Abstract:
The invention relates to a powershift gearbox comprising a first and a second gearbox block in which respectively one gear from a first or second set of gears associated with each gearbox block can be passed between an input shaft and an output shaft of the gearbox block. The inventive gearbox comprises a slip-free clutch arrangement between each gearbox block and a common drive shaft of the powershift gearbox, that enables the input shaft of the gearbox block to be coupled to the drive shaft either directly or by means of a common slippable clutch.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a U.S. National-Stage entry under 35 U.S.C. § 371 based on International Application No. PCT/EP2005/008286, filed Jul. 30, 2005, which was published under PCT Article 21(2) and which claims priority to German Application No. DE 10 2004 041 525.0, filed Aug. 27, 2004. 
     
     BACKGROUND 
       [0002]    The present invention relates to a power shift transmission in which the process of shifting between different gears is adapted to be effected without interrupting the tractive power, and also to a method of operating such a transmission. 
         [0003]    A conventional transmission of this type is described in DE 42 06 033 C2. This known transmission can be summarized as being one that is built up of two gear units in which different transmission ratios or gears are adapted to be set-up between an input shaft and an output shaft of each gear unit. Furthermore, the transmission comprises two drive shafts which are adapted to be driven by the same engine and two clutches which are respectively arranged between one of these drive shafts and the input shaft of a gear unit. 
         [0004]    When a shift between two gears is not about to take place, one of the two clutches is open and the other one is closed and torque is transferred from the engine via the closed clutch and the gear unit attached thereto to an output drive shaft common to both gear units. 
         [0005]    In order to effect a gear change with the known transmission, the new gear to be utilized is firstly preselected in the non-loaded gear unit, i.e. a torque-coupling connection is established in the gear unit between the output drive shaft and that part of its clutch facing the load-free gear unit. By contrast, the part of the self-same clutch on the engine side is driven directly by the drive shaft. The two parts of the clutch therefore rotate at different speeds, the ratio therebetween being determined by the transmission ratios of the currently engaged gear and the preselected gear. 
         [0006]    In order to actually engage the preselected gear, the clutch for the as yet load-free gear unit is gradually closed so that a portion of the engine torque is transferred therethrough, whilst the clutch of the currently loaded gear unit is gradually opened at the same time so that the moment transferred by this clutch becomes smaller. Since the two clutches are never open at the same time, the gear change is effected without an interruption of the tractive power, but nevertheless slippage between the parts of the clutches inevitably occurs for as long as the shifting process persists. 
         [0007]    Consequently, only clutches that are capable of slipping can be considered for such a transmission. The requisite size of the clutches is determined by the need for the entire engine moment to be transferred to the transmission when the clutch is closed; the two clutches thus require a considerable amount of space. Moreover, they contribute to a not insignificant degree to the costs of the transmission. 
       SUMMARY 
       [0008]    The object of the present invention is to produce a compact and inexpensively realizable power shift transmission for changing gear without interruption. 
         [0009]    In accordance with the invention, this object is achieved in that in a power shift transmission comprising a first and a second gear unit in which in each case a gear from a first and a second set of gears associated with each gear unit is adapted to be engaged between an input shaft and an output shaft of the gear unit, wherein there is provided between the gear unit and a common drive shaft of the power shift transmission a slip-free clutch arrangement which enables the input shaft of the gear unit to be coupled selectively either directly or via a common slippable clutch to the drive shaft. The power shift transmission in accordance with the invention thus manages to operate with just one slippable clutch; the slip-free clutch arrangements that are provided instead are comparatively compact and more economical to realize in comparison with a slippable clutch since they generally use interlocking rather than frictional engagement for the transmission of the torque. 
         [0010]    Preferably, each of the clutch arrangements comprises two slip-free clutches arranged at opposite ends of the input shaft of the gear unit associated with the clutch arrangement, of which the first is adapted to be directly coupled to the drive shaft and the second is adapted to be coupled to the drive shaft via the slippable clutch. 
         [0011]    Moreover, a compact construction is obtained in that the input shafts of the gear unit are in the form of hollow shafts that are coaxial with the drive shaft. 
         [0012]    Preferably, an intermediate hollow shaft that is adapted to be driven in rotary manner by the slippable clutch is arranged coaxially relative to the drive shaft in a gap between the two input shafts, and the second clutches serve in each case for connecting the intermediate hollow shaft to the input shafts of the gear units. 
         [0013]    For the purposes of obtaining a space-saving construction, it is also expedient for the slippable clutch to be arranged coaxially on the drive shaft. 
         [0014]    In order to connect the output drive side of the slippable clutch that is coaxial with the drive shaft to the second clutches mentioned above, there is preferably provided a bypass shaft which is offset in parallel with the drive shaft. 
         [0015]    A further slip-free clutch is preferably provided between each output shaft of a gear unit and a common output drive shaft of the power shift transmission. Of these clutches and insofar as a shift between two gears is not about to take place, one of the them is preferably always open so that the respective load-free gear unit does not have to be driven in rotary manner and thus too, no losses of energy are produced. 
         [0016]    The slip-free clutches are preferably in the form of claw clutches. 
         [0017]    The subject matter of the invention is also a method for operating a power shift transmission of the type described above. The gear-change process proceeds by firstly engaging a desired gear in a load-free gear unit, referred to hereinafter as the taking-over gear unit, so that the input shaft of the taking-over gear unit is driven in rotary manner by its output shaft and the momentarily still load-bearing gear unit, referred to hereinafter as the delivering gear unit. The taking-over gear unit is gradually coupled to the drive shaft by the slippable clutch. During the gradual coupling process, the load component flowing via the taking-over gear unit gradually increases until a time point is reached at which the delivering gear unit becomes load-free. At this point in time, the slip-free clutch which connects the delivering block to the drive shaft can be disengaged, and—at least provisionally—the new gear is engaged. 
         [0018]    Since a renewed gear change is not possible for as long as the taking-over gear unit is being driven by the slippable clutch, the input shaft of the taking-over gear unit is expediently coupled directly to the drive shaft and the slippable clutch is released again after the closure of the slippable clutch. 
         [0019]    In order to ensure that the slip-free clutch of the delivering gear unit disengages exactly at the time point when there is no load thereon, it suffices to exert a force on this clutch in the direction of a disengaging movement; as soon as the clutch becomes load-free, the parts thereof are moveable against each other and can yield to the force. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0020]    The present invention will hereinafter be described in conjunction with the following drawing figure, wherein like numerals denote like elements, and 
           [0021]    a.  FIG. 1  shows a heavily schematized illustration of a transmission in accordance with the invention in the course of several steps of a gear-changing process as well as the load fed through the gear unit during these steps; and 
           [0022]    b.  FIGS. 2 to 9  show the construction of a preferred exemplary embodiment of a transmission in accordance with the invention as well as the course of a gear-changing process in this transmission. 
       
    
    
     DETAILED DESCRIPTION 
       [0023]    The following detailed description of the invention is merely exemplary in nature and is not intended to limit the invention or the application and uses of the invention. Furthermore, there is no intention to be bound by any theory presented in the preceding background of the invention or the following detailed description of the invention. 
         [0024]      FIG. 1  is sub-divided into six parts, designated A to F, which respectively show the state of a power shift transmission driven by an engine  1  in a stationary operating state and in different stages of a gear shifting process. 
         [0025]    The power shift transmission comprises two gear units  2 ,  3  each having a respective input shaft  4  and  5  and an output shaft  6  and  7 . A given set of transmission ratios or gears is adapted to be set-up between the input shaft  4 ,  5  and the output shaft  6 ,  7  in each gear unit, whereby the gears of the first gear unit  3  correspond to the odd-numbered gears of the entire power shift transmission, thus e.g. the gears  1 ,  3 ,  5  of a six-gear transmission, and those of the second gear unit  2  correspond to the even-numbered gears, i.e. the gears  2 ,  4 ,  6 . 
         [0026]    On each input shaft  4 ,  5  there is arranged a respective claw clutch  8  and  9  which serves the purpose of directly or indirectly coupling the associated input shaft  4  or  5  via a friction clutch  11  to one of the drive shafts  10  that are driven by the engine  1 . The claw clutches  8 ,  9  can also exhibit an open position in which they do not connect the respective input shaft  4  or  5  to the engine  1 . 
         [0027]    The respective output shafts  6 ,  7  of the gear units  2 ,  3  are connectable via a further claw clutch  12 ,  13  to a common output drive shaft  14  of the power shift transmission. 
         [0028]    In part A thereof,  FIG. 1  shows the power shift transmission with an even-numbered gear, e.g. the second gear, engaged. The load flow from the engine  1  to the output drive shaft  14  is illustrated by a dash-dotted line extending along the loaded parts of the transmission. It leads from the engine  1  via the drive shaft  10  and the closed claw clutch  8  to the gear unit  2  in which the second gear is engaged, and from there via the closed claw clutch  12  to the output drive shaft  14 . The claw clutch  13  between the output drive shaft  14  and the gear unit  3  is open, in exactly the same manner as the friction clutch  11  to which the input shaft  5  of the gear unit  3  is connected by the clutch  9 . The gear unit  3  is not being driven. 
         [0029]    The two diagrams located laterally of the representation of the transmission in part A of  FIG. 1  illustrate in each case the load on the two gear units; the gear unit  2  is under a constant positive load, i.e. a load flowing from the engine  1  to the output drive shaft  14 , whereas that on the gear unit  3  disappears. 
         [0030]    In order to prepare for a gear change, the desired gear is firstly preselected in the non-loaded gear unit  3 . In principle, any gear in this gear unit  3  could be selected, but one generally selects one that neighbors the current gear, i.e. the first or third gear. Next, as is shown in part B, the clutch  13  is closed so that the gear unit  3  is set into rotation by its output drive shaft  7 . Namely, a part of the torque available at the output of the gear unit  2  is utilized for driving the gear unit  3 , and the torque present on the output drive shaft  14  is slightly smaller than the total torque at the output of the gear unit  2 , as is indicated by the dashed waveform in the upper diagram of part B. There is a weak load flow from the output to the drive side in the gear unit  3 , as is indicated in the lower diagram of part B. In this diagram, the load flow, directed there from the output to the drive side of the gear unit  3 , is illustrated with a negative prefix sign. Both sides of the friction clutch  11  are now being driven in rotary manner, the side facing the engine at the rotational speed of the drive shaft  10 , and the side facing the gear units at a rotational speed which is determined by the transmission ratio of the gears engaged in the gear units  2 ,  3 . 
         [0031]    Next, the friction clutch  11  is gradually closed as is shown in part C. In consequence, the load flow is distributed to the two gear units  2 ,  3 . If one considers the case of shifting from second into third gear as a concrete example, then one will appreciate that the rotational speed of the part of the clutch  11  on the engine side must be higher than that of the part connected to the gear unit  3 . The clutch  11  thus attempts to accelerate the gear unit  3  to a certain extent and to brake the gear unit  2 , whereby however, matching of the rotational speeds of the two parts of the clutch  11  is not possible as long as a gear is engaged in both gear units. The difference in rotational speed does however lead to the gear unit  3  taking ever more load from the gear unit  2  with increasing pressure of the clutch  11  until a time point is reached at which the load on the gear unit  2  disappears. At this point in time, the now load-free claw clutch  8  opens, and the stage in part D of  FIG. 1  is reached. The load flow now runs exclusively via the friction clutch  11  and the gear unit  3 . 
         [0032]    No further gear change can be initiated for as long as the clutch  11  is closed. However, since the drive shaft  10  and the side of the clutch  11  facing the transmission have equal rotational speeds, it is possible to establish a direct load connection between the drive shaft  10  and the input shaft  5  via the clutch arrangement  9 , as is shown in part E, so that the load flow from the engine  1  to the gear unit  3  is distributed over the direct path and the path via the friction clutch  11  as is illustrated in the diagrams. 
         [0033]    Subsequently, the connection of the input shaft  5  to the clutch  11  is removed as is shown in part F. The friction clutch  11  is now load-free again, and the gear unit  3  is driven directly by the engine  1 . After the friction clutch  11  has opened again, a further gear change can take place as is illustrated in parts A to F, whereby the roles of the gear units  2 ,  3  are exchanged in the course of this renewed gear change. 
         [0034]      FIG. 2  shows a more detailed scheme for the construction of a power shift transmission in accordance with the invention. The engine  1  is left out of this Figure, as well as out of those following. The friction clutch  11  is mounted coaxially on the drive shaft  10  of the transmission and it comprises a first disk which is rigidly mounted on the drive shaft  10  and incorporates axially displaceable clamping members  16  for clamping a second disk  17  which is fixed to a hollow shaft  18  that is coaxial with the drive shaft  10 . 
         [0035]    Two further hollow shafts coaxial with the drive shaft  10  form the input shafts  4  and  5  of the two gear units  2 ,  3 . An intermediate hollow shaft  19  which is arranged between the hollow shafts  4 ,  5  on the drive shaft  10  is connected to the hollow shafts  18  via a bypass shaft  20  that is parallel to the drive shaft  10  and pairs of interengaging gear wheels  21 ,  22  and  23 ,  24  on the shafts  18 ,  20 ,  19 . 
         [0036]    At the mutually remote ends thereof, the two input shafts  4 ,  5  carry an axial tooth structure  25  which, together with a pinion  26  rigidly mounted on the drive shaft  10  and an internally and axially toothed sleeve or operating collar  27  that is axially displaceable on the pinion  26 , form a claw clutch which is designated as a whole by  28  on the drive shaft  4  of the gear unit  2  and by  29  on the drive shaft  5  of the gear unit  3 . Furthermore, the claw clutches  28 ,  29  comprise in known manner a not shown synchronizing unit which serves the purpose of equalizing the rotational speed necessary for the engagement of the operating collar  27  with the axially toothed structure  25  by means of a non-positive coupling, in particular, by friction. This synchronizing unit can be understood as being a kind of positive pre-coupling; it differs from the likewise positive friction clutch  11  by virtue of its dimensions: whereas the friction clutch  11  is designed such as to enable it to transfer the entire torque of the engine, it suffices for the synchronizing unit that it can transfer a maximal amount of torque which is sufficient for overcoming inertia and friction in the associated gear unit  2  or  3 ; transmission of drive moment via a synchronizing unit to the running gear is not envisaged. The synchronizing unit can be formed, in particular, by synchronizing rings such as are to be found in conventional manual transmissions. They must be non-loaded for releasing the claw clutch  28  or  29 . 
         [0037]    A similar type of claw clutch is designated by  30  and  31  respectively at the opposite ends of the input shafts  4 ,  5 . These clutches  30 ,  31  each comprise a pinion  32  on the intermediate hollow shaft  19 , a sleeve or operating collar  33  that is axially displaceable on the pinion  32  and an axially toothed structure  34  on the drive shafts  4  and  5 . 
         [0038]    Furthermore, the drive shafts  4 ,  5  each carry in known manner three toothed gear wheels  35  of different size which respectively mesh with a complementary toothed gear wheel  36  which is connected by a hollow output drive shaft section  37  to a pinion  38  of a further claw clutch  39 ,  40 ,  41  or  42  in each case. In the case of the clutches  39 ,  42 , a sleeve  43  is displaceable between three positions, one in which it is seated only on a pinion  44  that is rigidly connected to the output drive shaft  14 , and two, in which it connects the pinion  44  to a neighboring axially toothed structure  38  on the right or on the left thereof. The clutches  41 ,  42  correspond in regard to the functioning thereof to the clutch  12  of  FIG. 1  and the clutches  39 ,  40  in the clutch  13 . 
         [0039]    A gear wheel on the intermediate shaft  20  which is not illustrated in the Fig. can be brought directly into engagement with one of the toothed gear wheels  36  of the gear unit  3  by bypassing the hollow shaft  5  in order to realize a reverse gear. 
         [0040]      FIG. 3  illustrates the force flow through the transmission of  FIG. 2  in the form of a thick dotted line. It runs from the drive shaft  10  via the closed clutch  29 , the toothed gear wheels  35 ,  36  of the first gear in the gear unit  3  and the claw clutch  39  to the output drive shaft  14 . The claw clutches  41 ,  42 ,  30  are open so that the gear unit  2  is at rest. 
         [0041]    In  FIG. 4 , the second gear is preselected as the gear that is to be newly engaged in the gear stage  2  by closing the clutch  41 . At the same time, the clutch  30  is closed so that the disk  17  of the friction clutch  11  is set to rotate via the gear unit  2 , the intermediate hollow shaft  19  and the bypass shaft  20 . 
         [0042]    In  FIG. 5 , the clamping members  16  of the friction clutch  11  are gradually closed so that the load is distributed over the two gear units  2 ,  3 . Consequently, as soon as the clutch  29  is load-free, the sleeve  27  thereof yields to a positioning force that was previously being exerted thereon and slides downwardly from the pinion  25  on the input shaft  5 , as shown in  FIG. 6 . In consequence, the gear unit  3  becomes load-free, and the load flow is effected completely over the gear unit  2 . The friction clutch  11  can now be closed completely as shown in  FIG. 7  in order to transfer the motive power of the engine without slippage. 
         [0043]    Since the input shaft  4  of the gear unit  2  is now being driven via the clutch  30  at exactly the same speed as the drive shaft  10 , the clutch  28  can be closed as shown in  FIG. 8 . The load is now distributed over the two clutches  28 ,  30 . In the next step, the friction clutch  11  is opened so that the clutch  30  is load-free and can be opened again. 
         [0044]    The force flow from the drive shaft  10  via the claw clutch  28  and the second gear unit  2  shown in  FIG. 9  then ensues. The clutches  29 ,  30 ,  31 ,  39 ,  40  are open so that the gear unit  3  is at a standstill. The next gear for a subsequent change of gear can now be preselected in this gear unit  3 . 
         [0045]    The operational sequence described above concerned a process of shifting up. Here, the rotational speed of the engine is lower after the shifting process than it was before, and the process of matching the rotational speed for the purposes of relieving the load on the claw clutch  29  was brought about by frictional losses, in particular, in the friction clutch  11  in the slipping state thereof, even when the engine is idling. The sequence of steps when shifting down is essentially the same as for the process of shifting up, with the only difference that the engine must be accelerated for the purposes of matching the rotational speed. 
         [0046]    While at least one exemplary embodiment has been presented in the foregoing detailed description of the invention, it should be appreciated that a vast number of variations exist. It should also be appreciated that the exemplary embodiment or exemplary embodiments are only examples, and are not intended to limit the scope, applicability, or configuration of the invention in any way. Rather, the foregoing detailed description will provide those skilled in the art with a convenient road map for implementing an exemplary embodiment of the invention, it being understood that various changes may be made in the function and arrangement of elements described in an exemplary embodiment without departing from the scope of the invention as set forth in the appended claims and their legal equivalents.