Abstract:
A servo-assistance device ( 10 ) for a shift mechanism of a motor vehicle transmission, with which a characteristic curve can be produced which, as a function of a manual shifting force or a shift phase, has ranges of different gradients or proportionality to the manual shifting force. The servo-assistance device ( 10 ) includes an element ( 20 ) which is acted upon by the manual shifting force to be assisted. To produce the characteristic curve, the servo-assistance device ( 10 ) has components ( 42, 44, 50, 52 ) that are actuated by a servo-pressure ( 60 ) in such manner that the element ( 20 ) maintains its current position.

Description:
This application is a National Stage completion of PCT/EP2007/060447 filed Oct. 2, 2007, which claims priority from German patent application serial no. 10 2006 048 254.9 filed Oct. 12, 2006. 
     FIELD OF THE INVENTION 
     The invention concerns a servo-assistance device for a shift mechanism of a motor vehicle transmission. 
     BACKGROUND OF THE INVENTION 
     Today&#39;s utility vehicles, such as buses and trucks, with their front control design, are characterized by spatial conditions for the accommodation of the transmission such that the transmission is necessarily a substantial distance away from the driver&#39;s seat. This distance is particularly large in the case of vehicles with under-floor or rear-mounted engines. Owing to the long and sometimes tight gearshift linkage used with mechanically shifted transmissions, exact gearshifts are often more difficult. 
     To allow the driver of a motor vehicle to devote his full attention to the traffic, he should so far as possible be relieved and assisted in all the activities needed for driving the motor vehicle. 
     Every driver knows how decisive the trouble-free operation of the gearshift system can be in difficult traffic situations. In this connection pneumatic shift-assisting devices for utility vehicles of any size can be helpful. 
     Servo-assistance devices known until now are partly fitted directly on the transmission itself and have a control rod and piston rod accessible from outside. The gearshift linkage is connected to the control rod. The assistance mechanism is activated by longitudinal movement of the control rod. This type of control is combined with two rod- or cable-pull shifts. A disadvantage in such cases is the sealing of the control and piston rod by bellows and the lack of lubrication. In trucks the location concerned is exposed to a lot of dirt. When the gearshift linkage ratio is changed, the beginning of the servo-assistance also changes or it has to be adapted by modifying the valve to suit the linkage ratio. The same applies to shifting aids that consist of a control valve and servo-cylinder separate from one another. The valve and cylinder are connected by spherical joints to the shift lever and a bracket, which is in turn fixed on the transmission. This arrangement has the additional disadvantage that at every shift the components move relative to the transmission and the chassis, so that the air lines connecting the valve and cylinder with one another can be perforated by chafing. 
     Such pneumatic shift aids with an open structure are known, consisting of a mechanical-pneumatic control element and a separate, pneumatic force element. A shift aid with an open structure is known from Loomann; Toothed-wheel transmissions; 2 nd  edition; Springer Verlag; 1988; p. 225. The control element is a mechanically actuated control valve, which is actuated by a shift rod. In this case the transmission of the selection movement for gearshifts takes place mechanically, directly in the transmission. During transmission of the shift movement the control valve is actuated and at the same time the manual shifting force is transmitted mechanically to the transmission by a lever. During this, the manual shifting force is additionally assisted pneumatically by a compressed air cylinder. The compressed air cylinder, a two-position cylinder with integrated hydraulic damper, forms the pneumatic force element. In this case the manual shifting force is not imaged directly proportionally. The distances between the control and force elements are long, and the structure takes up a lot of space. Damage to the compressed air lines between the control and force portions can hardly be avoided. 
     DE 195 39 472 A1 discloses a shifting mechanism with a servo-assistance device for a motor vehicle transmission. A control rod of the servo-assistance device is arranged axially movably within a piston and co-operates, via a gearshift linkage, with a shift lever. On the piston rod is arranged a piston which can be acted upon on both sides by a pressure medium, and the piston co-operates with means for shifting the gear-type variable-speed transmission. Axial displacement of the control rod in the piston rod can actuate control valves by means of actuating pistons. The shifting force exerted upon the control rod via the shift lever and gearshift linkage is transferred proportionally by the piston rod to the means for shifting the gear-type variable-speed transmission. During this, the two sides of the shift mechanism produce shift forces of different size, so that various gears of the vehicle&#39;s transmission can be shifted with different shifting forces. The different shifting forces are produced by different valve characteristics, given by differently sized piston faces of the actuating pistons and/or by different geometrical dimensions of the two opposed surfaces of the piston. 
     In the unpublished applications by the present applicant with file numbers 10 2006 006 651.0 and 10 2006 006 652.9, a shift mechanism with servo-assistance for a vehicle transmission is disclosed, which comprises means for selecting and shifting a gear of the transmission and a control rod of the servo-assistance device, which is acted upon by the manual shifting force to be assisted. In the unpublished application with file number 10 2006 006 651.0 elastic elements are provided in the shift mechanism in order to change or limit the manual shifting force that acts upon the servo-assistance device before the force passes into the servo-assistance device. In the unpublished application with file number 10 2006 006 652.9, spring elements are provided in order to change the manual shifting force that acts on the servo-assistance device within the servo-assistance device before and/or during the production of the servo-assistance force, and thereby to influence the servo-assistance device in its action. 
     To produce different servo-assistance forces, it is necessary to have different elastic compliances or elasticities. Depending on the design, this can be difficult to achieve and may require structural space that is often not available. 
     DE 10 2004 042 609 A1 describes characteristics for a servo-assistance device for a shift mechanism of a motor vehicle transmission. As a function of a manual shifting force or a shift phase the characteristic curves have sections with different gradients or proportionality to the manual shifting force. It is not disclosed in DE 10 2004 042 609 A1 how the characteristics can be realized by design. 
     SUMMARY OF THE INVENTION 
     The purpose of the present invention is to indicate a servo-assistance device for a shift mechanism of a motor vehicle transmission, by means of which a characteristic curve that has sections with different gradients or proportionality to the manual shifting force as a function of the manual shifting force or a shift phase, can be produced in a path-independent manner. 
     The objective of the invention is achieved by a servo-assistance device of the type concerned. 
     A servo-assistance device for a shift mechanism of a motor vehicle transmission comprises a control rod acted upon by a manual shifting force that is to be assisted, a piston rod which co-operates with a means for shifting a gear-type variable-speed transmission, and a valve comprising a valve piston and a valve slide. According to the invention the servo-assistance device comprises means for producing a characteristic curve which, as a function of the manual shifting force or a shift phase, has sections with different gradient or proportionality to the manual shifting force in a path-independent manner, i.e. without a change in the current position of the control rod. The means consist of a trailing piston and a trailing piston spring. The trailing piston spring is arranged in the servo-assistance device in such manner that it is in contact on one side with an abutment on the piston rod and on the other side it pushes the trailing piston against another abutment of the piston rod. The trailing piston is arranged in the piston rod in such manner that it surrounds both the control rod and the valve piston, which is arranged fixed on the control rod. Above a certain manual shifting force a regulated servo-pressure become large enough to push the trailing piston, in opposition to the force of the trailing piston spring, against the valve piston. Thus the manual shifting force or control rod force is opposed both by the pressure force acting directly on the valve piston and by the pressure force transferred, via the trailing piston, to the valve piston. Accordingly, beyond a certain set position the pressure force that opposes the manual shifting force or control rod force is correspondingly increased. This produces an inflection point in the characteristic curve, beyond which its gradient is smaller so that its course is less steep. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Below, the basic principle of the invention, which can be embodied in several ways, is explained in more detail considering an example, with reference to a drawing which shows: 
         FIG. 1 : Shift unit of the prior art 
         FIG. 2 : Sectioned view of an embodiment of the servo-assistance device in a neutral position 
         FIG. 3 : Sectioned view of an embodiment of the servo-assistance device in a section of its characteristic curve with a steep gradient 
         FIG. 4 : Sectioned view of an embodiment of the servo-assistance device in a section of its characteristic curve with a shallower gradient and 
         FIG. 5 : A valve characteristic curve. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  is a diagrammatic representation of the shift unit  2  of a motor vehicle, according to the prior art. Starting from a shift lever  4 , a shift rod  6  leads via a lever linkage  8  to a shift mechanism  11  with a pneumatic servo-assistance device  10 . The pneumatic servo-assistance device  10  has a connection line  12  that leads to a reservoir container  14  from which the pneumatic servo-assistance device  10  is supplied with compressed air. The lever linkage  8  comprises a first lever  16  preferably articulated to the shift rod  6 . The lever linkage  8  also comprises a second lever  18 , which in turn engages a control rod  20  arranged in the pneumatic servo-assistance device  10 . In addition, in the pneumatic servo-assistance device  10  is arranged a piston rod  22  in which a lever  24  engages, which is connected via a rotary shifting shaft  26  to a lever  28  in the vehicle&#39;s transmission  30 . The lever  28  engages in a shift rail  32  by means of which, in a known manner, 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 displace the shift rail  32  axially. During this axial movement the shift rail  32  preferably adopts three positions, namely two axial end positions, each corresponding to an engaged transmission ratio, and a central position between the end positions, which corresponds to a neutral setting of the transmission. 
       FIG. 2  shows a sectioned view of the servo-assistance device  10  according to the invention, in a neutral position. The servo-assistance device  10  comprises a control rod  20  and a piston rod  22 , as well as a cylinder  34  and a piston  36 . The control rod  20  of the servo-assistance device  10  is arranged to move axially within the piston rod  22  and co-operates, via a gearshift linkage, with a shift lever (see  FIG. 1 ). The piston  36  and the piston rod  22  are connected fixed to one another or made as one component. The piston rod  22  co-operates with means for shifting the gear-type variable-speed transmission (see  FIG. 1 ). A valve  56  consists of valve pistons  40 ,  48  and valve slides  38 ,  46 . The valve pistons  40 ,  48  are arranged fixed on the control rod  20 , so that actuation of the control rod  20  actuates them too along their axes and also in the axial direction relative to the control rod  20 . Between the valve pistons  40 ,  48  the valve slides  38 ,  46  are also arranged to move axially. The valve slides  38 ,  46  are held axially apart from one another by a spring element  54  and each rests against a valve seat of the piston rod  22 . According to the invention the servo-assistance device  10  has trailing pistons  42 ,  50  and trailing piston springs  44 ,  52 . The trailing pistons  42 ,  50  are for example made in the form of a hollow cylinder which, on the side opposite the trailing piston springs  44 ,  52 , has an abutment formed in the direction toward the control rod  20 . The abutment can for example be formed with the trailing pistons  42 ,  50  as one component. Likewise, it is conceivable for the abutment to consist, for example, of a securing ring. The trailing piston springs  44 ,  52  can for example be spiral springs or cup springs. In this case the trailing pistons  42 ,  50  are arranged so that they surround both the control rod  20  and the valve pistons  40 ,  48  and can move in the axial direction relative to the control rod  20 . The trailing piston springs  44 ,  52  rest at one end against an abutment of the piston rod  22  and at the other end against the trailing pistons  42 ,  50 . When the servo-assistance device  10  is not actuated, i.e. when the vehicle&#39;s transmission is in the neutral position, the valve slides  38 ,  46  rest against the valve seats of the piston rod and the trailing pistons  42 ,  50  are pressed by the trailing piston springs  44 ,  52  against corresponding abutments of the piston rod  22 . 
       FIG. 3  shows a sectioned view of the servo-assistance device  10  according to the invention for a regulation setting in the area of the steep characteristic curve. By virtue of a manual shifting force, the control rod  20  has been moved to the left in the plane of the drawing. Thus, the valve piston  40  fixed on the control rod  20  has also moved to the left. By means of the valve piston  40  the valve slide  38  is actuated, so that it moves clear of the valve seat of the piston rod  22  and opens the valve  56 . By virtue of an existing reservoir pressure  58  the open valve  56  controls a servo-pressure  60  in accordance with the prevailing control rod force. The pressure force acting due to the controlled servo-pressure  60  upon the trailing piston  42  is smaller than the force produced by the trailing piston spring  44 , so the trailing piston  42  still rests against the abutment of the piston rod  22  and the trailing piston spring  44  is not compressed. The manual shifting force or control rod force therefore only opposes the pressure force acting on the valve piston  40 , and this corresponds to a regulation setting in the area of the steep characteristic curve. 
     The effect is analogous when the control rod  20  is actuated to the right in the plane of the drawing. In that case the valve slide  46  is actuated by the valve piston  48 , whereby the valve slide  46  moves clear of the valve seat of the piston rod  22  and therefore opens the valve  56 . 
       FIG. 4  shows a sectioned view of the servo-assistance device  10  according to the invention for a regulation setting in the area of the less steep characteristic curve. In contrast to  FIG. 3 , in this case the manual shifting force is larger so the control rod  20  of the servo-assistance device  10  is moved farther to the left. Thus, the valve slide  38  is also pushed by the valve piston  40  farther to the left, so that the valve  56  opens more. This produces a correspondingly higher servo-pressure  60 . The controlled servo-pressure  60  is now large enough for the trailing piston spring  44  to be compressed by the trailing piston  42  so that the trailing piston  42  is pushed, against the force of the trailing piston spring  44 , toward the valve piston  40 . Thus, the manual shifting force or control rod force opposes both the pressure force acting directly on the valve piston  40  and the pressure force transmitted by the trailing piston  42  to the valve piston  40 . This increases the pressure force acting in opposition to the manual shifting force or control rod force and the characteristic curve shows an inflection point. The regulation position corresponds to the range in which the characteristic curve is less steep. The positions of the control rod  20  and the valve piston  40 ,  48  fixed to the control rod  20  do not change, so the “bent” characteristic curve is produced in a path-independent manner. 
     In turn, the situation produced by actuating the control rod  20  to the right in the plane of the drawing is analogous. 
       FIG. 5  shows the servo-force as a function of the control rod force. The valve characteristic curve produced by virtue of the servo-assistance device  10  according to the invention has an inflection point, beyond which the course of the characteristic is less steep so as to treat the components involved in a gearshift more gently. 
     If the control rod force is smaller than F_knick, then the controlled servo-pressure  60  is not large enough to move the trailing piston  42 ,  50  against the force of the trailing piston spring  44 ,  52 . The controlled servo-pressure  60  only acts upon the valve piston  40 ,  48 . Thus, the pressure force produced acts exclusively as a reaction force in opposition to the control rod force. This regulation position corresponds to the steep range of the characteristic curve. 
     If the manual shifting force is increased until the control rod force is larger than F_knick, then the trailing piston  42 ,  50  is pushed toward the valve piston  40 ,  48  against the force of the trailing piston spring  44 ,  52 . Accordingly, to the pressure force of the valve piston  40 ,  48  previously acting as a reaction force upon the control rod  20 , is added the pressure force acting from the trailing piston  42 ,  50  upon the valve piston  40 ,  48  (the pressure force exerted by the servo-pressure  60  on the trailing piston  42 ,  50 , less the force of the trailing piston spring). Thus, the range in which the control rod force is larger than F_knick corresponds to the less steep section of the characteristic curve. The inflection point of the characteristic curve can be determined as a function of the design of the trailing piston spring  44 ,  52 . 
     By virtue of the servo-assistance device  10  according to the invention a characteristic curve with a “bend” can be produced without changing the position of the control rod  20  and the valve piston  40 ,  48  connected fixed to the control rod  20 . The control rod  20  undergoes only a minimal adjustment movement which is not perceptible by a vehicle driver with his hand on the shift lever. 
     At the beginning of a shift operation sufficient servo-assistance force is ensured, such that during a synchronous phase a servo-assistance force is produced which does not overload the components involved in the shift or synchronization operations. 
     INDEXES 
     
         
           2  Shifting unit 
           4  Shift lever 
           6  Shift rod 
           8  Lever linkage 
           10  Servo-assistance 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 
           34  Cylinder 
           36  Piston 
           38  Valve slide 
           40  Valve piston 
           42  Trailing piston 
           44  Trailing piston spring 
           46  Valve slide 
           48  Valve piston 
           50  Trailing piston 
           52  Trailing piston spring 
           54  Spring element 
           56  Valve 
           58  Reservoir pressure 
           60  Controlled servo-pressure