ends, for the tripod joints,  FIGS. 6   a  and  b  showing side views,  FIG. 6   c  showing a longitudinal section in the plane B-B from  FIG. 6   b ,  FIG. 6   d  showing a cross section in the plane A-A from  FIG. 6   a  and  FIG. 6   e  illustrating a perspective side view; 
       FIG. 7  shows a top view of the position of the synchronizing shaft in the funnel-shaped orifice at the moment of rotation, where one of the groove-shaped recesses of the synchronizing shaft lies, in width, parallel to one of the sides of the nonagonal funnel; 
       FIG. 8  shows an abstract diagram of an illustrative power-branched transmission for a tractor with two hydrostatic axial piston machines according to  FIG. 1-6  operating selectively as a pump and as a motor; 
       FIG. 9  shows a greatly simplified transmission according to the diagram of  FIG. 8 , but without a continuous take-off shaft; 
       FIG. 10  shows, in various part figures  FIG. 10(   a   1 ) to  FIG. 10(   c ), the various shift positions of the transmission according to  FIG. 9  in the first forward driving step ( FIGS. 10(   a   1 )- 10 ( a   3 )), in the second forward driving step ( FIGS. 10(   b   1 )- 10 ( b   3 )) and in reverse drive ( FIG. 10(   c )); and 
       FIG. 11  shows the efficiency eta (curve A) and the percentage hydrostatic fraction of the power transfer (curve B) as a function of the driving speed v of the tractor with the power-branched transmission according to  FIG. 8 to 10 , in the case of a transmission input power of 195 kW at 1800 rev/min and a final speed of 62 km/h at 2100 rev/min. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1  illustrates a longitudinal section through a hydrostatic axial piston machine with an unpivoted cylinder block (pivot angle α=0) according to a preferred exemplary embodiment of the invention. The hydrostatic axial piston machine  10  comprises an elongate driven shaft  11 , a cylinder block  30 , a plurality of pistons  27  and a synchronizing shaft  23  for synchronizing the rotations of the driven shaft  11  and cylinder block  30 . The driven shaft  11  is subdivided over its length into differently machined portions which serve for mounting the shaft, for receiving transmission gearwheels and for receiving and actuating clutches when the hydrostatic axial piston machine  10  is part of a power-branched transmission, as illustrated diagrammatically by an example in  FIG. 7 . 
   At one end, which faces the cylinder block  30 , the driven shaft  11  is thickened and ends in a flange  12  concentric to the axis  38  of the driven shaft  11 . Nine circular-cylindrical bearing receptacles  32  are milled, uniformly distributed about the axis  38  on a reference circle, into the end face of the flange  12  (see also  FIGS. 4 and 5 ), spherical bearings  18  for the pivotable mounting of the pistons  27  being inserted into said bearing receptacles  32 . The axes of the bearing receptacles  32  are tilted a few degrees (for example, 5°) radially outward in relation to the axis  38  of the driven shaft  11 . The end face of the flange  12  is correspondingly machined to descend outward, so that, in the region of the bearing receptacles  32 , it runs perpendicularly with respect to the axes of these. 
   In the center of the flange  12  is provided a funnel-shaped orifice  13  ( FIGS. 4 ,  5 ) which merges further on, inside the driven shaft  11 , into a central bore  15  of stepped diameter. Three axially parallel bores  14 , which are part of a first tripod joint  22 , are introduced into the driven shaft  11  around the bore  15 , and so as to overlap partially with the bore  15 , and are arranged in each case so as to be rotated through 120°. Comparable bores are present, opposite them, in the cylinder block  30  and are part of a second tripod joint  24 . The two tripod joints  22  and  24  allow a rotationally fixed coupling of the synchronizing shaft  23  to the driven shaft  11  and to the cylinder block  30 , while at the same time ensuring a pivotability of the cylinder block  30  in relation to the flange  12  or the driven shaft  11 . For this purpose, according to  FIG. 6 , the synchronizing shaft  23  is equipped at each of the two ends with three radially oriented cylindrical tenons  34  which are arranged so as to be rotated through 120° and which, in the case of the first tripod joint  22 , extend from the central bore  15  through the laterally open overlap region into the adjacent bore  14 . A comparable engagement of the tenons  34  also takes place in the second tripod joint  24 . To reduce the play, in each case rings  17  ( FIG. 1 ), which are crowned on the outside, are drawn onto the tenons  34 . 
   When the cylinder block  30  is pivoted with respect to the flange  12 , the distance to be bridged between the cylinder block  30  and the flange  12  by the synchronizing shaft  23  changes. So that this distance change can be compensated, the synchronizing shaft  23  is mounted displaceably in the axial direction in the region of the first tripod joint  22 . The synchronizing shaft  23  is seated with its end facing the cylinder block  30  pivotably on a first pressure pin  25  which is inserted into the cylinder block  30  and which projects with a portion of its length out of the cylinder block  30 . So that the synchronizing shaft  23  does not come out of engagement with the cylinder block in the second tripod joint  24 , it is pressed against the second pressure pin  25  in the axial direction with prestress. A compression spring  19  accommodated in the bore  15  serves for generating the prestress and presses onto the synchronizing shaft  23  via an axially displaceable pressure piston  20  and a second pressure pin  21 . The pressure piston  20 , pressure pins  21 ,  25  and synchronizing shaft  23  have in each case a central oil duct. 
   The (cylindrical) cylinder block  30  has nine axially parallel cylinder bores  28  which are distributed uniformly about its axis  39  on a reference circle and which, like the bearing receptacles  32  of  FIG. 5 , are in each case at an angular distance of 40° from one another. The cylinder bores  28 , which have a diameter of about 26 mm in the example shown, are designed from the side facing the flange  12  as blind bores. The pistons  27 , which are mounted pivotably in the flange  12 , penetrate from this side into the cylinder bores  28 . For this purpose, each piston  27  has an elongate downwardly tapering piston shank  27 ′ which merges at the lower end into a spherical head  26 , by means of which it is mounted pivotably in the associated spherical bearing  18 . When the cylinder block  30  is pivoted upward out of the position shown in  FIG. 1  (pivot angle α=0), as illustrated in  FIG. 2 , the pistons  27  which lie above a mid-plane perpendicular to the drawing surfaces move further into their cylinder bores  28  and compress or displace through an orifice the medium located in these, whereas the pistons  27  lying below the mid-plane move further out of their cylinder bores  28  and expand the medium located in these or suck it in through an orifice. When the cylinder block  30  is pivoted downward according to  FIG. 3 , the pistons  27  lying above and below the mid-plane interchange their roles. The maximum piston stroke of the pistons  27  amounts to about 93 mm in the example shown. 
   When, in the case of a constant angle α=0, the driven shaft  11  and consequently, via the synchronizing shaft  23 , also the cylinder block  30  are rotated about their respective axes  38  and  39 , each of the nine pistons  27  runs through a complete stroke cycle per revolution, the top and bottom dead centers in each case being run through when the pistons and cylinder bores are respectively at the upper and lower (for α&gt;0; see  FIG. 2 ) or at the lower and upper (for α&lt;0; see  FIG. 3 ) vertex point of the rotational movement. The hydrodynamic axial piston machine  10  can in this case operate as a hydraulic pump when a drive takes place via the driven shaft  11  and a hydraulic medium is sucked in at the pistons  27  moving out of the cylinder bore  28  and is pressed out by the pistons moving into the cylinder bore  28 . The volumetric pumping capacity per revolution is in this case the higher, the larger the pivot angle α is. It may, however, also operate as a hydraulic motor when the cylinders are acted upon in each case, between top dead center and bottom dead center, with hydraulic medium which is under pressure, and the rotational movement occurring is picked up at the driven shaft  11 . The torque is in this case the higher, the larger the pivot angle α is. If, by contrast, high rotational speeds are to be achieved at the driven shaft  11 , the pivot angle α must be made small. In a power-branched transmission  40 , as illustrated diagrammatically in  FIG. 7 , the hydraulic power branch is formed by two hydrostatic axial piston machines H 1  and H 2  of the type shown in  FIG. 1  which are connected hydraulically to one another and which operate selectively as a pump and as a motor, depending on the speed range. 
   The working space in the cylinder bores  28  which is delimited by the pistons  27  is accessible from the outer end face of the cylinder block  30  through connecting orifices  29 . For activating the individual cylinders, there serves a rotationally fixed control disk, not illustrated in  FIGS. 1 to 3 , with corresponding orifices, on which the cylinder block  30  is supported axially with the outer end face via a plain bearing (a bearing bore  31  is provided for radial mounting in the cylinder block  30 ). Details of such a control are known and may be gathered from the publications initially mentioned. The same applies to the pivoting mechanism which is required in order to pivot the cylinder block  30  through the desired pivot angle α with respect to the flange  12 . 
   The hydrostatic axial piston machines known hitherto, such as are described in the publications initially mentioned, have a pivoting range which is limited by a maximum pivot angle α max  of 45°. As a result, the piston stroke per cylinder is limited, and consequently—with the overall size remaining the same—also the range of adjustment for the power. This leads, in particular to restrictions when the hydrostatic axial piston machines are to be used in power-branched transmissions. In the hydrostatic axial piston machines according to the present invention, this limitation is eliminated in that maximum pivot angles α max  of greater than 45°, preferably of up to 50°, are implemented. 
   A substantial limitation for the maximum pivot angle α max  of a hydrostatic axial piston machine according to  FIG. 1 to 3  is afforded by the synchronizing mechanism between the driven shaft  11  and cylinder block  30 . When the cylinder block  30  is pivoted through the pivot angle α, the synchronizing shaft  23  pivots out of the axis  38  of the driven shaft by about half the pivot angle. In order to provide room for this pivoting, according to  FIGS. 4 and 5 , in the center of the bearing receptacles  32  arranged on a reference circle the funnel-shaped orifice  13  is provided, the opening angle of which determines the pivoting range of the synchronizing shaft  23 . In the known hydrostatic axial piston machines, the orifice  13  has a conical design. The width of the orifice is then determined by a circle which is inscribed in the bearing receptacles  32 . In the present solution, by contrast, the space present between the adjacent bearing receptacles  32  is utilized in order to increase the possible pivoting range of the synchronizing shaft  23 . For this purpose, according to  FIG. 5 , in the angular range between adjacent bearing receptacles  32  bulges  33  are provided in the orifice, which extend beyond the inscribed circle. At the same time, according to  FIG. 6   d , the cross-sectional contour of the synchronizing shaft  23  is varied, deviating from the circle, such that, in cooperation with the bulges  33  in the orifice  13 , an increased pivoting range is obtained for the synchronizing shaft  23 . In this case, when the axial piston machine rotates, the synchronizing shaft  23  rolls in the manner of a gearwheel in a ring wheel on the edge contour, provided with the bulges  33 , of the orifice  13 . 
   The edge contour of the orifice  13  may basically be of wavy configuration, the wave crests lying between the bearing receptacles  32  and the wave troughs being arranged directly at the bearing receptacles  32 . Preferably, according to  FIG. 5 , the edge contour of the funnel-shaped orifice  13  is a polygon with a number of corners corresponding to the number of pistons  27 , that is to say a nonagon, the corners of the polygon being arranged in each case between adjacent pistons  27  or bearing receptacles  32  and forming a bulge  33 . By contrast, the cross-sectional contour of the synchronizing shaft  23  has a rotational symmetry which merges into itself as a result of rotation through 120°. This 120° rotational symmetry is generated by three groove-shaped recesses  35  in the synchronizing shaft  23  which run in the axial direction and are arranged in each case so as to be rotated through 120°. The ribs of the synchronizing shaft  23  which remain standing between the groove-shaped recesses penetrate synchronously into the bulges  33  of the orifice  13  in the case of corresponding angular orientation of the tripod joint  22 , when the synchronizing shaft rolls on the wall of the orifice  13  at a maximum pivot angle α max . On account of the 120° symmetry of the synchronizing shaft  23  and of the 40° symmetry of the orifice  13 , penetration takes place only at every third bulge  33 . A snapshot of the rolling operation, in which one of the groove-shaped recesses  35  of the synchronizing shaft  23  lies, in width, exactly parallel to one of the sides of the nonagonal funnel  13 , is reproduced in  FIG. 7 . 
   On account of the broadened pivoting range, the hydrostatic axial piston machine according to  FIG. 1 to 3  is particularly suitable for use in a power-branched transmission of a vehicle, in particular a tractor, which is driven by an internal combustion engine (diesel engine) and in which, on the one hand, a high torque has to be provided in a low driving speed range and, on the other hand, higher driving speeds are to become possible with good efficiency. A diagram of such a power-branched transmission is reproduced in  FIG. 8 , and a greatly simplified transmission according to the diagram of  FIG. 8  is shown in  FIG. 9  (in this case, however, the continuous take-off shaft  42  of  FIG. 8  is replaced by a drive shaft  56  ending in the stepped planetary gear  45 ; the torsion damper  49  is likewise absent). The power-branched transmission  40  illustrated is connected via a cardan shaft  41  and a torsion damper  49  to an internal combustion engine  50  which is symbolized by a piston/cylinder arrangement. A take-off shaft  42  runs through the power-branched transmission  40  and is connected at one end directly to the cardan shaft  41  and at the other end can be connected to a driven shaft  47  via a clutch  46 . Where a tractor is concerned, agricultural accessories can be driven by means of the driven shaft  47 . 
   Seated on the take-off shaft  42  is the large sun wheel z 1  of a stepped planetary gear  45  which comprises additionally double planet wheels z 2 , z 2 ′, a small sun wheel z 1 ′ and a ring wheel z 3 . The small sun wheel z 1 ′ is connected fixedly in terms of rotation via a hollow shaft to a further gearwheel z 6  which meshes with a gearwheel z 7 . The ring wheel z 3  is connected fixedly in terms of rotation to a gearwheel z 4  which, in turn, meshes with a gearwheel z 5 . The gearwheel z 5  can be connected via a clutch K 3  to the driven shaft  43  of a first hydrostatic axial piston machine H 1 . The gearwheel z 7  can be connected via a clutch K 2  to the driven shaft  44  of a second hydrostatic axial piston machine H 2 . The planet carrier ( 55  in  FIG. 8 ) of the double planet wheels z 2 , z 2 ′ is connected fixedly in terms of rotation to a gearwheel z 8  which meshes, on the one hand, with a gearwheel z 9  and, on the other hand with a gearwheel z 17 . The gearwheel z 9  can be connected by a hollow shaft and a clutch K 1  to the driven shaft  44  of the second hydrostatic axial piston machine H 2 . The gearwheel z 17  is part of a drive train  48  which is connected to the driven axles of the vehicle. In the transmission of  FIG. 9 , the power for vehicle propulsion is picked up at a corresponding driven wheel  54 . The powers transferred via the mechanical and hydraulic branches of the transmission are summed up at the planet carrier  55 . Furthermore, the take-off shaft  42  has seated on it fixedly in terms of rotation a gearwheel z  10  which can be connected to the driven shaft  43  of the first hydrostatic axial piston machine H 1  via an intermediate gearwheel z 12  and a further gearwheel z 11  by means of the clutch K 4 . 
   The two hydrostatic axial piston machines H 1  and H 2  are connected hydraulically to one another via two hydraulic lines  51  and  52  which are used in each case as an outward and a return line. A multiway valve  53  inserted into the hydraulic lines  51 ,  52  makes it possible to interchange the lines when the two axial piston machines H 1  and H 2  exchange their roles, that is to say when the axial piston machine operating as a pump is to operate as a motor, and vice versa. 
   The mode of action of the power-branched transmission  40  from  FIG. 8  or  FIG. 9  can be explained with reference to the part figures of  FIG. 10 . The part figures a 1  to a 3  relate in this case to a first forward driving step, the part figures b 1  to b 3  relate to a second forward driving step and the part figure c relates to reverse drive. For reasons of space, the designations of the individual transmission parts which are identical to the designations of  FIG. 9  are omitted. 
   At the commencement of the first forward driving step (slow forward drive;  FIG. 10(   a   1 )), the clutches K 1  and K 2  are engaged. The first hydrostatic axial piston machine H 1  operates, in the first forward driving step, as a pump and the second hydrostatic axial piston machine H 2  as a motor. The axial piston machine H 1  (pump) is first pivoted slowly out of the unpivoted state (pivot angle α=0) into the fully pivoted state (pivot angle α=α max ) which is reached in part  FIG. 10(   a   2 ). It thereby pumps more and more hydraulic fluid into the axial piston machine H 2  operating as a motor. The latter is pivoted to the maximum and therefore outputs a high torque at a slowly rising rotational speed. When the axial piston machine H 1  is pivoted to the maximum ( FIG. 10(   a   2 )), the axial piston machine H 2  is pivoted slowly back to zero ( FIG. 10(   a   3 )). In this case, its rotational speed rises, while the rotational speed of the axial piston machine H 1  and the transferring hydraulic power falls to zero at the end of the driving step. In  FIG. 11 , the first forward driving step corresponds to the speed range of between 0 and about 18 km/h, in which the fraction HP of transferred hydraulic power decreases linearly from 100% to 0%. 
   At the transition from the end of the first driving step ( FIG. 10(   a   3 )) to the start of the second driving step ( FIG. 10(   b   1 )), the clutch K 1  is disengaged and, instead, the clutch K 2  is engaged. Since the axial piston machine H 2  receives no torque in the case of a pivot angle of zero, the shift torque is virtually zero. Simultaneously with the actuation of the clutches K 1  and K 2 , the axial piston machine H 1  is changed over to motor operation and the axial piston machine H 2  to pump operation by the changeover of the multiway valve  53 . The same actions as in the first driving step then take place with a different ratio: first, with the motor H 1  fully pivoted, the pump H 2  is pivoted increasingly out of the un-pivoted state ( FIG. 10(   b   1 )), until it is likewise fully pivoted ( FIG. 10(   b   2 )). The motor H 1  is then pivoted back to zero ( FIG. 10(   b   3 )), it increases its rotational speed, and the transferred hydraulic power drops to zero. The second forward driving step corresponds to the speed range of between 18 km/h and 62 km/h in  FIG. 11 . The fraction of transferred hydraulic power rises in this case from 0% first to a maximum of about 30% (at 30 km/h) and then falls to 0% (at about 53 km/h) and for the speed lying above this remains at 0%. This type of transmission construction and transmission control results, according to  FIG. 11 , in an efficiency eta of the transmission which rises at the start very quickly to values of above 85% and at the highest driving speeds even reaches its maximum of about 90%. 
   For reverse drive ( FIG. 10(   c )), the clutches K 2  and K 3  are opened and the clutches K 1  and K 4  are closed. The axial piston machine H 1  operates as a pump and the axial piston machine H 2  as a motor. The motor H 2  is fully pivoted, while the pump H 1  is pivoted from the pivot angle zero.