TRW (trw.com) is launching its next generation SPR4 seat belt assembly for global applications, ranging from A-segment cars up through sport utility vehicles. The SPR4 (the letters stand for “Snake Pretensioner Retractor”) uses a plastic piston for transferring tensioning torque rather than a metal one.
Explains Norbert Kagerer, TRW vice president of Occupant Safety Systems engineering, “The lighter-weight plastic material allows the tensioning force to be generated more quickly than with conventional systems. Secondly, the damping behavior of the plastic snake allows the initial peak force, when impacting the pinion, to be significantly lower compared to conventional systems where two rigid steel elements impact on each other.”
That is, when the seat belt system is triggered by the vehicle sensors, a pyrotechnical gas generator releases gas that builds up pressure in the guiding tube, propelling the snake-like plastic piston into a pinion, that transmits torque to the belt retractor spool, thereby pretensioning the belt. This happens in 10 msec.