Patent ID: 7644811

Claim:
A lock-up device of a hydraulic torque transmitting apparatus including a front cover to which torque is input, an impeller fixed to the front cover to constitute a fluid chamber filled with a working fluid, and a turbine located opposite the impeller in the fluid chamber, the lock-up device comprising: a piston being located to divide a space between the front cover and the turbine into a first hydraulic chamber on a side of the front cover and a second hydraulic chamber on a side of the turbine, the piston being engageable with and disengageable from the front cover due to a differential pressure between the first and second hydraulic chambers, the piston including a plate-like main body having a piston surface formed on a first hydraulic chamber side, and a plurality of projections extending in the radial direction from the piston surface, each projection having a straight rotational front edge, a straight rotational rear edge, an arced radially outer edge, and an arced radially inner edge, the rotational rear edge intersecting a first straight line radially connecting a radially inner end thereof with a rotation center of the piston, or the rotational rear edge being inclined to the first straight line such that a radially outer end thereof is positioned rearward of the first straight line in the rotation direction, and the rotational front edge including a front wall surface extending from the piston surface and being inclined in the rotational direction with respect to the piston surface.