Front Belt Tension Limiter: Operation

The adaptive force limitation is based on a two-stage torsion bar (stage shaft). The torsion bar consists of the two head ends left and right, the stages and the center head. The belt force is transferred to the seatbelt roller. The seatbelt roller is connected to a sleeve that contains the torsion bar. There is a shaft ring with locking pawls on the sleeve. The locking pawls transfer the torque to the torsion bar.

Fig 1: Identifying Front Belt Tension Limiter Operation (1 Of 2)

Courtesy of BMW OF NORTH AMERICA, INC.

In the first stage, with the preset high level of force, the torque of the seatbelt roller is transferred via the locking pawls to the center head of the torsion bar.

If the seatbelt roller is turned relative to the fixed torsion bar, the force is transferred to the thicker part of the torsion bar. This produces the high power level. This is the function of a normal emergency locking retractor.

Fig 2: Identifying Front Belt Tension Limiter Operation (2 Of 2)

Courtesy of BMW OF NORTH AMERICA, INC.

In the event of a crash, the gas generator is ignited and a plunger moves out, shifting the shaft ring axially. The locking pawls are now no longer held by the sleeve and no longer transfer more torque to the center head of the torsion bar.

The belt force is now passed across the right-hand head end into the stage shaft and runs through the entire torsion bar. The lower diameter on the right-hand side means that the torsion bar is twisted further and thus the force is reduced to a lower level.