Patent ID: 7192015

Claim:
A plate spring, comprising: a spring member having a substantially circular-ring-shape and including a substantially externally conical upper side ( 12 , 22 , 32 ) and a substantially internally conical underside ( 13 , 23 , 33 ) between which there is positioned an outer annular edge ( 14 , 24 , 34 ) and an inner annular edge ( 15 , 25 , 35 ), wherein, in the unloaded condition, the surface layer ( 17 , 27 , 37 ) of the underside ( 13 , 23 , 33 ) has a higher inherent compressive stress than the surface layer ( 16 , 26 , 36 ) of the upper side ( 12 , 22 , 32 ), wherein, in the unloaded condition, a substantial portion of the surface layer ( 17 , 27 , 37 ) of the outer annular edge ( 14 , 24 , 34 ) and of the inner annular edge ( 15 , 25 , 35 ) has a higher inherent compressive stress than the surface layer ( 16 , 26 , 36 ) of the upper side ( 12 , 22 , 32 ).