Vehicle safety belts typically include a retractor mechanism which causes the belt to automatically wind onto a spring loaded reel when not in use. The retractor also insures that the belt remains flush against the person's body as the person changes seated positions, thus allowing the person to move freely without having to manually adjust the belt. In order to secure the person in the event of an emergency, the retractor also has a locking mechanism which senses the emergency condition and locks the reel, thus preventing further extension of the belt and keeping the person secured against the seat.
Typically a retractor responds in an emergency situation by sensing the deceleration of the vehicle, or the rotational acceleration of the reel. In an example of an acceleration sensing mechanism a freely rotating inertia element senses the belt unwinding, angular acceleration of the reel. As the reel accelerates, the rotation of the inertia element lags behind the rotation of the reel assembly. The relative change in position causes the inertia element to move a locking or braking mechanism into position and brake the reel. Some retractors, have locking means which respond to both the acceleration of the belt and the deceleration of the vehicle.
The locking mechanism in some prior art involves a ratchet attached to one or both sides of the reel which is surrounded by teeth. There is also either a bar or a pawl which is capable of locking the reel by engaging the ratchet teeth, and which moves into this locking position upon sensing the emergency condition. The ratchet teeth may be located either on the radial interior or exterior relative to the retractor housing. That is, the ratchet may be designed as a wheel with teeth pointing outward on the outside of the wheel or as a ring with teeth pointing inward on the inner circumference of the ring. Inwardly extending teeth or locking elements offer certain advantages, but such systems have other complexities concerning the mounting of components.