Seat Belt Tensioner: Description and Operation

FUNCTION OF PRETENSIONER
When a vehicle crashes with a certain degree of frontal impact, the gas generator will ignited an electrical firing signal from the SRSCM (Supplemental Restraint System Control Module).

Gas from the gas generator causes movement of the piston in the manifold case (cylinder), which operates the rack gear.

The rack gear, rotates a piston gear and a pinion rotates the planet gears.

Finally, the webbing is retracted by the rotation of the spool. Therefore, the pretensioner seat belt helps to reduce the severity of injury to the front seat occupant by retracting the seat belt webbing. This prevents the occupant from thrusting forward and hitting the steering wheel or the instrument panel when the vehicle crashes.





LOAD LIMITER
The load limiter is designed to relieve the impact force to an occupant chest's of the seat belt webbing when the occupant is restrained by the seat belt during a crash. If the crash force reaches a certain value, the torsion bar in the pretensioned seat belt will deform and cause the webbing to extracted from the seat belt, thus, relieving the impact force.