Abstract:
A drive mechanism for a mobile vehicle has one first hydraulic motor ( 9 ) and one second hydraulic motor ( 10 ) which are in permanent operative connection via a summarizing transmission ( 12, 14 ) and drive one output shaft ( 17 ). In order to make possible, driving a first hydromotor ( 9 ) above its admissible maximum rotational speed, the first hydromotor ( 9 ) is adjusted to zero displacement and hydraulically separated form the high pressure.

Description:
FIELD OF THE INVENTION 
   The invention relates to a drive mechanism for a mobile vehicle. 
   BACKGROUND OF THE INVENTION 
   Generic drive mechanisms are specially used for driving mobile vehicles such as working machines like wheel loaders. At low speed wheel loaders require high tractions and preferably a terminal speed of 40 km/h. In a first driving range, the working range, gear shifts are an obstacle since those gear shifts are time consuming and thus prolong the load cycle. 
   DE 39 07 633 C2 discloses a continuously variable hydrostatic drive system in which one output shaft is driven by a first hydromotor via a first transmission part and a second hydromotor via a second transmission part, likewise, drives the output shaft. In a first driving range, both transmission parts are connected with the output shaft so that the torque of both hydromotors acts upon the output shaft. By changing the displacement of one hydromotor, the output rotational speed can be additionally changed for changing the displacement of the pump. But this is possible only up to a maximum rotational speed of the hydromotors. For this reason, there is the reduction gear part of the second motor between a mechanical separating device which, upon reaching the maximum rotational speed of the hydromotor, separates it from the output shaft so that the other hydromotor can further be accelerated up to its maximum rotational speed. Before the separating device can be opened, the motor has to be adjusted to a displacement of nearly zero. But the coupling and uncoupling of the motor produces a shift jerk. 
   The problem on which this invention is based is to provide a drive mechanism for a mobile vehicle where, in a first driving range, the torque of two hydromotors acts upon the output shaft and where, in another driving range, the torque of only one hydromotor acts upon the output shaft whereby the shift jerk is to be minimized and the transmission simply constructed. 
   The problem is solved with a drive mechanism for mobile vehicles. 
   SUMMARY OF THE INVENTION 
   According to the invention, a first hydraulic motor is permanently connected with the output shaft via a first reduction gear part and another hydromotor, likewise, via another reduction gear part. The ratios of the transmission parts can be equal, preferably the ratios are different. 
   In one other embodiment, the first reduction gear part, which is connected with the first hydromotor, comprises one spur gear transmission, the output shaft of the hydromotor driving a first spur gear, which is operatively connected with a second spur gear non-rotatably connected with the output shaft. The second hydromotor drives the second reduction gear part which, likewise, consists of one first spur gear operatively connected with one other spur gear which is non-rotatably connected with the output shaft. It is also possible that the first hydromotor drives a first spur gear and the second hydromotor also drives one spur gear, both spur gears being operatively connected with one common spur gear which is non-rotatably connected with the output shaft. At least one hydromotor is designed with adjustable displacement, but both hydromotors are preferably of adjustable displacement. To start with a high torque, the pressure-medium supply lines of both hydromotors are connected with the pressure-medium exit of one pump and the hydromotors have a displacement which is above zero. If the flow rate of the pump is now increased, the motors are driven and they, in turn, drive the output shaft via their reduction gear parts. The output shaft is connected with a vehicle wheel which is also driven. If the maximum flow rate of the pump is reached, at least one hydromotor is adjusted in its displacement so that the rotational speed of the output shaft is further increased. This motor and the reduction gear part are preferably laid out so that the motor, when reaching its minimum displacement, achieves its maximum admissible rotational speed when the pressure-medium supply line is loaded with high pressure. The hydromotor is now adjusted to its zero displacement and the pressure-medium supply line is separated from the high pressure of the pump, the pressure medium exit. The pressure-medium supply line and the pressure-medium return line of this hydromotor are connected with the pressure of the suction side of the hydropump or, in case of closed circuit, with the pressure of the feed pump or with the return pressure from radiator to the transmission (tank), the lubrication pressure for the transmission. By the return pressure from the radiator being connected with the motor, the bearing points and seals of the motor are still less loaded, since this pressure is below the feed pressure. By reducing the displacement of the other motor, the rotational speed of the output shaft is further increased whereby the rotational speed of the first hydromotor also increases above its maximum admissible rotational speed. But the first hydromotor can be operated above its maximum admissible rotational speed since it has almost no displacement by the displacement adjusting device having adjusted the displacement to zero and the high pressure no longer abutting on the pressure-medium supply line. The hydromotor is preferably designed as radial piston motor whereby, through adjustment of the displacement to zero, no relative movement at all generates between the pistons and the cylinders, the motor thus having slight friction. When using a radial piston motor with crankshaft, the displacement adjustment is situated within the crankshaft such as in WO 99/17021, which is herein incorporated by reference when the hydromotor is adjusted to zero displacement, the crankshaft rotates concentrically and the pistons thus effect no stroke movement at all. The mechanical displacement adjustment shown in WO 99/17021 can also be hydraulic by the adjusting piston being adjusted via hydraulic pressure. These adjusting pistons are preferably connected with the high pressure whereby the adjustment is effected via the high pressure. But it is also possible to design the hydromotors as axial piston motors or to design the first hydromotor as radial piston motor and the second hydromotor as axial piston motor. At the maximum rotational speed of the output shaft, the first motor is separated from the high pressure, but mechanically connected with the output shaft and at its zero displacement. The second motor is adjusted to its minimum displacement and drives the output shaft. The hydromotors and the reduction are laid out so that the working range of the working machine is reached when the maximum admissible rotational speed of the first hydromotor has been reached. The drive range thus results exclusively via the second hydromotor. 
   It is obvious to the expert that when changing the flow rate of the pump, the pressure-medium supply line and the pressure-medium return line of the hydromotors turn around, since the pressure-medium return line is now loaded with high pressure and the pressure-medium supply line with low pressure. But when the hydromotor is disengaged, the line conveying the high pressure is always loaded with the low pressure or the return pressure from radiator to tank (transmission), the lubrication pressure, and separated from the high pressure. 
   In another embodiment, the adjusting device of the displacement is connected via a valve with the shut-off valve whereby, when the pressure-medium supply line is separated from the high pressure, the adjusting device for displacement of the motor is automatically separated from the high pressure and loaded with low pressure. It is thus ruled out that in a separated state, the adjusting device adjusts the hydromotor in direction of larger displacement. 
   By the motors not being situated upon the output shaft, it is possible to dispose the pressure-medium supply line for adjustment of the displacement in the crankshaft on one side thereof and connected it with the high pressure. The seal can thus be placed upon a small diameter whereby high rotational speeds are possible even with high pressure. 
   By the first hydromotor and the second hydromotor being in permanent mechanical operative connection with the output shaft and at a maximum rotational speed of the output shaft, one hydromotor being adjusted to zero displacement and its pressure-medium supply line and its pressure-medium return line being connected with the low pressure, the driving system needs no mechanical separating devices whereby no shift jerk can generate. By the motors being supplied with pressure medium also in disengaged state, the cylinders remain filled whereby, likewise, no shift jerk results during engagement. 

   
     BRIEF DESCRIPTION OF THE DRAWING(S) 
     The invention will now be described, by way of example, with reference to the accompanying drawings in which: 
       FIG. 1  is a hydraulics and transmission diagram of the drive mechanism; 
       FIG. 2  is a diagram of the displacement via the output rotational speed or speed; and 
       FIG. 3  is a diagram of the rotational speed via the output rotational speed or the vehicle speed. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 : 
   One hydrostatic pump  1 , which is variable in displacement and preferably has one electronic, rotational-speed dependent adjustment, conveys pressure medium into a common high-pressure line  2  and sucks it from a common low-pressure line  3 . One feed pump  4  sucks, via one filter  5  from a pressure-medium tank  6 , which is preferably the transmission housing, and delivers, on one side, to the common low-pressure line  3  and, on the other, to the feed pressure line  7 . The common high-pressure line  2  is connected via a first shut-off valve  8  with a first hydromotor  9  and with a second hydromotor  10 . The first hydromotor  9  is of an adjustable displacement and drives a first gear wheel  11  of a first reduction gear part  12  and the second hydromotor  10  drives a first gear wheel  13  of a second reduction gear part  14 . The first gear wheel  11  drives a second gear wheel  15  and the first gear wheel  13  drives a second gear wheel  16 , the second gear wheel  15  and the second gear wheel  16  being non-rotatably connected with an output shaft  17 . The output shaft  17  is connected with one vehicle wheel. The valve  8  has two shift positions, the common high-pressure  2  being connected in a first position with a pressure-medium supply line  18  and a pressure-medium return line  19  with the common low-pressure line  3 . In the second shift position of the valve  8 , the common high-pressure line  2  is separated from the pressure-medium supply line  18  and the common low-pressure line  3 , likewise, from the pressure-medium return line  19 . The pressure-medium supply line  18  and the pressure-medium return line  19  are connected with the feed pressure line  7 . It is also possible to connect a line  20  with the exit of a flush valve  21  whereby the pressure-medium supply line  18  and the pressure-medium return line  19  are, likewise, loaded with the pressure of the feed pump  4  but, in this shift position, the motor receives the hot oil that leaves the flush valve  21 . By the line  20  being connected with the line  7 , the motor receives the cool oil of the feed pump  4  in this shift position. One other possibility is to connect the line  20  with a return line  57  from a radiator  56  to the tank  6  whereby the pressure-medium supply line  18  and the pressure-medium return line  19  are loaded with a very low pressure. A valve  22  supplies a displacement adjusting device  23  always with high pressure while the valve  22  connects a line  24  either with a line  25  or a line  26 . The valve  22  is situated between the valve  8  and the first hydromotor  9  and thus, when the valve  8  is shifted to a first shift position, the displacement adjusting device  23  is automatically loaded with high pressure and when the valve  8  is shifted to its second shift position, the displacement adjusting device  23  is automatically loaded with low pressure. Thus it is ensured that in the second shift position of the valve  8  the first hydromotor  9  cannot be adjusted in its displacement. A valve  27  supplies a displacement adjusting device  28 , like the valve  22 , with high pressure. Valves  29  and  30  are designed as proportional valves and connected with an electronic control unit whereby the adjusting devices  28  and  23 , which are connected with valves  29  and  30 , can be controlled and the displacement of the hydromotors  9  and  10  can thus be regulated. Likewise, a valve  31  is connected with the electronic control device and actuates the valve  8  in order to hydraulically uncouple the first hydromotor  9  from the high pressure. 
   To start with the drive mechanism, the valve  8  is in its first shift position whereby the common high-pressure line  2  is connected with the pressure-medium supply line  18  and the displacement of the pump  1  is enlarged whereby a torque builds up in the first hydromotor  9  and in the second hydromotor  10  and the output shaft  17  is driven by the first transmission part  12  and the second transmission part  14 . By further increase of the displacement of the pump  1 , the rotational speed of the output shaft  17  accelerates whereby the vehicle speed increases. The displacement, at least of one motor or of the first motor or of the second motor or of both motors, is then reduced whereby the rotational speed of the output shaft  17  is further increased. When reaching the maximum admissible rotational speed of the first motor  9 , said first motor  9  is adjusted to zero displacement and the valve  31  is loaded by the electronic control unit so that the valve  8  is shifted to its second shift position and the pressure-medium supply line  18  is separated from the common high-pressure line  2 . The adjusting device  23  is also simultaneously separated from the high pressure via the valve  22 . Since the first hydromotor  9  has at this rotational speed zero displacement and no forces at all from the high pressure act upon the first hydromotor  9 , it is possible further to accelerate the first hydromotor  9  above its maximum admissible rotational speed. This occurs by the fact that the displacement of the second hydromotor  10  has also been further reduced until the second hydromotor  10  reaches its minimal displacement. At the maximum flow rate of the pump  1  and minimum displacement of the second motor  10 , the same as the first hydromotor  9  disengaged via the valve, the maximum rotational speed of the output shaft  17  is reached. When using a radial piston hydromotor with crankshaft and hydraulic adjustment in the crankshaft, the crankshaft rotates coaxially at maximum rotational speed whereby the pistons effect no stroke in the cylinders. The bearing forces of the hydromotor are sharply reduced, since the hydromotor is not loaded with the high pressure. It is thus unnecessary to disconnect the hydromotor mechanically from the output shaft  17 . The valves  31  and  8  are shafted so that in a currentless state the first hydromotor  9  is disengaged from the common high-pressure line  2 . The ratio of the first reduction gear part  12  and the ratio of the second reduction gear part  14  are laid out so that at the end of the working range of a wheel loader the first hydromotor  9  reaches its maximum admissible rotational speed and is uncoupled via the valve  8 . 
     FIG. 2 : 
   On the abscissa  32  is shown the vehicle speed or rotational speed of the output shaft  17  of  FIG. 1  is shown and on the ordinate  33  are shown the displacements of the pump  1  of the first motor  9  and of the second motor  10  shown in  FIG. 1 . At the origin  34  the vehicle is stationary. The first motor  9  is at its maximum displacement, which is shown with a line  35 . The second motor  10  is, likewise, at its maximum displacement shown with a line  36 . By enlarging the displacement of the pump  1 , the vehicle or the output shaft  17  accelerates, as shown with a line  37 . At a point  38 , the pump  1  has its maximum displacement which is not changed in the additional view to be seen on a line  39 . Starting from a point  40 , the displacement of the first motor diminishes as shown with a line  41 . Thereby the vehicle accelerates further. Starting from a point  42 , the displacement of the second motor  10 , likewise, becomes reduced, which is shown in a line  43 , whereby the vehicle accelerates further. At a point  44  the displacement of the first motor is adjusted to zero and the motor is separated from the high pressure via the valve  8  of  FIG. 1 . The rotational speed at the point  44  preferably corresponds to the maximum admissible rotational speed of the first hydromotor  9 . Starting from a point  45 , the second motor  10  alone is reduced in its displacement whereby the vehicle speed increases further, which can be seen in the line  46 . At a point  47 , the maximum speed of the vehicle is reached and the second motor  10  is at its maximum absorption volume. The rotational speed of the first motor  9  is at point  47  above its maximum admissible rotational speed. But the maximum admissible rotational speed is defined when the hydromotor is loaded with high pressure whereby the hydromotor can be accelerated above its maximum admissible rotational speed by its hydraulic disengagement and the adjustment of the hydromotor to zero absorption volume. 
     FIG. 3 : 
   On the abscissa  48  the speed or rotational speed of the output shaft  17  is also shown, as in  FIG. 2 . On an ordinate  49 , the rotational speeds of the pump  1  of the first motor  9  and of the second motor  10  are shown. The pump  1  is driven at maximum rotational speed, which can be understood from a line  50 . The rotational speed of the first hydromotor  9  increases by changing the absorption volume, as shown in  FIG. 2 , which is shown in a line  51 . The rotational speed of the second motor  10  increases by changing the displacement, as shown in  FIG. 2 , whereby the road speed of vehicle increases, as shown on a line  52 . The different gradients of the lines  51  and  52  result from the different ratios of the first transmission part  12  and of the second transmission part  14 . At a point  53 , the first motor  9  has reached its maximum rotational speed which in  FIG. 2  corresponds to the point  44 . Above the point  53 , the hydromotor  9  is operated with zero displacement and disengaged high pressure. But the rotational speeds of the first hydromotor  9  and of the second hydromotor  10  increase up to the maximum rotational speed of the output shaft  17 , points  54  and  55  showing the rotational speed at maximum output speed. 
   
     
       
             
           
             
             
             
           
         
             
                 
             
             
               Reference numerals 
             
             
                 
             
           
           
             
                 
             
           
        
         
             
                 
               1 
               pump 
             
             
                 
               2 
               common high-pressure line 
             
             
                 
               3 
               common low-pressure line 
             
             
                 
               4 
               feed pump 
             
             
                 
               5 
               filter 
             
             
                 
               6 
               pressure-medium tank 
             
             
                 
               7 
               feed-pressure line 
             
             
                 
               8 
               first shut-off valve 
             
             
                 
               9 
               first hydromotor 
             
             
                 
               10 
               second hydromotor 
             
             
                 
               11 
               first gear wheel 
             
             
                 
               12 
               first reduction gear part 
             
             
                 
               13 
               first gear wheel 
             
             
                 
               14 
               second reduction gear part 
             
             
                 
               15 
               second gear wheel 
             
             
                 
               16 
               second gear wheel 
             
             
                 
               17 
               output shaft 
             
             
                 
               18 
               pressure-medium supply line 
             
             
                 
               19 
               pressure-medium return line 
             
             
                 
               20 
               line 
             
             
                 
               21 
               flush valve 
             
             
                 
               22 
               valve 
             
             
                 
               23 
               displacement adjusting device 
             
             
                 
               24 
               line 
             
             
                 
               25 
               line 
             
             
                 
               26 
               line 
             
             
                 
               27 
               valve 
             
             
                 
               28 
               displacement adjusting device 
             
             
                 
               29 
               valve 
             
             
                 
               30 
               valve 
             
             
                 
               31 
               valve 
             
             
                 
               32 
               abscissa 
             
             
                 
               33 
               ordinate 
             
             
                 
               34 
               origin 
             
             
                 
               35 
               line 
             
             
                 
               36 
               line 
             
             
                 
               37 
               line 
             
             
                 
               38 
               point 
             
             
                 
               39 
               line 
             
             
                 
               40 
               point 
             
             
                 
               41 
               line 
             
             
                 
               42 
               point 
             
             
                 
               43 
               line 
             
             
                 
               44 
               point 
             
             
                 
               45 
               point 
             
             
                 
               46 
               line 
             
             
                 
               47 
               point 
             
             
                 
               48 
               abscissa 
             
             
                 
               49 
               ordinate 
             
             
                 
               50 
               line 
             
             
                 
               51 
               line 
             
             
                 
               52 
               line 
             
             
                 
               53 
               point 
             
             
                 
               54 
               point 
             
             
                 
               55 
               point 
             
             
                 
               56 
               radiator 
             
             
                 
               57 
               line