Patent ID: 8079454

Claim:
A friction engagement apparatus, comprising: a first friction plate supported on one side of a case; a rotation element rotatable with respect to the case; a second friction plate supported on a side of the rotation element opposite to the one side of the case; an annular piston slidably accommodated in the case and having a pressure receiving portion forming an annular oil pressure chamber together with the case between the case and the pressure receiving portion and a pressure imparting portion imparting pressure to the first and second friction plates in one axial direction with a thrust force from the pressure receiving portion; and an urging device intervening between the case and the piston and having a resilient member which resiliently urges the piston in the other axial direction opposite to the one axial direction; the piston having a first pressure receiving section formed by partly cutting off the pressure imparting portion in the circumferential direction and a second pressure receiving section extending in the circumferential direction to exclude the first pressure receiving section; the resilient member being constituted by an annular plate spring having an annular plate portion coaxially arranged with the annular piston and a plurality of warping arm portions radially extending from the annular plate portion, the annular plate spring having a bending stiffness in the one axial direction per unit of angular range of the annular plate spring, the stiffness in the range of the first pressure receiving section being larger than the stiffness in the range of the second pressure receiving section.