Metal crib structure for a rocking chair seating plane

An improved metal crib structure for a seating plane of an armchair, comprising:

• a fixed frame, constituted by two opposite lateral elements, and a first front element and a first back element, each of which lateral elements having an upwardly-projecting bearing with through holes on a horizontal axis for receiving pivoting bolts;
• a cradle-shaped seat support frame of the seating plane, oscillating about the horizontal axis, comprising two opposite v-shaped lateral elements and a second front element and a second back element which are both straight and welded to free ends of the v-shaped lateral elements, the v-shaped lateral elements being respectively rotatably and pivotally engaged to each said upwardly-projecting seat by pivoting bolts engaged in said through holes;
• two helix springs for opposing oscillation of the seat support frame mounted between the fixed frame and the seat support frame;
• a stiffening device for the helix springs, comprising, for each of the helix springs, means for adjustably blocking a top coil thereof in a trunconical seating on the seat support frame, a bottom coil thereof being blocked in a trunconical seating on the fixed frame wherein the means for adjustably blocking the top coil of each of the helix springs comprises:
• a lockscrew having a T-shaped positioning head and coupling with nuts associated with each said second front element and said second back element of the seat support frame, the lockscrew being situated in proximity to each helix spring of said two helix springs and passing through the second front element and second back element;
• a circular cap welded to and below each trunconical seating on the seat support frame for respectively constraining and end of said each helix spring, the circular cap being provided with a through hole and a threaded coupling for engaging the nuts;
• and
• manual lock means for blocking the seat support frame in a horizontal position.