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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.
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.
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
This application claims the priority of Japanese Patent Application 2005-340492 filed Nov. 25, 2005, which is incorporated herein by reference.
Number | Date | Country | Kind |
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2005-340492 | Nov 2005 | JP | national |
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.