Office chair with tiltable seat and back

Claim 1

A control mechanism for controlling the angular adjustment of a seat and backrest of a chair comprising: (a) a base member having a front end and back end and a longitudinal extent therebetween, said base member being adapted to be mounted on a leg support structure of a chair to be supported thereby in a generally horizontal plane, (b) a seat support member having a front end and a back end in a longitudinal extent therebetween, said seat support member overlying said base member, (c) sliding pivot means connected the front end of the seat support member to the front end of the base member for pivotal movement about a first horizontal axis which extends transversely of the base member and for sliding movement of the seat with respect to the base in the direction of the longitudinal extent of the base between a forward position and a rearward position, the pivotal movement permitting movement of the back end of the seat support with respect to the back end of the base member between an elevated position and a lowered position, (d) a backrest member pivotally mounted at the back end of the base member for movement about a second horizontal axis which extends parallel to said first horizontal axis, said backrest member having a first lever arm projecting from said second axis toward the front end of said base member and a second lever arm which projects upwardly from said second horizontal axis and is adapted to support a backrest at a predetermined height above said seat support member, said backrest member being movable between a first position in which the distal end of the first lever arm is in its lowered position and the distal end of the second lever arm is in a forward position overlying the seat portion ad a second position in which the distal end of the first lever arm is elevated and the distal end of the second lever arm is rearwardly displaced with respect to the seat member, (e) said first lever arm having a longitudinally elongated cam face extending toward its distal end, said cam face underlying said seat support member and having a point of contact bearing against the seat support member, said cam face having a profile such that the radius from the second axis to the point of contact decreases progressively as the seat moves away from its elevated position whereby the moment applied through the first lever arm about the second axis will decrease as the seat member moves from its raised position toward its lower position, (f) a link member having a first end and a second end, the first end of the link member being mounted on the base member for rotation about said second axis, the second end of the link member being pivotally connected to the back end of the seat support, said link arm extending forwardly from the second axis toward the first end of the base member when the seat member is in its lowered position and pivoting upwardly and rearwardly as the back end of the seat member is elevated to simultaneously move the seat support member toward its rearward position as it is elevated to reduce the change in the distance between the distal end of the second lever arm and the back end of the seat rest during relative movement therebetween.