Spring

A spring element comprising a longitudinally extending hollow body member of plastic material having a longitudinal axis with inner and outer convolutions along the length thereof, said inner and outer convolutions being of different thickness, said element being hollow with at least one end unenclosed, said convolutions having inner valleys and outer ridges, said valleys and ridges defined on radii which determine the thickness of said convolutions, and the wall portions extending between said convolutions are of tapering thickness, said element being precompressed along said longitudinal axis to a degree sufficient to cause the material thereof to exceed its yield point at least in said convolutions and give it a permanent set of decreased axial dimension in an unloaded condition and impart resilience thereto in the axial direction, the predominant set of said element occurring in the thinner walled convolutions, said convolutions being of essentially annular shape perpendicular to the longitudinal axis.