Seat Belt Pre-Tensioner; Introduction; Operation - Fiat Brava Service Manual

Bravo-Brava Bodywork
Seat belt pre-tensioner
70.
INTRODUCTION
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.
P4A037M01
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) over­
coming 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 decelera­
tion the oscillating mass is placed in such a po­
sition 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 py­
rotechnic 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 pis­
ton and on the other wrapped around a pulley
fitted on the same axis of the belt (11) winding
(10) roller, causes the rotary movement re­
quired 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 autho­
rized personnel.
Copyright by Fiat Auto
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