Abstract:
A multi-speed transmission having four gearsets, eight shafts and six shifting elements. The sun gear of gearset (P 2 ) couples, via shaft ( 5 ) and brake ( 05 ), the housing (G), and, via clutch ( 15 ) the drive shaft. The carriers of gearsets (P 2 , P 3 ) communicate, via shaft ( 4 ) and brake ( 04 ), with housing (G), and the sun gear of gearset (P 3 ) communicates, via shaft ( 8 ) and clutch ( 18 ), with the drive shaft. The ring gears of gearsets (P 2 , P 3 ) communicate, via shaft ( 6 ), with the ring gear of gearset (P 4 ) and the sun gear of gearset (P 1 ). The ring gear of gearset (P 4 ) and the sun gear of gearset (P 1 ) are connected with one another. The sun gear of gearset (P 4 ) communicates, via shaft ( 3 ) and brake ( 03 ), with housing (G). The carriers of gearsets (P 1 , P 4 ) communicate, via shaft ( 7 ) and clutch ( 17 ), with the drive shaft and the output shaft couples the ring gear of gearset (P 1 ).

Description:
This application claims priority from German patent application serial no. 10 2008 041192.2 filed Aug. 13, 2008. 
     FIELD OF THE INVENTION 
     The invention relates to a multi-step planetary transmission, in particular an automatic transmission for a motor vehicle. 
     BACKGROUND OF THE INVENTION 
     In the state of the art, automatic transmissions, particularly for motor vehicles, are comprised of planetary gear sets that are shifted by means of friction elements or, as the case may be, shifting elements such as clutches and brakes and are usually connected to a starting element that is subject to a slip effect and are optionally connected to a lockup clutch, such as a hydrodynamic torque converter or a fluid clutch. 
     Automatic transmissions of this kind are known, for example, from DE 199 49 507 A1 by the applicant: a multi-step transmission in which, on the driveshaft, two non-shiftable, front-mounted gear sets are provided, which on the output side produce two speeds, which in addition to the speed of the driveshaft, can optionally be shifted using a shiftable double planetary gear set that acts on the driveshaft by means of selective engagement of the shifting elements, so that in order to shift from one gear to the next higher or lower gear, only one of the two previously actuated shifting elements must in each case be engaged or disengaged. In this way, by using five shifting elements, seven forward gears can be achieved, and by using six shifting elements, nine or ten forward gears can be achieved. 
     Furthermore, from DE 102 13 820 A1, a multi-step automatic transmission with eight forward gears and one reverse gear is known, which comprises a first input path T1 of a first gear ratio; an input path T2 that has a greater gear ratio than the first input path T1; a planetary gear set of the Ravigneaux type with four elements, wherein the order of the four elements in a rotational speed diagram are: a first element, a second element, a third element, and a fourth element; a clutch C-2 that transmits rotation of the input path T2 to the first element S3; a clutch C-1 that transmits rotation of the input path T2 to the fourth element; a clutch C-4 that transmits rotation of the input path T1 to the first element; a clutch C-3 that transmits rotation of input path T1 to the second element C3; a brake B-1 that engages the fourth element; a brake B-2 that engages the second element; and an output element that is coupled to the third element S3. 
     Furthermore, a 9-gear multi-step transmission is known from DE 29 36 969 A1. It comprises eight shifting elements and four gear sets, wherein one gear set serves as a front-mounted transmission, and the main transmission has a Simpson set and an additional gear set that serves as a reversing gear. 
     Other multi-step transmissions are known from the applicant&#39;s DE 102005010210 A1 and DE102006006637 A1. 
     Automatically shiftable planetary vehicle transmissions in general have already been described many times in the state of the art and are subject to constant development and improvement. These transmissions should therefore require less complexity of construction, in particular a smaller number of shifting elements, and with sequential shifting, it should be possible to avoid double shifting, so that during shifting within defined gear groups, only one shifting element is engaged at any given time. 
     From the applicant&#39;s not yet published DE 102008000428.3, a multi-step planetary transmission is known, which has a drive and an output that are arranged in a housing. In the case of this known transmission, at least four planetary gear sets are provided, henceforth referred to as first, second, third, and fourth planetary gear set; at least eight rotatable shafts, henceforth referred to as driveshaft, output shaft, third, fourth, fifth, sixth, seventh, and eight shaft; as well as at least six shifting elements, comprising brakes and clutches, whose selective engagement produces different gear ratios between the drive and the output, so that preferably nine forward gears and one reverse gear can be achieved. 
     In this way, the first and the second planetary gear set, which are preferably designed as minus planetary gear sets, form a shiftable front-mounted gear set, and the third and the fourth planetary gear set form a main gear set. 
     In the known multi-step transmission, it is provided that the carriers of the first and second planetary gear set are coupled to each other via the fourth shaft, which is connected to an element of the main gear set; that the ring gear of the first planetary gear set is coupled to the sun gear of the second planetary gear set via the eighth shaft, which can be detachably connected via a first clutch with the driveshaft; and that the sun gear of the first planetary gear set can be coupled by means of the third shaft via a first brake to a housing and can be detachably connected via a second clutch with the driveshaft, in which case the ring gear of the planetary gear set can be coupled by means of the fifth shaft via a second brake to a housing of the transmission. In addition, the seventh shaft is permanently connected to at least one element of the main transmission and can be coupled via a third brake to a housing of the transmission, in which case the sixth shaft is permanently connected with at least one additional element of the main gear set and can be detachably connected via a third clutch with the driveshaft; the output shaft is permanently connected to at least one additional element of the third gear set. 
     In the known transmission, the fourth shaft is preferably permanently connected to the ring gear of the planetary gear set, in which case the sixth shaft is permanently connected to the ring gear of the fourth planetary gear set and the carrier of the third planetary gear set, and can be detachably connected via the third clutch to the driveshaft. In addition, the seventh shaft is permanently connected to the sun gears of the third and fourth planetary gear set and can be coupled via the third brake to a housing of the transmission. The output is supplied by means of the output shaft being permanently connected to the carrier of the fourth planetary gear set. In addition, the third and fourth planetary gear can be combined into, or, as the case may be, reduced to a Ravigneaux set with a common carrier and a common ring gear. 
     SUMMARY OF THE INVENTION 
     The object of the present invention is to propose a multi-step transmission of the type initially cited, which has at least nine forward gears and at least one reverse gear with sufficient gear ratio, in which the complexity of construction and installation size, in particular the installation length or, as the case may be, installation weight, are optimized, and in addition, the degree of efficiency is improved as far as drag and gearing loss are concerned. In addition, in the case of the multi-step transmission according to the present invention, there should be low torque ratios acting on the shifting elements. In particular, the transmission according to the present invention should be suitable for a front/transverse construction. 
     In them, a multi-step planetary transmission according to the present invention is disclosed, which has a drive and an output that are arranged in a housing. In addition, at least four planetary gear sets are provided, henceforth referred to as first, second, third, and fourth planetary gear set; at least eight rotatable shafts, henceforth referred to as driveshaft, output shaft, third, fourth, fifth, sixth, seventh, and eighth shaft; as well as at least six shifting elements comprising brakes and clutches, whose selective engagement produces different gear ratios between the drive and the output, so that preferably nine forward gears and one reverse gear can be achieved. The second and third planetary gear set are preferably combined into, or, as the case may be, reduced to one Ravigneaux set with a common carrier and a common ring gear. 
     According to the present invention, the second and the third planetary gear set form a shiftable front-mounted gear set, wherein the first and the fourth planetary gear set form the main gear set, in which the sun gear of the second planetary gear set, by means of the fifth shaft, can be coupled via a first brake to the housing and can be detachably connected via a first clutch with the driveshaft, in which case the carriers of the second and third planetary gear set can be coupled by means of the fourth shaft via a second brake to the housing, and the sun gear of the third planetary gear set can be detachably connected via the eighth shaft and a second clutch to the driveshaft. 
     Furthermore, the ring gears of the second and third planetary gear set are connected by means of the sixth shaft with the ring gear of the fourth planetary gear set and the sun gear of the first planetary gear set; the ring gear of the fourth planetary gear set and the sun gear of the first planetary gear set are connected to one another. According to the present invention, the sun gear of the fourth planetary gear set can be coupled by means of the third shaft via a third brake to a housing of the transmission, in which case the carriers of the first and the fourth planetary gear set can be detachably connected to the driveshaft by means of the seventh shaft and a third clutch, and in which case the driveshaft is connected to the ring gear of the first planetary gear set. 
     The embodiment according to the present invention of the multi-step transmission produces gear ratios that are suitable particularly for passenger cars, as well as produce a considerable increase in the overall transmission ratio spread of the multi-step transmission, which results in improved driving comfort and a significant reduction in consumption. 
     In addition, with the multi-step transmission according to the present invention, the complexity of construction is considerably reduced due to the small number of shifting elements. With the multi-step transmission according to the present invention, it is advantageously possible to start the vehicle with a hydrodynamic converter, an external starting clutch, or with other suitable external starting elements. It is also potentially possible to enable a starting operation by means of a starting element that is integrated into the transmission. A shifting element is preferred that is actuated in a first forward gear and in reverse gear. 
     What is more, with the multi-step transmission according to the present invention, there is a good degree of efficiency in the main driving gears with regard to drag and gearing losses. 
     Furthermore, there are low torque ratios in the shifting elements and in the planetary gear sets of the multi-step transmission, which advantageously reduces wear in the multi-step transmission. In addition, the low torque ratios allow correspondingly smaller dimensions, which leads to a reduction in the required installation space and a corresponding reduction in the costs. Moreover, there are also low speeds in the shafts, the shifting elements, and the planetary gear sets. 
     Besides, the transmission according to the present invention is conceived in such a way that it can be adapted to different drivetrain embodiments, both as regards the direction of power flow and also in terms of dimensions. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be described in more detail below on the basis of the figures. They show: 
         FIG. 1 : A schematic view of a preferred embodiment of a multi-step transmission according to the present invention; 
         FIG. 2 : An example of a shift pattern for a multi-step transmission according to  FIG. 1 , 
         FIG. 3 : A diagram illustrating the progressive ratio phi corresponding to a respective transmission ratio i of a gear in the transmission shown in  FIG. 1 ; and 
         FIG. 4 : A diagram illustrating the degree of efficiency corresponding to the respective gear for the transmission shown in  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  illustrates a multi-step transmission according to the present invention with a driveshaft  1 , an output shaft  2 , and four planetary gear sets P 1 , P 2 , P 3 , and P 4 , which are arranged in a housing G. In this case, the second and third planetary gear set P 2 , P 3  form a shiftable front-mounted gear set; the first and the fourth planetary gear set P 1 , P 4  form the main gear set. 
     Here the first, the second, and the fourth planetary gear set are designed as minus planetary gear sets, in which the third planetary gear set P 3  is designed as a plus planetary gear set. As is known, a simple minus planetary gear set comprises a sun gear, a ring gear, and a carrier, on which the planet gears, which mesh respectively with sun gear and ring gear, are rotatably mounted. In this way, the ring gear, with the carrier held in place, rotates in the opposite direction of the sun gear. In contrast, a simple plus planetary gear set comprises a sun gear, a ring gear, and a carrier, on which internal and external planet gears are rotatably mounted, in which case all internal planet gears mesh with the sun gear and all external planet gears with the ring gear, in which case each internal planet gear respectively meshes with one external planet gear. In this way, the ring gear, with the carrier held in place, has the same direction of rotation as the sun gear. 
     In the example shown in  FIG. 1 , the second and the third planetary gear set P 2 , P 3  are combined into, or, as the case may be, reduced to one Ravigneaux set with a common carrier and a common ring gear. 
     According to further embodiments, the planetary gear sets P 2  and P 3  can be designed as separate planetary gear sets. In the shown exemplary embodiment, which is particularly suitable for a front/transverse design, the planetary gear sets P 1 , P 4 , which form the main set, seen axially, are arranged on the same gear set level, in which case the first planetary gear set P 1 , seen radially, is arranged above the fourth planetary gear set P 4 , which results in the total of four planetary gear sets P 1 , P 2 , P 3 , P 4 , seen in spatial terms from an axial perspective, are limited to three gear-set levels. Here, the ring gear of the fourth planetary gear set P 4  is connected to the sun gear of the first planetary gear set P 1 . 
     As can be seen from  FIG. 1 , six shifting elements are provided, namely three brakes  03 ,  04 ,  05 , and three clutches  15 ,  17 , and  18 . The spatial arrangement of the shifting elements can be arbitrary and is only limited by the dimensions and external shape. 
     With these shifting elements, selective shifting of nine forward gears and one reverse gear can be achieved. The multi-step transmission according to the present invention has a total of eight rotatable shafts, namely the shafts  1 ,  2 ,  3 ,  4 ,  5 ,  6 ,  7 , and  8 , in which case the driveshaft comprises the first shaft and the output shaft the second shaft of the transmission. 
     According to the present invention, in the multi-step transmission according to  FIG. 1 , it is provided that the sun gear of the second planetary gear set P 2  can be coupled by means of the fifth shaft  5  via a first brake  05  to the housing G of the transmission and detachably connected via first clutch  15  to the driveshaft  1 , in which case the carrier of the Ravigneaux set, or, as the case may be, the carrier of the second and the third planetary gear set P 2 , P 3 , can be coupled by means of the fourth shaft  4  via a second brake  04  to the housing G of the transmission. Furthermore, the sun gear of the third planetary gear set P 3  can be detachably connected via the eighth shaft  8  and a second clutch  18  to the driveshaft  1 . 
     According to the invention, the ring gear of the Ravigneaux set, or, as the case may be, the ring gears of the second and third planetary gear set P 2 , P 3 , are connected by means of the sixth shaft  6  to the ring gear of the fourth planetary gear set P 4  and the sun gear of the first planetary gear set P 1 , in which case the ring gear of the fourth planetary gear set P 4  and the sun gear of the first planetary gear set P 1  are preferably connected to one another by means of gear teeth. Furthermore, the sun gear of the fourth planetary gear set P 4  can be coupled by means of the third shaft  3  via a third brake  03  to the housing G of the transmission, in which case the carriers of the first and the fourth planetary gear set P 1 , P 4  are detachably connected by means of the seventh shaft  7  and a third clutch  17  to the driveshaft  1 , and in which case the output shaft  2  is connected to the ring gear of the first planetary gear set P 1 . 
     Seen radially, the first clutch  15  is preferentially arranged below the second planetary gear set P 2 ; the second clutch  18 , seen radially, is arranged below the third planetary gear set P 3 . In addition, the first and second brake  05 ,  04 , seen axially, are arranged side by side, in which case the three clutches  15 ,  18 , and  17 , seen axially, are arranged in the order: “first clutch  15 —second clutch  18 —third clutch  17 .” 
       FIG. 2  shows an exemplary shift pattern of a multi-step transmission according to  FIG. 1 . For each gear, three shifting elements are engaged. The respective ratios i of the individual gear ratios and the gear transmissions determined on that basis, or, as the case may be, the progressive ratios phi to the next higher gear, can be derived from the shift pattern, in which case the value 10.016 represents the spread. 
     Typical values for the standard ratios of the planetary gear sets P 1 , P 2 , P 3 , and P 4  are respectively −1.60, −1.80, 2.50, and −2.20. It can be seen from  FIG. 2  that by shifting sequentially, double shifts, or, as the case may be, group shifts, can be avoided, because two adjacent gear ratios use two shifting elements in common, in which case a total of nine forward gears and one reverse gear can be achieved. It is also seen that a wide spread is achieved with small gear transition ratios. 
     The second clutch  18  and/or the third brake  03  are preferably designed as claw-type shifting elements. That reduces the drag torque of the transmission, which is generated due to the short distance between the friction surfaces of a frictional shifting element in an unengaged state and the oil, or, as the case may be, the air/oil mixture contained in it. The other clutches  15 ,  17  and the other brakes  04 ,  05  are preferably designed as frictional shifting elements, or as the case may be disk shifting elements. 
     The first forward gear is produced by engaging the second clutch  18  and the second and third brake  04  and  03 ; the second forward gear by engaging the first and third brake  05 ,  03 , and the second clutch  18 ; the third forward gear by engaging the third brake  03  and the first and second clutch  15 ,  18 ; the fourth forward gear by engaging the third brake  03  and the third and second clutch  17 ,  18 ; the fifth forward gear by engaging the three clutches  15 ,  17 , and  18 ; the sixth forward gear by engaging the brake  05  and the third and second clutch  17 ,  18 ; the seventh forward gear by engaging the second brake  04  and the third and second clutch  17 ,  18 ; the eighth forward gear by engaging the first and second brake  05 ,  04  and the third clutch  17 ; and the ninth forward gear is achieved by engaging the second brake  04  and the first and third clutch  15 ,  17 . 
     As can be seen from the shift pattern, the reverse gear is achieved by engaging the third and second brake  03 ,  04  and the first clutch  15 . 
     Because the third and second brake  03 ,  04  are engaged in the first forward gear and in the reverse gear, these shifting elements (designed as frictional shifting elements, or, as the case may be, as disk shifting elements) can be used as starting elements. 
     According to the present invention, different gear transition ratios can also be produced using the same shift pattern, depending on the gearshift logic, thereby enabling variations specific to use, or, as the case may be, vehicle. 
     A diagram illustrating the progressive ratio phi corresponding to the respective gear ratio i of a gear for the transmission shown in  FIG. 1  that corresponds to the values shown in  FIG. 2  is the subject of  FIG. 3 . 
     On the basis of the diagram shown in  FIG. 4 , which shows the degree of efficiency eta depending on the respective gear of an transmission according to the present invention, it is obvious that the configuration of the second and the third planetary gear set as a Ravigneaux set results in an improvement in the degree of efficiency in the second and in the sixth forward gear. 
     According to the present invention, it is possible to provide additional freewheels at any suitable site in the multi-step transmission, for example between a shaft and the housing, or for the purpose of connecting two shafts, if required. 
     According to the present invention, an axle differential and/or a distributor differential can be arranged on the drive side or on the output side. 
     In the context of an advantageous further development, the driveshaft  1  can be disconnected from a drive engine by means of a clutch element, as required, in which case a hydrodynamic converter, a hydraulic clutch, a dry starting clutch, a wet starting clutch, a magnetic powder clutch, or a centrifugal clutch can be used as a coupling element. It is also possible to arrange a starting element of that type in the direction of the power flow behind the transmission, in which case the driveshaft  1  is permanently connected to the crankshaft of the engine. 
     What is more, the multi-step transmission according to the present invention enables the arrangement of a torsion vibration damper between the engine and the transmission. 
     In the context of an additional embodiment of the invention, which is not shown, a wear-free brake, for example a hydraulic or electric retarder or similar, can be arranged on each shaft, preferably on the driveshaft  1 , which is particularly significant for use in commercial motor vehicles. Furthermore, a power take-off can be provided for driving additional assemblies on each shaft, preferably on the driveshaft  1  or the output shaft  2 . 
     The frictional shifting elements employed in the design can be power shift clutches or brakes. In particular, friction-locking clutches or brakes can be used, such as disk clutches, band brakes, and/or conical clutches. 
     An additional advantage of the multi-step transmission presented here is that an electric machine can be mounted on each shaft as a generator and/or additional electrical machine. 
     It is self-evident that every constructive variation, in particular every spatial arrangement of the planetary gear set and the shifting elements as such and in relation to each other, as far as technically feasible, is covered by the scope of protection of the existing claims, without influencing the function of the transmission as specified in the claims, even if these variations are not explicitly illustrated in the figures or mentioned in the description. 
     Reference Numerals 
     
         
           1  first shaft, driveshaft 
           2  second shaft, output shaft 
           3  third shaft 
           4  fourth shaft 
           5  fifth shaft 
           6  sixth shaft 
           7  seventh shaft 
           8  eighth shaft 
           03  third brake 
           04  second brake 
           05  first brake 
           15  first clutch 
           17  third clutch 
           18  second clutch 
         P 1  first planetary gear set 
         P 2  second planetary gear set 
         P 3  third planetary gear set 
         P 4  fourth planetary gear set 
         i gear ratio 
         phi progressive ratio 
         G housing 
         eta degree of efficiency