Shock absorber for a back rest of a chair

A shock absorber operable for properly absorbing shock caused when a user sits on a chair and then reclines against a back rest of said chair, said back rest including a pair of support arms for turnably supporting said back rest at upper ends thereof, lower ends of said support arms being pivotally held by a first transversely extending shaft at a rear end part of a base frame fixedly mounted on an upper end of a foot column for said chair, comprising:

• a mounting frame having a front end which is turnably supported by a second transversely extending shaft fixedly secured to said base frame, said second transversely extending shaft extending through said mounting frame;
• a pair of spring mounting rods comprising an elongated U-shaped spring mounting rod having parallel straight portions and an engagement portion integrally bridged between rear ends of said straight portions, said spring mounting rods having front ends which are received in said mounting frame while slidably extending through a rear wall of said mounting frame in a longitudinal direction, said front ends of said spring mounting rods being fixedly secured to a spring support portion slidably arranged on said mounting frame, said spring mounting rods further having rear ends which are connected to each other via an engagement portion bridged therebetween;
• a pair of compression springs characterized by a bias force, said springs mounted on said spring mounting rods in said mounting frame between said spring support portion and said rear wall of said mounting frame;
• an engagement piece adapted to turn about said first transversely extending shaft together with said support arms, said engagement piece being slidably engaged with said engagement portion bridged between the rear ends of said spring mounting rods;
• a guide member slidably bridged between said spring mounting rods at the rear ends of the spring mounting rods outside of said rear wall of said mounting frame, said guide member having a nut which is threadedly engaged with an adjustment bolt;
• and
• a handle fixedly secured to an outer end of said adjustment bolt, said handle being located outside of said base frame to rotate said adjustment bolt, rotation of said handle causing said guide member nut to travel along said adjustment bolt and turnably displacing said mounting frame, thereby repositioning said guide member relative to said engagement piece and adjusting said bias force;
• wherein:
• when said mounting frame is turnably displaced in a downward direction by rotating said handle in a predetermined direction so as to allow said engagement portion of said spring mounting rods to assume a position on the rear surface of said engagement piece near said first transversely extending shaft, a weak shock absorbing mode is exhibited for said back rest, while when said mounting frame is turnably displaced in an upward direction by rotating said handle in an opposite direction so as to allow said engagement portion of said spring mounting rods to assume the position on the rear surface of said engagement piece away from said first transversely extending shaft, a strong shock absorbing mode is exhibited for said back rest;
• and
• irrespective of whether the engagement portion is located near said first transversely extending shaft to impart weak shock absorbing to the back rest, or away from said first transversely extending shaft to impart strong shock absorbing, said compression springs have a substantially consistent initial biasing force, so that shock absorbing is consistently weak immediately after a user reclines and then gradually enlarges, whereby a user can enjoy a pleasant resting attitude in both the weak shock absorbing mode and the strong shock absorbing mode.