In modern belt retractors, both a reversible belt tensioning unit and an irreversible belt tensioning unit are used for driving the belt reel in wind-up direction, when a vehicle condition is regarded as critical, in order to eliminate a slack in the safety belt and bring the vehicle occupant into an upright position. The reversible belt tensioning unit is configured as a pretensioner, which in the case that the critical vehicle situation does not lead to an accident returns the belt reel to its starting position. The irreversible belt tensioning unit, however, only is activated in actual accident situations, which subsequently require the replacement of the entire belt retractor.
In addition, modern belt retractors include a force limitation system which limits the force maximally exerted by the belt onto the body of the vehicle occupant, in that it permits a certain withdrawal of belt webbing from a defined force acting in the belt webbing. If the irreversible belt tensioning unit now is activated as long as the reversible belt tensioning unit still is coupled with the belt reel, the succeeding limitation of force not only requires to overcome the resistance provided by the force limitation system, but also to turn back the reversible belt tensioning unit against its direction of action. As a result higher forces than desired and provided by the design of the force limitation system are generated in the safety belt.
This invention creates a belt retractor, in which during a force limitation phase after a tensioning effected by the irreversible belt tensioning unit, the reversible belt tensioning unit reliably is separated from the belt reel, but is again operable after such force limitation phase.