Seat belt retractor

A seat belt retractor for a vehicle including a reel shaft for winding up a webbing, a frame rotatably supporting both ends of said reel shaft, lock means disposed in between said frame and said reel shaft and arranged to allow rotation of said reel shaft under normal circumstances and to be activated, when need arises, so as to prevent rotation of said reel shaft in at least a direction in which said webbing is unwound, deceleration sensing means operating when deceleration exceeding a predetermined value acts on the vehicle, and means for activating said lock means in response to the operation of said deceleration sensing means, wherein the improvement is characterized that:

• said lock activating means has a lock means activating member that rotates together with said reel shaft under normal circumstances and that activates said lock means by relative rotation caused between the same and said reel shaft by the operation of said deceleration sensing means;
• said deceleration sensing means has a casing and an actuator that is provided on said casing so as to be pivotable between an inoperative position where it allows rotation of said lock means activating member and an operative position where it prevents rotation of said lock means activating member in at least the direction in which said webbing is unwound, said actuator including a lever and a pawl provided on said lever, and an inertia member that is accommodated in said casing and held in an erect position under normal circumstances so as to set said actuator in said inoperative position and that tilts when deceleration exceeding a predetermined value acts on the vehicle so as to set said actuator into said operative position;
• said inertia member has a large-mass portion as a top portion thereof, a small-mass portion as a bottom portion thereof, and an actuating projection formed on said top portion to pivot said lever toward said operative position when said inertia member tilts;
• said large-mass portion of said inertia member has a frustoconical configuration, and the bottom portion of said small-mass portion of said inertia member has a cylindrical or columnar configuration;
• said inertia member is formed so that when it tilts to such an extent that said inertia member abuts on a front wall of said casing, an outer peripheral side surface of said large-mass portion of said inertia member substantially coincides with the front wall of said casing;
• a pair of horizontally extending ridges having a trapezoidal cross-sectional configuration are formed on said front and rear walls, respectively, of said casing, and a pair of recesses having a trapezoidal cross-sectional configuration, which are fitted with said pair of ridges, respectively, are formed on said housing of said deceleration sensing means, which is provided on said frame, wherein said trapezoidal cross-sectional configuration of said ridges and that of said recesses are substantially similar to each other, and the trapezoidal cross-section of said ridges is little larger than that of said recesses.