US Patent 8646839 Adjustment device for chairs

An adjustment device for chairs, comprising a main body (2), which can be engaged onto a supporting shank (3) of a chair; a prop element (6) mounted above the main body (2), which can be associated to a seat for a user; a backrest (10) mounted onto the main body (2), which can be associated to a back (11) for a user; and articulation means (14) for movably engaging the backrest (10) and the prop element (6) to the main body (2), the articulation means (14) comprise at least one lever (17) pivoting around the main body (2) in a first mounting site and having at least two lever portions (17a, 17b) hinged to the prop element (6) in a second mounting site and to the backrest (10) in a third mounting site, respectively, so as to obtain a given movement relation between the backrest (10) and the prop element (6).