Abstract:
A two-step multi-step transmission in which a first planetary gear set ( 1 ) has two radially nested transmission sub-assemblies. The first planetary gearset has an inner sun gear ( 11 ), inner planetary gears ( 12 ), an outer sun gear ( 14 ) which is rotationally fixed to the inner ring gear ( 13 ), outer planetary gears ( 15 ), an outer ring gear ( 16 ), and a carrier ( 17 ) on which the inner planetary gears ( 12 ) and the outer planetary gears ( 15 ) are mounted. The inner ring gear ( 13 ) and the outer sun gear ( 14 ), respectively, are rotationally fixed to a second, traditionally designed planetary gear set ( 2 ) and can be locked via a brake ( 42 ).

Description:
This application is a National Stage completion of PCT/EP2013/055056 filed Mar. 13, 2013, which claims priority from a German patent application serial no. 10 2012 207 017.6 filed Apr. 27, 2012. 
     FIELD OF THE INVENTION 
     The invention concerns a multi-stage transmission in a planetary construction, in particular for a motor vehicle, comprising:
         a first planetary gear set having an inner sun gear, planetary gears which mesh with the inner sun gear, an inner ring gear which meshes with the inner planetary gears, an outer sun gear which is connected in a rotationally fixed manner with the inner ring gear, outer planetary gears which mesh with the outer sun gear, an outer ring gear which meshes with the outer planetary gears, and a carrier on which the inner planetary gears and outer planetary gears are mounted, and   a second planetary gear set with a sun gear, inner planetary gears which mesh with the sun gear, outer planetary gears which mesh with the inner planetary gears, and a ring gear which meshes with the outer planetary gears, and a carrier on which the planetary gears are mounted.       

     Such multi-gear transmissions are preferably utilized as automatic transmissions for motor vehicles, whereby the effective flow of force in the respective gear steps within the planetary sets is defined through selective activation of the shift elements. Hereby, the planetary sets in an automatic transmission are also usually connected to a starting element, if desired with a bridging clutch, which provides a slippage effect, such as for instance a hydrodynamic torque converter or a turbo coupling. 
     BACKGROUND OF THE INVENTION 
     A two-step multistep transmission of the previously discussed type, in which a first planetary gear set comprises of two radially nested transmission sub-assemblies, which is coupled with a second planetary set in a traditional construction, is known through U.S. Pat. No. 5,429,557. A disadvantage in this known multi step transmission is the nesting of the particular parts, especially the coupling of the two gear sets is in comparison rather complex. 
     SUMMARY OF THE INVENTION 
     Therefore, the task of the invention is to show an improved multistep transmission and in particular, the coupling between the two planetary gear sets needs to be simplified. 
     The task of the invention is solved through a two-step multistep transmission of the above mentioned art, in which
         the sun gear of the first planetary gear set can be coupled through a first clutch with a drive,   the carrier of the second planetary gear set can be coupled by a first brake with the housing,   the sun gear of the second planetary gear set is connected in a rotationally fixed manner with the drive and the ring gear of the second planetary gear set is connected in a rotationally fixed manner with the inner ring gear and the outer sun gear of the first planetary gear set,   the inner ring gear and the outer sun gear of the first planetary gear set can be coupled through a second brake with the housing,   the carrier of the first planetary gear set can be coupled through a third brake with the housing,   the carrier of the first planetary gear set can be coupled through a second clutch with the drive, and   the outer ring gear of the first planetary gear set is connected in a rotationally fixed manner with the output.       

     Through the selected configuration, it can be achieved that the shift elements, meaning the brakes and clutches at the transmission side, are well accessible from the outside. Thus, actuators such as for instance electric motors, hydraulic valves, hydraulic pumps or similar can be positioned near the shift elements which reduces mechanical and/or hydraulic losses, and less energy might be needed for the actuation of the shift elements. Also, less energy might be needed to keep the shift element in each of its actual operating modes, meaning disengaged or engaged, where, for instance, the pressure losses are also lower through shorter pipes. It is also possible to position the shift elements on the housing and thus at least partially rotationally fixed, whereby gaskets which connect a fixed pipe with a rotating pipe, can be completely or partially omitted. 
     Also, the spatial positioning of the well accessible shift elements simplifies the replacement of the traditional, hydraulically actuated disc clutches or disc brakes with, for instance, electro-mechanically or electro-hydraulically actuated brakes and clutches, which in comparison can be actuated very simply. Easily accessible shift elements are, on one hand, brakes, which connect a shaft with the housing in a rotationally fixed manner, but also shift elements and outer positioned shafts of the multistep transmission, preferably at a drive shaft or output shaft, where in comparison the needed hydraulic fluid for the activation can easily be supplied. 
     Beside this characteristic, the named multistep transmission also has at the same time good gear efficiency, low stress on parts, in particular low planetary set and shift element torques, low absolute and relative rotational speeds, as well as a lower need for construction space. The latter enables the multistep transmission to be realized with lower weight and lower cost. In particular, the length of the proposed transmission is extremely short. Therefore, the multistep transmission is preferably constructed as a drive with a front-transverse configuration. In particular, the transmission can also be utilized, due to its short length, in hybrid drives (for instance combustion engines and electric machines). Finally, the multistep transmission also enables a good gear ratio sequence, meaning a well applicable gradation of the gear steps. 
     In general, the named multistep transmission has two planetary gear sets, two clutches, and three brakes. By actuating two shift elements (brakes and clutches), six forward gears and one reverse gear can be shifted. 
     A hydrodynamic torque converter, a hydrodynamic clutch, an additional starting clutch, an integrated starting clutch or brake, and/or an additional e-machine can serve as starting elements. 
     The geometric positioning (sequencing) of the individual gear sets and shift elements is freely selectable, as long as the specifications of the elements can handle it. Thus, the position of individual elements can be moved. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Additional, advantageous variations of the multistep transmission result from the description in connection with the drawings. For a better understanding of the invention, it is further explained based on the following drawings. Each show in a simplified format: 
         FIG. 1  a first schematically presented variation of a multistep transmission in accordance with the invention, and 
         FIG. 2  a section cut through a multistep transmission in accordance with the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     By way of introduction it should be noted that in the different embodiments described like parts have like reference numbers or same component names, the disclosure contained in the description can be usefully applied to the same parts with the same reference numbers or same component names. Also, the wording of positions which has been used in the description, such as for instance top, bottom, side etc., refers to the specific and presented drawings and can be applied accordingly to new positions in the case of a change of the position. In addition, also individual features or combinations of features of the presented and described drawings of the different embodiments can represent by themselves either inventive or the invention related solutions. 
       FIG. 1  shows a first schematically presented variation of a multistep transmission in accordance with the invention and with two planetary gear sets. 
     The first planetary gear set is comprised of an inner sun gear  11 , inner planetary gears  12  which mesh with the inner sun gear  11 , an inner ring gear  13  which meshes with the inner planetary gears  12 , and outer sun gear  14  which is connected in a rotationally fixed manner with the inner ring gear  13 , outer planetary gears  15  which mesh with the outer sun gear  14 , and an outer ring gear  16  which meshes which the outer planetary gears  15 , as well as a carrier  17  on which the inner planetary gears  12  and outer planetary gears  15  are mounted. 
     In this example, the second planetary gear set  2  is designed as a plus-gear set. It is therefore comprised of a sun gear  21 , inner planetary gears  22  which mesh with the sun gear  21 , the inner planetary gears  22  mesh with the outer planetary gears  23  which mesh with the ring gear  24 , as well as a carrier  25  on which the planetary gears  22 ,  23  are mounted, The chosen construction advantageously results in a relatively low rotational speed of the planetary gears  22  and  23  of the second planetary gear set. 
     In the multistep transmission of  FIG. 1 
         the sun gear  11  of the first planetary gear set  1  can be coupled, via a first clutch  31 , with a drive  51 ,   the carrier  25  of the second planetary gear set  2  can be coupled, via a first brake  41 , with the housing  53 ,   the sun gear  21  of the second planetary set  2  is connected in a rotationally fixed manner with the drive  51 , and the ring gear  24  of the second planetary gear set  2  is connected in a rotationally fixed manner with the inner ring gear  13  and with the outer sun gear  14  of the first planetary set  1 ,   the inner ring gear  13  and the outer sun gear  14  of the first planetary gear set  1  can be coupled, via a second brake  42 , with the housing  53 ,   the carrier  17  of the first planetary gear set  1  can be coupled, via a third brake  43 , with the housing  53 ,   the carrier  17  of the first planetary gear set  1  can be coupled, via a second clutch  32 , with the drive  51  and   the outer ring gear  16  of the first planetary gear set  1  is connected in a rotationally fixed manner with an output  52 .       

       FIG. 1  also presents a journal  54  which is positioned between the first planetary gear set  1  and the second planetary gear set  2 , a stator  55  of an electric machine which is connected in a rotationally fixed manner with the housing  53  of the multistep transmission, a rotor  56  of the electric machine is connected in a rotationally fixed manner with the drive  51 , and an optional vibration damper  57  which is added into the drive train. Finally, also a third clutch  33  is provided to selectively utilize the electric machine  55 ,  56 , or a combustion engine (not shown), for the creation of the movement. Naturally, the clutch  33  can be omitted if only the electric machine  55 ,  56  is provided on the drive side. Generally, the clutch  33  can also be omitted when a combustion engine is used, as it will be explained later. 
     The following gears can be shifted by the activation of the named elements
         a first gear through a first clutch  31  and third brake  43 ,   a second gear through first clutch  31  and second brake  42 ,   a third gear through first clutch  31  and first brake  41 ,   a fourth gear through first clutch  31  and second clutch  32 .   a fifth gear through second clutch  32  and first brake  41 ,   a sixth gear through second clutch  32  and second brake  42 , and   a reverse gear through first brake  41  and third brake  43 ,       

     wherein each of the elements not mentioned are not activated or rather disengaged. 
     In an especially preferred embodiment of the multistep transmission, the gear ratio is:
         of the first planetary gear set  1  between the inner sun gear  11  and inner ring  12  is equal to −2.5 and   of the first planetary gear set  1  between the outer sun gear  14  and outer ring gear  16  is equal to −1.46 and   of the second planetary gear set  2  between the sun gear  21  and the ring gear  24  equal to +2.5.       

     Hereby, an especially advantageous stepping of the gears arises. 
     All shift elements in principle can be friction or form-fitted. However, it is an advantage if, as presented in this example, the third brake  43  is designed as a claw brake and the first clutch  31  and the first brake  41  as a friction shift element. It is also hereby an advantage that the first clutch  31  and the first brake  41  are designed as starting elements. Thus, no frictional losses occur at the third brake  43 . At the same time, a starting element in the form of a seperate clutch  33 , as it is commonly used, can be omitted. The transmission is therefore constructed especially short. Referring to the above mentioned shift positions, the first clutch  31  is used as the starting element in the first gear and the first break  41  as the starting element in the reverse gear. Advantageous the support factor (ratio between the clutch or brake torque, respectively, and the transmission input torque) of the first clutch  31  in this transmission configuration is equal to 1, and the support factor of the first brake  41  is even reaching &lt;1. 
     It is also possible that the third brake  43  is designed as friction shift element and the first clutch  31  as a claw clutch. It is hereby also an advantage if the third brake  43  is designed as a starting element. Thus, no friction losses occur at the first clutch  31 . At the same time, the starting element in the form of a seperate clutch  33 , as they are commonly used, can again be omitted. Also in this case, the construction of the transmission is especially short. 
     In both above named cases, the application of claw clutches or claw brakes, respectively, is not necessarily mandatory. Instead, also the application of low load friction elements can be assumed. 
       FIG. 2  illustrates a sectional view of a multi-step transmission which relates to the schematically presented embodiment in  FIG. 1 . 
     In this example, a spur gear transmission step is again provided at the output side. It can also be seen in  FIG. 2  that the outer ring gear  16  of the first gear set  1  drives the first spur gear  58  which meshes with the second spur gear  59  and which has a journal  54 . In this construction of the multistep transmission, an advantage arises that, through the journal of the first spur gear  58 , the outer ring gear  16  of the first gear set  1  is axially secured. In addition and through the selected construction, the inner ring gear  13  of the first gear set  1  which is combined with the outer sun gear  14 , and the ring gear  24  of the second gear set  2 , can be axially secured without any large construction effort. 
     It is also particularly favorable when the connections  60 , for connecting an external oil supply of the transmission as shown in  FIG. 2 , are designed through one or more bores  61 , positioned in the driveshaft  51  and brought to the drive side, and several bores  62  brought to the output side. For instance, the flow of the pressure oil, lubrication oil, and/or lubrication oil can take place through one or several bore(s)  61  which are brought to the drive side, as well as one or several additional longitudinal bore(s)  62  in the driveshaft  51 . Thus, radial bores at the side facing away from the drives, as they are commonly used, can be omitted. Hereby, the construction length of the transmission can be further reduced. 
     REFERENCE CHARACTERS 
     
         
           1  First Planetary gear Set 
           2  Second Planetary gear Set 
           11  inner Sun Gear of the first planetary gear set 
           12  inner planetary gears of the first planetary gear set 
           13  inner ring gear of the first planetary gear set 
           14  outer sun gear of the first planetary gear set 
           15  outer planetary gears of the first planetary gear set 
           16  outer ring gear of the first planetary gear set 
           17  carrier of the first planetary gear set 
           21  sun gear of the second planetary gear set 
           22  inner planetary gears of the second planetary gear set 
           23  outer planetary gears of the second planetary gear set 
           24  ring gear of the second planetary gear set 
           25  carrier of the second planetary gear set 
           31  first clutch 
           32  second clutch 
           33  third clutch 
           41  first brake 
           42  second brake 
           43  third brake 
           51  drive/driveshaft 
           52  output 
           53  housing 
           54  journal 
           55  Stator electric machine 
           56  rotor electric machine 
           57  vibration damper 
           58  first spur gear 
           59  second spur gear 
           60  connections for oil supply 
           61  oil bore 
           62  oil bore