Abstract:
A dual clutch transmission includes a first and a second input shaft provided with a first and second input arrangement, respectively, a centre shaft, a countershaft, and a first and a second primary gear step. The first and second input shaft are coaxially arranged with the centre shaft and can transfer torque from the first and second input means to the countershaft via the first and the second primary gear step. The first primary gear step including a first input gearwheel and a first driven gearwheel. The first input gearwheel is rotationally fixed with the first input shaft. The first input gearwheel is arranged upon the centre shaft.

Description:
BACKGROUND AND SUMMARY 
       [0001]    The present invention relates to the arrangement of gearwheels in a dual clutch transmission, especially a dual clutch transmission for on-road vehicles. 
         [0002]    In a dual clutch transmission with coaxial input and output shafts, a small main pilot bearing is arranged between the centre shaft and the first input shaft. The pilot bearing is subjected to large axial forces in some gear speeds. The large axial forces originate from the helical gears, which when transferring torque are subjected to a gear mesh force. These large axial forces are a problem especially in gear speeds when there will be no relative rotation in the main pilot bearing. Large axial forces and no relative rotation are very unfavourable operating conditions for a small bearing. Fretting wear may occur, which will lead to premature damage of the bearing. 
         [0003]    U.S. Pat. No. 5,181,431 discloses example of prior art. 
         [0004]    A solution o the problem would be to increase the size of the bearing. There is however no room available for a larger bearing. 
         [0005]    It is desirable to provide an inventive gear arrangement in a dual-clutch transmission, said gear arrangement aims to decrease the load upon the pilot bearing in situations where there is no relative rotation in the bearing. 
         [0006]    According to an aspect of the invention, a dual clutch transmission comprises a first and a second input shaft provided with a first and second input means respectively, a centre shaft, a countershaft and a first and a second primary gear step. The first and second input means are normally a first and a second clutch. The first and second input shafts are coaxially arranged with the centre shaft and can transfer torque from said first and second input means to said countershaft via said first and said second primary gear step. This is a well known configuration, used in several dual clutch transmissions. 
         [0007]    The first primary gear step comprises a first input gearwheel and a first driven gearwheel, wherein said first input gearwheel is rotationally fixed with said first input shaft. According to the invention, the first input gearwheel is arranged upon said centre shaft, instead of upon the first input shaft as previously known. 
         [0008]    By arranging the first input gearwheel upon the centre shaft instead of the first input shaft, as previously known, axial gear mesh forces that act on the first primary gearwheel will be taken up and carried by the main shaft, whereby the pilot bearing is subjected to less axial load. 
         [0009]    It is preferred that the first input shaft is connected to the first input gearwheel through a connection allowing an axial play, whereby the connection not is able to transfer any axial load. The connection comprises a first part belonging to the input shaft and a second part belonging to the centre shaft. One such preferred connection is a spline coupling. A spline coupling could be integrated, whereby it comprises mating clutch teeth on the first input shaft and first primary cut gearwheel  332 . The spline coupling could alternatively comprise a bridging element that mates with clutch teeth on the first input shaft and the first primary cut gearwheel. However, the connection is always engaged. An alternative coupling could be axially directed dog clutch teeth. 
         [0010]    It is further preferred that said first input shaft arranging said first input gearwheel upon conical roller bearings, which can be subjected to relatively high axial loads. 
         [0011]    Alternatively, the first input gearwheel is arranged upon cylindrical roller bearings, which could be preferred in a cost perspective relative conical roller bearings. 
         [0012]    It is further advantageous that said first input gearwheel is arranged upon said centre shaft such, that it can be axially displaced between a distanced axial position and a contact axial position upon the centre shaft, but always rotationally fixed to said first input shaft. In the distanced axial position, the first input gearwheel is axially distanced from the first part of said connection and in the contact axial position, an axial contact between the first input gearwheel and the first part of said connection is enabled. In the contact position, axial forces acting upon the input gear wheel can thereby be transferred to the first input shaft, whereby the pilot bearing is further relived from axial load. 
         [0013]    It is foreseen that the first input gearwheel is displaced into said contact axial position when it transfers a torque load in a first direction. The axial displacement is due to gear mesh forces acting upon the first input gearwheel. 
         [0014]    The first input gearwheel and the first driven gearwheel are provided with helical gear teeth, whereby an axial displacement of the first input gearwheel, from said distanced axial position to the contact axial position, is actuated when the first input gearwheel transfers a torque load to the driven gearwheel. 
         [0015]    To achieve the axial sliding of the first input gearwheel it is mounted such upon a first and a second roller bearing that it can slide thereupon. The first input gearwheel is thereby provided with a radially inwards directed protrusion, which is arranged such between said first and second roller bearing that said axial displacement is allowed. The radially inwards directed protrusion thereby has an axial extension which is shorter than the corresponding distance between the first and the second roller bearing. 
         [0016]    In a further embodiment of the invention the first input gearwheel is provided with a synchronisation mechanism or a dog clutch enabling a rotational connection between said first input gearwheel and said centre shaft. By closing the synchronisation mechanism or the dog clutch a direct connection between the first input shaft and the centre shaft is achieved. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]    The present invention will now be described in detail with reference to the figures, wherein: 
           [0018]      FIG. 1  shows a dual clutch transmission according to the prior art; 
           [0019]      FIG. 2  shows a first embodiment of the inventive dual clutch transmission, and 
           [0020]      FIGS. 3   a, b  shows a second embodiment of the inventive dual clutch transmission. 
       
    
    
     DETAILED DESCRIPTION 
       [0021]      FIG. 1  shows a prior art dual clutch transmission  200 , when it is in engine drive. The dual clutch transmission  200  is arranged inside the main housing  102 , there are a first input shaft  221  and a second input shaft  222 . The first input shaft  221  can be rotationally driver by the first clutch disc set  213 , and the second input shaft  222  can be rotationally driven by the second clutch disc set  214 . 
         [0022]    According to the prior aft first primary gear teeth  232  are rotationally fixed to or integral with the first input shaft  221  and second primary gear teeth  230  are rotationally fixed to or integral with the second input shaft  222 . A centre shaft  224  is coaxial with the input shafts  221  and  222  and a countershaft  223  is parallel thereto. The second input shaft  222  is suspended in the clutch housing  201  by an input shaft bearing  225 . Between the centre shaft  224  and the first input shaft  221  a main pilot bearing  229  is arranged. An adequate suspension of the centre and input shafts is achieved with two bearings between the input shafts  221  and  222 . 
         [0023]    On the countershaft  223 , the second primary gearwheel  131  meshes with the second primary gear teeth  230  of the second input shaft  222 . A primary countershaft loose gearwheel  233  meshes with the first primary gear teeth  232  of the first input shaft  221 . A secondary countershaft loose gearwheel  235  meshes with the second secondary loose gearwheel  134  on the main shaft  224 . 
         [0024]    In  FIG. 1  the transmission  200  is shown with the top sixth gear active (dashed line  6   g ) and the fifth, direct, gear inactive. The primary countershaft loose gearwheel  233  is rotationally connected to the countershaft  223  by a first countershaft tooth clutch  248 , secondary countershaft loose gearwheel  235 , and second countershaft tooth clutch  249 . In addition, the direct tooth clutch  240  rotationally connects the first input shaft  221  and centre shaft  224 . In  FIG. 1  the second clutch disc set  214  is engaged. Power can thereby be transferred to the centre shaft  224  via the second input shaft  222 , second primary gear teeth  230 , second primary gearwheel  131 , countershaft  223 , second countershaft tooth clutch  249 , secondary countershaft loose gearwheel  235 , first countershaft tooth clutch  248 , primary countershaft loose gearwheel  233 , first primary gear teeth  232 , first input shaft  221 , and direct tooth clutch  240 , 
         [0025]    The arrows F 1 , F 2  show the axial gear mesh forces that act on the first primary gear teeth  232  and the second primary gear teeth  230 , respectively, when the engine is driving the vehicle. It can be concluded that the first input shaft  221  will be pushed to the right. Thereby, the small main pilot bearing  229  will be subjected to large forces. Since the direct tooth clutch  240  is engaged, there will be no relative rotation in the main pilot bearing  229 . Large axial forces and no relative rotation are very unfavourable operating, conditions for a small bearing. Fretting wear may occur, which will lead to premature damage of the bearing. 
         [0026]    Now, in the following embodiments of the invention are shown and described in conjunction with  FIG. 2  and  FIGS. 3   a  and  b,  simply by way of illustration of some modes of carrying out the invention. 
         [0027]      FIG. 2  shows a first embodiment of the inventive dual clutch transmission  300  in engine drive. The dual clutch transmission  300  comprises a first and a second input shaft  32   322  a centre shall  324  and a countershaft  223 . 
         [0028]    The inventive dual clutch transmission  300  corresponds to the prior art transmission  200  disclosed in  FIG. 1 . However, the first input shaft  321  is inventively not provided with any gear teeth, instead a first primary cut gearwheel  332  is provided on bearings  332   b,    322   c  and carried by the main shaft  324 . Further, the first primary cut gearwheel  332  is connected rotationally to the first input shaft  321  by a first primary spline coupling  321   s,  whereby an axial play is allowed in the spline coupling  321   s.  Axial gear mesh forces (arrow F 1 ) that act on the first primary cut gearwheel  332  will be carried by the main shaft  324  instead of the main pilot bearing  229 . Axial gear mesh forces (arrow F 2 ) that act on the second primary cut gearwheel  230  will be taken up by the bearing  225 . 
         [0029]    In  FIG. 2  the transmission  300  is shown operating with the sixth gear active ( 6   g ), whereby the dog clutch  240  is closed and there is no relative rotation in the main pilot bearing  229 . For the case of engine driving the vehicle, arrows F 1 , F 2  indicate the directions of the axial gear mesh forces F 1 , F 2  that act on the first primary cut gearwheel  332  and second primary gear teeth  230 , respectively. As can be seen, the main pilot bearing  229  is not subjected to axial gear mesh forces. Thereby, the risk of having fretting, wear damages is greatly reduced. 
         [0030]    The first primary cut gearwheel  332  is rotationally connected to the main shaft  324  through the dog clutch  240 , whereby, the bearings  332   b,    332   c  will not be subjected to relative rotation. The bearings  332   b    332   c  will carry the load from the gear mesh force F 1 , but they are considerably larger than the main pilot bearing  129 , whereby problems with fretting wear can be avoided. Hence, by separating the first primary cut gearwheel  332  from the first input shaft  321  and arranging it on the main shaft  324 , the risk for fretting wear damages can be avoided for the main pilot bearing  229 . 
         [0031]    The first primary spline coupling  321   s  provides a constant rotational connection between the first input shaft  321  and the first primary cut gearwheel  332 . The spline coupling  321   s  allows an axial play between the first primary cut gearwheel  332  and the first input shaft  321 . Axial forces can thereby not be transferred from the centre shaft  324  to the first input shaft  321  or vice versa. 
         [0032]    In conclusion, the dual clutch transmission  300  enables the use of the rear primary gear stage  233 ,  332  as a secondary gear stage, i.e., transfer power from the countershaft to the main shaft, and that without having the main pilot bearing  229  subjected to fretting wear. 
         [0033]    The most common way to suspend gearwheels that are simultaneously subjected to load and relative rotation is tapered roller bearings arranged opposed each other. Axial forces can be transferred in both directions between gearwheel and shaft. In  FIG. 2  the arrows F 1 , F 2  indicate axial gear mesh forces that act on the second primary gear teeth  230  and first primary cut gearwheel  332  when the engine is driving the vehicle. For opposite power flow, engine braking, the axial force directions will be reversed, whereby the main pilot bearing  229  could be subjected to large axial forces while still having no relative rotation therein. This poses a risk for fretting wear, but could be acceptable, since the engine braking torque is usually significantly smaller than the maximum driving torque, and engine braking is considerably less frequent than engine driving. 
         [0034]    However, a direct transfer of axial force from the first primary cut gearwheel  332  to the first input shaft  321  would be of advantage. Then, the main pilot bearing  229  could be relieved from axial gear mesh forces. 
         [0035]    In  FIGS. 3   a  and  b  an alternative embodiment of the inventive transmission is shown, in which said risk for fretting wear at engine braking is reduced. A modified dual clutch transmission  400  comprises an inventive first primary cut gearwheel  432  that is carried on the centre shaft  424  by cylindrical roller bearings  432   b, c.    
         [0036]    Cylindrical roller bearings may allow some relative axial displacement. However, in a carefully designed cylindrical bearing arrangement, the axial force transfer can be controlled. In the dual clutch transmission  400 , this is done in the following way: The first primary cut gearwheel  432  has an inwardly directed protrusion  440  between the cylindrical roller bearings  432   b,    432   c.  When the first primary cut gearwheel  432  is subjected to axial gear mesh forces directed to the right, as indicated by the arrow F 3  in  FIG. 3   a,  the inwardly directed protrusion  440  will come in contact with the right cylindrical roller bearings  432   c.  The axial gear mesh forces will then be transferred to the main shaft  424  and taken up by the bearing  128  between the centre shaft  424  and the housing  102 . This corresponds to the case of the engine driving the vehicle, in analogy with  FIGS. 1 and 2 . 
         [0037]    At engine braking, an arrow F 4  in  FIG. 3   b  shows the direction of the axial gear mesh force on the first primary cut gearwheel  432 . The gear mesh force will push the first primary cut gearwheel  432  to the left. The inwardly directed protrusion  440  is designed in such a way that it will not, come in contact with the left of the cylindrical roller bearings  432   b.  Instead, the first primary cut gearwheel  432  will come in axial contact with the first input shaft  321 , to which the axial gear mesh forces will be transferred directly. Thereby, the main pilot bearing  229  will be relieved from axial gear mesh forces also during engine braking. 
         [0038]    In the embodiments shown in  FIGS. 2 ,  3   a  and  3   b  the spline coupling  321   s  comprises mating clutch teeth on the first input shaft  321  and first primary cut gearwheel  332 ,  432 . This is illustrated schematically in  FIGS. 2 ,  3   a  and  3   b.    
         [0039]    Alternatively, the spline coupling  321   s  comprises a bridging element (not shown) that mates with clutch teeth on the first input shaft  321  and the first primary cut gearwheel  332 / 432 . 
         [0040]    Reference signs mentioned in the claims should not be seen as limiting the extent of the matter protected by the claims, and their sole function is to make claims easier to understand. 
         [0041]    As will be realised, the invention is capable of modification in various obvious respects, all without departing from the scope of the appended claims. Accordingly, the drawings and the description thereto are to be regarded as illustrative in nature, and not restrictive.