Abstract:
A self-propelled forage harvester includes front and rear sets of driven ground wheels. The front set of wheels is driven through a transmission including an input shaft driven by a variable displacement motor and coupled to the front set of wheels by a gear train including one planetary gear set, (in one embodiment), and two planetary gear sets, (in a second embodiment). In both cases, the transmission is shiftable so as to establish at least first and second gear ratios. The shifting of the transmission is accomplished automatically by a control arrangement which takes into account the operating mode of the harvester so that a ratio for producing the correct velocity range for harvesting is established when the harvester is in a working operating mode, and a higher velocity range is established when the harvester is in a transport operating mode. Other operating parameters are sensed and used to automatically control hydraulic pump and/or motor displacement, and for controlling engine speed.

Description:
FIELD OF THE INVENTION  
         [0001]    The invention concerns the drive system of a self-propelled working machine, with an internal combustion engine that can be operated to drive elements in contact with the ground for the forward propulsion of the working machine over a transmission with at least two selectable gear ratios, where a control arrangement is connected with an actuator that is arranged to select the gear ratios of the transmission.  
         BACKGROUND OF THE INVENTION  
         [0002]    As a rule, self-propelled working machines, particularly harvesting machines, are driven hydrostatically. Normally a transmission that can be shifted through a number of gear ratios is inserted between the axle drive and the hydraulic motor. In the state of the art, these transmissions are shifted manually. In the working or harvesting operation, as a rule, only a few of the available gear ratios are required (the first and/or second gear), While during the transport of the working machine or operating on public roads, the normal operation is faster so that the other gear ratios are needed. A particular disadvantage lies in the fact that shifted transmissions for harvesting machines require that the machine be brought to a complete stop before a gear shift can be performed. With normal motor vehicles operating on roads, synchronized transmissions are used in order to have the speed of one transmission shaft conform to the speed of another, so as to perform smooth gear shifts. In harvesting machines, the synchronization is not used for conformity of velocity, but for the conformity of the position or the phase in order to provide the assurance that the gear teeth that are to be brought into mesh are properly aligned with each other. Much time is lost by the stopping and restarting, and thereby dangerous situations can develop, particularly in traffic on the road, in case a gear shift becomes necessary.  
           [0003]    EP 0 819 562 A proposes an electronic control arrangement that automatically brings the harvesting machine to a standstill after a gear change input of an operator and then shifts the gears. While a shift lever may be saved and the operator relieved of the task of shifting, it is nevertheless necessary to bring the machine to a standstill before the shifting process, which has the aforementioned disadvantages.  
           [0004]    EP 1 052 388 A proposes a harvesting machine in which the rotational speed of the drive engine is controlled as a function of an automatically recognized operating mode. During harvesting, the operation is performed at a fixed rotational speed, while during transport operations or operation on public roads, the rotational speed of the engine is a function of the immediate velocity. Here, too, the immediate gear ratio is selected manually.  
           [0005]    The problem underlying the invention is seen in the need to make available an improved drive system for a self-propelled working machine.  
         SUMMARY OF THE INVENTION  
         [0006]    According to the present invention, there is provided an improved propulsion drive transmission for a self-propelled working machine.  
           [0007]    An object of the invention is to provide a propulsion drive transmission having at least two selectable gear ratios and for these ratios to be automatically selected.  
           [0008]    The foregoing object is achieved in a machine having the following structure. Specifically, in a self-propelled working machine an internal combustion engine drives the elements that engage the ground over a transmission with selectable gear ratios that are used for the forward propulsion of the working machine and which usually are wheels or track-laying chains. The gear ratio of the transmission can be selected by an actuator that is controlled by a control arrangement. It is proposed that the control arrangement automatically selects the gear ratio of the transmission, that is, independently of any gear ratio selection input from an operator.  
           [0009]    In this way, a manual actuation of a shift control lever and a manual input of the gear ratio is omitted, which considerably simplifies the operation of the working machine for the operator. The corresponding operating elements in the operator&#39;s cab can be omitted so that the former can be configured for easier operation and the manufacturing cost can be reduced.  
           [0010]    Preferably, the control arrangement is supplied with information regarding the operating mode in which the working machine is operating at that particular time. On the basis of this information, the control arrangement can select the appropriate gear ratio of the transmission.  
           [0011]    The operating mode can be recognized on the basis of an operating mode selector arrangement that can be actuated by the operator manually or by a foot switch or acoustically or by any desired other manner. The control arrangement is supplied with information about the operating mode selected by the operator by means of the operating mode selection arrangement. In the case of a harvesting machine, the operating mode selector arrangement may be a field/road selector switch, known in itself, with which the harvesting machine can be switched between a harvesting operating mode and a road operating mode. Alternatively, or in addition, it would be conceivable that the operating mode be recognized on the basis of the condition of an element of the working machine. If this element is in its working position, for example, a cutter head lowered into its harvesting position, it can be recognized that the working machine is in a working operating mode. With respect to an automatic recognition of the operating mode, reference is made in addition to the disclosure of EP 1 052 388 A, whose disclosure is incorporated into the present supporting documents by reference.  
           [0012]    In a preferred embodiment, the gear ratio is selected by the control arrangement as a function of whether the working machine is in a working operating mode or in a road or transport operating mode. During the working operation (for example during harvesting), the greater part of the power made available by the internal combustion engine is required, as a rule, for the working operation, so that the forward propulsion is performed only at reduced speed compared to the transport operation. An obvious solution, therefore, is to make available fewer gear ratio steps during the working operation than for the operation on roads. During reverse operation, a smaller region of operating speeds, and therefore fewer gear ratios of the transmission are required, so that it is useful to select the gear ratios on the basis of the direction of the operation as well. As a rule, it is sufficient to make available only a single fixed gear ratio during the working operation or during reverse operation. The control arrangement then automatically selects this gear ratio when the working machine is in the working operating mode or in reverse operation. On the other hand, during the transport operation or transport on the road, the control arrangement can select among several (as a rule, all) available gear ratios.  
           [0013]    Self-propelled working machines, particularly harvesting machines, are usually driven hydraulically. The internal combustion engine drives a hydraulic pump that pressurizes a hydraulic fluid, that in turn, drives a hydraulic motor. The latter drives the elements used for the forward propulsion over the aforementioned transmission of the working machine with a gear ratio automatically selected by the control arrangement. Such hydraulic drives are distinguished by elasticity and reliability.  
           [0014]    In such working vehicles, the hydraulic pump and or hydraulic motor, as a rule, are equipped with an adjustable swash plate in order to reposition the displacement and thereby the rotational speed of the hydraulic motor and the forward propulsion velocity. While it would be conceivable to perform the repositioning of the hydraulic pump and/or the hydraulic motor swash plates manually, it is particularly advantageous that the control arrangement also reposition the swash plate(s) of the hydraulic pump and/or the hydraulic motor. Then the control arrangement can control not only the gear ratio of the transmission, but also the forward propulsion velocity of the working vehicle, for example, automatically or on the basis of an input from the operator or regulate on the basis of a forward propulsion signal of a sensor. In order to minimize the number of gear ratios of the transmission, an embodiment is preferred in which the control arrangement controls the hydraulic pump as well as the hydraulic motor, since this combination makes possible a relatively wide range of speeds without any additional transmission gear ratios. In this way, one or more transmission gear ratios could be omitted.  
           [0015]    In a preferred embodiment, the control arrangement is arranged to select the gear ratio of the transmission on the basis of operating data of the hydraulic pump and/or the hydraulic motor. Thereby, the gear ratio can be selected by the control arrangement automatically in such a way that the allowable limit data of the hydraulic pump and the hydraulic motor are maintained. In actual fact, the transmission may be a function of the pressure in the hydraulic system. If the pressure exceeds a fixed threshold value, the control arrangement can select the next smaller gear ratio in which a lower pressure exists in the hydraulic system. Alternatively or in addition, the gear ratio selected by the control arrangement may be a function of the forward propulsion velocity. If this exceeds a certain limit value, the control arrangement selects the next higher gear ratio in each case.  
           [0016]    Furthermore, it is proposed that the control arrangement can also provide rotational speed of the main drive engine as an input. Thereby, a higher rotational speed can be applied as a target value in the working operation, in which a higher power level (and hence a higher rotational speed) is required than in a transport or road operation, in which a lower rotational speed is sufficient and in which the internal combustion engine operates more frugally and with lower noise output. Thereby, in the working operation, a lower, fixed rotational speed for the internal combustion engine is provided as an input. Alternatively, the operator may select a rotational speed where the entire rotational speed range of the internal combustion engine or a particularly useful partial region of it, is available. In operation over public roads or in a transport operation, on the other hand, an obvious solution is to provide a fixed rotational speed as an input for the internal combustion engine (that is lower, as a rule, than in the working operation) or to control it as a function of the immediate load on the internal combustion engine. A control of the rotational speed of the internal combustion engine, as a function of the load, may also be useful during the working operation.  
           [0017]    In the over-the-road or transport operating mode, the forward propulsion velocity of the working machine can be provided as an input, as a rule, by an operator by means of an operating element. For this purpose, a manually controlled operating lever and/or a pedal may be available to the operator. During the working operation, the forward propulsion velocity of the working machine can also be proved as an input by the operator by means of the operating element. An obvious solution is to provide at the operator in both operating modes with the operating element in the available region of movement. In the over-the-road transport operating mode, the range of velocities that can be attained is larger than in the working operating mode in which a rather sensitive velocity input is possible. In addition or alternatively, there is the possibility of the controlling of the forward propulsion velocity of the working machine in the working operating mode as a function of its load.  
           [0018]    The invention is particularly appropriate for harvesting machines. An obvious solution is to automatically deactivate the working arrangements, that is, in particular, the crop conveying and/or the crop processing arrangements in the over-the-road or transport operating mode. In the working (harvesting) operating mode, on the other hand, all arrangements are preferably deactivated that are required only during operations on the road, such as, for example, the road traffic illumination.  
           [0019]    The drive system described so far could be applied to transmissions that require the working machine to be brought to a standstill before a change in the gear ration of the transmission is performed. In order to make possible an automatic shifting of the gear ratio, a control arrangement could be used that automatically brings the machine to a standstill before the actual gear shifting process, and then permits it to drive off. Due to the disadvantages associated with this process, however, a simpler and more advantageous solution is preferred. As an alternative, or in addition to the aforementioned object, it is therefore proposed that the gear ratio of the transmission be changed while the working machine is operating. Thereby, the stopping of the working machine before the change in gear ratio could be omitted.  
           [0020]    An appropriate transmission could be a transmission known in itself from automobiles that is equipped with synchronizing arrangements. However, due to the relatively high torques that must be transmitted by working machines, such transmissions are expensive. For the present application, a transmission is preferred that includes a planetary transmission. The planetary transmission includes a sun gear and a planet carrier. There is the possibility of blocking the planetary transmission in itself at a gear ratio, while the planets mesh with and roll on the sun gear in the other gear ratio.  
           [0021]    These possible transmissions can be attained in various ways. In one embodiment, the planet carrier is coupled in on gear ratio with the output shaft or the input shaft. If the planets mesh with the planets of another planetary transmission that is connected so as to transmit torque over another gear with the output or input shaft, the planetary transmission is locked in itself. The result is a transmission gear ration of 1:1. On the other hand, if the planet carrier is arrested, the torque is transmitted over the gear, the planet sets, and the sun gear (or the inverse), so that a gear reduction ratio or step-up ratio can be attained.  
           [0022]    In another embodiment, the planetary transmission includes a ring gear, a planet and a sun gear. One of these elements is used as the drive and another as the output. The third element is driven at different rotational speeds, as function of the gear ratio selected, particularly in that it is connected so as to drive one of the other elements or is stopped. In a preferred embodiment, the ring gear is coupled with the sun gear in a first gear ratio and is held in a stationary position in a second gear ratio. Here, the planet carrier may be used as an input or an output, while the sun gear is used correspondingly an output or an input. The advantage of this solution lies in the fact that only one clutch that can be disengaged is required between the third element (here the ring gear) and one of the other elements (here the sun gear), and a brake is required for the third element.  
           [0023]    In the embodiments of the planetary transmission described, synchronization can be omitted, since all gears are constantly in mesh.  
           [0024]    Finally, it should be noted that in place of the internal combustion engine described above, a fuel cell could also be used where an electric motor connected to it electrically provides the drive power. Such arrangements are also intended to fall under the protection of the claims of this application. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0025]    The drawings show an embodiment of the invention that shall be described in greater detail in the following.  
         [0026]    [0026]FIG. 1 shows a schematic side view of a self-propelled working machine.  
         [0027]    [0027]FIG. 2 is a schematic representation of the drive system for the working machine of FIG. 1.  
         [0028]    [0028]FIG. 3 is a schematic representation of an alternative embodiment of the transmission of FIG. 2. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0029]    Referring now to FIG. 1, there is shown a self-propelled working machine  10  in the form of a self-propelled forage harvester supported on a frame  12  that is carried on front wheels  14  and rear, steerable wheels  16 . The working machine  10  is controlled from an operator&#39;s cab  18  from which a harvested crop take-up arrangement  20  can be viewed by the operator. Crop, for example, corn, grass or the like, taken up from the ground by means of the crop take-up arrangement  20  is conducted by four intake rolls  30 ,  32 ,  34 ,  36  to a chopper drum  22 , that chops the crop into small pieces and delivers it to a conveying arrangement  24 . The crop leaves the working machine  10  over a rotating discharge duct  26  to an accompanying trailer. Between the chopper drum  22  and the conveying arrangement  24 , a post-chopper reduction arrangement including two interacting rolls  28  is arranged by means of which the conveyed crop is conducted tangentially to the conveying arrangement  24 .  
         [0030]    The drive power required for the forward propulsion and for the operation of the working machine  100  is made available by an internal combustion engine  38 , which is usually a diesel engine. An electronic control arrangement  40  is arranged in the operator&#39;s cab  18 .  
         [0031]    [0031]FIG. 2 shows a schematic representation of the drive system of the working machine  10 . The internal combustion engine  38  drives a shaft  42 , on which a clutch  44  is arranged, that can be engaged and disengaged by an electromagnet  46 . Furthermore, the shaft  42  drives a hydraulic pump  48  and a supply pump  50  that is used to supply the hydraulic systems of the working machine  10  except for the hydraulic motors used for the forward propulsion drive. The clutch  44  is arranged within a belt pulley about which a drive belt  52  circulates that also circulates about a first belt pulley  54  and a second belt pulley  56 . The first belt pulley  54  is connected to drive the conveying arrangement  24 , while the second belt pulley  56  is connected to drive the chopper drum  22 . The power to drive the harvested crop take-up arrangement  20  and the intake rolls  30 ,  32 ,  34 , and  36  is taken from the chopper drum  22 . A part of the drive power or all of it for the intake rolls  30 ,  32 ,  34 , and  36  can also be made available by an additional hydraulic motor that is driven by the supply pump  50  or another hydraulic pump that is connected so as to be driven by the internal combustion engine  38  and that drives the intake rolls over a planetary transmission that is driven, in addition, from the chopper drum  22 , as was described by DE 198 12 500 A. In this way, the length of cut can be provided as a continuous input. But it would also be conceivable to use a transmission that could be shifted in steps for the intake rolls.  
         [0032]    The hydraulic pump  48  is connected by hydraulic lines  58  with a hydraulic motor  64 . The hydraulic motor  64  is connected over a transmission  66  so as to drive the front wheels  14 . Between the transmission  66  and the front wheels  14 , a differential gearbox  108 , and in each case, a drive shaft  92  are arranged which drive the wheels  14  over axle end drives (not shown). An embodiment would also be conceivable in which each front wheel is associated with a hydraulic motor  64 .  
         [0033]    A second hydraulic motor  68  is also connected with the hydraulic pump  48  over lines carrying hydraulic fluid (not shown for the sake of clarity). The second hydraulic motor  68  is connected so as to drive the rear wheels  16 , where a differential gearbox may also be provided or a hydraulic motor  68  is associated with each rear wheel  16 .  
         [0034]    The transmission  66  includes an input shaft  70  that is driven directly by the hydraulic motor  64  and that carries a cylindrical clutch disk  72 . Furthermore, a sun gear  74  of a planetary transmission  76  is fastened to the input shaft  70 . A planet carrier  78  of the planetary transmission  76  is connected to a gear  82  over a hollow shaft  80 , that is arranged coaxially to the input shaft  70  and is supported in bearings on it, free to rotate, and the planet carrier  78  is coupled over a further gear  84  to an intermediate shaft  86 , so as to carry or transfer torque. The intermediate shaft  86  drives a gear  88  that meshes with a gear  90 , which drives a differential gearbox  108 , which drives two output shafts  92 . Each of the output shafts  92  is connected so as to drive one of the front wheels  14 .  
         [0035]    The ring gear  94  of the planetary transmission  76  can be brought into engagement and out of engagement with the clutch disk  72  by a clutch that can be actuated. Furthermore, a brake arrangement  98  is arranged between the housing  100  of the transmission  66  and the ring gear  94 , with which the ring gear  94  can be held in a stationary position.  
         [0036]    If the brake  98  is disengaged, the ring gear  94  is stopped. Then the hydraulic motor  64  drives the sun gear  74  over the input shaft  70 , whereby the planetary transmission  76  produces a gear reduction ratio. The output drive is performed over the planet carrier  78 , the hollow shaft  80 , the gear pair  82 ,  84 , the intermediate shaft  86 , the gear pair  88 ,  90 , and the output shaft  92 , and the axle end drive to the front wheel  14 .  
         [0037]    If the brake  98  is released and the clutch  96  is engaged, the ring gear  94  is driven over the input shaft  70  by the hydraulic motor  64  at the same rotational speed with which the sun gear  74  is also driven. Therefore, the speed difference between the ring gear  94  and the sun gear  74  is less than in the operating mode described above, in which the ring gear  94  is held in a stationary position and the planetary transmission  76  is blocked in itself. The result is a gear ration of 1:1. Otherwise, the output drive is performed as described in the previous paragraph.  
         [0038]    In this way, the result is that with the clutch  96  disengaged and the brake  98  applied, a first gear ratio is obtained; and with the clutch  96  engaged and the brake  98  released, a second gear ratio is obtained that provides a higher gear ratio, and thereby a higher forward propulsion velocity with a given rotational speed of the hydraulic motor  64 , than the first gear ratio. The shift process can also be performed when the working machine  10  is operating, since the gears of the planetary transmission  76  are in mesh at all times and no synchronization is required. Any stopping of the working machine  10  for the performance of a shift process can be omitted.  
         [0039]    The control arrangement  40  controls the elements of the drive system that can be controlled and is connected to a series of sensors. For this purpose, a bus  102  is provided, which as a rule can be either an LBS or a CAN bus, with which the control arrangement  40  is connected over a microprocessor or any other appropriate control circuit. The elements that can be controlled and the sensors are also equipped for data transmission over the bus  102  with microprocessors or other appropriate control circuits. Any other desired sensors or actuators can also be connected to the bus  102 . Furthermore, several or all of the sensors and/or actuators can be connected directly to the control arrangement  40  or connected over a communications unit with the bus  102 .  
         [0040]    The bus  102  is connected with an electronic control unit  104  of the internal combustion engine  38 . Furthermore, the bus  102  is connected with the electromagnet  46  for engaging and disengaging the clutch  44  for the drive of the chopper drum  22  and the conveying arrangement  24  and with an electromagnetic actuator  106  that is arranged for the repositioning of the swash plate of the hydraulic pump  48 . The actuator  106  makes it possible to change the flow rate made available by the hydraulic pump continuously or in steps. Furthermore, it makes it possible to reverse the direction of flow of the hydraulic fluid so that the hydraulic motor  64  can be changed from forward to reverse operation. A rotational speed sensor  110  that detects the rotational speed of the output shaft  92  of the transmission  66 , for example, optically or magnetically, is connected with the bus  102  as is an electromagnetic actuator  112  that is arranged for the engagement and disengagement of the clutch  96 . A pressure sensor  114  connected by a flange to the hydraulic motor  64  detects the pressure of the hydraulic fluid existing at the hydraulic motor  64  and is connected with the bus  102 , as is an electromagnetic actuator  116  that is arranged to reposition the swash plate of the hydraulic motor  64 . The actuator  116  makes it possible to change the inflow flow rate of the hydraulic motor  64  and thereby reposition the rotational speed of the input shaft  70  at a given flow rate of the hydraulic fluid. Another electromagnetic actuator  118  is used to apply and release the brake  98  and is connected with the bus  102 . The second hydraulic motor  68  is also equipped with an electromagnetic actuator  120  that repositions the swash plate of the second hydraulic motor  68  and is connected with the bus. The second hydraulic motor  68  is equipped with a pressure sensor  122  to detect the pressure existing at the second hydraulic motor  68  and with a rotational speed sensor  124  to detect the output rotational speed of the second hydraulic motor  68 , each of which is connected with the bus  102 . In place of the aforementioned electromagnetic actuators  46 ,  106 ,  108 ,  112 ,  116 ,  118 , and/or  120 , hydraulically controlled actuators could also be used that are controlled electromagnetically over appropriate valves.  
         [0041]    Finally, the bus  102  is also connected with a first switch  126 , a second switch  128 , a third switch  134 , and a potentiometer  130  as well as an operating element  136  that can also be configured in the form of a potentiometer, which are arranged in the operator&#39;s cab  18  within comfortable reach of the operator. The first switch  126  is used to switch between the working or harvesting mode and a road or transport mode. The second switch  128  is used to engage or disengage the all-wheel drive. The potentiometer  130  is used to detect the position of an operating lever  132  that can be pivoted and/or slid that is used to provide input for the forward propulsion velocity. The third switch  134  is used to turn the operating arrangements on and off by means of the clutch  44 . The operating element  136  is used to provide the input for the rotational speed of the internal combustion engine  38 .  
         [0042]    The method of operation of the working machine  10  and the drive system described is as follows:  
         [0043]    The control arrangement  40  tests regularly, for example, every ten milliseconds, whether the first switch  126  is in the position associated with the road or transport operation. It would also be conceivable, that in place of the position of the first switch  126  to detect the operating position or mode of a working element of the working machine  10 , for example, a sensor tests whether the crop take-up arrangement is raised (non-operating position) or lowered (operating position).  
         [0044]    If the first switch  126  is not in the road or transport operating mode position, the working machine  10  is therefore in the working (harvesting) operating mode; then the clutch  96  of the planetary transmission  76  is disengaged by the actuator  112 , and shortly thereafter, the brake  98  is applied by the actuator  118 . Thereby, the first gear ratio of the transmission  66  is selected permanently, since the forward propulsion velocity range made available by it is sufficient for the harvesting operation. The internal combustion engine  38  is brought to a rotational speed appropriate for the working operation by the control arrangement  40  over the electronic control unit  104  that may be provided as a fixed input or may be changed by the operator by means of the operating element  136 . In the working operating mode, the control arrangement  40  makes it possible for the operator to engage the clutch  44  over the third switch  134  and the electromagnet  46 , so that the working operating arrangements (conveying arrangement  24 , chopper drum  22 , intake rolls  30 - 36 , and crop take-up arrangement  20 ) can be put into operation.  
         [0045]    In the working operating mode, the control arrangement  40  controls the actuators  106 ,  116 , and  120  of the hydraulic pump  48  and the hydraulic motors  64 ,  68  corresponding to a target velocity provided as an input by the operating lever  132 , that can be provided as an input by the operator. The operating lever  132  can be pivoted forward for forward operation and to the rear for operation in reverse. On the basis of the potentiometer  130 , actuators  106 ,  116 , and  120  are controlled by the control arrangement  40 . If the operating lever  132  is in a reverse operating position, then the control arrangement  40  switches the hydraulic pump  48  into a reverse position over the actuator  106 . During the reverse operation, any actuation of the third switch  134  is ignored so that the working arrangements are automatically switched into a non-operating mode. In both directions of operation, the range of movement of the operating lever  132  corresponds to the entire velocity range available in the working operating mode. There are no end regions of the operating range of the operating lever  132  that can effect a change in the velocity, and a sensitive adjustment of the velocity is possible. The signals of the pressure sensors  114 ,  122  and the velocity sensors  110 ,  124  are considered by the control arrangement  40  during the adjustment of the actuators  106 ,  116 , and  120 , and are used to attain the desired forward propulsion velocity of the working machine  10 , yet to avoid a spinning of the wheels  14 ,  16  or backward rotation during all external operating conditions. In this connection, reference is made to the disclosure of DE 102 11 779 A which is incorporated by reference into the present supporting documents.  
         [0046]    It should be noted that it is also conceivable during the working operation to detect the crop throughput by any desired sensor, for example, by the position of the intake roll  36  that can move upward against the force of a spring, and to use it in place of the signal from the potentiometer  130  for the control of the forward propulsion velocity.  
         [0047]    If the second switch  128  is in the non-all-wheel operating position, then the control arrangement  40  arranges for a further valve (not shown) to separate the second hydraulic motor  68  from the hydraulic lines  62  so that it revolves freely with the wheels  16 .  
         [0048]    If on the other hand, the control arrangement  40  finds that the first switch  126  is in the road or transport operating position, the control arrangement  40  ignores the third switch  134  or brings it automatically by means of a further actuator into the position that corresponds to a disengaged clutch  46 . Thereby, in road or transport operation, no activation of the working arrangement is possible.  
         [0049]    Thereafter, the control arrangement  40  tests whether the potentiometer  130  is in a position that corresponds to reverse operation. If that is the case, the actuator  106  is brought into a position for reverse operation and the clutch  96  is or remains disengaged, and shortly thereafter, the brake  98  is applied so that the transmission  66  remains in the first gear ration position or reaches that position. The reverse operating velocity range that can be provided by the operating lever  132  corresponds to the entire range of movement corresponding to the reverse operation in which the operating lever  132 , as a rule, is moved to the rear. The control arrangement  40  controls the actuators  106 ,  116 , and  120  considering the signals of the sensors  110 ,  114 ,  122 , and  124  and the potentiometer  130 , as described above. The second hydraulic motor  68  can be activated or deactivated by the second switch  128 .  
         [0050]    However, if the operating lever  132  is in a position that corresponds to a forward operation and the first switch is in the position corresponding to the road or transport operating mode, then the control arrangement  40  detects the actual forward propulsion velocity of the working vehicle on the basis of the signals of the velocity sensors  110 ,  124 . If it lies below a predetermined threshold value, the clutch  96  is or remains disengaged or deactivated with the use of the actuators  112 ,  118 , and shortly thereafter, the brake  98  is applied or activated. The transmission  66  is thereby placed in the first gear ration condition. If the actual velocity, however, is below the predetermined threshold value and the pressure sensor  114  signals an allowable pressure, the brake  98  is released or deactivated and the clutch  96  is engaged by the particular actuators  112 ,  118  so that the second gear ration of the transmission  66  is selected. Thereby, the full velocity range is available from the transmission  66  in the road or transport operating mode. The forward velocity range that can be provided as an input by the operating lever  132  corresponds to its entire operating range in the forward operating direction, in which the operating lever  132 , as a rule, is moved forward. While no such sensitive adjustment is possible in the working and reverse operating range, however, a selection of all available forward and reverse operating velocities is available. In the road or transport operating mode, the forward propulsion velocity and/or the reverse operating velocity of the working machine  10  could also be provided as an input by a pedal.  
         [0051]    The control arrangement can perform a comparison between the immediate operating velocity that is detected by the velocity sensors  110 ,  124  and an allowable maximum operating velocity. When the maximium velocity is reached or exceeded, inputs for any increase in the velocity are ignored or the velocity is automatically reduced. With respect to this, reference is made to the disclosure of DE 101 09 775 A which is incorporated by reference into the present supporting documents.  
         [0052]    With both gear ratio steps of the transmission  66 , the control arrangement  40  considers the signals from the velocity sensors  110 ,  124  and the pressure sensors  114 ,  122  in the road or transport operating mode in forward and reverse operation, in order to attain the desired forward propulsion velocity, but to prevent a spinning or backward rotation of the wheels  14 ,  16 , as was explained above on the basis of the working operation. Furthermore, the control arrangement  40  provides for an automatic transition from the second into the first gear ration, in which the clutch  96  is disengaged, and preferably shortly thereafter, the brake  98  is applied, as soon as the forward propulsion velocity measured by the velocity sensors  110 ,  124  falls below the aforementioned threshold value or the pressure sensor  114  detects an excessively high value. The threshold value for a downshift may also be somewhat lower than the threshold value for an upshift in order to avoid this hysterisis frequent, unnecessary shifting of the clutch  96  and the brake  98  in the velocity range close to the threshold value.  
         [0053]    In the selection of the gear ratio, the control arrangement also considers the pressure of the hydraulic fluid existing at the hydraulic motors  6 ,  68 . If it exceeds an established threshold value, for example, during operation at an upward incline, then the control arrangement  40  automatically provides a transition from the second to the first gear ratio so that no limit values of the hydraulic motors  64 ,  68  are exceeded and the latter do not overheat excessively.  
         [0054]    As already noted above, the second hydraulic motor  68  can be turned on and off by the second switch  128 . It would also be conceivable to omit the second hydraulic motor  68  and the second switch  128 .  
         [0055]    The rotational speed of the internal combustion engine  38  is also provided as an input in the road or transport operating mode by the control arrangement  40  over the electronic control unit  104 . It can be adjusted to a fixed value or a value provided, as input by the operator, within a range that as a rule is limited compared to the available rotational speed range, by means of the operating element  136  that is smaller than in the working operating mode in order to reduce the fuel consumption and the noise emission. It would also be conceivable to control the rotational speed of the internal combustion engine  38  as a function of its load that can be calculated by the control arrangement  40 , for example, on the basis of the pressure sensor  114 ,  122  and the velocity sensors  110 ,  124  or determined on the basis of data stored in the electronic control unit  104 .  
         [0056]    [0056]FIG. 3 shows another embodiment of a transmission  66 ′ that can be used in place of the transmission  66  of FIG. 2. The transmission  66 ′ differs essentially only in the configuration of the planetary transmission  76 ′ from the configuration according to FIG. 2, while the remaining elements are the same. The transmission  66 ′ includes a gear  138  arranged on the input shaft  70  that meshes with a set of planets  140  which is arranged on a planet carrier  78 . Furthermore, the planet carrier  78 , which retains the planet set  140 , is coupled with a disk  142  which can be brought into engagement with the brake  98  and that can be coupled with the input shaft  70  over the clutch  96 . In addition, the planet carrier  78  is provided with a second set of planets  144 , whose planets mesh with the sun gear  74 , which is attached to the hollow shaft  80  which is used as an output drive. Each of the planets of the planet sets  140  and  144  are connected to each other so as to transmit torque.  
         [0057]    If the brake  98  is applied by the actuator  118  and the clutch  96  is disengaged by the actuator  112 , the planet carrier  78  is held stationary. The torque is transmitted over the input shaft  70 , the gear  138 , the planets of the planet set  140 , the gears of the planet set  144 , and the sun gear  74  to the hollow shaft  80 , and from there further to the wheels  14 , as described above. The planet carrier  78  remains stationary.  
         [0058]    If the brake  98  is released and the clutch  96  is engaged, the torque is transmitted over the input shaft  70  over the clutch  96  to the planet carrier  78 , from there over the second planet set  144  to the sun gear  74  and to the hollow shaft  80 , and from there further to the wheels  14 , as described above. In this case, the planetary transmission  76 ′ is blocked in itself, while with the brake  98  applied and the clutch  96  engaged, the result is a gear reduction ratio. With this embodiment, the first gear ratio corresponds thereby to a brake  98  that is applied and a disengaged clutch  96 , while the second gear ratio corresponds to a brake  98  that is released and a clutch  96  that is engaged. In this embodiment, the engagements of the clutch  96  and application of the brake  98  are also, in each case, delayed by a time interval in order to prevent a blocking of the transmission  66 .  
         [0059]    Having described the preferred embodiment, it will become apparent that various modifications can be made without departing from the scope of the invention as defined in the accompanying claims.