Oscillation system for chairs

An oscillation system for chairs, comprising:

• a frame configured for a base of a chair;
• a backrest support mounted on the frame to oscillate about an axis of rotation;
• a seat support mounted on the frame and distinct from said backrest support, wherein said seat support comprises a first support element structured to be coupled to the seat and coupled to said framed to assume, given an oscillation position of the backrest support, a plurality of relative positions with respect to the frame depending on, and by effect of, a weight force applied to the first support element, said relative positions having different vertical heights of the first support element with respect to the frame,
• a first elastic system operatively interposed between said frame and said backrest support, comprising a first elastic element structured to oppose an elastic reaction to an oscillation of the backrest support about said axis of rotation with respect to an at rest position of the backrest support in the absence of oscillation forces,
• an adjustment system structured to position a first anchoring end of said first elastic element depending on said relative position of the first support element with respect to the frame, with said backrest support in said given oscillation position,
• wherein said adjustment system comprises a lever member having a fulcrum fixed to said frame and defining a fulcrum axis the lever member being physically connected to said first support element to be rotated by the first support element during a variation of the relative position, and being physically connected to said first anchoring end of the first elastic element wherein said rotation of the lever member induces a displacement of the first anchoring end, wherein said fulcrum axis is non-coincident with said axis of rotation of the backrest support.