REACTION FORCE ADJUSTMENT MECHANISM FOR CHAIR BACKREST REACTION FORCE MECHANISM

To enable weight saving and down-sizing for a reaction force adjustment mechanism in a chair backrest reaction force mechanism.SOLUTION: A chair backrest reaction force mechanism generates, by compression of a reaction force spring, a force for pushing back a backrest linked with backward tilting of the backrest. In connection with this mechanism, a reaction force adjustment mechanism includes: a reaction force adjustment cam 51 which provides displacement in stretching direction for the reaction force spring; and a reaction force adjustment operation shaft 7 (70) which rotates the reaction force adjustment cam 51 to change over a cam displacement amount that is provided for the reaction force spring. Also, a reaction force adjustment mechanism is provided in which, by the rotation operation of the reaction force adjustment operation shaft 7 (70), the cam displacement amount provided by the reaction force adjustment cam 51 is switched, with the initial compression amount of the reaction force spring adjusted. The reaction force adjustment cam 51 has a cam basic circle 51a that is smaller in diameter than the minimum shaft diameter of the shaft of the reaction force adjustment operation shaft 7 (70), for example, the shaft diameter of a tip end shaft part 55, with the position of the cam basic circle 51a made a starting point or a terminal point.SELECTED DRAWING: Figure 14 【課題】椅子の背凭れ反力機構の反力調整機構の軽量化並びに小型化を可能とする。【解決手段】背凭れの後傾に連動して背凭れを押し戻そうとする力を反力ばねの圧縮により発生させる椅子の背凭れ反力機構にあって、反力ばねに対し伸縮方向の変位を与える反力調整カム５１と、反力調整カム５１を回転させて反力ばねに付与されるカム変位量を切り替える反力調整操作軸７（７０）とを含み、反力調整操作軸７（７０）を回転操作することで反力調整カム５１によって付与されるカム変位量が切り替えられ、反力ばねの初期圧縮量を調整する反力調整機構を備え、反力調整カム５１は、反力調整操作軸７（７０）の軸部の最小軸径例えば先端軸部５５の軸径よりも小径のカム基礎円５１ａを有し、該カム基礎円５１ａの位置を起点または終点とするものである。【選択図】図１４