The invention relates to a belt tensioner for a belt retractor comprising a load limiter. The invention further relates to a method of tensioning a seat belt and subsequent load limitation.
A belt tensioner is intended to tension the seat belt in a crash so that the vehicle occupant takes part as quickly as possible in the overall deceleration of the vehicle. By tensioning the seat belt especially the impacts of the film spool effect (belt webbing loosely rolled on the belt reel) and the belt slack (webbing that is not in close contact with the vehicle occupant) are reduced which have a negative influence on the restraint of the vehicle occupant.
A load limiter provides that the load of the seat belt acting on the vehicle occupant after tensioning does not excessively increase. In order to relieve the vehicle occupant, after locking the belt reel the belt reel is nevertheless allowed to rotate in the webbing unwinding direction (opposite to the tensioning direction), the rotation being decelerated, however, by energy conversion, i.e. by performing work (e.g. twisting a torsion rod).
In the case of most known belt retractors comprising a load limiter it is necessary to adjust a locking pawl prior to load limitation for locking the belt reel. However, the adjustment usually is not effectuated simultaneously with the end of the tensioning operation but with a time delay. This delay causes undesired decrease of the webbing load before the load limitation starts.