Seat-belt pretensioner coupling

A pretensioner coupling for a seat belt retractor for a motor vehicle, the retractor having a belt shaft and a pretensioner drive for winding the belt shaft, the pretensioner coupling comprising:

• the belt shaft having a coupling neck forming a notch,
• the pretensioner drive having a drivewheel journalled for rotation on the coupling neck, the drivewheel forming a recess,
• a coupling latch having an engagement region and a pin positioned on opposite ends of an arm, the latch movable between a release position and an engagement position, the coupling latch engagement region engaging both the notch and the recess producing a load-transmitting rotational connection between the belt shaft and the drivewheel when in the engagement position, and movable out of the engagement position and into the release position wherein the engagement region disengages coupling between the recess and the notch to permit relative rotation between the belt shaft and the drivewheel, and
• an inertial mass mounted on the belt shaft and being rotationally arranged in relation to the belt shaft, the inertial mass forming a cam track in the form of a spiral, the pin positioned in the cam track wherein when the pretensioner drive is activated to drive the belt shaft, the drivewheel rotates relative to the inertial mass, moving the pin in the cam track and orienting the engagement region to the engaged position and the pin reaching an end of the cam track forcing the inertial mass to rotate with the belt shaft, and upon the pretensioner drive no longer driving the drivewheel, the inertial mass is rotating faster than the belt shaft causing the pin to move in the cam track to move the latch to the release position.