Chair having automatically adjustable backrest

A chair having a seat, a backrest and support legs, the backrest being automatically adjustable for thereby preventing various backbone-related diseases, comprising:

• upper, intermediate and lower plates installed between the seat and the support legs at a certain interval therebetween in upper and lower directions;
• a hydraulic pressure-applying means installed between the intermediate plate and the lower plate in such a manner that an elastic operation is achieved in the upper and lower directions, the hydraulic pressure-applying means including a hydraulic pressure-applying cylinder operating in such a manner that when a user sits on a chair, the intermediate plate is elastically compressed by weight of the user, and a hydraulic pressure is applied to a reciprocation operation cylinder installed between the upper plate and the intermediate plate, and when the user gets up from the seat, the compressed state of the intermediate plate is recovered to its original state, so that the hydraulic pressure supplied to the reciprocation operation cylinder is applied to the hydraulic pressure applying cylinder;
• a guide box installed between the upper plate and the intermediate plate and horizontally installed on an upper surface of the intermediate plate in forward and backward directions;
• a backrest moving means including:
• the reciprocation operation cylinder horizontally installed in an interior of the guide box, and a piston rod extends as hydraulic pressure is inputted into the reciprocation operation cylinder and discharged from the hydraulic pressure-applying cylinder of the hydraulic pressure-applying means, and when the hydraulic pressure-applying cylinder collects hydraulic pressure the pressure is discharged from the reciprocation operation cylinder, the piston rod retracts;
• and
• a main operation member horizontally installed and passing through a movement elongated hole formed at both sides of the guide box in a direction perpendicular to the guide box and connected with a front end of the piston rod of the reciprocation operation cylinder and forwardly and backwardly moved within a moving range of the movement elongated hole of the guide box based on the forward and backward movements of the piston rod;
• a backrest frame connected at both ends to the main operation member of the backrest moving means and forwardly and backwardly moved together with the main operation member;
• and
• an assistant operation member movably installed at a rear side between the upper plate and the intermediate plate and forwardly and backwardly moved together with the backrest frame in a state that both ends of the same are connected with the backrest frame.