When accidents occur, several systems incorporated in the seat belt assembly go into operation simultaneously. In less time than it takes for a human being to blink an eye, the seat belt system goes into action to control the energy load the occupant endures on impact. In the latest designed motorized seat belts, there are also functions to warn the driver, as the seat belt motor gives a tug on the belt when there is an immediate dangerous situation.
Every Second Counts
Here is a timeline on how seatbelts work. “T” stands for the time of collision:
- T+0.003 Seconds — Sensors detect the impact
Collision sensors that are attached to the vehicle detect the collision, a signal is then sent to the ECU (Electronic Control Unit). When the speed of the vehicle suddenly drops because of impact, a separate vehicle sensitive locking feature located in the retractor detects the negative acceleration (rate of deceleration) and locks the webbing to keep it from further extraction. This restrains the forward movement of passenger. The vehicle sensitive locking feature also goes into operation when the driver applies the brakes suddenly or turns the steering wheel sharply.
- T+0.015 Seconds — Evaluation of the impact
The ECU (Electronic Control Unit) processes the signal from the satellite sensors and diagnoses the severity of the impact. If the impact is identified by the ECU to be a collision, the ECU sends a signal to activate the pre-tensioners (gas emitting devices) attached to the seat belt.
- T+0.020 Seconds — The pre-tensioners activate
The pre-tensioners remove some of the slack between the passengers and the belts, thereby restraining the passengers in their seats more effectively.
- T+0.040 Seconds — Load limiter goes into action
As the occupant moves forward, the seat belt load limiter activates and is allowed to move webbing from the housing to help absorb the early weight burden on the occupant.
- T+0.060 Seconds — Energy Movement
The energy of movement of the passenger’s body is absorbed by the crushing of the vehicle, the load limiter, and the airbags.
- T+0.120 Seconds — Absorption of the energy of the passenger’s movement is now completed
Seat belts are the primary device for passenger safety. The “airbag” is a “supplemental restraint system”. As the meaning of the name suggests, the airbag plays a supplementary role in restraining and protecting passengers and are most effective when the occupants have their seat belts fastened. Buckle up and stay safe!