Trigger mechanism for belt pretensioner

A trigger mechanism for a belt pretensioner in a safety belt restraining system for a vehicle, comprising:

• a movably mounted sensor mass which is held in a rest position, said sensor mass being movable into at least two trigger positions which are situated on either side of the rest position, a first one of the trigger positions being associated with a head-on vehicle collision and a second one being associated with a collision in which the vehicle is rammed from behind, said sensor mass being pivotal between the first and second triggering positions in a common plane, said sensor mass being formed by a movable assembly which includes a biasing pressure spring and a guide tube accommodating said pressure spring to provide a mechanical belt pretensioner drive unit, said guide tube being pivotally mounted at a first end thereof on a base fixedly connected to a housing of a belt retractor;
• and
• a two-armed locking lever pivotally mounted on said guide tube and having a first, shorter arm which comprises at an end thereof a holding nose which engages an end face of a piston subjected to said pressure spring, and a second, longer arm which has a free end supported on a holding surface provided on said base.