Abstract:
A torsional vibration damper hub for a vehicle clutch includes a first row of springs extending substantially in the clutch rotating direction and disposed in openings formed in a first inner annular plate and a first pair of outer annular plates; a second row of springs extending substantially in the clutch rotating direction and is disposed in openings formed in the second inner annular plate and the second pair of outer annular plates; the first and second row of the springs being arranged on a same radius from a center of rotation of the clutch; the springs in the first row of springs arranged in one first axial level are axially overlapping the springs in the second row of springs arranged in a second axial level with the purpose to decrease a total axial length of the clutch. In an alternative embodiment the damper hub is substantially identical to a dual damper hub in conventional twin-plate clutches.

Description:
BACKGROUND AND SUMMARY 
     The present invention relates to vehicle transmissions, and more particularly to a system for improved performance of damper hubs in dry plate clutches. 
     Dry plate clutches are used in manual and automated vehicle transmissions to facilitate start-off from rest and disengage the transmission from the engine at gear shifts. In general, there is a damper hub integrated in a dry plate clutch. Adequately designed, this damper hub reduces torsional vibrations from the engine and spares the transmission. A damper hub usually has a number of helical springs arranged circumferentially on the driven disc that transfers torque from the engine flywheel to the input of the transmission. Some designs are shown in DE-10220205. 
     For heavy road vehicles, such as heavy trucks and buses, there has been a long-lasting trend towards more powerful engines. This poses technical challenges on the damper hubs to withstand higher engine torques and more severe torsional vibrations. For conventional simple damper hubs there is a limit on how large engine torques that can be handled. That limit has been increased by the use of helical springs of larger diameter that are located on a larger radius from the axis of rotation of the damper hub. Further such increases would imply a reduction of the friction lining area of the driven disc. That would, in turn, reduce the energy-absorbing ability as well as the life of the clutch. An alternative would be to increase the outer diameter of the clutch, which would make it difficult to fit the clutch in the chassis. Thus, these would be impractical ways to increase the torque capacity of the damper hub. 
     For higher engine torques, twin-disc clutches are frequently used. A typical design is shown in U.S. Pat. No. 6,782,985. Another example is shown by U.S. Pat. No. 1,935,459. In a twin-disc clutch there are two driven discs connected in parallel. Each of these discs has a damper hub. Thereby, each damper hub will be subjected to half the engine torque. Hence, very high engine torques can be handled by a twin-disc clutch. Unfortunately, twin-disc clutches are in general less attractive in terms of length, weight and cost. Moreover, compared to single-disc clutches they are more difficult to control. That makes twin-disc clutches unsuited for automated transmissions. 
     Another way to handle the torsional vibrations of powerful engines is by using a dual mass flywheel. In such a design, the flywheel is divided into two parts with a resilient damper in between. One design can be seen in WO-9427062. In a dual mass flywheel the damper springs can be located in a very efficient way. Thereby, it has the potential to handle large engine torques. It also allows the use of a single-disc clutch, which is of advantage for automated transmissions. On the other hand, dual mass flywheels are heavy and expensive. Since the flywheel is divided, the clutch is also likely to have reduced thermal capacity. 
     Another known solution for higher engine torques is to arrange two axially separated rows of springs in a single disc clutch. Thereby, the torsional vibration handling capacity of a twin-disc clutch is combined with the lower weight and cost along with ease of control of a single-disc clutch. Examples of such damper hubs in single disc clutches are disclosed in DE 19528319, WO9200470, U.S. Pat. No. 4,475,640, DE4040606 and U.S. Pat. No. 5,145,463. The cost for manufacturing these single-disc clutches are still higher compared to a conventional single-disc clutch with one row of springs. There is therefore a need to further slim the design. Another problem with known art is that two parallel rows of springs increases the axial length compared to a solution with a single row of springs. 
     So, there is a need for a single-disc dry plate clutch with increased ability to handle torsional vibrations but without the disadvantages regarding weight, cost and total axial length of prior art. In a first embodiment of the invention a design is provided where a torsional vibration damper hub for a vehicle clutch comprises a hub splined to a shaft; first and second inner annular plates rotatably fitted to an outer periphery of the hub, first and second pairs of outer annular plates arranged at both sides of said first and second inner annular plate respectively; first row of springs extending substantially in the clutch rotating direction and disposed in openings formed in said first inner annular plate and said first pair of outer annular plates; second row of springs extending substantially in the clutch rotating direction and disposed in openings formed in said second inner annular plate and said second pair of outer annular plates, each first and second pair of the outer annular plates being connected to the adjacent inner annular plate by said springs; said first and second pair of outer annular plate being connected to the hub; a torque input member including one single friction plate fixed to said first and second inner annular plate. The invention is characterized, according to an aspect thereof, in that the springs in said first row of springs arranged in one first axial level are axially overlapping the springs in said second row of springs arranged in a second axial level with the purpose to decrease a total axial length of said clutch. That will give a very compact design whose axial space requirements would not be significantly larger than for a conventional single-disc clutch. 
     In a second embodiment of the invention an axial distance between geometrical centres of one spring in said first row and one spring in said second row, is larger than one half of the outer diameter of said springs. In this context the diameter of said springs of said first and second row can be chosen to be substantially equal or substantially different, thus, in the latter case the axial total length of said clutch can be decreased even further. 
     In a further embodiment of said invention said damper hub is substantially identical to a dual damper hub in conventional twin-plate clutches. This embodiment of the invention can also be used in a torsional damper hub without having springs in a first and second row that are axially overlapping (as in the embodiment above), but instead having two rows of springs that are not axially overlapping. This would result in a fairly compact and cost-effective solution. 
     In a further embodiment of said invention said first and second row of said springs are arranged on a substantially same radius from centre of rotation of said clutch. Arranging said rows on the same radius would give the opportunity to maximize the possibility to handle high engine torques by the clutch. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be exemplified by means of the enclosed drawings. 
         FIG. 1  shows a schematic longitudinal section of a conventional single-disc dry plate clutch with an integrated damper hub according to prior art. 
         FIG. 2  shows an axial view of the conventional driven disc in  FIG. 1 . 
         FIG. 3  shows a typical relationship between torque and relative rotational displacement for a damper hub like the one in  FIG. 1 . 
         FIG. 4  shows a schematic longitudinal section of a conventional twin-disc clutch with a dual damper hub according to prior art. 
         FIG. 5  shows a schematic longitudinal section of a clutch whose damper hub has two axially separated rows of springs according to prior art. 
         FIG. 6  shows a schematic longitudinal section of a driven disc according to prior art whose damper hub is composed of two damper hubs from conventional single-disc clutches. 
         FIG. 7  shows a schematic longitudinal section of a driven disc according to the invention whose damper hub is substantially a dual damper hub from a conventional twin-disc clutch. 
         FIG. 8  shows a schematic longitudinal section of a driven disc according to the invention where the two rows of springs are partially overlapping each other axially. 
         FIG. 9  shows an alternative embodiment to the embodiment in  FIG. 8 , but where the damper hub is substantially a dual damper hub from a conventional twin-disc clutch. 
         FIG. 10  shows an axial view from the left of the driven disc of  FIG. 8  or  9 . 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows a simplified longitudinal section of a single-disc dry plate clutch  101  of prior art. There, a clutch cover assembly  102  is fastened to a flywheel  103  by means of screws  104 . The clutch cover assembly  102  is composed of a clutch cover  105 , a diaphragm spring  106  and a pressure plate  107 . The diaphragm spring has fingers  106   f  extending radially inwards. A coupling device (not shown) rotationally connects the pressure plate  107  to the clutch cover  105  and allows a limited axial relative motion. Furthermore, there is a driven disc  110  that is composed of a friction plate  111  and a damper hub  112 . An inner plate  113  of the damper hub  112  is connected fixedly to the friction plate  111 . Spring packs  114  are carried in windows  113   w  in the inner plate  113 . The spring packs  114  are also carried by outer plates  115  in corresponding windows  115   w . Each spring pack  114  can be composed of a single helical spring or of two or more helical springs placed inside each other. The outer plates  115  are connected via a symbolically shown pre-damper  116  to an inner hub  117 . Finally, the inner hub  117  is axially moveable but rotationally fixed to an input shaft  120  of a not shown transmission. 
     The clutch  101  is controlled by an actuator  121  whose piston  121   p  via a release bearing  122  pushes the fingers  106   f  of the diaphragm spring  106  for disengaging the clutch. 
       FIG. 2  shows an axial view of the driven disc  110 . It can be seen that there is a number of spring packs  114  arranged in corresponding windows  115   w  of the outer plates  115 . In general, the larger the number of spring packs is, the larger torques can be handled by the driven disc. 
     When the clutch  101  is engaged, the diaphragm spring  106  urges the pressure plate  107  to clamp the friction plate  111  of the driven disc  110  towards the flywheel  103 . Thereby, torque can be transferred from the flywheel  103  via the friction plate  111  to the inner plate  113 . A relative angular motion between the inner plate  113  and the outer plates  115  will compress the spring packs  114 . Thereby, at each instant the torque that is transferred is dependent on the compression of the spring packs  114 . A large torque corresponds to a large compression, and vice versa. On the outer plates  115 , the forces from the compressed spring packs  114  are carried by shoulders  115   s  between the windows  115   w . For strength reasons, the shoulders  115   s  must be fairly wide. This will limit the width and number of windows  115   w  and the torque that can be handled. The corresponding applies to windows  113   w  of the inner plate  113 . 
       FIG. 3  shows a typical relationship between transferred torque M and relative angular motion a between the inner plate  113  and the inner hub  117 . At low levels of torque, area  331 , the pre-damper  116  is active and allows a fairly large relative motion. That will reduce rattling noise from the gear meshes of the transmission when the engine is idling. At a certain point  332  the spring packs  114  start to compress. The transferred torque will then increase substantially linearly with the relative angular motion along line  333  up to a stop torque  334  where further compression of the spring packs  114  is mechanically blocked. The corresponding relative angular motion is referred to as the stop angle  335 . In order to handle higher engine torques, as indicated by line  336 , an increase is required of both the stop torque  334  and the stop angle  335 , that is, the compression of the spring packs  114 . Due to the strength reasons mentioned, such an increase is hardly feasible in a conventional single-disc clutch. 
       FIG. 4  shows a twin-disc dry plate clutch  401  according to prior art. There, two substantially identical driven discs  410   a ,  410   b  are connected to a common inner hub  417 . Each of the driven discs  410   a  and  410   b  has a damper hub  412   a ,  412   b  of similar design as the damper hub  112  in  FIG. 1 . A splined joint  417   s  allows a limited axial motion of the driven disc  410   b  relative the driven disc  410   a . Thereby, dimensional tolerances and wear can be compensated for. Furthermore, between the friction plates  411   a ,  411   b  of the driven discs  410   a ,  410   b  there is an intermediate pressure plate  408  that is rotationally connected to the clutch cover  405 . Basically, the number of spring packs is doubled in a twin-disc clutch  401  compared to a conventional single-disc clutch  101 . Thus, higher engine torques can be handled. However, a twin-disc clutch  401  is considerably more expensive and heavy. In addition, more axial space is required, and the increased inertia resulting from the two friction plates  411   a ,  411   b  burdens the shift system. Moreover, twin-disc clutches are known to be more difficult to control in a precise way. 
       FIG. 5  shows a single-disc clutch  501  according to prior art having two axially separate rows of spring packs  514   a ,  514   b  in the damper hub  512  of the driven disc  510 . The number of spring packs is doubled, giving a potential to handle large engine torques. In terms of weight and driven disc inertia, the clutch  501  substantially does not share the disadvantages of the twin-disc clutch  401 . On the flywheel  503 , the friction surface  503   f , which faces the friction plate  511  of the driven disc  510 , is axially separated from the abutment  503   c  that supports the clutch cover  505 . Then, clutch covers from single-disc clutches may be used. On the other hand the damper hub  512  is purposely designed for a single disc clutch with two rows of spring packs, which means increased manufacturing costs compared to a conventional single disc clutch with one row of spring packs. 
       FIG. 6  shows an embodiment according to prior art. The damper hub  612  of the driven disc  610  is composed of a first damper hub  612   a  and a second damper hub  612   b . The inner plates  613   a ,  613   b  of the damper hubs  612   a ,  612   b  are connected to the friction plate  611  by a connecting element  609 . Both damper hubs  612   a ,  612   b  are substantially identical to damper hubs in single-disc clutches. Thereby, high-volume parts can be used. The damper hubs  612   a ,  612   b  can be of equal or different size. If of different size, as shown in  FIG. 6 , they can be packaged in a more compact way. 
       FIG. 7  shows an embodiment of the invention. The damper hub  712  of the driven disc  710  is substantially identical to a dual damper hub from a twin-disc clutch, like in  FIG. 4 . A connecting element  709  joins the inner plates  713   a ,  713   b  to the friction plate  711 . 
     The damper hubs  512 ,  612  and  712  in  FIGS. 5 to 7  require fairly large amounts of axial space.  FIG. 8  shows an embodiment of the invention where the two rows of spring packs  814   a ,  814   b  of the driven disc  810  are partly overlapping each other. That gives a very compact design in axial direction. The overlapping of the rows of spring packs  814   a ,  814   b  implies some requirements on the relative location in angular direction of the spring packs. In order to avoid interference, the spring packs of one of the rows must be located between the spring packs of the other row. This is shown in  FIG. 10 . There, the spring packs of row  814   b  are located in the same angular positions as the shoulders  815   s  of the outer plate  815  that carries the other row of spring packs  814   a . As was discussed earlier, the shoulders  815   s  of the outer plate  815  (as well as the corresponding shoulders of the other plates that carry the spring packs) need a certain width in angular direction for strength reasons. 
       FIG. 9  shows an alternative embodiment of the embodiment in  FIG. 8 . Here, the damper hub  912   a  and  912   b  of the driven disc  910  is substantially identical to a dual damper hub from a twin-disc clutch, like in  FIG. 4 . A connecting element  909  joins the inner plates to the friction plate. Thus, a more standardized and cheaper design can be used.  FIG. 10  can be used for an axial view from the left of the driven disc of the embodiment in  FIG. 9 , as well. 
     The design in  FIGS. 8 ,  9  and  10  makes use of the width of the shoulders  815   s  for the other row of spring packs. In total, a larger part of the periphery can be used for spring packs. This is obvious when comparing  FIGS. 2 and 10 ; there are six spring packs in  FIG. 2 , whereas there are eight in  FIG. 10 . That gives a potential to handle larger input torques. 
     The partial overlapping of the rows of spring packs  814   a ,  814   b  could be quantified by the centre distance  828  in axial direction. That centre distance should preferably be larger than one half of the outer diameter  829  of the spring packs. Thereby, there will be sufficient space available for inner plates  813   a ,  813   b  to carry the rows of spring packs  814   a ,  814   b . Corresponding applies to the embodiment of  FIG. 9 . 
     In an alternative embodiment of the embodiments in  FIGS. 8 and 9  respectively, one of the rows of spring packs could comprise springs with smaller diameter compared to the springs in the other row. This would make it possible to further decrease the total axial length of the clutch. 
     Another advantage in using the hub from a twin-disc clutch in the embodiments of  FIGS. 7 and 9  is that the splined joint (corresponding to  417   s  in  FIG. 4 ) admits easier accommodation to dimensional tolerances for the connecting element ( 709  and  909  respectively). 
     The invention should not be deemed to be limited to the embodiments described above, but rather a number of further variants and modifications are conceivable within the scope of the following patent claims.