Abstract:
A power split transmission for a working machine, such as a wheel loader, for example, has a hydrostatic and a mechanical power branch which are summed with one another via a summation gear ( 12 ). A reversing gear ( 7 ) is connected downstream of the summation gear ( 12 ) and a gear shifting mechanism ( 20 ) is connected downstream of the summation gear ( 12 ).

Description:
[0001]    This application is a National Stage completion of PCT/EP2008/060764 filed Aug. 15, 2008, which claims priority from German patent application serial no. 10 2007 047 194.9 filed Oct. 2, 2007. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The invention concerns a power split transmission. 
       BACKGROUND OF THE INVENTION 
       [0003]    Power split transmissions of this kind have one hydrostatic power branch and one mechanical power branch, which are summed via a summation gear in order to drive an output means. 
         [0004]    The hydrostatic-mechanical transmission with power split disclosed in DE 28 54 375 A1 has two shift ranges, and the continuously variable control of the transmission is carried out by means of the hydrostatic power branch. 
       SUMMARY OF THE INVENTION 
       [0005]    The invention has as an object the creation of a power split transmission with at least two operating ranges, wherein at least two operating ranges are available in one forward travel direction and in one reverse travel direction, and the transmission is characterized by a simple control of the hydrostatic power branch. 
         [0006]    The object is attained with a power split transmission of the specified kind, which has the characteristic features of the main claim. 
         [0007]    The power split transmission has one hydrostatic power branch and one mechanical power branch, which are summed in a summation gear. A reversing gear, by means of which the direction of rotation of the transmission input shaft can be reversed depending on the desired direction of travel, is connected upstream of the summation gear. 
         [0008]    The summation gear is preferably configured as a planetary gear set for use with the power split transmission in a working machine, for example, a wheel loader, wherein the outer central gear of the planetary gear set is operatively connected to the output shaft of the reversing gear, the sun gear of the planetary gear set is operatively connected to the first hydrostatic unit, and the planetary gear carrier of the planetary gear set is operatively connected to a downstream gear shifting mechanism. The gear shifting mechanism is configured herein as a spur gear unit, whereby the axle offset needed for the wheel loader can be obtained in this combination. 
         [0009]    In one embodiment of the invention, a first clutch for forward travel is arranged in the reversing gear coaxially with respect to the input shaft of the power split transmission and the planetary gear set, the gear shifting mechanism and the clutch for reverse travel are arranged outside of the input shaft of the power split transmission, that is, not coaxial thereto. The second hydrostatic unit is operatively connected to the planetary gear carrier of the summation gear, either directly or via a spur gear stage, and is arranged preferably next to the first hydrostatic unit. The first and second hydrostatic units have a common component, by means of which the displacements of the first and of second hydrostatic units can be adjusted simultaneously, wherein the first and second hydrostatic units are configured as hydrostatic units in a transverse axis design. The displacements of the first and of the second hydrostatic unit are configured such that in the first operating mode of the power split transmission with rotating input shaft and stopped output shaft, the common component adjusts the first and the second hydrostatic units in such a way that the first hydrostatic unit, which is operatively connected to the sun gear, is adjusted to zero displacement, and the second hydrostatic unit, which is directly connected in operative connection to the planetary gear carrier, is adjusted to its maximum displacement. In a second mode of operation of the power split transmission at maximum possible rotational speed of the output drive, the displacement of the first hydrostatic unit is adjusted to its maximum displacement, and the displacement of the second hydrostatic unit is adjusted to zero displacement, whereby that the total power is transferred purely mechanically. If the standard transmission is to be shifted from a first gear to a second gear, then the two gear clutches are actuated to engage, wherein at least one of these gear clutches is kept in a slip mode until the load has been transferred from the one gear clutch to the other gear clutch, wherein the gear ratio, that is, the displacement, is reduced during this transfer via the common component by the progressive ratio between the first and the second gear. The power split transmission can be shifted without interruption of tractive force by means of this procedure during range shifting. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    Additional features are disclosed in the description of the figures. In the figures: 
           [0011]      FIG. 1  shows a kinematic diagram of a power split transmission with the working pump arranged coaxial with respect to the input shaft; and 
           [0012]      FIG. 2  shows a power split transmission with the working pump not arranged coaxially with respect to the input shaft. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0013]      FIG. 1 . 
         [0014]    A drive engine  1  drives the input shaft  2  of the power split transmission. The input shaft  2  is connected in a rotationally fixed manner to the spur gear  3 . The spur gear  3  is operatively connected to the spur gear  4 , wherein the clutch for forward travel  5  and the clutch for reverse travel  6  and the spur gear  3  and the spur gear  4  form the reversing unit  7 . The spur gear  8 , which can be connected to the spur gear  4  via the clutch for reverse travel  6 , and the spur gear  9 , which can be connected to the spur gear  3  via the clutch for forward travel  5 , are operatively connected to the spur gear  10 . The spur gear  10  is connected in a rotationally fixed manner to the outer central gear  11  of the planetary gear set  12 . The planetary gear set  12  forms the summation gear for the mechanical power branching and the hydrostatic power branch. The inner central gear  13  of the planetary gear set  12  is connected in a rotationally fixed manner to the first hydrostatic unit  14 . The planetary gear carrier  15  is connected in a rotationally fixed manner to the spur gear  16 . The spur gear  16  is operatively connected to the spur gear  17 , wherein the spur gear  17  is connected in a rotationally fixed manner to the second hydrostatic unit  18  and the gear clutch  19  for the second gear of the reduction gear  20 . The spur gear  17  is operatively connected to spur gear  21 , which can be connected to spur gear  23  via the gear clutch  22 . The spur gear  17  can be connected to spur gear  24  via the gear clutch  19 . The spur gear  24  is operatively connected to spur gear  23 . The spur gear  23  drives the spur gear  25  and the latter drives in turn the output drive  26 . The first hydrostatic unit  14  and the second hydrostatic unit  18  are configured as hydrostatic units in a transverse axis design, wherein the displacements can be adjusted via a common component  27  and the hydrostatic units  14  and  18  are arranged adjacent to each other. A working pump  28 , as well as a feed and lubrication pressure pump  29 , is arranged coaxially with respect to the input shaft  2 . For startup in forward travel direction with rotating input shaft  2 , the clutch for forward travel  5  is engaged, the clutch for reverse travel  6  is open, the gear clutch  19  is disengaged, and the gear clutch  22  is engaged. The first hydrostatic unit  14  is adjusted to zero displacement and the second hydrostatic unit  18  is adjusted to its maximum displacement. In this situation, the output drive  26  is at standstill. If the common component  27  is now adjusted, then the first hydrostatic unit  14  is adjusted from its zero displacement and pumps pressurizing medium to the second hydrostatic unit  18 , which is connected to the first hydrostatic unit  14  in a closed circuit. The output drive  26  begins to rotate. At the maximum possible adjustment of the common component  27 , the first hydrostatic unit  14  is at its maximum displacement and the second hydrostatic unit  18  is at zero displacement. The total power is transferred herein exclusively via the mechanical power branch to the output drive  26 . If the second gear is to be selected, then the gear clutch  19  is likewise operated in an engaging direction toward the engaged gear clutch  22 , wherein either the gear clutch  19  or the gear clutch  22  or both gear clutches are kept in a slip mode until the common component  27 , the first hydrostatic unit  14 , and the second hydrostatic unit  18  are adjusted in such a manner that the displacements and thus the rotational speed of the hydrostatic units  14  and  18  are adapted to the new gear ratio. The gear clutch  22  is subsequently fully disengaged and the gear clutch  19  is fully engaged. The input drive can now be further modified in its output speed by adjusting the common component  27 . 
         [0015]      FIG. 2 : 
         [0016]    This figure differs from  FIG. 1  exclusively in that the working pump  28  and the feed and lubrication pressure pump  29  are in drive connection with the spur gear  4 , whereby the working pump  28  and the feed and lubrication pressure pump  29  can be configured with a smaller displacement, since the rotational speed of the spur gear  4  is greater than the rotational speed of the spur gear  3 . It is also possible to only connect the working pump  28  to the spur gear  4  and the feed pump  29  to the input shaft  2 , or the working pump  28  to the input shaft  2  and the feed pump  29  to the spur gear  4 . 
       LIST OF REFERENCE NUMERALS 
       [0000]    
       
           1  Engine 
           2  Input shaft 
           3  Spur gear 
           4  Spur gear 
           5  Clutch for forward travel 
           6  Clutch for reverse travel 
           7  Reversing unit 
           8  Spur gear 
           9  Spur gear 
           10  Spur gear 
           11  Outer central gear 
           12  Planetary gear set 
           13  Inner central gear 
           14  First hydrostatic unit 
           15  Planetary gear carrier 
           16  Spur gear 
           17  Spur gear 
           18  Second hydrostatic unit 
           19  Gear clutch 
           20  Reduction gear 
           21  Spur gear 
           22  Gear clutch 
           23  Spur gear 
           24  Spur gear 
           25  Spur gear 
           26  Output drive 
           27  Common component 
           28  Working pump 
           29  Feed pump