Introduction and operation

INTRODUCTION AND OPERATION

The seat belt PRE-TENSIONER is a device integrated in the reel which, in the case of a frontal impact, recovers the inevitable lengthening of the belt due to the action of the weight of the body of the person in the seat, ensuring that they adhere to the backrest.

It is, in effect, vital that the belt remains adhering as closely as possible to the body of the person in order to gradually absorb the kinetic energy assumed during the impact.

The lengthening of the belt can be due to the following causes:

• - delay in the operation of the inertia locking device;
• - stretching of the belt fibres;
• - wrapping of the belt around the reel (spooling effect);
• - garments of a certain thickness which increase the distance between the belt and the chest.

Diagram showing operation of pre-tensioner

OPERATION

The pre-tensioner fitted on the vehicle is equipped with a bracket (see page 38) which has the task of making the system active.

This takes place because the end of the above mentioned bracket acts on the pulley (14) overcoming the spring (12) loading and pushing the pulley (14) tooth (13) in the direction of the arrow shown in detail in the diagram, releasing the oscillating mass (1) thereby allowing it to act.

Sudden deceleration, caused by an impact, causes the advance of the oscillating mass (1) and the compression of the opposing spring (2). In the case of sufficiently rapid deceleration the oscillating mass is placed in such a position that the gas generator (4) is released from its retaining system (3) and moves downwards through the action of the thrust spring (5) and impacts with the percussion-pin (6) which causes the engagement of the ignition charge This gives rise to the combustion of the pyrotechnic charge which activates the main charge and the development of the gas in the chamber (7) which flows through the special opening to the piston (8) causing the rapid movement along the expansion chamber.

The metal rod (9), fixed on one side to the piston and on the other wrapped around a pulley fitted on the same axis of the belt (11) winding (10) roller, causes the rotary movement required for the latter to rewind.

At the end of the operation the reel locks in the maximum return position for the actual belt on the body of the occupant of the appropriate seat.

The locked belt is an indication that the pre-tensioner has been activated, or of a malfunction of the reel. In both cases, the device must be replaced by authorized personnel.