Abstract:
A shifting device ( 11, 40 ) with a servo-assistance mechanism ( 10 ), in particular for a transmission ( 30 ) of a motor vehicle which comprises an assembly for selecting and engaging a gear of the transmission ( 30 ) and an element ( 20 ) upon which acts a manual shifting force to be assisted. The servo-assistance mechanism ( 10 ) can produce a performance characteristic curve which, as a function of the manual shifting force or a shift phase, has ranges with different gradients or proportions with respect to the manual shifting force. Elements ( 88, 90, 106, 108 ), upon which a servo-pressure acts, are present in the servo-assistance mechanism ( 10 ) for producing the characteristic. In the control system of the servo-assistance mechanism ( 10 ), path limitation units and at least one elastic element ( 84, 110 ) are provided, upstream from and/or in the servo-assistance mechanism ( 10 ), to limit the servo-assistance force in a path-dependent manner for each gate or gear.

Description:
[0001]    This application is a National Stage completion of PCT/EP2008/056458 filed May 27, 2008, which claims priority from German patent application serial no. 10 2007 026 422.6 filed Jun. 6, 2007. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The invention concerns a shifting device with a servo-assistance mechanism. 
       BACKGROUND OF THE INVENTION 
       [0003]    In such shifting devices the rotary movement of the gearshift rod serves to pre-select the individual shift gates and does not generally require any large shifting forces. The axial, longitudinal movement of the shift rod serves to engage the gear desired and, particularly in the case of transmissions for heavy vehicles and vehicles in which the driving seat is some distance away from the transmission, requires larger shifting forces. 
         [0004]    Present-day utility vehicles, such as omnibuses and trucks, with their forward control design, provide transmission filling space such that the transmission is necessarily far removed from the driving seat. The distance is particularly large in vehicles with under-floor or rear-mounted engines. Owing to the long and sometimes stiffly moving shift linkage in mechanically shifted transmissions, accurate gearshifts are often made more difficult. 
         [0005]    To allow the driver of a motor vehicle to focus his attention completely on the road traffic, he must be assisted and supported as much as possible in all the activities required for driving the motor vehicle. 
         [0006]    Every driver knows how decisively important it can be, to be able to operate the transmission without difficulty in difficult traffic situations. In this respect pneumatic shift aids for utility vehicles of any size can be helpful. 
         [0007]    Previously known servo-shifting devices are partially built directly onto the transmission and comprise a control rod accessible from outside and piston rods. The shift linkage is connected to the control rod. The assistance mechanism is activated by longitudinal movement of the control rod. This type of control is found in combination with two rod or cable shifts. A disadvantage here is the sealing of the control and piston rods by bellows and the lack of lubrication. In trucks the point concerned is exposed to much dirt. When the shift linkage ratio is changed, the beginning of the servo-assistance also changes or it has to be adapted to the linkage ratio by modifying the valve. The same applies to shift aids consisting of a control valve and a servo-cylinder that are separate from one another. The valve and cylinder are connected, via ball joints, to the shift lever and a cantilever, which is in turn fixed on the transmission. This arrangement has the added disadvantage that during every shift the components move relative to the transmission and the vehicle&#39;s chassis, so the air lines by which the valve and the cylinder are connected to one another can be perforated by chafing. 
         [0008]    Such pneumatic shift aids are also known in a divided configuration, consisting of a mechanical-pneumatic control portion and a separate, pneumatic force portion. A shift aid of divided configuration is known from Loomann; Zahnradgetriebe (Geared transmissions); 2nd edition; Springer Verlag; 1988; p. 225. The control portion is a mechanically actuated control valve, which is actuated by the shift linkage. In this case movement of the selection lever during gearshifts is transmitted mechanically directly to the transmission. When the shifting movement is transmitted, the control valve is actuated and at the same time the manual shifting force is transferred by a lever to the transmission. The manual shifting force is additionally assisted pneumatically by a compressed-air cylinder. This compressed-air cylinder, which is a two-position cylinder with an integrated hydraulic damper, forms the pneumatic force portion. In this case the manual shifting force is not reflected directly proportionally. The paths between the control and force portions are long, and the structure takes up considerable space. Damage of the compressed-air lines between the control and force portions cannot be avoided. 
         [0009]    DE 195 39 472 A1 discloses a shifting device with a servo-assistance mechanism for a motor vehicle transmission. A control rod of the servo-assistance mechanism that can move axially within a piston rod is provided, and co-operates via a shift linkage, with a shift lever. On the piston rod is arranged a piston that can be acted upon on both sides by a pressure medium, and the piston rod co-operates with means for shifting the geared change-speed transmission. When the control rod is moved axially in the piston rod, control valves can be actuated by actuating pistons. The shifting force exerted, via the shift lever and shift linkage, on the control rod is transferred proportionally, via the piston rod, to the means for shifting the geared change-speed transmission. During this the two sides of the shifting device produce shifting forces of different size, so that different gear steps of the vehicle&#39;s transmission can be engaged with different shifting forces. The different shifting forces are enabled by different valve characteristics, produced by actuating piston areas of different size and/or by different geometrical dimensions of the two surfaces arranged on opposite sides of the piston. 
         [0010]    From DE 10 2006 006 651 A1 a shifting device with a servo-assistance mechanism for a motor vehicle transmission has been made known, which comprises means for shifting and selecting a gear step of a transmission and a control rod of the servo-assistance mechanism, upon which a manual shifting force that is to be supported acts. Elastic elements are provided in the shifting device to change or limit the manual shifting force that acts on the servo-assistance mechanism before the force is applied to the servo-assistance unit. 
         [0011]    DE 10 2006 006 652 A1 also discloses a shifting device with a servo-assistance mechanism for a motor vehicle transmission, which comprises means for shifting and selecting a gear step of a transmission and a control rod of the servo-assistance mechanism upon which a manual shifting force to be supported acts. Spring elements are provided in the shifting device to modify the manual shifting force applied to the servo-assistance mechanism within the servo-assistance mechanism before and/or during the production of the servo-assisting force. 
         [0012]    To produce different servo-assisting forces, different compliances or elasticities are needed. Depending on the design this can be complicated to realize and can require structural space which is often not available. 
         [0013]    DE 10 2004 042 609 A1 proposes performance characteristics for a servo-assistance mechanism of a shifting device of a motor vehicle transmission. As a function of a manual shifting force or a shift phase the characteristics have sections with different gradients or proportionality to the manual shifting force. DE 10 2004 042 609 A1 does not disclose how the characteristics can be realized by design means. 
       SUMMARY OF THE INVENTION 
       [0014]    The purpose of the present invention is to describe a shifting device with a servo-assistance mechanism, with which a servo-characteristic cruve, which has sections of a different gradient or proportionality to the manual shifting force as a function of the manual shifting force, can be realized in a path-independent manner and with which a corresponding servo-assistance force can be associated with each gate or gear, with no need for additional structural space. 
         [0015]    The shifting device with a servo-assistance mechanism according to the invention, in particular for a motor vehicle transmission, comprises means for selecting and engaging a gear step of the transmission, and an element upon which a manual shifting force that is to be supported acts. This element can consist, for example, of a control rod of the servo-assistance mechanism. According to the invention, the servo-assistance mechanism comprises means for producing a performance characteristic which, as a function of the manual shifting force or a shift phase, has sections with different pitches or proportionality to the manual shifting force, and in the control of the servo-assistance mechanism a path limitation and at least one elastic element are provided upstream from and/or in the servo-assistance mechanism to limit a servo-assistance force for each gate and each gear in a path-dependent manner. 
         [0016]    The means for producing the performance characteristic are arranged in the servo-assistance mechanism in such a manner that the characteristic can be produced by a servo-pressure acting on the means in a path-independent manner, i.e. without any change of the current position of the control rod. 
         [0017]    By virtue of the path limitation in the control system of the servo-assistance mechanism, the relative movement between the control rod and a piston rod of the servo-assistance mechanism is restricted. This path limitation is so designed that a servo-characteristic cruve and therefore a servo-assistance force can be used conditionally, whereby for each gate or each gear a corresponding servo-assistance force can be produced. In this, for example, a servo-characteristic cruve can be used which is formed by virtue of an elastic element in the components upstream from or in the servo-assistance mechanism or a combination thereof. For example, the necessary elasticity can be realized by a spring element arranged on the control rod of the servo-assistance mechanism. 
         [0018]    In addition, the shifting device comprises a shifting shaft and a hollow shaft. The servo-assistance mechanism comprises a valve with a valve piston and a valve slide, and the piston rod of the servo-assistance mechanism co-operates with means for shifting the transmission. 
         [0019]    In a preferred embodiment of the shifting device according to the invention, the means for producing the servo-characteristic cruve consist of a trailing piston that co-operates with the valve and a trailing piston spring connected to the trailing piston. The trailing piston spring can be in the form, for example, of a spiral spring or a cup spring. 
         [0020]    In a particularly preferred embodiment of the shifting device according to the invention, the trailing piston is arranged inside the piston rod in such manner that the trailing piston surrounds the control rod and the valve piston radially, and is in contact with an abutment of the piston rod. The trailing piston spring rests on one side against an abutment of the piston rod and on the other side is connected to the trailing piston. 
         [0021]    For example, the trailing piston can be in the form of a hollow cylinder which, in its actuated condition, acts upon a stop element arranged on the control rod. Likewise, it is conceivable for the trailing piston to be formed as a hollow cylinder and to have an abutment formed on the side opposite the trailing piston spring in the direction toward the control rod, so that in its actuated condition the trailing piston rests with its abutment against the valve piston. Above a given manual shifting force, a regulating servo-pressure becomes high enough for the trailing piston to be pushed, in opposition to the force of the trailing piston spring, against the valve piston or against the abutment on the control rod, which constitutes the actuated condition of the trailing piston. Thus, the manual shifting force or control rod force opposes both the pressure force acting directly on the valve piston and also the pressure force transmitted by the trailing piston to the valve piston or to the stop element arranged on the control rod. Accordingly, beyond a certain regulating position, the pressure force that opposes the manual shifting force or the control rod force is correspondingly increased. This produces an inflexion point in the characteristic curve, beyond which its gradient is smaller so that it is less steep. The position of the inflexion point of the characteristic can be varied as a function of the design of the trailing piston spring. 
         [0022]    In a particularly advantageous embodiment of the shifting device according to the invention, means are provided in the servo-assistance mechanism by which the servo-assistance force can be limited to at least two different servo-force levels. 
         [0023]    In a preferred embodiment of the shifting device according to the invention, the path limitation in the control of the servo-assistance mechanism can be realized by a groove in the hollow shaft and at least one pin that passes through the groove. 
         [0024]    In a further embodiment of the shifting device according to the invention, the path limitation in the control of the servo-assistance mechanism can be realized by a groove in the shifting shaft and at least one pin that passes through or engages in the groove. 
         [0025]    In these cases the groove in the hollow shaft or the shifting shaft can be designed such that in the axial direction of the shifting shaft there is rotational clearance between the pin and the groove which is of different size for each gate or each gear. For example, the rotational clearance on one side of the groove can be of a size different from those on the opposite side of the groove, in order to produce different servo-assistance forces in different shift positions within the same shift gate of the transmission. 
         [0026]    In another embodiment the relative movement between the control rod and the piston rod of the servo-assistance mechanism is limited by a bore in the shifting shaft, for example a slewed bore. By virtue of this bore, corresponding clearance are produced between the shifting shaft and a pin arranged in the bore, whereby corresponding rotational clearance are produced between the shifting shaft and the hollow shaft and the servo-assistance force can accordingly be limited. 
         [0027]    Thus, the path limitation enables a mechanical bridging of the servo-assistance mechanism whereby the servo-assistance force can be limited as a function of the gate or gear. The means for limiting the path of the control rod of the servo-assistance mechanism can also be located elsewhere in the shifting device or unit, and can then be designed differently. 
         [0028]    Thus, by virtue of the shifting device with its servo-assistance mechanism according to the invention an “inflected” servo-characteristic cruve can be produced without changing the position of the control rod, and the path limitation means provided in the control of the servo-assistance mechanism can limit the actuation of the control rod of the servo-assistance mechanism relative to the piston rod in a path-dependent or gear-dependent manner, whereby a corresponding servo-assistance force can be produced in the path-dependent or gear-dependent way. 
         [0029]    Accordingly, by virtue of the shifting device and servo-assistance mechanism according to the invention, at the beginning of a shift a sufficiently large servo-assistance force can be ensured, such that during a synchronous phase the servo-assistance force produced does not overload the components or synchronizers involved in the shift operations. For example, the shifting device according to the invention can be used in transmissions with shift linkages or cable shifts. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0030]    Below, the basic principle of the invention, which can be embodied in many forms, is explained in greater detail as an example with reference to a drawing, which shows: 
           [0031]      FIG. 1 : Shifting unit of the prior art; 
           [0032]      FIG. 2 : Sectional view of an embodiment of the shifting device; 
           [0033]      FIG. 3 : Sectional view of another embodiment of the shifting device; 
           [0034]      FIG. 4 : Sectional view of another embodiment of the shifting device; 
           [0035]      FIG. 5 : Sectional view of an embodiment of the servo-assistance mechanism; 
           [0036]      FIG. 6 : Sectional view of another embodiment of the servo-assistance mechanism; 
           [0037]      FIG. 7 : Sectional view of an embodiment of the servo-assistance mechanism, for producing two different servo-assistance force levels; and 
           [0038]      FIG. 8 : Sectional view of an embodiment of the servo-assistance mechanism, for producing three different servo-assistance force levels. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0039]      FIG. 1  is a sketch showing the shifting unit  2  of a motor vehicle, according to the prior art. From a shift lever  4  a shift rod  6  leads, via a lever deflection  8  to a shifting device  11  with a pneumatic servo-assistance mechanism  10 . The pneumatic servo-assistance mechanism  10  comprises a connection line  12  leading to a reservoir container  14 , from which the pneumatic servo-assistance mechanism  10  is supplied with compressed air. The lever deflection comprises a first lever  16 , which is preferably articulated to the shift rod  6 . The lever deflection  8  comprises a second lever  18 , which in turn engages in a control rod  20  arranged in the pneumatic servo-assistance mechanism  10 . Furthermore, in the pneumatic servo-assistance mechanism  10  a piston rod  22  is provided, in which there engages a lever  24  which is connected, via a rotating shifting shaft  26 , to a lever  28  in the transmission  30  of the vehicle. The lever  28  engages in a shift rail  32  by means of which, in a known way, transmission ratios of the transmission can be engaged. Movement of the lever  24  is converted by the shifting shaft  26  into movement of the lever  28 , so that the lever  28  can move the shift rail  32  axially. By virtue of that movement the shift rail  32  adopts preferably three positions, namely two axial end positions, each respectively corresponding to an engaged transmission ratio, and a central position between the end positions, which corresponds to a neutral setting of the transmission. 
         [0040]      FIG. 2  shows a sectioned view of the shifting device  40  according to the invention. A shifting shaft  44  is fitted to rotate about its axis within a hollow shaft  52 . The hollow shaft  52  is fitted within the shifting device  40  to rotate about the axis of the shifting shaft  44 . On its circumference the shifting shaft  44  is provided with grooves  54  along its axis. In these grooves  54  in the shifting shaft  44  engage pins  56  which are mounted to rotate in needle sockets  58  in the lever  18 . The pins  56  are arranged in the grooves  54  in such manner that axial displacement of the shifting shaft  44  along its axis is possible and rotation of the shifting shaft  44  about its axis causes the lever  18  to pivot. 
         [0041]    With a roller  60 , for example arranged to rotate freely on a bolt, the lever  18  engages in a pocket  70  in the control rod  20  and enables a low-friction transfer of the manual shifting force to the pneumatic servo-assistance mechanism  10 . A servo-assistance force produced by the servo-assistance mechanism  10  is transmitted by the piston rod  22  to the lever  24  (see  FIG. 1 ) and from there to the hollow shaft  52 . 
         [0042]    By a path limitation in the control system of the servo-assistance mechanism  10 , different servo-assistance forces can be produced. In this case the path limitation means are designed such that there are corresponding rotational clearance between the shifting shaft  44  and the hollow shaft  52 , which are produced by grooves  48  in the hollow shaft  52  and pins  56 . By an appropriate design of the groove  48  in the hollow shaft  52 , different rotational clearance can be produced between the shifting shaft  44  and the hollow shaft  52 . The servo-assistance force is maintained until the rotational clearance between the shifting shaft  44  and the hollow shaft  52  is eliminated. Thereafter, the manual shifting force is transmitted directly to the hollow shaft  52  and the control rod  20  moves no farther relative to the piston rod  22  of the servo-assistance mechanism  10 . In this embodiment the hollow shaft  52  and the lever  18  are arranged so that they can move axially relative to one another. 
         [0043]      FIG. 3  shows another embodiment of the shifting device  40 . In contrast to the embodiment of  FIG. 2 , this embodiment has a pin  38  which is arranged fixed or, via a bearing  62 , in the shifting shaft  44 . The lever  18  has two continuous axial grooves  46  and a bore  42 . The pin  38  is arranged in the grooves  46  in such manner that the shifting shaft  44  can be displaced axially along its axis and rotation of the shifting shaft  44  about its axis causes the lever  18  to pivot. The pin  38  can be fitted thanks to the bore  42 . The hollow shaft  52  and the shifting shaft  44  are arranged so that they can move axially relative to one another, preferably with the shifting shaft  44  being able to undergo this axial movement while the hollow shaft  52  cannot move axially. The path limitation in the control system of the servo-assistance mechanism  10  is designed such that appropriate rotational clearance exist between the shifting shaft  44  and the hollow shaft  42 , which are realized by the grooves  48  in the hollow shaft  52  and the pin  38 . 
         [0044]      FIG. 4  shows a section through the shifting shaft  44  in the area of the lever  28 , which engages with projections  74  in shift rails  32  not shown here (see  FIG. 1 ). The lever  28 , the hollow shaft  52  and the shifting shaft  44  are penetrated by at least one pin  66 . In this area the hollow shaft  52  has two slots  64 . The width of the slots  64  in the circumferential direction of the hollow shaft  52  corresponds approximately to the diameter of the pin  66 . The pin  66  is held in position, for example, by retaining rings  72 . In this area the shifting shaft  44  has a bore, for example a slewed bore. By virtue of the slewed bore, corresponding clearance s, s′ are produced between the shifting shaft  55  and the pin  66 . Due to these clearance s, s′ corresponding rotational clearance  68 ,  68 ′ are produced between the shifting shat  44  and the hollow shaft  52 . The clearance s, s′ can be designed such that the same rotational clearance  68 ,  68 ′ or different rotational clearance  68 ,  68 ′ are produced on the two sides between the shifting shaft  44  and the hollow shaft  52 . Thus, the clearance s, s′ can also be used for a path-dependent limitation of the servo-assisting force. 
         [0045]    The slewed bore through the shifting shaft  44 , in which the pin  66  is located, can also be made as a slot whereby clearance s, s′ of different size can be produced in the axial direction of the shaft  44 . Furthermore, instead of the slewed bore a groove can be provided in the shifting shat  44  to produce clearance s, s′ of different size between the shaft  44  and the pin  66 , and in this case the pin  66  is split. By an appropriate design of the slot or groove a corresponding servo-assistance force can be produced for each gate or gear, and then the selection movement is transmitted to the lever  28  not by the shifting shaft  44 , but by another element (not shown here). 
         [0046]      FIG. 5  is a sectional representation of part of the servo-assistance mechanism  10 . The servo-assistance mechanism  10  comprises a control rod  20  and a piston rod  22  as well as a cylinder  76  and a piston  78 . The control rod  20  of the servo-assistance mechanism  10  can move axially within the piston rod  22  and co-operates, via a shift linkage, with a shift lever (see  FIG. 1 ). The piston rod  22  co-operates with means for shifting the geared change-speed transmission (see  FIG. 1 ). The piston  78  and the piston rod  22  are permanently connected together, or made as one component. A valve  98  consists of valve pistons  82 ,  104  and valve slides  80 ,  102 . In this case the valve pistons  82 ,  104  are arranged to move axially on the control rod  20  against the spring force of spring elements  84 ,  110 , and when the servo-assistance mechanism  10  is not actuated, are held by the spring elements  84 ,  110  in their starting position. In this case the spring elements  84 ,  110  are made as spiral springs and can, for example, even be arranged in the servo-assistance mechanism under pre-stress. Between the valve pistons  84 ,  104  the valve slides  80 ,  102  are also arranged axially movably. The valve slides  80 ,  102  are held axially apart by a spring element  100  and each rests on a valve seat of the piston rod  22 . The servo-assistance mechanism  10  has two trailing pistons  88 ,  106  with trailing piston springs  90 ,  108 . In this case the trailing pistons are made in the form of a hollow cylinder and are arranged in the servo-assistance mechanism in such manner that they surround both the control rod  20  and the valve pistons  82 ,  104 , and can move in the axial direction relative to the control rod  20 . The trailing piston springs  90 ,  108  are here made as spiral springs and rest at one end against an abutment of the piston rod  22  and at their other end against the respective trailing pistons  88 ,  106 . The abutment on the piston rod  22  is here in the form of a retaining ring, although it can be formed integrally with the piston rod. When the servo-assistance mechanism  10  is not activated, the valve slides  80 ,  102  rest against the valve seats of the piston rod  22  and the trailing pistons  88 ,  106  and are pushed against corresponding abutments of the piston rod  22  by the trailing piston springs  90 ,  108 . On the valve pistons  82 ,  104  there can also be arranged additional elements  96 ,  112 , for example in the form of disks. In the servo-assistance mechanism are arranged stop elements  92 ,  114 , in this case held by retaining rings, and the spring element  84 ,  110  are fixed on the control rod  20 . The stop elements  92 ,  114  are here pot-shaped and have abutments  94 ,  116 . 
         [0047]    If the control rod  20  is moved to the left in the plane of the drawing by a manual shifting force, the valve piston  82  arranged on the control rod  20  is also moved to the left. The valve piston  82  actuates the valve slide  80 , moving it clear of the valve seat of the piston rod  22  so that the valve  98  opens. By virtue of an existing reservoir pressure, the open valve  98  regulates a servo-pressure corresponding to the prevailing control rod force. If, now, the control rod  20  is moved further to the left by increasing the manual shifting force, then because of the force equilibrium reached the valve piston  82  and valve slide  80  remain in their previously reached open positions while the control rod  20  moves farther relative to the two of them and the spring element  84  is compressed farther. This establishes a new force equilibrium, producing a corresponding course of the servo-force. Thus, by virtue of the spring element  84  an internal servo-pressure limitation can be achieved. 
         [0048]    If the pressure force acting on the trailing piston  88  due to the regulated servo-pressure is smaller than the force produced by the trailing piston spring  90 , then the trailing piston  88  remains in contact with the abutment of the piston rod  22  and the trailing piston spring  90  is not compressed. Thus, the manual shifting force or control rod force opposes only the pressure force acting on the valve piston  82 , which corresponds to a regulated setting in the range of the steep servo-characteristic curve. 
         [0049]    If, now, the control rod  20  is moved by a larger manual shifting force even farther to the left in the plane of the drawing, then the valve slide  80  too is pushed by the valve piston  82  farther to the left so that the valve  98  opens more. This results in a correspondingly higher servo-pressure. If the regulated servo-pressure is now large enough for the trailing piston spring  90  to be compressed by the trailing piston  88  and the trailing piston  88  to be pushed, against the force of the trailing piston spring  90 , onto the abutment  94  of the stop element  92 , then the manual shifting force or control rod force opposes both the pressure force acting directly on the valve piston  82  and the pressure force transmitted by the trailing piston  88  and the stop element  92  to the control rod  20 . The pressure force opposing the manual shifting force or control rod force is thereby increased and the characteristic curve develops a point of inflexion. This regulated setting corresponds to the range in which the characteristic is flatter. The positions of the control rod  20  and the valve piston  82  do not change, so the “bent” characteristic can be produced in a path-independent manner. 
         [0050]    If the regulated servo-pressure now becomes so high that the valve piston  82  is pressed, against the spring force of the spring element  84 , onto the stop element  92 , then the course of the servo-characteristic curve will correspond to a servo-characteristic curve shape such that the valve piston  82  is in a fixed position on the control rod  20 . 
         [0051]    The behavior is analogous if the control rod  20  is actuated to the right in the plane of the drawing. In that case the valve slide  102  is actuated by the valve piston  103 , whereby the valve slide  102  is raised clear of the valve seat of the piston rod  22  and thus opens the valve  98 . 
         [0052]      FIG. 6  shows a sectioned view of part of another embodiment of the servo-assistance mechanism  10 . This embodiment functions in the same way as the embodiment described with reference to  FIG. 5 . In contrast to the embodiment shown in  FIG. 5 , however, the trailing piston  88  made as a hollow cylinder has on its side opposite to the trailing piston spring  90  an abutment formed in the direction of the control rod  20 . The abutment can for example be made integrally with the trailing pistons  88 ,  106 , as one component. Here too, the trailing piston  88  is arranged so that it surrounds both the control rod  20  and the valve piston  82 , and can be moved in the axial direction relative to the control rod  20  against the spring force of the spring element  90 . Above a certain manual shifting force the regulated servo-pressure becomes high enough for the trailing piston  88  to be pushed, against the force of the trailing piston spring, onto the valve piston  82 . Thus, the manual shifting force or control rod force opposes both the pressure force acting directly on the valve piston  82  and the pressure force transmitted to the valve piston  82  by the trailing piston  88 . Accordingly, beyond a certain regulation setting the pressure force that opposes the manual shifting force or control rod force is correspondingly increased, and this produces an inflexion point in the servo-characteristic cruve beyond which its gradient is smaller so that it is flatter. In this case the axial movement of the valve piston  82  is limited by an abutment  86 , for example in the form of a retaining ring arranged in a fixed position on the control rod  20 . 
         [0053]      FIG. 7  shows a sectioned view of an embodiment of the servo-assistance mechanism  10  according to the invention, in which a servo-assistance force can be limited to two different servo-assistance force levels. In the description below the indexes used are the same as in the description of  FIG. 5 . 
         [0054]    In contrast to  FIG. 5  the servo-assistance mechanism  10  has a further spring element  118  and further stop elements  120 ,  122  and  124 . The stop element  124  is arranged fixed on the control rod  20  of the servo-assistance mechanism  10 . Here, the spring element  84  is arranged on the control rod  20  in such manner that when the servo-assistance mechanism  120  is not actuated it is fixed between the valve piston  82  and the stop element  124 . A stop disk  122  between the stop element  124  and the spring element  84  can serve as an abutment surface for the spring element  84 . The stop element  120  is pot-shaped, comprises a radially outward-extending abutment  128  and is arranged, in the radial direction relative to the control rod  20 , between the spring element  84  and the spring element  118 . Relative to the control rod  20 , the spring element  118  is arranged in the radial direction between the stop elements  92  and  120  and, when the servo-assistance mechanism  10  is not actuated, is fixed between the stop element  92 , that also serves as the contact surface for the trailing piston  88 , and the abutment  128  of the stop element  120 , whereby the stop element  120  is brought into contact with the stop element  124 . 
         [0055]    If the control rod  20  of the servo-assistance mechanism  10  is moved to the left in the drawing plane by a manual shifting force, the valve  98  is opened by the valve piston  82 . With the valve  98  open, when a pressure level is reached which is higher than the spring force exerted by the spring element  84  on the valve piston  82 , the valve piston  82  moves relative to the control rod  20  until the path limitation in the control system of the servo-assistance mechanism  10  becomes effective, i.e. until the servo-assistance mechanism  10  is mechanically bridged (see the earlier description). This produces a first servo-force level, which acts for example in the gate comprising gears 5/6 of a 6-gear main transmission. 
         [0056]    If another gate is now selected, for example the gate for gears 3/4 of the 6-gear main transmission, a second servo-force level can be produced. For this, the path limitation in the control system of the servo-assistance mechanism  10  is designed such that, for example, between the shifting shaft and the groove in the hollow shaft there is a larger rotational clearance in the gate for gears 3/4 than in the gate for gears 5/6 of the 6-gear main transmission (see the earlier description), whereby a larger actuation path is available for the control rod  20  of the servo-assistance mechanism  10  before the servo-assistance mechanism  10  is mechanically bridged and the valve piston  82  moves farther relative to the control rod  20  until it comes up against the stop element  120 . If the manual shifting force is now increased further, the valve piston  82  does not move any farther relative to the control rod  20  until a servo-force level is reached that corresponds to the sum of the spring forces exerted by the spring elements  118  and  120  on the valve piston  82 . If the manual shifting force is now increased still more, then because of the higher servo-force level the valve piston  82  moves relative to the control rod  20  until, here too, the servo-assistance mechanism  10  is mechanically bridged due to the path limitation in the control system of the servo-assistance mechanism  10 . 
         [0057]    By virtue of the arrangement of the stop element  120  and the spring element  118  in the servo-assistance mechanism  10 , the servo-assistance force can be limited to two different servo-force levels without any increase of the dimensions of, or the structural space occupied by the servo-assistance mechanism  10 . 
         [0058]      FIG. 8  reproduces the servo-assistance mechanism  10  according to  FIG. 7 , with the difference that in the axial direction of the control rod  20  a further stop element  126 , in this case of annular shape, is arranged between the two stop elements  92  and  120 . Starting from  FIG. 7 , from the gate for gears 3/4 the gate for gears 1/2 of the 6-gear main transmission is selected. Here, the path limitation in the control system of the servo-assistance mechanism  10  is designed such that in the gate for gears 1/2 there is an even larger rotational clearance than in the gate for gears 3/4 of the 6-gear main transmission. 
         [0059]    If the manual shifting force is now increased still more, the valve piston  82  moves relative to the control rod  20  until it encounters the stop element  126  via the stop element  120  and can therefore move no farther relative to the control rod  20 . In this way a third servo-force level can be produced, which corresponds for example to the reservoir pressure present in the motor vehicle. Accordingly for example, the 1 st  gear can be engaged with an unrestricted reservoir pressure. 
       INDEXES 
       [0000]    
       
           2  Shifting unit 
           4  Shift lever 
           6  Shift rod 
           8  Lever deflection 
           9  End-stop 
           10  Servo-assistance mechanism 
           11  Shifting device 
           12  Connection line 
           14  Reservoir container 
           16  Lever 
           18  Lever 
           20  Control rod 
           22  Piston rod 
           24  Lever 
           26  Shifting shaft 
           28  Lever 
           30  Vehicle transmission 
           32  Shift rail 
           38  Pin 
           40  Shifting device 
           42  Bore 
           44  Shifting shaft 
           46  Groove 
           48  Groove 
           52  Hollow shaft 
           54  Groove 
           56  Pin 
           58  Needle socket 
           60  Roller 
           62  Bearing 
           64  Slot 
           66  Pin 
           68  Rotational clearance 
           68 ′ Rotational clearance 
           70  Pocket 
           72  Retaining ring 
           74  Projection 
           76  Cylinder 
           78  Piston 
           80  Valve slide 
           82  Valve piston 
           84  Spring element 
           86  Stop element 
           88  Trailing piston 
           90  Training piston spring 
           92  Stop element 
           94  Abutment 
           96  Disk 
           98  Valve 
           100  Spring element 
           102  Valve slide 
           104  Valve piston 
           106  Trailing piston 
           108  Trailing piston spring 
           110  Spring element 
           112  Disk 
           114  Stop element 
           116  Abutment 
           118  Spring element 
           120  Stop element 
           122  Stop element 
           124  Stop element 
           126  Stop element 
           128  Abutment 
         s Clearance 
         s′ Clearance