a rotary power transmission device includes a torque converter , which transmits rotary power between coaxial power input and power output shafts , and a lock up clutch between the power input and power output shafts , which includes first and second plate elements which can be biased towards or away from one another . the first clutch plate element is connected to the power input shaft . in a generally annular space defined between the outer part of the generally toroidal torque converter and the outer part of the second clutch plate element there is disposed a rotary damper which rotationally couples together the second clutch plate element and the power output shaft and provides a certain amount of rotary damping therebetween . the rotary damper includes two outer damper plate elements sandwiched on either side of one inner damper plate element , the outer damper plate elements being connected together , and the combination of them being rotationally connected to the second clutch plate element , while the inner damper plate element is rotationally connected to the power output shaft . two of the damper plate elements are flat annuli lying in said annular space , and one is subtantially a flat disk whose central portion is connected to the power output shaft . the inner damper plate element is pierced with several circumferentially extending slots in which are received compression coil springs , and opposing these slots there are formed in the outer damper plate elements corresponding circumferentially extending slot shaped apertures into which the sides of these springs project . around the entire peripheries of the slot shaped apertures in the outer damper plate elements there extend raised lips , which project from the planes of the outer damper plate elements in the direction away from the inner damper plate element . these lips reinforce the outer damper plate elements , and optionally may taper outwards so as snugly to receive the sides of the compression coil springs .