Abstract:
A multistage transmission with eight forward and one reverse gear, including input and output shafts, planetary gearsets, gear stages, shift elements and shafts. Input shaft couples the carrier of gearset (P 1 ) and, via clutch ( 15 ), can couple shaft ( 5 ) that couples the sun gear of gearset (P 3 ) and, via clutch ( 58 ), can couple shaft ( 8 ) connected to the ring gear of gearset (P 2 ). The ring gear of gearset (P 1 ) couples shaft ( 6 ) connected to the sun gear of gearset (P 2 ). Shaft ( 3 ) couples the sun and ring gears of respective gearsets (P 1 , P 3 ) and can couple, via brake ( 03 ), housing (G). The carrier of gearset (P 2 ) couples shaft ( 4 ) which couples gear stage (S 1 ) and can couple the output shaft. The carrier of gearset (P 3 ) couples shaft ( 7 ) which can couple, via gear stage (S 2 ) and clutch ( 27 ), the output shaft. Clutch ( 56 ) can couple shafts ( 5  and  6 ).

Description:
This application is a National Stage completion of PCT/EP2013/050340 filed Jan. 10, 2013, which claims priority from German patent application serial no. 10 2012 201 689.9 filed Feb. 6, 2012. 
     FIELD OF THE INVENTION 
     The present invention relates to a multi-stage transmission, in particular an automatic transmission for a motor vehicle. 
     BACKGROUND OF THE INVENTION 
     According to the prior art, automatic transmissions, particularly for motor vehicles, comprise planetary gear sets that are shifted using friction elements or shift elements such as clutches and brakes, and typically are connected to a start-up element, such as a hydrodynamic torque converter or a fluid coupling, that is subject to a slip effect and is provided optionally with a lock-up clutch. 
     Such automatic transmissions have been described numerous times in the prior art and are continually undergoing further development and improvement. These transmissions have a sufficient number of forward gears as well as at least one reverse gear, and a suitable transmission ratio which is highly favourable for motor vehicles with a high overall gear ratio as well as a favourable gear interval. Furthermore, they enable a high startup transmission ratio in the forward direction and contain a direct gear. In addition, an automatic transmission requires moderate complexity and few shift elements, wherein only one shift element is engaged and one shift element is disengaged during sequential shifting. 
     Such an automatic transmission is known, for example, from DE 199 12 480 B4 by the applicant. It comprises three single-carrier planetary gear sets, as well as three brakes and two clutches for shifting six forward gears and one reverse gear, an input shaft and an output shaft, wherein the carrier of the first planetary gear set is continuously connected to the ring gear of the second planetary gear set, and the carrier of the second planetary gear set is continuously connected to the ring gear of the third planetary gear set, and the input shaft is directly connected to the sun gear of the second planetary gear set. 
     Furthermore, in the known transmission, the input shaft is connectable by means of the first clutch to the sun gear of the first planetary gear set, and by means of the second clutch to the carrier of the first planetary set, wherein the sun gear of the first planetary gear set is connectable by means of the first brake to a housing of the transmission, and the carrier of the first planetary gear set is connectable by means of the second brake to the housing of the transmission, wherein the sun gear of the third planetary gear set is connectable by means of the third brake to the housing of the transmission. The input shaft of the transmission is continuously connected to the carrier of the third planetary gear set and the ring gear of the first planetary gear set. 
     SUMMARY OF THE INVENTION 
     The present invention addresses the problem of presenting a multi-stage transmission that, viewed axially, requires less installation space, thus enabling the transmission to be installed, and optionally, the transmission to be hybridized, as a front-transverse system. Furthermore, the efficiency is improved in regard to drag and gearing losses. 
     This object is attained according to the invention by the features described below. 
     Accordingly, a multi-stage transmission is proposed which has an input shaft and an output shaft that are disposed in a housing. Furthermore, the transmission comprises three planetary gear sets hereinafter termed the first, second and third planetary gear sets, at least eight rotatable shafts, hereinafter termed the input shaft, output shaft, third, fourth, fifth, sixth, seventh and eighth shafts, a first and a second spur gear stage in connection with a countershaft disposed parallel to the longitudinal axis of the planetary gear sets and serving as an output shaft for the transmission, and five shift elements comprising a brake and clutches, whose selective engagement produces different transmission ratios between the input shaft and the output shaft such that preferably eight forward gears and one reverse gear can be implemented. 
     The planetary gear sets are preferably designed as minus planetary gear sets. As is well known, a simple minus planetary gear set comprises a sun gear, a ring gear, and a carrier on which the planetary gears are rotatably carried, the planetary gears meshing with the sun gear and the ring gear. As a result, when the carrier is fixed, the ring gear has a direction of rotation that is opposite that of the sun gear. In contrast, a simple plus planetary gear set comprises a sun gear, a ring gear and a carrier, on which inner and outer planet gears are rotatably carried, wherein all inner planet gears mesh with the sun gear and all outer planet gears mesh with the ring gear, and each inner planet gear meshes with only one outer planet gear. As a result, when the carrier is fixed, the ring gear has the same direction of rotation as the sun gear, and a positive stationary transmission ratio results. 
     Furthermore, preferably two of the planetary gear sets of the transmission are disposed above one another from a radial perspective to thereby achieve a very short overall length, whereby the transmission can be installed in a front-transverse design in passenger cars. 
     Advantageously, the second planetary gear set, viewed radially, is disposed above the first planetary gear set, wherein the planetary gear sets, viewed axially, are disposed in the sequence of the first planetary gear set/second planetary gear set, third planetary gear set. In the context of additional embodiments of the invention, the axial sequence of the planetary gear sets can be arbitrary; in addition, the planetary gear sets can also be disposed sequentially from an axial perspective. 
     According to the invention, the input shaft is connected to the carrier of the first planetary gear set and, by means of a first clutch, is releasably connectable to the fifth shaft that is connected to the sun gear of the third planetary gear set and is releasably connected by means of a second clutch to the eighth shaft connected to the ring gear of the second planetary gear set, wherein the ring gear of the first planetary gear set is connected to the sixth shaft connected to the sun gear of the second planetary gear set. 
     Furthermore, the carrier of the second planetary gear set is connected to the fourth shaft, which is connected by means of the first spur gear stage to the output shaft. 
     Instead of, or in addition to, the first spur gear stage, the fourth shaft can be releasably connectable to the output shaft by means of a chain, belt or one or more other components. 
     Furthermore, the carrier of the third planetary gear set is connected to the seventh shaft which is releasably connectable to the output shaft by means of the second spur gear stage and a third clutch. The seventh shaft can be connected thereby to a first spur gear of the second spur gear stage whose second spur gear is releasably connectable by means of the third clutch to the output shaft. Alternatively, the seventh shaft can be releasably connectable by means of the third clutch to the first spur gear of the second spur gear stage, wherein the second spur gear of the second spur gear stage is connected to the output shaft. 
     Instead of, or in addition to, the second spur gear stage, the fourth shaft can be releasably connectable to the output shaft by means of a chain, belt or one or more other components. 
     The third shaft of the transmission is connected to the sun gear of the first planetary gear set and the ring gear of the third planetary gear set, and can be coupled by means of the brake to the housing of the transmission. 
     According to the invention, a fourth clutch is provided, the engaging of which can block the second planetary set. The fourth clutch is designed as a clutch which releasably connects the sixth shaft to the fifth shaft such that the second planetary gear set can be blocked by coupling the ring gear of the second planetary gear set to the sun gear of the second planetary gear set. 
     The embodiment of the multi-stage transmission according to the invention results in transmission ratios that are particularly suitable for passenger vehicles, and in a greater overall gear ratio of the multi-stage transmission, thereby improving driving smoothness and significantly reducing fuel consumption. 
     Furthermore, design complexity is significantly reduced with the multi-stage transmission according to the invention due to a low number of shift elements. Using the multi-stage transmission according to the invention, it is advantageously possible to perform a start-up using a hydrodynamic converter, an external start-up clutch, or any other suitable external start-up element. It is also conceivable to perform a start-up using a start-up element integrated in the transmission. Preferably, a shift element that is actuated in the first forward gear and in the reverse gear is suitable for this. 
     Moreover, the multi-stage transmission, according to the invention, results in good efficiency in the main drive gears with respect to drag losses and gearing losses. 
     Furthermore, low torques are present in the shift elements and in the planetary gear sets of the multi-stage transmission, thereby advantageously reducing wear in the multi-stage transmission. In addition, the low torques make it possible to utilize correspondingly low dimensions, thereby reducing the necessary installation space and related costs. Furthermore, the rotational speeds of the shafts, shift elements, and planetary gear sets are low. 
     In addition, the transmission according to the invention is designed such that the required installation space, viewed axially, is small, which enables hybridization of the transmission as a front-transverse system. 
     The output shaft is preferably connected to a differential that distributes the torque to the drive wheels. A planetary gear set can be disposed between the input shaft of the differential and the output shaft. This can increase the transmission ratio from the output axle to the output axle of the differential, whereby the transmission ratio of the aforementioned spur gear stages and/or mechanical components can be designed smaller, whereby design advantages such as smaller axial spacings or smaller tooth widths can be incorporated. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention is described in greater detail in the following, as an example, with reference to the attached figures. They show: 
         FIG. 1 : a schematic view of a preferred embodiment of a multi-stage transmission according to the invention; 
         FIG. 2 : a schematic view of a second preferred embodiment of a multi-stage transmission according to the invention; 
         FIG. 3 : an example of a shift pattern for a multi-stage transmission according to  FIGS. 1 and 2 ; 
         FIG. 4 : a diagrammatic view of an electric machine disposed on the input drive shaft of the multi-stage transmission according to the invention; 
         FIG. 5 : a diagrammatic view of an electric machine disposed axially parallel and connected to the input shaft according to the invention; and 
         FIG. 6 : a diagrammatic view of a clutch disposed between an internal combustion engine and the multi-stage transmission according to the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  shows a multi-stage transmission according to the invention with a first planetary gear set P 1 , a second planetary gear set P 2 , and a third planetary gear set P 3 , a first spur gear stage S 1  and a second spur gear stage S 2  in connection with a countershaft disposed parallel to the longitudinal axis of the coaxially-disposed planetary gear sets P 1 , P 2 , P 3 , the countershaft serving as an output shaft  2  of the transmission which is disposed in a housing G. 
     In the depicted example, the second planetary gear set P 2 , viewed radially, is disposed above the first planetary gear set P 1 , whereby the overall length of the transmission is advantageously reduced. Viewed axially, the planetary gear sets P 1 , P 2 , P 3  are disposed in the sequence of the first planetary gear set P 1 /second planetary gear set P 2 , third planetary gear set P 3 . 
     At least one planetary gear set of the transmission can be designed as a plus planetary gear set if the carrier and ring gear connection are exchanged and, simultaneously, the value of the stationary transmission ratio is increased by 1 in comparison to the design as a minus planetary gear set. 
     As shown in  FIG. 1 , five shift elements are provided, namely, one brake  03  and four clutches  15 ,  58 ,  27  and  56 . The spatial arrangement of the shift elements can be arbitrary, and is limited only by the dimensions of the outer design. The clutches and the brake of the transmission are preferably designed as friction shift elements or lamellar shift elements. 
     Selective shifting of eight forward gears and one reverse gear can be implemented using these shift elements. The multi-stage transmission according to the invention has a total of at least eight rotatable shafts, wherein the input shaft forms the first shaft  1 , and the output shaft forms the second shaft  2  of the transmission. 
     According to the invention, in the multi-staged transmission according to  FIG. 1 , the input shaft  1  is connected to the carrier of the first planetary gear set P 1  and, by means of a first clutch  15 , is releasably connectable to the fifth shaft  5  that is connected to the sun gear of the third planetary gear set P 3  and is releasably connected by means of a second clutch  58  to the eighth shaft  8  connected to the ring gear of the second planetary gear set P 2 , wherein the ring gear of the first planetary gear set P 1  is connected to the sixth shaft  6  connected to the sun gear of the second planetary gear set P 2 . The third shaft  3  of the transmission is connected to the sun gear of the first planetary gear set P 1  and the ring gear of the third planetary gear set, and can be coupled by means of the brake  03  to the housing G of the transmission. 
     With reference to  FIG. 1 , the carrier of the second planetary gear set P 2  is connected to the fourth shaft  4  that is connected to a first spur gear ST 1  of the first spur gear stage S 1  whose second spur gear ST 2  is connected to the output shaft  2 . 
     Furthermore, the carrier of the third planetary gear set P 3  is connected to the seventh shaft  7  which is connected to a first spur gear ST 3  of the second spur gear stage S 2  whose second spur gear ST 4  is releasably connected by means of the third clutch  27  to the output shaft  2 . 
     Furthermore, a fourth clutch  56  is provided in the transmission according to the invention and releasably connects the sixth shaft  6  to the fifth shaft  5 . Viewed axially, the first, second and fourth clutch  15 ,  58 ,  56  can be disposed next to each other and have a common outer clutch disk carrier. 
     The transmission shown in  FIG. 2  differs from the embodiment according to  FIG. 1  in that the seventh shaft  7  of the transmission is releasably connectable by means of the third clutch  27  to the first spur gear ST 3  of the second spur gear stage S 2 , wherein the second spur gear ST 4  of the second spur gear stage S 2  is connected to the output shaft  2 . 
     In the exemplary embodiments according to  FIGS. 1 and 2 , an additional spur gear stage S 3  is downstream from the transmission and is connected to a differential D of the vehicle. 
     Within the scope of one advantageous development of the invention, it is possible for only the second clutch  58  and third clutch  27  to be designed as friction shift elements capable of permanent slip, whereby measures against irregular rotation are irrelevant. 
       FIG. 3  shows an example of a shift pattern of a multi-stage transmission according to  FIGS. 1 and 2 . Three shift elements are engaged for every gear. The shift pattern shows, as examples, the particular transmission ratios i of the individual gear steps and, to be determined therefrom, the gear increments or step changes φ to the next higher gear, wherein the value 8.040 is the transmission ratio spread. 
     In the portrayed example, the values for the stationary transmission ratio of the planetary gear set designed as minus planetary gear sets P 1 , P 2 , P 3  are respectively −1,720, −1,630 and −1,580, wherein the transmission ratios of the first and second spur gear stage S 1 , S 2  are respectively 1,000 and 1,970.  FIG. 3  shows that only one shift element must be engaged and one shift element must be disengaged when shifting sequentially, since two neighbouring gear steps jointly use two shift elements. It is also shown that a large transmission ratio spread is attained with small gear increments. 
     The first forward gear is attained by engaging the brake  03  and the first and third clutch  15 ,  27 ; the second forward gear is attained by engaging the brake  03  and the third and fourth clutch  27 ,  56 ; the third forward gear is attained by engaging the first, third and fourth clutch  15 ,  27 ,  56 ; the fourth forward gear is attained by engaging the second third and fourth clutch  58 ,  27 ,  56 ; the fifth forward gear is attained by engaging the first, second and third clutch  15 ,  58 ,  27 ; the sixth forward gear, which is designed as a direct gear, is attained by engaging the first, second and fourth clutch  15 ,  58 ,  56 ; the seventh forward gear is attained by engaging the brake  03  and the first and second clutch  15 ,  18 ; and the eighth forward gear is attained by engaging the brake  03  and the second and fourth clutch  58 ,  56 ; wherein the reverse gear is attained by engaging the brake  03  and the second and third clutch  58 ,  27 . 
     Since the brake  03  and the third clutch  27  are engaged in the first forward gear and in the reverse gear, these shift elements can be used as start-up elements. 
     According to the invention, different gear increments also result from the same gear pattern depending on the shift logic, thereby making it possible to implement an application-specific or vehicle-specific variation. 
     In principle, an electric machine or an additional drive source can be disposed on each shaft of the transmission according to the invention. In a particularly advantageous development of the invention, an electric machine can be provided that is disposed on the input drive shaft  1  and is directly connected to the input drive shaft  1 , as shown in  FIG. 4 . A fifth clutch can be provided by means of which the internal combustion engine can be decoupled from the transmission, whereby all gears of the transmission can be advantageously driven exclusively electrically, as shown in  FIG. 6 . 
     As an alternative to directly connecting the electric machine to the input shaft, the electric machine can be disposed axially parallel to the input shaft  1  and connected to the input shaft  1  by means of a gear stage or a chain drive, diagrammatically shown as element D, as shown in  FIG. 5 , wherein a fifth clutch can be optionally provided by means of which the internal combustion engine can be decoupled from the transmission. 
     According to the invention, it is possible to provide additional freewheels at each suitable location of the multi-staged transmission, for example, between a shaft and the housing, or possibly to connect two shafts. 
     According to the invention, an axle differential and/or a distributor differential can be disposed on the drive side or on the output side. 
     Within the scope of advantageous developments, the drive shaft  1  can be separated from a drive motor, as needed, by a clutch element, wherein a hydrodynamic converter, a hydraulic clutch, a dry start-up clutch, a wet start-up clutch, a magnetic powder clutch, or a centrifugal clutch can be used as the clutch element. It is also possible to dispose such a start-up element in the power flow direction behind the transmission wherein, in this case, the drive shaft  1  is continuously connected to the crankshaft of the drive motor. 
     The multi-stage transmission, according to the invention, also makes it possible to situate a torsional-vibration damper between the drive motor and the transmission. 
     Within the scope of a further, not represented, embodiment of the invention, a wear-free brake, for instance, a hydraulic or electric retarder or the like, can be disposed on each shaft, preferably on the drive shaft  1  or the output shaft  2 , which is of special significance for use in commercial vehicles in particular. Furthermore, a power take-off drive can be provided on each shaft, preferably on the input shaft  1  or the output shaft  2 , for driving additional assemblies. 
     The friction shift elements that are used can be designed as power shiftable clutches or brakes. In particular, force locking clutches or brakes can be used, for instance, lamellar clutches, band brakes, and/or cone clutches. Furthermore, the individual shift elements can be designed as form-locking switch elements. 
     REFERENCE CHARACTERS 
     
         
           1  First shaft, input shaft 
           2  Second shaft, output shaft 
           3  Third shaft 
           4  Fourth shaft 
           5  Fifth shaft 
           6  Sixth shaft 
           7  Seventh shaft 
           8  Eighth shaft 
           03  Brake 
           15  First clutch 
           27  Third clutch 
           56  Fourth clutch 
           58  Second clutch 
         D Differential 
         G Housing 
         P 1  First planetary gear set 
         P 2  Second planetary gear set 
         P 3  Third planetary gear set 
         S 1  First spur gear stage 
         S 2  Second spur gear stage 
         S 3  Third spur gear stage 
         ST 1  First spur gear of the first spur gear stage S 1   
         ST 2  Second spur gear of the second spur gear stage S 1   
         ST 3  First spur gear of the second spur gear stage S 2   
         ST 4  Second spur gear of the second spur gear stage S 2   
         i Transmission ratio 
         φ Step change