Abstract:
A vehicle includes a variable speed hydrostatic transmission which drives through a gear transmission, having selectable gear ratios, to provide propulsion power to a pair of drive wheels. The hydrostatic transmission is used as a brake to supplement the braking ability of the vehicle. The gear transmission is provided with a positioning plate which is mounted for pivoting between respective positions corresponding to different transmission gear ratios in response to a gear shift lever being moved between corresponding positions. Provided for ensuring that the gear transmission is not shifted to neutral while the vehicle is in motion, and the consequent loss of the ability of the hydrostatic transmission to provide braking, is a blocking arrangement, including a vehicle motion responsive sensor, that operates to prevent the movement of the positioning plate, so long as the vehicle is in motion.

Description:
The invention concerns the drive system of an agricultural vehicle, with a hydraulic motor that drives elements in contact with the ground for the propulsion of the vehicle, in particular wheels and/or crawler chains, with an intervening gearbox, where the hydraulic motor is able to act as a brake upon the elements in contact with the ground. 
     BACKGROUND OF THE INVENTION 
     In agricultural vehicles and front harvesting attachments, such as cutter heads and corn pickers, a tendency can be see towards ever more powerful, larger and therefore heavier units. For reasons of safety of operation and of traffic, there s also the necessity to dimension the brakes of the vehicles correspondingly. In order to avoid having to increase the size and thereby the cost of the conventional friction brakes, that are, as a rule, disk or drum brakes, and act mechanically on the axles of the wheels, there is also the possibility of utilizing the braking effect of a hydrostatic drive of the vehicle, whose swash plate is brought into the null position. 
     In a publication (VkB1. official part, volume 21-1998, dated Oct. 5, 1998, page 1226 and following) guidelines are forth for brake systems of vehicles with hydrostatic drives in which it has been established that such vehicles are admitted to traffic over roads only if this drive cannot be uncoupled during the operation and operates as a hydrostatic brake system or is a component of at least one brake system. It is not disclosed how the uncoupling can be prevented. 
     EP 0819562 A describes a gearbox shifting system for a harvesting machine in which the user can push a button associated with the desired gear ratio. If the button is pushed, a mechanical brake, independent of the gearbox, is applied automatically by a control arrangement, which brings the harvesting machine to a stop. If the harvesting machine is stopped, which can be determined on the basis of the expiration of a pre-determined time interval that is required in the general case for the braking of the harvesting machine, or by means of a speed sensor, then the shifting process is performed. The reason for this is the fact that the hydrostatic gearbox of the harvesting machine can only be shifted when the machine is stopped. The disadvantage is the high cost. 
     U.S. Pat. No. 4,572,340 describes a blocking arrangement for an automatic transmission, which permits a shift between a neutral, forward and reverse position of the selector lever of the automatic transmission only if the service brakes of the vehicle are applied. The blocking arrangement locks the selector lever for such a time until the brakes are applied. 
     The problem underlying the invention is seen as that of creating an agricultural vehicle with a hydrostatic traction drive that is safe in traffic and low in cost. 
     SUMMARY OF THE INVENTION 
     According to the present invention there is provided an improved drive system for the ground-engaging propulsion elements of an agricultural vehicle wherein the drive system embodies a hydrostatic transmission coupled to a gear transmission. 
     An object of the invention is to provide a mixed hydrostatic and gear transmission that uses the hydrostatic transmission for braking the ground wheels with a simple inexpensive way of controlling the drive system so that gear ratio changes in the gear transmission cannot be done unless the vehicle is at a standstill. 
     A more specific object of the invention is to provide a drive system as set forth in the previous object wherein the shifting assembly for the gear transmission includes a device which blocks the shifting operation if the propulsion wheels are rotating. 
     Yet a more specific object is to provide a drive system as set forth in the immediately preceding object wherein the device which blocks the shifting operation includes a controller which is responsive to the pressure developed by a lubrication pump for completing a circuit for energizing a solenoid that positions a locking pin in a lock position any time the propulsion wheels are being driven by the hydrostatic transmission or when they are coasting. 
     These and other objects of the invention will become apparent from a reading of the ensuing description together with the appended drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic side view of a harvesting machine. 
     FIG. 2 is a schematic view of a drive system for the harvesting machine. 
     FIG. 3 is a plan view of the cove plate of the gear box of the harvesting machine. 
     FIG. 4 is a vertical sectional view taken along line  4 — 4  of FIG.  3 . 
     FIG. 5 is a plan view of a cover plate of the gearbox according to a second embodiment. 
     FIG. 6 is a vertical section view taken along line  6 — 6  of FIG.  5 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to FIG. 1, there is shown a harvesting machine  10 , which is in the form of a self-propelled forage harvester including a frame  12  that is supported on a front pair of driven ground wheels  14  and a rear pair of steerable wheels  16 . The operation of the harvesting machine  10  is controlled from an operator&#39;s cab  18  from which a crop intake arrangement  20  can be easily seen. Crop taken up from the ground by the crop intake arrangement  20 , for example, corn, grass or the like is conducted to a chopper drum  22  that chops it into small pieces and conducts it to a conveyor arrangement  24 . The crop leaves the harvesting machine  10  to an accompanying trailer through a discharge duct  26  that is mounted for swinging horizontally. Between the chopper drum  22  and the conveyor arrangement  24  there extends a post-chopper reduction arrangement  28 , through which the conveyed crop is conducted tangentially to the conveyor arrangement  24 . Further details of the harvesting machine  10  do not need description since this is known in itself. the harvesting machine  10  is shown here only as an example in the form of a forage harvester; the invention can also be applied to any desired self-propelled agricultural vehicles, such as combines, tractors, telescoping loaders and the like. 
     FIG. 2 shows the drive system of the harvesting machine  10  in a simplified, schematic form. The front wheels  14  are respectively connected through first and second shafts  32  and  32 ′ with the output side of a gearbox  30 . It is to be understood that for some vehicles the rear wheels  16  could also be connected to be driven by the gearbox  30 . The input side of the gearbox  30  is connected through a third shaft  36  with a hydraulic motor  40 . For its part, the hydraulic motor  40  is coupled by two hydraulic oil pressure/return lines  46  and  48  with a hydraulic pump  44 , which is connected so as to be driven through a fourth shaft  50  with the main engine  52  of the agricultural vehicle  10 . The main engine  52  is arranged to drive the hydraulic pump  44  which supplies the hydraulic motor  40  with pressurized oil over one of the lines  46  or  48  while the other line acts to convey return oil to the pump  44  in a closed system. The hydraulic motor  40  converts the oil pressure into a rotation of the shaft  36 , which, for its part, drives the wheels  14  through the gear box  30  and the shafts  32  and  32 ′. The speed of the shaft  36  is controlled by means of a swash plate  42  of the hydraulic pump  44 , which can be controlled over a cable pull  60  and a speed selector lever  62  arranged in the cab  18  for manipulation by the operator of the vehicle. The direction of operation of the vehicle can be reversed by reversing the direction of the flow of the oil in the lines  46  and  48  through appropriate hydraulic valves (not shown) actuated by a corresponding control switch located in the cab  18 , or the pump  44  itself could be reversible by having a swash plate which is pivotable to opposite sides of a neutral position. As an alternate construction, the gearbox  30  could be provided with a reverse gear arrangement. The gearbox  30  is provided with three differing gear ratios, that can be selected by means of a shift lever  58  in the cab  18 , the shift lever forming one component of a simple and low cost configuration of the shifting arrangement. In this shifting arrangement, the shift lever  58  is connected over a cable pull  56  with another lever  54  which, for its part, is connected so as to drive a positioning plate  74  arranged in the interior of the housing of the gearbox  30 . Alternatively, the application of an electro-mechanical or hydraulic device is conceivable that also transmits the motion of the shift lever  58  to the repositioning plate  74 . 
     The positioning plate  74  is employed, in a way known in itself, but is not shown in any further detail in the drawings, to select the gear ratio of the gearbox  30 , that is, to select the gears of the gearbox  30  that establish a driving connection between the third shaft  36  on the input side, and the first and second shafts  32  and  32 ′ on the output side. As can be seen in FIG. 4, gears  94  and  98  are rigidly connected to the third shaft  36 , and a gear  92  is rigidly fixed to a countershaft  104 , while, in a manner described for example in EP 0819562 A, operation of the positioning plate  74  can selectively establish, by means of a sliding coupling device  91  associated with a gear  90  mounted for free rotation about the shaft  36 , and by means of a sliding coupling  97  located between and associated with gears  96  and  100  mounted for free rotation about the countershaft  104 , which of the three gears  92 ,  96  and  100  is connected, so as to transmit torque. The countershaft  104 , for its part, drives the first and second shafts  32  and  32 ′ through a gear  106 , fixed to the counter shaft  104  and meshed with a gear  108  coupled for driving a differential gear set  102 , that, in turn, is coupled to the first and second shafts  32  and  32 ′. During the shifting process, the lever  54  and the positioning plate  74 , connected to it so as to drive, is pivoted in the plane of FIG. 2 about a pivot axis  80 . The positioning plate  74  can occupy one of three different operating positions in which, in each case, a different gear ratio is selected. 
     If the positioning plate  74  is in a position between two adjacent operating positions, then the gearbox  30  is in a neutral or idle position, in which there is no driving connection between the shafts  32  and  32 ′ and the third shaft  36 . In this case, a forward propulsion cannot be performed by the hydraulic motor  40  nor can it develop any braking action, the latter action being obtainable by placing the swash plate  42  in a position that corresponds to a lower speed than the current vehicle speed. In order to prevent the gearbox  30  from being shifted into the neutral position during operation, ensuring that the braking effect of the hydraulic motor  40  is always available during operation and can be used as a service brake for the vehicle  10 , a blocking arrangement is provided which locks the shift arrangement of the gearbox  30  in case the vehicle is not stopped. The blocking arrangement can be arranged to lock the manually operable shift lever  58  and/or the mechanical linkage  54  and  56  through which the shift lever  58  is operationally connected to the positioning plate  74 . The last two named solutions are particularly advantageous since in the cab  18 , in which the shift lever  58  is arranged, frequently little space is available while in the vicinity of the gearbox  30  as a rule adequate space is available and the blocking arrangement can be attached without any problems to repositioning arrangements already present on the gear-box  30 . A further conceivable blocking arrangement would be to connect the shift lever  58  mechanically to a further device, independent of the repositioning unit or repositioning plate  74  of the gearbox, which can be locked by the by the blocking arrangement. This latter type of blocking arrangement has the advantage that it can be applied at any desired location of the vehicle. In the case where an electro-mechanical or hydraulic device is used between the shift lever  58  and the positioning plate  74 , it in appropriate to permit the blocking arrangement to interact with the shift lever in order to mechanically indicate to the user that no shifting is possible when the vehicle is moving. 
     The blocking arrangement of the preferred embodiment is of the type for locking the repositioning plate  74  and includes an electronic control arrangement  68 , that is connected through a line  66  with a sensor  64 , which may be a pressure responsive switch. The sensor  64  is attached to a lubricating oil pump  34  of the gearbox  30 , and detects the oil pressure that is made available by the lubricating oil pump  34 . The lubricating oil pump  34  is known in itself and is connected so as to be driven by one of the shaft  36 , when the latter is driven by the motor  40 , or by the shafts  32  or  32 ′, when the vehicle is coasting. In any event, the lubricating pump  34  is used to supply oil for lubricating the gearbox  30 . Alternatively, the sensor  64  could be placed, as shown in dashed lines in FIG. 2, for sensing the pressure of the oil being delivered by the pump  44  to the motor  40 , or by the motor  40  to the pump  44 . Therefore, the lubricating oil pump  34  is operable to generate oil pressure anytime the front wheels  14  are rotating, and the presence of pressure in one or the other of the lines  46  and  48  indicates that the wheels  14  are either being driven by the motor  40  or that the vehicle  10  is coasting with the motor  40  being driven as a pump by the rotating wheels  14 . In either case, if the sensor  64  determines that the vehicle is moving, the pressure switch closes and completes a circuit through the control arrangement  68  so as to send electric current through a line  72  coupled to a solenoid of a locking element  70 , which operates to extend a pin  82 , against the force of a return spring  81  (see FIG. 4) so that a shoulder portion  83  is received in a selected one of three complementary shaped openings  76  of the positioning plate  74  located at a fixed radius about the shaft  80  (see FIG. 3) and connected together by a narrow guide path  79 . The shoulder portion  83  of the pin  82  blocks the positioning plate  74  mechanically and prevents the shift lever  58  from being moved. Thereby the gearbox  30  cannot be brought into the neutral position. If, on the other hand, the sensor  64  determines that the vehicle is stopped, the sensor  64  acts to effect an open switch so that an open circuit is established resulting in the blocking control arrangement  68  discontinuing current flow to the locking element  70  so that its solenoid is deactivated permitting the return spring  81  to retract the shoulder portion  83  from the opening  76  and bring a neck portion  77  of the pin  82  into register with the guide slot  79 . The operation of the shift lever  58  and a movement of the positioning plate  74  into a different operating position can now be accomplished without any problem. Once the vehicle  10  begins to move, the blocking control  68  will again be energized through operation of the sensor  64  so that the locking element  70  is energized to cause the pin  82  to once again be extended into the opening  76  that is associated with the newly selected speed ratio. The positioning plate  74  is retained in each of its operating positions by means of a detent arrangement comprising a ball  84  releasably loaded, by a spring  85 , into a respective one of three hemispherical detent seats or recesses  89  respectively located in the positioning plate  74  so as to register with the ball  84  when the plate  74  is positioned for selecting a desired gear ratio. 
     FIG. 4 shows a vertical section through the gearbox  30  and here it can be seen that the third shaft  36  (FIG. 4) is rotated through 90 ° as compared to FIG. 2 (where it was shown in offset position for the sake of simplicity). In addition, brakes  110  associated with the shafts  32  and  32 ′ can be seen, which make available an additional braking effect to that of the hydraulic motor  40 . 
     FIGS. 5 and 6 show a gearbox  30  according to a second embodiment of the invention. Parts that correspond to the parts shown in FIGS. 2 through 4 are designated by the same reference numerals. Most of these parts are identical so that for these parts a more detailed description is omitted. In the second embodiment of the invention, a positioning plate  74 ′ is provided that, like the plate  74  described above, is supported in bearings so as to pivot about the pivot axis  80 . Also like the plate  74 , the plate  74 ′ is provided with a detent arrangement comprising the three hemispherical detent element seats or recesses  89  located at equal distances from the pivot axis  80  for selective engagement by the detent ball  84 . While shifting the positioning plate  74 ′ rotates it about the pivot axis  80  and forces the ball  84  upward (as seen in FIG. 6) against a detent element  84 ′ that is biased toward the ball  84  by the force of the spring  85 . A detent is therefore provided for selectively retaining the positioning plate  74  in three rotational positions, each of which corresponds to an operating position with a specific gear ratio of the gearbox  30 . In the second embodiment of the invention, the locking element  70  is arranged to block the movement of the ball  84  from a selected one of the seats  89 , so that a pivoting of the positioning plate  74 ′ about the pivot axis  80  is prevented. Thus, the positioning plate  74 ′ differs from the plate  74  in that it does not have an arcuate slot for the reception of the locking pin  82 . Specifically, the locking element  70  is arranged such that the blocking pin  82  is located in a bore that registers with the top of the detent element  84 ′ when the detent ball  84  is received in the seat  89 . Actuation of the locking element  70  results in the pin  82  being projected above the detent element  84 ′ so as to capture the ball  84  and prevent movement of the positioning plate  74 ′. If the locking element  70  is not activated, the element  84 ′ and therewith the ball  84  can move freely so that a shifting process is possible. A significant difference as compared with the embodiment shown in FIGS. 3 through 5 lies in the fact that the locking element  70 , that is also equipped with an electromagnet, is oriented parallel to the positioning plate  74 ′ and to the upper side of the gearbox  30 . Therefore the second embodiment requires less space and can be applied more easily to vehicles already in mass production or even to existing agricultural vehicles. 
     It should be noted that in place of an electromagnetically actuated locking element  70 , a hydraulically actuated locking element could be employed. Also, other sensors in place of the sensor  64  could be used for the detection of the movement of the vehicle, such as, for example, a speed sensor  112  (FIG. 2) interacting with one of the wheels  14  or even a non-contacting sensor, for example an optical sensor  114 , shown schematically in FIG. 1, could be used to sense relative movement between the ground and the vehicle  10 . In either case, the sensor  112  or the sensor  114  operates to send a signal to the blocking control arrangement  68  when the vehicle  10  is moving relative to the ground.