Patent ID: 8678509

Claim:
A seat belt retractor comprising: a seat belt; a spool that retracts the seat belt; a locking member that corotates with the spool in a normal state and that is inhibited from rotating in a seat-belt withdrawing direction in the event of an emergency so as to produce relative rotation with respect to the spool; a lock mechanism that locks the rotation of the locking member in the seat-belt withdrawing direction in the event of the emergency; an energy absorption mechanism that is provided between the spool and the locking member and that limits a load applied to the seat belt during the relative rotation between the spool and the locking member; and a pretensioner that operates in the event of the emergency so as to rotate the spool in a seat-belt retracting direction, wherein the pretensioner includes at least a case, a pipe attached to the case, a predetermined number of balls that are movably provided in the pipe and that transmit a force for rotating the spool in the seat-belt retracting direction, a gas generator that generates gas in the event of the emergency, a piston that is movably provided in the pipe and that is moved by pressure of the gas so as to push the balls, a ring gear that is at least rotatably provided in the case and that has a plurality of internal teeth on an inner periphery and a plurality of pressed portions on an outer periphery, the pressed portions being pressed by the balls, and a pinion that is provided at a spool side member in the case, that has external teeth to be meshed with the internal teeth, and that rotates the spool, wherein the piston includes an exhaust passage that allows a side of the piston close to the gas generator to communicate with a side of the piston close to the balls, wherein the exhaust passage is defined by an exhaust hole extending in an axial direction of the piston, and an exhaust groove provided at an end of the piston in contact with the balls and extending in a radial direction of the piston, the exhaust groove communicating with the exhaust hole, and wherein the piston includes a piston body and a cylindrical piston ring fitted on a shaft portion of the piston body, an axial direction length of the piston ring is shorter than an axial direction length of the shaft portion, and the exhaust passage is provided in the piston body.