Patent Publication Number: US-2007119674-A1

Title: Lockup clutch

Description:
FIELD OF THE INVENTION  
      This invention relates to improvements in a lockup clutch, which is useful in a torque converter of an automatic transmission and can inhibit a shudder which take place by a stick-slip. 
    
    
     DESCRIPTION OF THE BACKGROUND  
       FIG. 1  is a cross-sectional view showing the fundamental construction of a torque converter equipped with a lockup clutch. A torque converter  10  comprises a pump section  2 , a turbine section  3 , and a stator  4 .  FIG. 1  also illustrates a front cover  21 , an outer shell  22 , blades  23  and an inner core  24  of the pump section  2 , an outer shell  32 , blades  33  and an inner core  34  of the turbine section  3 , and a turbine hub  35 .  
      Also shown in  FIG. 1  are a clutch piston  5 , a damper support portion  51 , a damper  52 , a damper plate  53  and a damper spring  54  of the lockup clutch. The outer shell  32  of the turbine section  3 , the turbine hub  35  and the damper plate  53  are joined together by a rivet  55 . Designated at numeral  56  is a hole located centrally in the piston  5 , and through the hole  56 , the turbine hub  35  extends. Numeral  7  indicates a friction lining bonded on a clutch-engaging portion of the clutch piston  5 . It is to be noted that this friction lining may be bonded on the side of the front cover  21 . Numeral  6  designates a coupling welded on the front cover  21  and adapted to connect the torque converter to an engine. An alternate long-and-short dash line X-X indicates a central axis of the whole torque converter.  
      In recent years, there is an increasing tendency toward actuating a lockup clutch even from a low vehicle-speed range to improve the fuel economy. Reflecting this tendency, slip control is increasingly performed these days. In such slip control, the lockup clutch involves a problem in that a clutch piston and a damper or the like act as a shuddering body and a resilient body, respectively, and vibrations called a shudder occur as a result of a stick-slip of a friction lining.  
      A friction lining is formed from fibers, a filler, a friction modifier, and a resin dissolved in a solvent. As the solvent evaporates, the resin located inside the green friction lining progressively moves together with the solvent close to the surface. A high resin-proportion layer with the resin contained at a high concentration is, therefore, formed near the surface of the friction lining. The friction lining has a high coefficient of friction when the slip rotation speed is low, but a low coefficient of friction when the slip rotation speed becomes high. In other words, the μ-ν characteristics of the friction lining have a negative gradient, thereby causing a shudder by a stick-slip when the clutch is brought into engagement. It is to be noted that the expression “slip rotation speed” as used herein means a difference in rotation speed between a member with a friction lining bonded thereon and another member having a counterpart friction surface.  
      It has, therefore, been a conventional practice to remove the high resin-proportion layer from the surface of the friction lining and further to subject the friction lining to cutting on the side of its outer circumference such that the surface is rendered smooth. This cutting is usually performed in automatic transmission fluid. The automatic transmission fluid, therefore, adheres to the product, leading to a deterioration in the working environment. Moreover, the need for the cutting results in higher manufacturing cost.  
      It was, therefore, contemplated to divide a friction lining into a radially-outer-side friction lining and a radially-inner-side friction lining, to make the proportion of a filler higher in the radially-outer-side friction lining than in the radially-inner-side friction lining to provide the μ-ν characteristics of the radially-outer-side friction lining with a positive gradient, in other words, such that the coefficient of friction becomes higher with the slip rotation speed to inhibit a shudder which takes place by a stick-slip; and further, to make higher the proportion of fibers in the radially-inner-side friction lining to provide the radially-inner-side friction lining with flexibility and to increase the sealing property and contact area of its friction surface and hence the coefficient of static friction, thereby making the lock-up capacity greater.  
      As a result of research, it became clear that among fillers, it is diatomaceous earth that is effective for the inhibition of a shudder. Recently, there is a tendency toward referring a combination of a filler and a friction modifier simply as “a filler”.  
      It was also found that, even if the proportion of fibers is not made specifically higher in a radially-inner-side friction lining than in a radially-outer-side friction lining, the coefficient of static friction of the radially-inner-side friction lining can still be made higher than that of the radially-outer-side friction lining by adjusting the proportion of the filler including a friction modifier.  
       FIG. 2  is a front view of the friction lining  7  bonded on the piston  5  as viewed at a friction surface of the piston  5 , while  FIG. 3  is a cross-sectional view taken in the direction arrows III-III of  FIG. 2 .  FIGS. 2 and 3  show a radially-outer-side friction lining  71 , a radially-inner-side friction lining  72 , and a seam  73  between the friction lining  71  and the friction lining  72 .  
       FIGS. 4 and 5  illustrate characteristics of the above-mentioned friction linings.  FIG. 4  shows a positive gradient, and the slip rotation speed and the torque vary in the same direction.  FIG. 5  shows a negative gradient, the coefficient of static friction is large, and as the slip rotation speed becomes lower, the torque increases conversely. It has, therefore, been the conventional practice to provide the radially-outer-side friction lining with μ-ν characteristics of positive gradient, for example, by increasing the proportion of diatomaceous earth in its filler and also to provide the radially-inner-side friction lining with a larger coefficient of static friction by adjusting the composition of its filler including a friction modifier. Keeping a step with the recent move toward lock-up clutches of higher performance, however, there is an outstanding demand for further functional enhancements.  
     SUMMARY OF THE INVENTION  
      In one aspect of the present invention, there is thus provided a lockup clutch for a torque converter, wherein a friction lining is divided into a radially-outer-side friction lining and a radially-inner-side friction lining, μ-ν characteristics of the radially-outer-side friction lining have a positive gradient to inhibit occurrence of a shudder which takes place by a stick-slip, the radially-inner-side friction lining is equipped with a large coefficient of static friction to provide large torque capacity during a clutch engagement, and the radially-outer-side friction lining is provided with plural grooves formed with intervals therebetween formed in a circumferential direction.  
      In another aspect of the present invention, there is also provided a lockup clutch for a torque converter, wherein a friction lining is divided into a radially-outer-side friction lining and a radially-inner-side friction lining, μ-ν characteristics of the radially-outer-side friction lining have a positive gradient to inhibit occurrence of a shudder which takes place by a stick-slip, the radially-inner-side friction lining is equipped with a large coefficient of static friction to provide large torque capacity during a clutch engagement, and the radially-inner-side friction lining is provided with at least one groove which is in communication with a lockup release chamber in the lockup clutch.  
      In a further aspect of the present invention, there is a lockup clutch for a torque converter, wherein a friction lining is divided into a radially-outer-side friction lining and a radially-inner-side friction lining, μ-ν characteristics of the radially-outer-side friction lining have a positive gradient to inhibit occurrence of a shudder which takes place by a stick-slip, the radially-inner-side friction lining is equipped with a large coefficient of static friction to provide large torque capacity during a clutch engagement, and the radially-outer-side friction lining has a shape of a rectangle combined with a segment of a circle the chord of which lies on a side of the rectangle such that as viewed in an axial cross-section of the lockup clutch, the radially-outer-side friction lining is thicker at a radial center thereof and becomes thinner toward a radially outer end and radially inner end thereof while defining a bow-shaped contour.  
      In a still further aspect of the present invention, there is also provided a lockup clutch for a torque converter, wherein a friction lining is divided into a radially-outer-side friction lining and a radially-inner-side friction lining, μ-ν characteristics of said radially-outer-side friction lining have a positive gradient to inhibit occurrence of a shudder which takes place by a stick-slip, said radially-inner-side friction lining is equipped with a large coefficient of static friction to provide large torque capacity during a clutch engagement, and said radially-outer-side friction lining is provided with a continuous groove formed alongside an entire circumference thereof in the circumferential direction.  
      By providing the radially-outer-side friction lining with plural grooves formed with intervals therebetween in a circumferential direction or a continuous groove formed alongside an entire circumference thereof in the circumferential direction, a hydraulic pressure over a contact surface of the radially-outer-side friction lining becomes uniform in a circumferential direction, thereby making a shudder still harder to occur. By providing the radially-inner-side friction lining with plural radial grooves which are in communication with the lockup release chamber, the discharge of oil from the friction lining to a low-pressure side at the time of a clutch engagement is promoted, thereby making an oil film hard to be formed on the friction surface so that a transmittable torque can be increased.  
      By forming the radially-outer-side friction lining into the shape of the rectangle combined with the segment of the circle the chord of which lies on the side of the rectangle, the contact pressure becomes more uniform over the friction surface of the radially-outer-side friction lining, thereby making it possible to further heighten the effect of preventing the occurrence of a shudder.  
      Reflecting the recent demand for improvements in fuel economy, there is an increasing demand for lighter and smaller lockup clutches, and hence, an ever increasing demand for friction surfaces of greater torque capacity and also for smaller and lighter pistons. By adequately providing the radially-outer-side friction lining and/or the radially-inner-side friction lining with the corresponding at least one groove or making the improvements in the shape of the radially-outer-side friction lining, the resulting lockup clutch can meet these demands. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a cross-sectional view showing the fundamental construction of a torque converter equipped with a lockup clutch.  
       FIG. 2  is a front view of a piston in the lockup clutch as viewed at a friction surface of the piston.  
       FIG. 3  is cross-sectional view of the piston, taken in the direction of arrows III-III of  FIG. 2 .  
       FIG. 4  is a diagram showing a positive gradient of a friction lining.  
       FIG. 5  is a diagram showing a negative gradient of a friction lining.  
       FIG. 6  is a fragmentary front view of a piston in a lockup clutch according to a first embodiment of the present invention.  
       FIG. 7  is a fragmentary front view of a piston in a lockup clutch according to a second embodiment of the present invention.  
       FIG. 8  is a fragmentary cross-sectional view of a piston and front cover in a lockup clutch according to a third embodiment of the present invention.  
       FIG. 9  is a fragmentary cross-sectional view of a piston and front cover in a lockup clutch according to a fourth embodiment of the present invention, in which a seal member is arranged.  
       FIG. 10  is a fragmentary front view similar to FIG.  6 , and illustrates a modification of the first embodiment shown in  FIG. 6 , in which a radially-outer-side friction lining is provided with a continuous groove formed alongside an entire circumference thereof in the circumferential direction. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION AND PREFERRED EMBODIMENTS  
      Reflecting the recent demand for improvements in fuel economy, there has been an increasing demand for lighter and smaller lockup clutches. As a result, a piston has become still smaller and lighter so that upon a clutch engagement, the piston is flexed at a central part thereof toward a front cover. When a radially-outer-side friction lining is made thicker, the radially-outer-side friction lining firstly begins to come into contact with the front cover in an initial stage of a clutch engagement to perform slip control. At the time of completion of the clutch engagement, the radially-outer-side friction lining and a radially-inner-side friction lining are both brought at their entire surfaces into close contact with the front cover so that the torque capacity increases.  
      Although the radially-outer-side friction lining has its own sealing effect, the exhibition of sealing effect can be assured further when a seal member is arranged between the radially-outer-side friction lining and the radially-inner-side friction lining. By this sealing effect, oil is prevented from flowing around to the radially-inner-side friction lining. The seal member can, therefore, exhibit its effect for an increase in the torque capacity at the time of the completion of a clutch engagement.  
      With reference to  FIGS. 6 through 9 , lockup clutches according to the first to fourth embodiments of the present invention will hereinafter be described. In  FIGS. 6 through 9 , those elements of the lockup clutches which are the same as or equivalent to their corresponding elements in  FIG. 2  are shown by the same reference numerals.  
       FIG. 6  illustrates the first embodiment of the present invention, in which the present invention is applied to a radially-outer-side friction lining  71 . The radially-outer-side friction lining  71  which performs slip control is provided with plural grooves  74  formed with intervals therebetween in a circumferential direction. Owing to the provision of the plural grooves  74 , a hydraulic pressure over a contact surface of the radially-outer-side friction lining  71  becomes uniform in a circumferential direction, thereby making a shudder still harder to occur.  
       FIG. 7  depicts the second embodiment of the present invention, in which a radially-inner-side friction lining  72  is provided with radial grooves  75  which are in communication with the side of a lockup release chamber. Designated at numeral  76  is a continuous groove formed alongside an entire circumference of the radially-inner-side friction lining  72  in the circumferential direction such that oil over the contact surface of the radially-inner-side friction lining  72  can be smoothly collected into the grooves  75 . It is, however, to be noted that the groove  76  is not essential but is optional. Owing to the provision of the grooves  75 , the oil over the friction surface is allowed to promptly flow to a low-pressure side at the time of completion of a clutch engagement, thereby leaving the oil only in a small amount on the friction surface and resulting in increased torque capacity. Instead of arranging the radial grooves  75 , a space defined between the radially-outer-side friction lining  71  and the radially-inner-side friction lining  72  may be used as a continuous groove extending alongside entire circumferences of the radially-outer-side and radially-inner-side friction linings  71 , 72  such that the continuous groove is in communication with the plural radial grooves  75 . It is to be noted that, although the radially-outer-side friction lining and radially-inner- side friction lining are both provided with their corresponding grooves in  FIG. 7 , only one of the radially-outer-side friction lining and radially-inner-side friction lining may be provided with its corresponding grooves.  
       FIG. 8  illustrates the third embodiment of the present invention, in which a radially-outer-side friction lining  71  has a shape of a rectangle combined with a segment of a circle the chord of which lies on a side of said rectangle such that as viewed in an axial cross-section of the lockup clutch, the radially-outer-side friction lining  71  is thicker at a radial center thereof and becomes thinner toward a radially outer end and radially inner end thereof while defining a bow-shaped contour. By forming the radially-outer-side friction lining  71  as described above, the contact pressure becomes uniform in the circumferential direction, and the sealing effect that prevents the oil from flowing around to the radially-inner-side friction lining is increased. The radially-outer-side friction lining  71 , therefore, exhibits its effect for the prevention of the occurrence of a shudder and for the increase in the torque capacity during a clutch engagement.  
       FIG. 9  shows the fourth embodiment of the present invention, in which a seal member  8  is arranged between a radially-outer-side friction lining  71  and a radially-inner-side friction lining  72 . In  FIG. 9 , the radially-outer-side friction lining  71  is shown thicker than the radially-inner-side friction lining  72 . It is to be noted that in all the above embodiments, the radially-outer-side friction lining can preferably be made thicker than the radially-inner-side friction lining. The provision of the seal member  8  further assures the exhibition of the above-mentioned effect that prevents the high-pressure oil from flowing around to the radially-inner-side friction lining  72 .  
       FIG. 10  illustrates a modification of the above-described first embodiment of the present invention. Instead of arranging the plural grooves  74 , the radially-outer-side friction lining  71  is provided with a continuous groove  78  formed alongside the entire circumference thereof in the circumferential direction. This modification can bring about similar advantageous effects as the first embodiment.  
      This application claims the priority of Japanese Patent Application 2005-340492 filed Nov. 25, 2005, which is incorporated herein by reference.