The present invention relates to a wet type clutch to be actuated by a hydraulic pressure.
This kind of clutch is made to effect torque transmission by bring a friction member and a frictional surface facing the friction member into facial contact with each other. In a known torque transmission system for forcing the friction member and the frictional surface to contact, the friction member is adhered to either a piston or the surface facing the piston, and the frictional surface is exemplified by the surface facing the friction member, so that the piston is moved forward to engage with that surface. In this system, the piston itself acts as a torque transmission member, as exemplified in a lockup clutch of a torque converter.
FIGS. 16 and 17 show a torque converter 1 which is equipped with a lockup clutch of the prior art. This torque converter 1 has its entirety defined liquid-tight by a front cover 2 and a shell 3a of the pump impeller 3 and is filled up with a fluid. The torque converter 1 is arranged with a turbine runner 4, a stator 5, and a piston 6 of the lockup clutch. To such a surface of the piston 6 as is located at the circumferential edge portion and faces the inner surface of the front cover 2, there is adhered a porous friction member 7 which is formed into an annular shape having a constant width. Across this piston 6, there are formed a first oil chamber 8 at the side of the front cover 2 and a second oil chamber 9 at the side of the turbine runner 4. In case the lockup clutch is to be applied, the fluid is fed to the second oil chamber 9 and discharged from the first oil chamber 8. As a result, the internal pressure in the second oil chamber 9 exceeds that in the first oil chamber 8 so that the piston 6 is pushed toward the front cover 2 to bring its friction member 7 into contact with the front cover 2 as a countermember while shutting the communication between the first oil chamber 8 and the second oil chamber 9 by that friction member 7. As a result, leakage of the fluid from the second oil chamber 9 to the first oil chamber 8 is substantially eliminated to establish a large pressure difference between the two oil chambers 8 and 9 so that the friction member 7 of the piston 6 is pushed to contact with the front cover 2. In other words, the lockup clutch is applied to transmit the torque of the engine directly to the (not-shown) input shaft of the gear transmission mechanism from the front cover 2 through the piston 6.
As in the lockup clutch described above, however, the wet type clutch, which is to be applied by shutting the communication between the higher pressure oil chamber and the lower pressure oil chamber with the friction member and the frictional surface, has a tendency to be given a lower torque capacity for the fed oil pressure than that of the ordinary wet type clutch, in which clutch discs and clutch plates alternately arranged are pushed by a piston. Specifically, such lockup clutch as in the prior art is constructed such that the porous friction member 7 is used and impregnated with the fluid. As a result, the higher pressure of the second oil chamber 9 at the time of applying the clutch propagates to the fluid in the friction member 7, too. This pressure applied to the portion of the friction member 7 acts in the direction to move the piston 6 away from the front cover 2 thereby to reduce the force to applying the clutch.
More specifically, the force acting to move the piston 6 apart from the front cover 2 takes a higher level (as will be called the "apply pressure"), as seen from FIG. 17, at the portion of the friction member 7 contacting with the second oil chamber 9 having a higher pressure and gradually takes the lower level as goes away from the second oil chamber 9, until it takes substantially the same level (as will be called the "drain pressure") as that in the first oil pressure 8 at the portion contacting with the first oil chamber having a lower pressure. The so-called "sum of reactions", as is distributed due to the porous material of the friction member 7, lowers the force to apply the clutch so that the apparent coefficient .mu. of friction of the friction member 7 drops to reduce the torque capacity.
In order to increase this torque capacity, it is conceivable to increase the size of the clutch or to raise the oil pressure for applying the clutch. The large-sized clutch will cause problems that its mountability on a vehicle becomes poor and that its responsiveness to the operation is deteriorated. On the other hand, the rise in the oil pressure for the clutch application will increase the deformation of the shell or the like of the torque converter to cause another problem that an offset contact with the clutch surface occurs.