Abstract:
To make operating a hydraulic drive mechanism above its admissible maximum rotational speed without separating the mechanical connection to the drive wheel possible, the motor ( 1 ) is adjusted in displacement to almost no displacement and the pressure-medium supply line ( 2 ) and the pressure-medium return line ( 3 ) operate a hydraulic drive with at least a hydrostatic radial piston motor ( 7, 9 ) exclusively with the supply pressure ( 20 ) connected.

Description:
FIELD OF THE INVENTION 
   The invention relates to a hydraulic drive mechanism for mobile vehicles. 
   BACKGROUND OF THE INVENTION 
   Hydraulic drive mechanisms for mobile vehicles are specially used for mobile machinery such as excavators, wheel loaders or also graders in which an internal combustion engine drives a hydraulic pump the flow rate of which supplies the hydraulic motors with pressure medium which, in turn, are connected with drive wheels which they drive. Axial piston motors of variable displacement are often used in order to change the rotational speed of the drive wheels. But in the axial piston motors, the maximum rotational speed is limited and to exceed the rotational speed would damage the motor. 
   U.S. Pat. No. 4,069,886 discloses a hydraulic drive mechanism for a mobile vehicle in which the front wheels can be driven via a respective motor and the rear wheels are directly driven by the internal combustion engine via a mechanical transmission. Since the rotational speed of the hydraulic motors is limited, the front wheels can be used only within a limited working range and above the working range have to be separated from the vehicle wheel via mechanical clutches. The motors are for this purpose entirely uncoupled mechanically. It is thereby possible, via the drive system proper of the vehicle, to operate the front wheels at a higher rotational speed than the admissible rotational speed of the hydromotors. The hydromotors are stationary here. 
   DE 39 07 633 C2 discloses a hydraulic drive mechanism for a mobile vehicle which serves as exclusive drive system; one internal combustion engine driving a hydraulic pump the flow rate of which supplies two hydromotors with pressure medium connectable with an output shaft via a reduction gear. Since the hydromotors are connected with the output shaft via different reduction gears, there is danger that one hydromotor is operated beyond its maximum rotational speed. For this reason, one clutch is disposed between the output shaft and one hydromotor in order that the hydromotor can be mechanically uncoupled. 
   The problem on which this invention is based is to provide a hydraulic drive mechanism for mobile vehicles in which at least one hydraulic motor drives one output shaft and also can remain connected with the output shaft even above a maximum rotational speed of the motor by the motor being operable above its maximum rotational speed. 
   SUMMARY OF THE INVENTION 
   According to the invention, the hydraulic drive mechanism has at least one motor, the displacement of which is adjustable and can be changed down to zero. In this state, the hydromotor has almost no displacement whereby, on one side, it absorbs no flow except the leakage of the pump flow and, on the other, it generates no rotational speed. In addition, the pressure-medium supply line and the pressure-medium return line from the pump to the motor are closed and the pressure-medium supply line and the pressure-medium return line of the hydraulic motor are short circuited and loaded with a low pressure. This is preferably the lubrication pressure of the transmission (return pressure toward radiator). It is also possible to use the feed pressure. Thereby only a very low pressure acts upon the motor on its high-pressure and low-pressure sides whereby all parts of the motor are pressurized with low stresses thereby producing a low resistance torque whereby the motor can be operated at a very high rotational speed. The motor is preferably designed as a radial piston motor having a crankshaft which is in operative connection with the pistons. By the motor being adjusted to a zero displacement so as to have almost no displacement, the eccentricity of the crankshaft is compensated whereby the crankshaft of the motor extends concentrically with its drive system and the pistons effect almost no lifting movement in the cylinders. The radial piston motor is preferably designed like the radial piston motor in WO99/17021 A1, the hydromotor being preferably adjusted not mechanically, but hydraulically whereby the adjusting pistons in the crank shaft are adjusted via hydraulic pressure. The hydraulic pressure is preferably connected with the pressure-medium supply line of the motorwhereby the motor is automatically adjusted to the zero displacement when the pressure-medium supply line is disconnected. The motor is preferably first adjusted to zero displacement and then separated from the high pressure. 
   In another embodiment, in the disengaged state of the hydraulic motor, the pressure-medium supply line and the pressure-medium return line are connected with the pressure-medium exit of a feed pump or the lubrication pressure of the transmission (return pressure toward radiator) whereby the leakage of the hydraulic motor is permanently replenished with cool pressure medium. An overheating of the motor is thus prevented. By virtue of the hydraulic separation from the high pressure and the adjustment to zero displacement of the hydraulic motor, it is no longer needed mechanically to separate the motor from the output whereby no engagement jerks result when uncoupling or recoupling. By the motor remaining loaded with feed pressure or lubrication pressure, the cylinders do not empty whereby no jerk results when shifting. 
   In one other embodiment, the hydraulic motor is configured as additional drive mechanism for front wheels such as a grader, the main drive results via the internal combustion engine and a reduction gear; the hydraulic motors are engaged during a defined working range and above the admissible rotational speed of the hydromotor which is hydraulically connected with the feed pressure or lubrication pressure and is adjusted to zero absorption volume whereby the hydraulic motor can be operated above its rotational speed. 
   In another embodiment, at least two hydraulic motors are connected with one output shaft via a summarizing transmission and form the drive mechanism of the vehicle. Both motors are permanently in mechanical operative connection with the output shaft and are adjustable in their absorption volume. Between the output shaft and the hydromotor can be located reduction gears of a different ratio. If the hydromotor is operated above its admissible rotational speed, it has no displacement and its supply and return lines are connected with the feed pressure or lubrication pressure. Also it is possible to operate the motors in open circuit. 
   By the motors not being disposed upon the output shaft, it is possible to place the pressure-medium supply line for adjustment of the displacement in the crankshaft on one side thereof and connect it with the high pressure. The sealing can thus be placed on a small diameter whereby high rotational speeds are possible even with high pressures. 
   The invention thus provides a hydraulic drive mechanism which can be operated also above its maximum admissible rotational speed and needs no mechanical disengagement devices. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will now be described, by way of example, with reference to the accompanying drawings in which: 
       FIG. 1  is a diagram of a hydraulic motor with a reduction step; 
       FIG. 2  is a diagram of a summarizing transmission with two hydraulic motors, and  FIG. 2A  is a diagram of a summarizing transmission showing the hydraulic motors as radial piston motors; 
       FIG. 3  is a diagram of a summarizing transmission with two hydraulic motors having one shiftable gear step; and 
       FIG. 4  is a hydraulic circuit diagram of the inventive drive mechanism. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 : 
   One hydraulic motor  1  of adjustable displacement has one pressure-medium supply line  2  and one pressure-medium return line  3 . The pressure-medium supply line  2  and the pressure-medium return line  3  are connected with a pressure-medium source (not shown). An output  4  of the motor  1  is connected with a reduction gear  5 . An output  6  of the reduction gear  5  is connected with one vehicle wheel. If the output  4  is operated above a defined rotational speed, the pressure-medium supply line  2  and the pressure-medium return line  3  of the motor  1  are connected with the feed pressure or lubrication pressure of the pressure-medium source and the motor  1  is adjusted so as to have almost no displacement. The displacement and the shift valve can be controlled via an electronic control unit. The defined rotational speed is the maximum admissible rotational speed of the hydromotor when loaded with high pressure. 
     FIG. 2 : 
   One first motor  7  drives one first reduction gear  8  and one second motor  9  drives one second reduction gear  10 . The pressure-medium supply lines  2  and the pressure-medium return lines  3  are connected in parallel with the pressure-medium source. The first reduction gear and the second reduction gear can have the same ratio, but it is advantageous to design the reduction gears with different ratios. The ratios are preferably designed so that in a first working range the first motor  7  and the second motor  9  are connected in parallel with the delivery exit of the pressure medium source and the suction inlet of the pressure-medium source, the delivery exit being connected with the pressure-medium supply line and the suction inlet with the pressure-medium return line. By adjusting the volume of the hydraulic pump of the first motor  7  and of the second motor  9 , it is possible to change the rotational speed of the output shaft  11 , which is connected both with the first reduction gear  8  and with the second reduction gear  10 . The hydromotor and the ratio are preferably designed so that the motor, which in the second drive range has no more displacement and is operated above its maximum rotational speed, reaches its maximum rotational speed at the end of the first drive range. This motor is then adjusted to almost no displacement and the pressure-medium supply line  2  and the pressure-medium return line  3  are hydraulically connected with the feed pressure or lubrication pressure of the pressure-medium source. It is also possible to connect the pressure-medium supply line  2  and the pressure-medium return line  3  with a line which has the same pressure as the suction inlet of the pressure-medium source. In the second drive range the displacement of the still remaining motor is then further reduced and the other motor operated above its admissible rotational speed without mechanically separating the two motors from the output shaft  11 . 
     FIG. 3 : 
   The first motor  7 , having a crankshaft  36  and the second motor  9 , having a crankshaft  36  are connected with an output shaft  11 , respectively, via the first reduction gear  8  and a second reduction gear  10 . As diagrammatically shown in  FIG. 2A , both the first and the second motors ( 7 ,  9 ) are radial piston motors ( 38 ,  39 ). The first reduction gear  8  is shiftably effected via a shift device  12 , the output shaft  11  being connectable via the shift device  12  with the first motor  7  via a first reduction  13  or via a second reduction  14 . The shift device  12  is preferably designed as a synchronizer device and an engagement of the shift device is effected when the motor  7  has almost no displacement and is hydraulically separated from the circuit. The shift device  12  is barely loaded in this state inasmuch as only the inert masses of the gear wheels and shafts have to be accelerated, since the pistons ( 40 ) effect no stroke motion in the cylinders ( 35 ) and do not rotate. During the gear shift, the pressure-medium supply line  2  and the pressure-medium return line  3  are connected with the feed pressure or lubrication pressure whereby the bearing forces of the hydromotor become further reduced. In order to achieve the greatest torque possible during the starting operation, the starting is effected with both motors and full displacement, the greater reduction  13  being engaged via the shift device  12 . By changing the displacement of the pressure-medium source and of the first motor  7  and of the second motor  9 , the rotational speed of the output shaft  11  is increased. When reaching a maximum rotational speed of the first motor  7  or of the second motor  9 , a shift is made via the shift device  12  to the lower ratio  14  whereby, during the maximum output rotational speed of the output shaft  11 , the first motor  7  drives the output shaft  11  via the second reduction ratio  14  and the second motor  9  has no displacement and is hydraulically connected with the low pressure or lubrication pressure. 
     FIG. 4 : 
   One pressure-medium source  15  is driven by a prime mover (not shown). A delivery exit  16  is connected with the pressure-medium supply line  2  of the first motor  7  via a valve  17 . A suction inlet  18  is connected with the pressure-medium return line  3  of the first motor  7  also via the valve  17 . The delivery exit  16  and the suction inlet  18  are connected with the pressure-medium supply line  2  and the pressure-medium return line  3  of the second motor  9  via a valve  19 . Depending on the flow rate of the pressure-medium source  15 , the delivery exit  16  and the suction inlet  18  can interchange. The valve  17  and the valve  19  are connected with the low pressure via a line  20  or  23 . The lines  20  and  23  are preferably connected with the exit of a feed pump  21  whereby cooled pressure medium reaches the line  20 . But it is also possible to connect the line  20  with the exit line of a flush valve  22  whereby the line  20  can, likewise, be loaded with low pressure. The valve  19  is also connected with the low pressure, likewise, via its line  23 . In addition, it is possible to connect the lines  20  and  23  with a return line  30  from a radiator  31  to a tank  29 . An electronic control unit (not shown) is connected with valves  24 , valves  25  being designed as proportional valves. The proportional valves  25  load a displacement adjusting device  26  thus adjusting the displacement of the hydromotor  7  and  9 . By valves  27  supplied with pressure by a valve  28  connected with the pressure-medium supply line  2  and the pressure-medium return line  3  being situated between the displacement adjusting device  26  and the proportional valves  25 , it is possible by shifting the valves  25  automatically to adjust the displacement of the first motor  7  or the second motor  9  so that it has almost no displacement. The motor  7  or  9  is preferably at first adjusted via the valve  27  to zero displacement and then the valve  17  or  19  is shifted so that the pressure-medium supply line  2  and the pressure-medium return line  3  is connected with the line  20  and thereby the adjusting device  26  is also connected with the line  20  whereby the displacement of the motor cannot be enlarged. But if valve  17  or valve  19  is shifted so that the pressure-medium supply line  2  and the pressure-medium return line  3  is respectively connected with the delivery exit  16  of the pressure-medium source  15  and the suction inlet  18 , then a valve  28  takes care that the adjusting device  26  is loaded with the high pressure whereby an adjustment of the motor  7  or  9  is possible. 
   In one other embodiment of the invention, the first motor  7 , the second motor  9 , the same as the reduction gear  5  and the valves, are located within the transmission housing which, at the same time, constitutes the tank  29 . The leakage of the hydromotors thus directly reaches the transmission housing and mixes with the pressure medium in the tank. The hydromotors are preferably situated above the pressure medium level of the tank whereby loss by churning is prevented. 
   REFERENCE NUMERALS 
   
       
         1  motor 
         2  pressure-medium supply line 
         3  pressure-medium return line 
         4  output 
         5  reduction gear 
         6  output 
         7  first motor 
         8  first reduction gear 
         9  second motor 
         10  second reduction gear 
         11  output shaft 
         12  shift device 
         13  first reduction 
         14  second reduction 
         15  pressure-medium source 
         16  delivery exit 
         17  valve 
         18  suction inlet 
         19  valve 
         20  line 
         21  feed pump 
         22  flush valve 
         23  line 
         24  electronic control unit 
         26  proportional valves 
         26  displacement adjusting device 
         27  valve 
         28  valve 
         29  tank 
         30  return line 
         31  radiator