Seat belts stop you during a crash primarily through a combination of immediate tightening and controlled release of webbing, preventing ejection and reducing impact forces.
The Two Key Mechanisms of Seat Belt Functionality
Seat belts employ two key technologies to effectively restrain occupants during a collision: crash tensioners and force limiters.
1. Crash Tensioners: Instantaneous Tightening
- Mechanism: When a collision is detected, crash tensioners (also called pretensioners) are activated. These devices rapidly tighten the seat belt webbing around the occupant's body.
- How it works: These pretensioners typically use small pyrotechnic charges (like tiny controlled explosions) or electric motors to quickly retract the slack in the seat belt.
- Purpose: The primary goal is to minimize the occupant's forward movement during the initial stages of the crash, preventing them from slamming into the steering wheel, dashboard, or windshield.
2. Force Limiters: Controlled Webbing Release
- Mechanism: Force limiters are designed to release some of the seat belt webbing in a controlled manner as the occupant moves forward.
- How it works: Force limiters are built into the seat belt mechanism and allow a specific amount of webbing to spool out gradually when the force exerted on the belt reaches a certain threshold. This is often achieved through a calibrated twisting or tearing mechanism.
- Purpose: By releasing the webbing slightly, force limiters reduce the peak forces exerted on the occupant's chest. This helps to minimize the risk of chest injuries such as broken ribs or internal organ damage.
In Summary
The combination of crash tensioners and force limiters allows seat belts to:
- Immediately restrain the occupant, minimizing forward movement.
- Control the forces exerted on the occupant's body during the crash, reducing the risk of serious injury.
Without these features, the risk of severe and fatal injuries in a car accident would be significantly higher.