Auxiliary Systems: SRS Systems: Safety systems
This Video Sequence is
not available in the free
CDX eTextbook
 
Click this icon for more information about CDX Automotive Resource Kit Plus
This Knowledge Check is
not available in the free
CDX eTextbook
 
Click this icon for more information about CDX Automotive Resource Kit Plus
This Lab Experiment is
not available in the free
CDX eTextbook
 
Click this icon for more information about CDX Automotive Resource Kit Plus
This Workshop Activity Sheet is not available in the free CDX eTextbook
 
Click this icon for more information about CDX Automotive Resource Kit Plus
This Assessment Checklist is not available in the free CDX eTextbook
 
Click this icon for more information about CDX Automotive Resource Kit Plus
This Handout Activity Sheet is not available in the free CDX eTextbook
 
Click this icon for more information about CDX Automotive Resource Kit Plus

Topic IntroductionHelp

Seat belt pre-tensioners

Summary
There are two types of seat belt pre-tensioner, mechanical & electrical. Seat belt pre-tensioners are used to tighten the seat belt in a severe frontal accident.
Seat belt pre-tensioners

Seatbelt pre-tensioners are used to tighten the seatbelt in a severe frontal accident. Both mechanical and electronic control systems are available. The most common type relies upon an explosive charge that is detonated electronically by a sensor within the seatbelt tensioning mechanism.

This explosion moves a piston that pulls on a steel cable causing the belt to tighten by approximately 4 inches or 100 millimeters. The design allows for the belt to tension before the occupant has moved forward in the seat.

Mechanical systems rely on inertia to move a sensing mass. This releases a spring to pull on a cable, thus tightening the belt.

Once the pre-tensioner has triggered, a ratchet prevents the seatbelt from loosening. When the seat belt is removed from the buckle, it cannot be reinserted, and the assembly should be replaced.

Rip stitching is used on seat belts in conjunction with an air bag and seat belt pre-tensioners. During a collision the pre-tensioners initially pull the seat belt tight, however the stitching gradually tears to allow the occupant to move forward into the air bag at a controlled rate.

For safety reasons, these belts must be replaced once they have had their stitching ripped. Manufacturers generally fit warning labels within the fold to indicate the belt is to be replaced when the label is revealed.