Patent ID: 7278600

Claim:
A seat belt retractor for winding a seat belt, comprising: a spool for winding the seat belt, a motor for generating rotational torque to rotate the spool, a power transmission mechanism for transmitting the rotational torque of the motor to the spool to wind the seat belt onto the spool, said power transmission mechanism having a low reduction ratio mechanism for transmitting the rotational torque of the motor to the spool in a low reduction ratio transmission mode, a high reduction ratio mechanism for transmitting the rotational torque of the motor to the spool in a high reduction ratio transmission mode, and a power transmission blocking mode which disconnects a path from the motor to the spool to prevent transmission of the rotational torque of the motor to the spool, and a power transmission mode switching mechanism for switching the power transmission mechanism in one of the low reduction ratio transmission mode, the high reduction ratio transmission mode and the power transmission blocking mode, wherein said power transmission mode switching mechanism switches the power transmission mechanism in one of the low reduction ratio transmission mode and the high reduction ratio transmission mode according to tension of the seat belt, and switches the power transmission mechanism in one of the low and high reduction ratio transmission modes to the power transmission blocking mode when the motor rotates in a direction of unwinding the seat belt, wherein said power transmission mode switching mechanism switches the power transmission mechanism to the low reduction ratio transmission mode when the tension of the seat belt is smaller than a predetermined value, and switches the power transmission mechanism to the high reduction ratio transmission mode when the tension of the seat belt is greater than the predetermined value, and wherein said power transmission mode switching mechanism comprises a clutch gear having external teeth, a rotary shaft for rotationally supporting the clutch gear, a clutch arm pivotally supported on a frame and having a supporting groove capable of supporting the rotary shaft along the supporting groove, and a friction spring formed on the frame and engageable with the clutch arm.