Abstract:
The present invention provides a wet multi-plate clutch comprising a clutch case and hub disposed on a same axis, a first friction engagement element disposed in the clutch case, a second friction engagement element disposed on the hub, and a piston for applying an axial load to the first and second friction engagement elements, and wherein a recessed groove is formed in a surface of the piston opposing to the first and second friction engagement elements, and a wave spring is disposed within the recessed groove, and only a portion of the opposed surface at an outer diameter side or an inner diameter side of the recessed groove urges the respective friction engagement elements.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a wet multi-plate clutch used in an automatic transmission. More particularly, the present invention relates to an improvement in an urging arrangement for friction engagement elements.  
           [0003]    2. Related Background Art  
           [0004]    A wet multi-plate clutch has mainly been used in an automatic transmission since a torque transmitting property of such a clutch from a clutch releasing condition to a clutch lockup (frictional engagement) condition is very smooth. Further, in order to damp transmission shock of the automatic transmission, there has frequently been used a technique in which an elastic member is interposed between a piston and a friction engagement element.  
           [0005]    [0005]FIG. 10 is an axial sectional view showing an example of a conventional piston. An elastic member, i.e., a wave spring  40  for damping transmission shock is disposed between urging surfaces  51  and  52  of a piston  50 . Although the wave spring is arranged so that it is embedded into the urging surfaces  51 ,  52 , the reason why such an arrangement is adopted is that an axial dimension of the wet multi-plate clutch is reduced.  
           [0006]    However, in the wet multi-plate clutch having the above-mentioned arrangement, since the urging surfaces  51 ,  52  are flush with each other, when the piston urges the friction engagement elements, surface pressure distribution of a lockup load is widened, so that there arises a phenomenon (referred to as “shudder phenomenon” hereinafter) in which transmitting torque is varied repeatedly in friction engagement.  
         SUMMARY OF THE INVENTION  
         [0007]    Accordingly, an object of the present invention is to provide a wet multi-plate clutch that can suppress a shudder phenomenon, while maintaining a reduced axial dimension of the wet multi-plate clutch.  
           [0008]    To achieve the above object, the present invention provides a wet multi-plate clutch comprising a clutch case and hub disposed on a same axis, a first friction engagement element disposed in the clutch case, a second friction engagement element disposed on the hub, and a piston for applying an axial load to the first and second friction engagement elements, and wherein a recessed groove is formed in a surface of the piston opposing to the first and second friction engagement elements, and a wave spring is disposed within the recessed groove, and only a portion of the opposed surface at an outer diameter side or an inner diameter side of the recessed groove urges the respective friction engagement elements.  
           [0009]    Further, in the present invention, the contact portion of the piston for urging the friction engagement element may be substantially line-shaped. Further, in the present invention, the urging surface of the piston may be arc-shaped. Here, the “urging surface” means a surface provided on the piston and adopted to contact with the friction engagement element when a lockup load is applied to the friction engagement elements.  
           [0010]    In a preferred embodiment of the present invention, a separator plate is used as the first friction engagement element and a friction plate is used as the second friction engagement element. Further, a fundamental construction of the piston is substantially the same as that of the above-mentioned conventional piston. In addition, the separator plates are installed within the clutch case through splines for axial movement, and the friction plates are installed on the hub through splines for axial movement. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]    [0011]FIG. 1 is an axial sectional view of a wet multi-plate clutch according to an embodiment of the present invention;  
         [0012]    [0012]FIG. 2 is a front view of the wet multi-plate clutch according to the embodiment of the present invention;  
         [0013]    [0013]FIG. 3 is a front view of a piston according to a first embodiment of the present invention;  
         [0014]    [0014]FIG. 4 is a front view of a piston according to a first embodiment of the present invention;  
         [0015]    [0015]FIG. 5 is an enlarged view of main part in a second embodiment of the present invention;  
         [0016]    [0016]FIG. 6 is an enlarged view of main part in a third embodiment of the present invention;  
         [0017]    [0017]FIG. 7 is an enlarged view of main part in a fourth embodiment of the present invention;  
         [0018]    [0018]FIG. 8 is a front view of a wave spring;  
         [0019]    [0019]FIG. 9 is an axial sectional view of the wave spring;  
         [0020]    [0020]FIG. 10 is an axial sectional view of a conventional piston;  
         [0021]    [0021]FIG. 11 is a graph showing a test result of a conventional wet multi-plate clutch;  
         [0022]    [0022]FIG. 12 is a graph showing a test result of a wet multi-plate clutch according to a first embodiment of the present invention; and  
         [0023]    [0023]FIG. 13 is a graph showing a test result of a wet multi-plate clutch according to a second embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0024]    The present invention will now be fully explained in connection with embodiments thereof with reference to the accompanying drawings. Incidentally, in the drawings, the same elements are designated by the same reference numerals.  
         [0025]    [0025]FIGS. 1 and 2 show a wet multi-plate clutch according to an embodiment of the present invention. FIG. 1 is a sectional view taken along the line  1 - 1  in FIG. 2, and FIG. 2 is a front view of the clutch.  
         [0026]    In a wet multi-plate clutch  1 , a clutch case  2  and a hub (not shown) are disposed on a same axis. Splines  6  are formed on an inner periphery of an outer cylinder  21  of the clutch case  2 , and separator plates  4  are provided on the splines  4  and friction plates  5  are provided on splines formed on an outer periphery of the hub (not shown), and the separator plate and the friction plates are arranged alternately. A backing plate  8  having an axial thickness greater than those of the separator plates  4  is disposed in an axial open end of the clutch case  2 . These friction engagement elements are prevented from being dislodging and are restricted for axial movement by means of a stop ring  18 .  
         [0027]    On the other hand, a piston  30  is disposed adjacent to an axial closed end of the clutch case  2 . The piston  30  is provided with a wave spring  40  disposed in a surface (of the piston) contacting with the separator plate (friction engagement element)  4 . The piston is shifted to the left (FIG. 1) by supplying pressurized oil to an oil chamber  13  defined between the piston and an inner wall of the clutch case  2 . The pressurized oil is supplied through an oil hole  17 . Incidentally, in order to prevent leak of the operating pressurized oil, a seal ring  33  is provided on the piston  30  and a seal  16  is provided on an inner cylinder  22  of the clutch case  2 . Further, when the pressurized oil is released from the oil chamber  13 , the piston  30  is returned by a repelling force of a return spring  14 , thereby releasing the friction engagement elements. The return spring  14  is supported by a snap ring  19  through a canceller  11 .  
         [0028]    [0028]FIGS. 3 and 4 show a first embodiment of the present invention. FIG. 3 is a front view of the piston  30  and FIG. 4 is an axial partial sectional view of the piston  30 . As mentioned above, the piston is provided at its outer periphery with the seal ring  33 . Further, the piston is also provided at its outer periphery with a substantially annular urging surface  31  opposing to the friction engagement element such as the separator plate  4  in an axial direction and adapted to apply an axial urging force. Further, at an inner periphery side of the piston, there is provided a substantially annular surface  34  having an axial length smaller than that of the urging surface  31  (i.e., height smaller than that of the urging surface).  
         [0029]    A substantially annular recessed groove  32  opened in a confronting relationship to the separator plate  4  is provided between the urging surface  31  and the surface  34 . A substantially annular wave spring  40  is disposed in the recessed groove  32 .  
         [0030]    As apparent from FIG. 4, the wave spring  40  and the urging surface  31  can contact with the separator plate  4 , but the surface  34  does not contact with the separator plate  4 . With this arrangement, surface pressure distribution of a lockup load is limited to a small area, with the result that resonance is hard to occur.  
         [0031]    [0031]FIG. 5 shows a second embodiment of the present invention. FIG. 5 is an enlarged view showing a part of the urging surface and the wave spring  40 . In this second embodiment, contrary to the first embodiment, the urging surface  31  is provided at an inner periphery side of the recessed groove  32 , and a surface  35  at an outer periphery side does not contact with the separator plate  4 . Also in this second embodiment, similar to the first embodiment, the surface pressure distribution of the lockup load is limited to the small area, with the result that the resonance is hard to occur.  
         [0032]    [0032]FIGS. 6 and 7 show third and fourth embodiments of the present invention, respectively. In the third embodiment shown in FIG. 6, an urging surface  36  is provided at an outer diameter side of the recessed groove  32  and an axial sectional configuration of the urging surface is semi-circular. With this configuration, the urging surface is line-contacted with the friction engagement element to greatly reduce a contact area, thereby limiting the surface pressure distribution of the lockup load to a smaller area, with the result that the resonance is hard to occur.  
         [0033]    On the other hand, in the fourth embodiment shown in FIG. 7, an urging surface  37  is provided at an inner diameter side of the recessed groove  32  and an axial sectional configuration of the urging surface is substantially triangular, and a tip end is chamfered. Also in this embodiment, a contact area to the friction engagement element can be reduced, thereby limiting the surface pressure distribution of the lockup load to a small area, with the result that the resonance is hard to occur.  
         [0034]    [0034]FIGS. 8 and 9 show the wave spring used in the various embodiments in detail. FIG. 8 is a front view and FIG. 9 is an axial sectional view. The substantially annular wave spring  40  has mountain portions  41  and valley portions  42  alternately along a circumferential direction, so that, when these mountain portions  41  and valley portions  42  are urged to be flattened, an elastic force is generated.  
         [0035]    A comparison test regarding the conventional wet multi-plate clutch and the wet multi-plate clutch according to the present invention was carried out. The test was performed in such a manner that, after the friction plates and inertia (inertia member) are rotated at a predetermined number of revolutions by means of an electric motor, a predetermined lockup load is applied to the friction engagement elements immediately after the driving from the electric motor is interrupted. It was examined how the transmitting torque is changed during a time period from when the lockup load is inputted to when the number of revolutions becomes zero. ATF (Automatic Transmission Fluid) is used as lubricating oil. Test conditions are as follows:  
         [0036]    Initial number of revolutions: 3000 rpm  
         [0037]    Oil temperature: 100° C.  
         [0038]    Lockup load: 300 kgf  
         [0039]    Amount of lubricating oil: 2.0 l/min  
         [0040]    Lubricating method: axis core oil supplying method  
         [0041]    Lubricating oil: ATF  
         [0042]    Inertia: 0.49 kg ·m 2    
         [0043]    Lockup time: about 1 sec  
         [0044]    [0044]FIG. 11 shows the test result of the conventional wet multi-plate clutch, FIG. 12 shows the test result of the wet multi-plate cultch according to the first embodiment of the present invention, and FIG. 13 shows the test result of the wet multi-plate clutch according to the second embodiment of the present invention. Graphs indicate the lockup load and the transmission torque. Here, observing waveforms of the transmitting torques, it can be ascertained that, in the conventional case, amplitude of the torque, i.e., shudder is generated during a time period from when the lockup load is inputted to when the number of revolutions become zero. However, in the first and second embodiments of the present invention, it can clearly be recognized that there is substantially no shudder.  
         [0045]    The present invention is carried out as the above-mentioned embodiments and achieves the effect that a wet multi-plate clutch which has a small axial dimension and in which a shudder phenomenon can be suppressed.