Abstract: A safety device, particularly usable for mechanisms for lifting a back of a chair or armchair of the type that comprising a blade-like element which is slidingly associated between a first and a second supporting plate for a back; the blade-like element protrudes to the rear of, and above, a seat and has a first recess for a rack with two mutually opposite and offset sets of teeth, which interacts with a complementarily toothed ratchet element which cooperates with elements for forcing the locking of the sliding of the blade-like element as a consequence of a sudden movement imparted to the back in an upward or downward direction.