Chair backrest device

Abstract

The invention is capable of maintaining the planar view curve profile of the elastic membranous member of the back-contacting surface while enabling the maintenance of a stable and comfortable seated posture. The side frame members ( 13 C) of a backrest frame ( 13 ) are formed substantially as obtuse angles with the bend towards the front when viewed from the side. The upper frame member ( 13 A) is formed, in planar view, as an arc that protrudes towards the back. An elastic membranous member ( 14 ) is stretched so that when viewed from the side, the intermediate position curves gently towards the front, substantially forming an obtuse angle. Paired left and right shape-maintaining members ( 17 ), each slanting outward and upward, connect the lower frame member ( 13 B) near the left and right sides to the bends ( 13 a ) in the left and right side frame members. The lower parts of both the left and right sides of the elastic membranous member are supported by the shape-maintaining members.

What is claimed is:  
     
         1 . The backrest of a chair, the backrest comprising a substantially rectangular backrest frame comprising upper, lower, right-side and left-side frame members and an elastic membrane stretched in the backrest frame,
 wherein the right-side and left-side frame members of the backrest frame are bent at a bent point so that a middle of each of the frame members gently projects forward when viewed from a side, the elastic membrane being stretched so that a middle is curved at a curved point forward when viewed from the side, the backrest further comprising a pair of shape-maintaining members each of which is provided obliquely outward and upward between the bent portion of each of the right and left side members and part of the lower frame member close to each of the right and left side frame members, the elastic membrane being supported at lower ends of right and left side portions by the shape-maintaining member.   
     
     
         2 . The backrest of  claim 1  wherein the curved point of the elastic membrane is higher than the bent point of the side frame. 
     
     
         3 . The backrest of  claim 1  wherein the backrest frame is molded together with the shape-maintaining members and the elastic membrane. 
     
     
         4 . The backrest of  claim 1  wherein the upper frame member is formed like an arc gently projecting backward when viewed from an above. 
     
     
         5 . The backrest of  claim 1  wherein the upper frame member is formed straight when viewed from the above. 
     
     
         6 . The backrest of  claim 1  wherein the lower frame member is formed like an arc gently projecting backward when viewed from the above.