1.  Technical Field
The present invention relates to a power transmission device, particularly to a power transmission device disposed between a torque converter main body and a front cover coupled to an engine-side member in order to transmit torque outputted from a turbine of the torque converter main body to a transmission-side member. 
2.  Background Art
The torque converters normally include a front cover and a torque converter main body.  The front cover is coupled to an engine-side member, while the torque converter main body is coupled to the front cover.  Torque inputted through the front cover is transmitted towards a transmission through the torque converter main body.  Torque converter main body includes an impeller, a turbine and a stator.  The impeller is coupled to the front cover.  The turbine is coupled to a transmission-side member.  The stator is configured to regulate the flow of operating fluid returning from the turbine to the impeller. 
Patent Literature 1 describes an exemplary torque converter structured as described above.  In the torque converter, a damper mechanism including torsion springs is disposed between the turbine and the transmission-side member.  In the torque converter of this type, torque is transmitted from the turbine to the transmission-side member through the damper mechanism. 
Further, Patent Literature 2 describes an exemplary damper mechanism employed for a lock-up device of a torque converter.  In the damper mechanism, reduction in stiffness and expansion in a torsion angle for the torsion springs are herein required to eliminate and attenuate variation in torque inputted thereto from the engine.  In view of this, the damper mechanism described in Patent Literature 2 includes torsion springs disposed on the radial inner part thereof and those disposed on the radial outer part thereof.  Further, the radially inner side torsion springs and the radially outer side torsion springs are configured to act in series through an intermediate member.