Operation

OPERATION

The front seat belt system incorporates both driver and passenger Seat Belt Tensioner (SBT) retractors. At the onset of an impact event each tensioner uses a pyrotechnic device, which is triggered simultaneously with the front airbags to rapidly retract the seat belts. With the slack removed, the occupant's forward motion in an impact will be reduced as well the likelihood of contacting interior components. After an impact that deploys the airbag, the seat belt tensioner assembly must be replaced. The Occupant Restraint Controller (ORC) module monitors the seat belt tensioners circuit resistance and reports active and stored DTCs if any problem is found. Follow all of the safety procedures when servicing tensioner.





The Seat Belt Tensioners (SBT) are equipped with a force limitation device which reduces the force of the belt on the passenger's body, without affecting the protective function of the seat belt during an impact.

SEAT BELT TENSIONER - FUNCTIONALITY





In the event of a collision, the Occupant Restraint Controller (ORC) puts out an electrical signal for triggering the Seat Belt Tensioner (SBT) after a certain delay.

The Squib in the gas generator is triggered and the resulting combustion gas moves the piston (2) in the gas generator cylinder (3) up at high speed. The pull cable (4) connected to the piston is tensioned causing the cable pulley (5) to turn. Inertia and centrifugal force cause the pawls (7) in the cable pulley (5) to engage in the teeth in the toothed ring (6).

A positive connection is then present between the gas generator piston and the seat belt retractor shaft via the pull cable (4), the cable pulley (5) and toothed plate pulling and thereby tensioning the seat belt (1).

FORCE LIMITATION





After tensioning the seat belt, the gas pressure in the gas generator cylinder (1) decreases. The pull cable (2) is relieved and the force on the cable pulley (4) reduced.

The force exerted by the passenger on the seat belt then changes the direction of pull and the seat belt (8) attempts to unwind. This is prevented by the outer locking pawls (3) in the toothed ring (5). The change in the direction of pull simultaneously causes a change in the direction of rotation of the retractor shaft. Inertia and centrifugal force cause the outer locking pawls (3) on the toothed ring (5) to engage in the inner teeth on the locking ring (7).

As soon as a locking pawl (3) is in contact with the teeth on the locking ring (7) the latter turns until it locks up. The arrested locking ring (7) causes the torsion rod (6) to arrest on one side. The torsion rod (6) forms the heart of the retractor shaft and therefore takes up the tension forces acting on the seat belt.

As soon as the force on the seat belt exceeds 4 kN, the torsion rod (6) deforms, counteracting a further increase in the load and limiting the force.

As soon as the force on the seat belt decreases, the locking pawls in the toothed ring and the pawls on the cable pulley return to their initial position as the seat belt is retracted. The locking and retracting functions of the inertia reel are effective again.





Triggering of the emergency tensioning retractors is indicated by the open viewing window (1) on the tensioning system housing

In the event of another collision, the normal protective function of the seat belt is still available. The retractor shaft is locked up by the locking system without tensioning and force limitation so that the entire force is taken up.