Patent ID: 8579376

Claim:
A device for adjusting the height of the seat back of an office chair, comprising: a) a seat body having chair feet at a bottom thereof; b) a frame body formed in an L shape with a vertical portion and a bottom portion, the bottom portion thereof being pivotally attached to a rear side of the seat body, the frame body having a left and right vertical portion both of which are parallel to each other and located at a position facing to a front of the the chair, a longitudinal slot being formed at the front or rear side of the vertical portions, a longitudinal projecting body being positioned at the center of the longitudinal slot such that an elongated ring-shaped turning slide is formed within the longitudinal slot by means of the longitudinal projecting body, one side of the longitudinal projecting body having a plurality of engaging teeth with engaging spaces between the teeth directed substantially upward; c) a backrest unit mounted at the front side of the frame body and having at the rear side thereof two slide sleeves facing to the longitudinal slot of the frame body and movable up and down on the vertical portions, the slide sleeves each include a mounting hole at the side facing to the engaging teeth; and d) a engaging unit having an engaging body and a torsion spring, the engaging body having a shank and a crank at the front side of the shank, the end of the crank being provided with a projection insertable into the engaging spaces between the engaging teeth, the rear end of the shank fitting into the mounting hole of the slide sleeves, the torsion spring being mounted on the shank, the torsion spring having a front arm resting on the surface of the crank and a rear arm fixed at the periphery of the mounting hole of the slide sleeves, wherein the crank of the engaging body is subject to a rotational bias from the torsion spring such that the projection at the end of the crank contacts the side of the longitudinal projecting body when the slide sleeves are adjusted alternatively in an upward direction and a downward direction; wherein, when the projection rises to the prearranged position, the projection is engaged into the engaging spaces between the engaging teeth by means of the rotational bias of the torsion spring; wherein, when the projection rises to the top of the longitudinal projecting body, the projection rotates over the longitudinal projecting body to the side opposite to the engaging teeth, the slide sleeve falls with the projection freely sliding along the side opposite to the engaging teeth, whereby the height of the backrest unit is adjustable from a lower position to a higher position, and, when the projection reaches the highest position, the backrest unit automatically falls back to the lowest position, and the height adjustment may be done according to the requirement of the user once again.