Abstract:
The invention relates to a hydrostatic traveling mechanism ( 1 ) which comprises a hydraulic pump ( 4 ), a first hydraulic engine ( 8 ) that is linked with the hydraulic pump ( 4 ) via a hydraulic work circuit ( 2 ) and that drives a first drive train ( 17 ). The traveling mechanism further comprises a second hydraulic engine ( 10 ) that is linked with the hydraulic pump ( 4 ) via a hydraulic work circuit ( 2 ) and that drives a second drive train ( 19 ). The inventive traveling mechanism is also provided with a third hydraulic engine ( 23 ) that is coupled with the first drive train ( 17 ) and a fourth hydraulic engine ( 24 ) that is coupled with the second drive train ( 19 ). The third hydraulic engine ( 23 ) and the fourth hydraulic engine ( 24 ) are linked with each other via a hydraulic secondary circuit ( 3 ) that is independent of the work circuit ( 2 ).

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a hydrostatic drive for driving various types of vehicles. 
     2. Discussion of the Prior Art 
     A hydrostatic drive according to the precharacterising clause of Claim 1 is known from EP 0 547 947 A1. In the case of the hydrostatic drive disclosed in this publication, two vehicle wheels lying opposite on a vehicle axle are driven in each case by two hydraulic motors arranged in pairs on a common shaft. The hydraulic fluid delivered in a working circuit by a hydraulic pump branches upstream of the hydraulic motors arranged in pairs. Whereas the hydraulic fluid from the outlet of one of the two hydraulic motors arranged in pairs flows back directly to the hydraulic pump, the outlet of the other hydraulic motor arranged on the same shaft is connected to the hydraulic pump via a further hydraulic motor in each case, these further hydraulic motors driving vehicle wheels of another vehicle axle. In the case of the hydrostatic drive disclosed in this publication, no measures are provided to prevent a slip occurring at one of the mutually opposite vehicle wheels which considerably reduces the efficiency of the drive. 
     EP 0 505 254 A1 discloses a hydrostatic drive in which all the hydraulic motors driving different vehicle wheels are connected in parallel to the hydraulic pump. Speed sensors are provided on the output shafts of the individual hydraulic motors. As a function of the speeds determined at the individual output shaft, the amount of pressure fluid flowing through the assigned hydraulic motors can be regulated by adjustable, throttled branch valves, so that possible speed differences are equalised and in particular steering or exact straight-line driving permitted. However, this arrangement has only limited use for equalising a slip at one of the vehicle wheels. 
     EP 0 378 742 A2 discloses a hydrostatic drive in which a first and second drive train are completely separated from each other on cornering, the first drive train having a first hydraulic pump and a first hydraulic motor and the second drive train having a second hydraulic pump and a second hydraulic motor. In order to permit as exact a straight-line driving as possible, the hydraulic motors can be mechanically connected to each other on the one hand by means of a mechanical coupling on straight-line driving. On the other hand, the separated hydraulic working circuits are hydraulically connected to each other by valves on straight-line driving. A measure for preventing the slip at one of the two drive trains is not disclosed in this publication. 
     DE-A 20 26 910 discloses the arrangement of a first hydraulic pump, a first hydraulic motor, a second hydraulic pump and a second hydraulic motor in series in a common working circuit. Although in the case of this drive a slip is largely avoided owing to the hydraulic rigid coupling between the two hydraulic motors, the efficiency of this kind of drive is substantially reduced owing to the series arrangement of the two hydraulic motors. 
     SUMMARY OF THE INVENTION 
     The object on which the invention is based is to provide a hydrostatic drive for driving a plurality of drive trains, in which a slip at one of the drive trains is prevented without substantially reducing the efficiency. 
     The invention is based on the finding that it is advantageous to provide two hydraulic motors mechanically coupled to each other on each drive train, in each case one of the hydraulic motors being arranged in a working circuit and serving for the direct drive of the assigned drive train, whereas the other two hydraulic motors are hydraulically connected to each other via a secondary circuit. If a slip occurs at the first drive train, the associated hydraulic motor arranged in the secondary circuit works as a pump and generates a braking pressure in the secondary circuit. Since the speed of the hydraulic motor, arranged in the secondary circuit, of the second drive train is limited, the braking pressure built up in the secondary circuit reduces the speed at the first drive train. This avoids a situation where the slip occurring at the first drive train uses an excessively large amount of pressure fluid in the working circuit. The hydraulic power of the working circuit can therefore act undiminished on the second drive train which is not subjected to a slip. 
     Advantageous developments of the invention are specified in the subclaims. 
     Advantageously, switching valves are arranged in the secondary circuit in such a way that two of the hydraulic motors arranged in pairs on the drive trains are hydraulically interconnected in the secondary circuit, in the manner described above, only when a slip actually occurs. As long as no slip occurs, these hydraulic motors are, in contrast, connected via the switching valves to the working circuit, so that the output torque increases. The switching valves can be driven, for example, electrically via a control unit which determines the occurrence of a slip for example by a comparison of the drive-train speeds detected by means of speed sensors or by detection of a pressure drop at the hydraulic motors situated in the working circuit. 
     The hydraulic fluid can be fed into the secondary circuit by direct connection to a feed line via appropriate nonreturn valves. Alternatively, it is possible to use the low pressure of the working circuit as feed pressure for the secondary circuit. The feed is then expediently effected via a suitable switching valve for the pressure change. 
     The invention is also suitable for three, four or more drive trains. In this case, each drive train has two hydraulic motors, in each case one hydraulic motor being connected to the working circuit and another to the secondary circuit. It is also possible to provide a plurality of secondary circuits. The lines of the secondary circuit can be connected via a throttle, as a result of which a limited slip is allowed between the drive trains and thus the steering of the vehicles is facilitated. 
    
    
     BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS 
     The invention is described in more detail below with reference to the drawing, in which: 
     FIG. 1 shows a basic hydraulic circuit diagram of a first exemplary embodiment of the invention; 
     FIG. 2 shows a basic hydraulic circuit diagram of a second exemplary embodiment of the invention; 
     FIG. 3 shows a basic hydraulic and electrical circuit diagram of a third exemplary embodiment of the invention; 
     FIG. 4 shows a basic hydraulic circuit diagram of a fourth exemplary embodiment of the invention; 
     FIG. 5 shows a basic hydraulic circuit diagram of a fifth exemplary embodiment of the invention; and 
     FIG. 6 shows a basic hydraulic circuit diagram of a sixth exemplary embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 shows a basic hydraulic circuit diagram of a first exemplary embodiment of the invention. The hydrostatic drive  1  according to the invention consists of a working circuit  2  and a secondary circuit  3 . A preferably adjustable and reversible hydraulic pump  4  is arranged in the working circuit  2 . A first connection  6  of the hydraulic pump  4  is connected via a first working line  5  to a first connection  7  of a first hydraulic motor  8 . Furthermore, the first connection  6  of the hydraulic pump is connected via the first working line  5  to a first connection  9  of a second hydraulic motor  10 . A second connection  11  of the first hydraulic motor  8  and a second connection  12  of the second hydraulic motor  10  are connected via a second working line  13  to a second connection  14  of the hydraulic pump  4 . 
     The first hydraulic motor  8  is connected via a first output shaft  15  to a first drive train  17 , which comprises a first vehicle wheel  16  in the exemplary embodiment illustrated. The second hydraulic pump  10  is connected via a second output shaft  18  to a second drive train  19 , which comprises a second vehicle wheel  20  in the exemplary embodiment illustrated. The vehicle wheels  16  and  20  are opposite vehicle wheels of a common vehicle axle in the exemplary embodiment. The arrows  21  and  22  indicate the direction of rotation of the vehicle wheels  16  and  20 . The hydraulic motors  8  and  10  do not necessarily have to drive opposite vehicle wheels of a common vehicle axle. The drive trains  17  and  19  may also be used for example to drive two tracks of a tracked vehicle. 
     A third hydraulic motor  23  is mechanically coupled to the first drive train  17 . In contrast, a fourth hydraulic motor  24  is mechanically coupled to the second drive train  19 . The first hydraulic motor  8  and the third hydraulic motor  23  are preferably arranged on the common output shaft  15  as double hydraulic motors. In the same way, the second hydraulic motor  10  and the fourth hydraulic motor  24  are preferably arranged on the common second output shaft  18  as double hydraulic motors. 
     The third hydraulic motor  23  and the fourth hydraulic motor  24  are hydraulically interconnected by the secondary circuit  3  in such a way that a first connection  25  of the third hydraulic motor  23  is connected via a first secondary line  26  to a first connection  27  of the fourth hydraulic motor  24  and a second connection  28  of the fourth hydraulic motor  24  is connected via a second secondary line  29  to a second connection  30  of the third hydraulic motor  23 . The secondary circuit  3  is thus designed as a closed hydraulic circuit independent of the working circuit  1 . 
     To feed hydraulic fluid both into the working circuit  1  and into the secondary circuit  3 , use is made of a feed pump  31  which is coupled to the hydraulic pump  4  and which draws pressure fluid from a tank  32  and feeds it into a feed line  33 . To limit the pressure in the feed line  33 , use is made of a pressure-limiting valve  34  which connects the feed line  33  to the tank  32 . 
     The feed line  33  is connected via a first nonreturn valve  35  to the first working line  5  and via a second nonreturn valve  36  to the second working line  13 . The pressure fluid is thus fed in each case into that respective working line  5  or  13  which is carrying low pressure at the time. Arranged parallel to the nonreturn valves  35  and  36  are pressure-limiting valves  37  and  38  in order to limit the pressure in the respective working line  5  or  13  carrying high pressure at the time. 
     The first secondary line  26  is connected via a third nonreturn valve  39  to the feed line  33 , whereas the second secondary line  29  is connected via a fourth nonreturn valve  40  to the feed line  33 . As a result, hydraulic fluid is fed into the respective secondary line  26  or  29 , carrying low pressure at the time, of the secondary circuit  3 . 
     The hydrostatic drive  1  according to the invention works as follows: 
     When neither the drive train  17  nor the drive train  19  is subjected to a slip, the first hydraulic motor  8  and the second hydraulic motor  10  receive substantially the same amount of pressure fluid, so that the vehicle wheels  16  and  20  of the two drive trains  17  and  19  rotate at substantially the same speed. Consequently, the third hydraulic motor  23  and the fourth hydraulic motor  24  also rotate at substantially the same speed, so that no braking pressure is built up in the secondary circuit  3 . 
     If, however, the first drive train  17  for example is subjected to a slip, in that the vehicle wheel  16  spins on a surface with poor grip, the speed of the vehicle wheel  16  would increase considerably without the measure according to the invention, since the vehicle wheel  16  encounters no resistance. The increased speed would increase the amount of pressure fluid flowing to the hydraulic motor  8 , so that the pressure fluid flows substantially via the first hydraulic motor  8  and only to a far lesser extent via the second hydraulic motor  10  and thus the drive via the second drive train  19  would be less effective. 
     According to the invention, the drive trains  17  and  19  are, however, hydraulically connected to each other by the third hydraulic motor  23  and the fourth hydraulic motor  24  via the secondary circuit  3 . The increase in the speed at the first output shaft  15  leads to an increase in the speed of the third hydraulic motor  23 , which works as a pump and builds up a braking pressure either in the first secondary line  26  or the second secondary line  29  depending on the direction of rotation of the vehicle wheel  16 . Since the speed of the fourth hydraulic motor  24  and hence the amount of pressure fluid flowing through this hydraulic motor  24  is determined by the speed of the vehicle wheel  20  which is in firm engagement with the surface, the fourth hydraulic motor  24  is not accelerated by the braking pressure, but rather the speed of the third hydraulic motor  23  and thus the speed of the first output shaft  15  adapts to the speed of the second output shaft  18 . A substantially uniform distribution of the volumetric flow flowing in the working circuit  2  between the first hydraulic motor  8  and the second hydraulic motor  10  is therefore preserved and the drive via the second drive train  19  remains effective. 
     The first secondary line  26  and the second secondary line  29  of the secondary circuit  3  can be connected to each other via a throttle  41 . The throttle  41  permits a throttled cross-flow between the first secondary line  26  and the second secondary line  29  and thus a slight, limited slip between the vehicle wheels  16  and  20 . As a result, the steering of the vehicle is permitted or facilitated. 
     FIG. 2 shows a second exemplary embodiment of the hydrostatic drive  1  according to the invention. In all the figures of the drawing, elements which are identical or correspond to each other are provided with the same reference symbols, so that repeated description in this regard is unnecessary. 
     The difference from the exemplary embodiment already described with reference to FIG. 1 consists, in the case of the exemplary embodiment illustrated in FIG. 2, in that the nonreturn valves  39  and  40  for feeding the hydraulic fluid into the respective secondary line  26  or  29  carrying low pressure at the time are connected via a pressure-controlled 3/3-way switching valve  50  to the respective working line  5  or  13  carrying low pressure at the time. The valve  50  is in connection both with the first working line  5  and with the second working line  13  and compares the pressures prevailing in the working lines  5  and  13  with each another. If high pressure is present in the working line  5  and low pressure in the working line  13 , the valve  50  assumes the valve position  51 , so that the working line  13  carrying low pressure is connected via the valve  50  and one of the two nonreturn valves  39  or  40  to the secondary circuit  3 . If, conversely, high pressure is present in the second working line  13  and low pressure in the first working line  5 , the valve  50  assumes the valve position  52 , so that the first working line  5  carrying low pressure is connected via the valve  50  and one of the two nonreturn valves  39  and  40  to the secondary circuit  3 . A direct connection to the feed line  33  is not necessary in the case of this embodiment. 
     FIG. 3 shows a third exemplary embodiment of a hydrostatic drive according to the invention. 
     In contrast to the exemplary embodiment already described with reference to FIG. 1, in the case of the exemplary embodiment illustrated in FIG. 3 a first 3/2-way switching valve  60  and a fourth 3/2-way switching valve  63  are provided in the first secondary line  26  of the secondary circuit  3 , whereas a second 3/2-way switching valve  61  and a third 3/2-way switching valve  62  are provided in the second secondary line  29  of the secondary circuit  3 . 
     If the valves  60  to  63  are in their first valve position  60   a,    61   a,    62   a  and  63   a  illustrated in FIG. 3, the secondary circuit  3  is closed. The secondary circuit  3  works as described with reference to FIG. 1 to counteract a slip at one of the two drive trains  17  and  19 . If, however, the valves  60  to  63  are in their other switching position  60   b,    61   b,    62   b  and  63   b  as the case may be, the first connection  25  of the third hydraulic motor  23  is connected to the first working line  5  and the second connection  30  of the third hydraulic motor  23  is connected to the second working line  13 . Correspondingly, the first connection  27  of the fourth hydraulic motor  24  is then connected to the second working line  13  and the second connection  28  of the fourth hydraulic motor  24  to the first working line  5 . The valves  60  to  63  are in the switching position  60   b  to  63   b  as long as no slip occurs at the drive trains  17  and  19 . This has the advantage that for the first drive train  17  both the first hydraulic motor  8  and the third hydraulic motor  23  are available and for the second drive train  19  both the second hydraulic motor  10  and the fourth hydraulic motor  24  are available and thus the torque which can be generated is relatively high. If a slip occurs at one of the two drive trains  17  and  19 , the valves  60  to  63  are switched over by a suitable control signal. 
     The valves  60  to  63  are driven in the exemplary embodiment illustrated in FIG. 3 via an electrical control signal which is supplied to electromagnets via an electrical control line  64 . The electrical control signal is generated by a control device  69  which is connected to two speed sensors  70  and  71 . The first speed sensor  70  determines the speed n 1  of the first output shaft  15 . Correspondingly, the second speed sensor  71  determines the speed n 2  of the second output shaft  19 . If the difference n 1 −n 2  of the speeds n 1  and n 2  exceeds a preset threshold valve, this indicates a slip at one of the two drive trains  17  and  19 . The valves  60  to  63  are then correspondingly switched over by the control device  69 . 
     FIG. 4 shows a fourth exemplary embodiment of a hydrostatic drive  1  according to the invention, in which a third drive train  70  is provided in addition to the first drive train  17  and the second drive train  19 . The three drive trains  17 ,  19  and  70  are used, for example, to drive three different vehicle wheels, which are not illustrated in FIG. 4. A fifth hydraulic motor  72  and a sixth hydraulic motor  73  are situated on an output shaft  71 . A first connection  74  of the fifth hydraulic motor  72  is connected via the first working line  5  to the first connection  6  of the hydraulic pump  4 . In contrast, a second connection  75  of the fifth hydraulic motor  72  is connected to the second connection  14  of the hydraulic pump  4 . The first hydraulic motor  8 , the second hydraulic motor  10  and the fifth hydraulic motor  72  are thus connected in parallel in the working circuit  2 . 
     In contrast, the sixth hydraulic motor  73  is connected via the secondary circuit  3  to the third hydraulic motor  23  and the fourth hydraulic motor  24 . For this purpose, a first connection  76  of the sixth hydraulic motor  73  is in connection via the first secondary line  26  of the secondary circuit  3  with the first connection  25  of the third hydraulic motor  23  and the first connection  27  of the fourth hydraulic motor  24 . In contrast, a second connection  77  of the sixth hydraulic motor  73  is in connection via the secondary line  29  of the secondary circuit  3  with the second connection  30  of the third hydraulic motor  23  and the second connection  27  of the fourth hydraulic motor  24 . The hydraulic motors  23 ,  24  and  73  are therefore coupled to one another via the secondary circuit  3  and, in the procedure already described, prevent a slip at the vehicle wheels driven via the drive trains  17 ,  19  and  70 . 
     In the case of this exemplary embodiment, depending on the direction of rotation of the hydraulic motors  23 ,  24  and  73 , one of the hydraulic motors  23 ,  24  or  73  must be designed in such a way that its absorbing volume is the same size as the sum of the absorbing volumes of the other two hydraulic motors. In the exemplary embodiment illustrated in FIG. 4, the fourth hydraulic motor  24  for example has an absorbing volume twice the size of that of the third hydraulic motor  23  and the sixth hydraulic motor  73 . 
     In the exemplary embodiment illustrated in FIG. 4., there is once again provided a throttle  41  which permits a slight cross-flow between the first secondary line  26  and the second secondary line  29 , so that the steering of the vehicle is facilitated. It is, however, also possible to dispense with the throttle  41  if a particularly rigid coupling of the drive trains  17 ,  19  and  70  is desired. 
     FIG. 5 shows an exemplary embodiment of a hydrostatic drive  1  according to the invention for driving four drive trains  17 ,  19 ,  70  and  80 . The individual drive trains  17 ,  19 ,  70  and  80  drive different vehicle wheels for example. The first drive train  17  has the first hydraulic motor  8  and the third hydraulic motor  23 , whereas the second drive train  19  has the second hydraulic motor  10  and the fourth hydraulic motor  24 . Whereas the first hydraulic motor  8  and the second hydraulic motor  10  are connected to the working circuit  2  in the same way as illustrated in FIG. 1, the third hydraulic motor  23  and the fourth hydraulic motor  24  are connected to each other crosswise in the same way as illustrated in FIG. 1 via a first hydraulic secondary circuit  3 . In a corresponding fashion, the third drive train  70  has a fifth hydraulic motor  72  connected to the working circuit  2  and a sixth hydraulic motor  73  arranged in a second secondary circuit  78 . A first connection  74  of the fifth hydraulic motor  72  is in this case connected via the first working line  5  to the first connection  6  of the hydraulic pump  4 , whereas a second connection  75  of the fifth hydraulic motor  72  is connected via the second working line  13  to the second connection  14  of the hydraulic pump  4 . 
     The fourth drive train  80  has a seventh hydraulic motor  81  arranged in the working circuit  2  and an eighth hydraulic motor  82  arranged in the second secondary circuit  78 . In this case, a first connection  83  of the seventh hydraulic motor  81  is connected via the first working line  5  to the first connection  6  of the hydraulic pump  4 , whereas a second connection  84  of the seventh hydraulic motor  81  is in connection via the second working line  13  with the second connection  14  of the hydraulic pump  4 . The seventh hydraulic motor  81  in this case drives an output shaft  85 . 
     The connection of the sixth hydraulic motor  73  to the eighth hydraulic motor  82  is effected in a manner corresponding to the connection of the third hydraulic motor  23  to the fourth hydraulic motor  24 , i.e. a first connection  76  of the sixth hydraulic motor  73  is connected to a first connection  86  of the eighth hydraulic motor  82 , whereas a second connection  87  of the eighth hydraulic motor  82  is in connection with a second connection  77  of the sixth hydraulic motor  73 . If the running direction of the hydraulic motors  23  and  24  on the one hand and  73  and  82  on the other hand is the same, the connections of these hydraulic motors connected crosswise in each case. 
     In the case of the exemplary embodiment illustrated in FIG. 5, the first secondary circuit  3  is completely separated from the second secondary circuit  78 . The first drive train  17  and the second drive train  19  can drive for example the vehicle wheels of a first vehicle axle, whereas the third drive train  70  and the fourth drive train  80  drive the vehicle wheels of a second vehicle axle. A throttle  41 , illustrated in FIG. 1, can be provided in each of the secondary circuits  3  and  78 , in order to allow a slight slip for the purpose of facilitating the steering of the vehicle. 
     The feeding of the pressure fluid from the feed line  33  into the secondary circuit  78  is effected via two nonreturn valves  88  and  89 . 
     FIG. 6 shows another exemplary embodiment of a hydrostatic drive  1  according to the invention for driving four drive trains  17 ,  19 ,  70  and  80 . Elements which have already been described with reference to FIG. 5 have been provided with the same reference symbols, so that repeated description in this respect is unnecessary. 
     In contrast to the exemplary embodiment illustrated in FIG. 5, in the exemplary embodiment illustrated in FIG. 6 the third hydraulic motor  23 , the fourth hydraulic motor  24 , the sixth hydraulic motor  73  and the eighth hydraulic motor  82  are not arranged, paired in series, in two separated secondary circuits but, altogether in series, in a single secondary circuit  3 . For this purpose, the first connection  25  of the third hydraulic motor  23  is connected via a first secondary line  92  to the first connection  27  of the fourth hydraulic motor  24 , the second connection  28  of the fourth hydraulic motor  24  is connected via a second secondary line  93  to the first connection  76  of the sixth hydraulic motor  73 , the second connection  77  of the sixth hydraulic motor  73  is connected via a third secondary line  94  to the first connection  86  of the eighth hydraulic motor  82  and the second connection  87  of the eighth hydraulic motor  82  is connected via a fourth secondary line  95  to the second connection  30  of the third hydraulic motor  23 . 
     For feeding pressure fluid, the first secondary line  92  is in connection with the feed line  33  via a nonreturn valve  39 , the second secondary line  93  via a nonreturn valve  88 , the third secondary line  94  via a nonreturn valve  89  and the fourth secondary line  95  via a nonreturn valve  40 . 
     Whereas the exemplary embodiment illustrated in FIG. 5 only prevents a slip of vehicle wheels arranged in pairs, for example on a common vehicle axle, in the exemplary embodiment illustrated in FIG. 6 a slip of all the vehicle wheels is avoided in the manner of a four-wheel drive. It may be advantageous to provide a switching valve (not illustrated in the drawing) to be able to switch over between the circuit configuration illustrated in FIG.  5  and the circuit configuration illustrated in FIG.  6 . 
     The invention is not restricted to the exemplary embodiments illustrated. The valves  60  to  63  can also be driven hydraulically instead of electrically. The measured quantity employed to detect a slip at one of the two drive trains  17  and  19  can also be the pressure drop at the first hydraulic motor  8  or the second hydraulic motor  10 . Too small a pressure drop indicates an excessive speed of the hydraulic motor  8  or  10  and thus a slip at the respective drive train  17  or  19 .