A seat structure comprises a base configured to be a chair base assembly; a seat support located above the base, wherein a front part of the seat support has rotary connection with the front part of the base through a transition link; a chair back assembly having rotary connection with a rear part of the base through a pivot; a lever driving member linked between the chair back assembly and the seat support, and having rotary connection with a rear part of the seat support, wherein the lever driving member and the seat support are approximately on the same surface. When the chair back assembly rotates rearward around the pivot from the initial position, the rear part of the seat support is raised diagonally to the rear through the lever driving member, meanwhile the transition link and the front part of the seat support are pulled up.

An adjusting mechanism is provided for selectively setting a restoring force acting on a backrest of a chair. In a biasing unit of the adjusting mechanism, a spring segment is disposed between a head end and a follower end to bias the follower end to the remote position. The follower end is angularly movable on a cam surface between distal and proximate positions. In response to movement of a backrest of the chair from a normal position to an inclined position to permit the follower end to be forced by the cam surface to move from the remote position to a close position, two different restoring forces can be set by adjusting the follower end between the distal and proximate positions.

A body support structure includes a frame and a membrane attached to the frame. The membrane includes elastomeric filaments. The membrane includes different regions with different stiffness. The membrane may form a part of a seat or a backrest.

A device for adjusting the tilt of a chair backrest having a backrest frame rotatable with respect to the base frame of the chair is provided. The device includes elastic means for returning the backrest and/or countering the rotation thereof, in addition to means for controlling the preload of the elastic means. The means for controlling the preload include at least one first connecting rod and at least one first crank. The first connecting rod is hinged to the first crank and constrained to the elastic means so as to move the point of application of the returning and/or countering force with respect to the backrest. Adjusting means to adjust the position of the first crank determine at least two distinct stable positions of the first crank corresponding to two distinct positions of the mentioned point of application of the returning and/or countering force with respect to the base frame.

An oscillation mechanism for chairs includes a support frame, a structure coupled to the support frame to rotate about a first axis, an elastic element interposed between the support frame and the structure and configured to oppose a reaction force to an oscillation of the structure. The reaction force generates a reaction moment acting on the structure with respect to the first axis. An oscillation adjustment system is able to vary the reaction moment with the oscillation being unchanged. The adjustment system includes a pin movably coupled to the structure and a movement system of the pin for varying the distance between the pin and the first axis. The elastic element is directly fastened to the pin so that the reaction force is transmitted to the structure with the application point corresponding to the pin and with direction and orientation unchanged.

A tiltable chair includes a flexible seat body including an integrally formed seat back and seat surface supported on a seat frame disposed on upper parts of legs, and the seat back connected to a back frame allowing tilting rearwards, the position of the connection being in an area around the upper part of the pelvis of a person seated thereon. A guide rail having an arc-shaped groove is fixed to the seat frame, and guide members which engage and roll in the arc-shaped grooves are pivotably supported on a shaft bracket of the rear surface of the seat surface. The arc-shaped groove has a high center so that the seat surface undergoes curved motion forward and upward such that the ischium is positioned lower than the lower part of the knees when the seat back tilts rearwards.

An item of seating furniture, in particular an office chair, has a seat carrier for a horizontal seat member, and a backrest carrier for a vertical backrest. The backrest carrier is constructed in a substantially U-shaped manner with two vertically orientated U-shaped members and with a horizontal upper U-shaped member. Wherein, along at least one rotation axis which extends below the seat member, a first rotary joint and a second rotary joint are provided for the lateral tilting movability of the backrest with respect to the seat carrier. The first and second rotary joints are arranged spaced apart from each other along the rotation axis.

A tilt control system includes a spring adjuster and compression spring, which may be removed in an unstressed condition. The tilt control system may also include a variable back stop having curved transition surfaces between the stop surfaces. A lumbar compensator may be integrated into a backrest, which may be coupled with a quick release to the base and actuated by sliding engagement with the seat.

A body support structure includes a frame and a membrane attached to the frame. The membrane includes elastomeric filaments. The membrane has a first region with a first stiffness and a second region with a second stiffness. The second stiffness is greater than the first stiffness. The membrane may form a part of a seat or a backrest.

A pivoting mechanism with a first member, a second member pivotally engaged with the first member to pivot about a pivot axis, and pivoting resistance mechanism with a spring housing disposed in the first member spaced from the pivot axis, a spring disposed in the spring housing, and a deflecting member that projects from the second member spaced from the pivot axis to engage the spring disposed in the spring housing. The first member can be a chair mechanism body, and the second member can be a pivoting support, such as for a chair bottom or back. A height adjustment mechanism has a movable member, and a locking member, such as a screw, can be operative as a height locking mechanism with a first, restraining condition wherein actuation of the movable member is prevented and a second, non-restraining condition wherein actuation of the movable member is permitted.