How exactly do seat belt systems work when accidents occur?
We would like to give an example of how seat belts equipped with a pretensioner and load limiter work.
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. By retracting some webbing as the collision is occurring, the pretensioner enhances the protective functions of the belt by removing some slack and helping restrain the passenger. The load limiter absorbs and softens the load on the occupant as the passenger moves forward because of inertia. 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, that there is immediate dangerous situation. (a pre-impact warning).
*This is one example of how pretensioners and load limiters operate. The figures are for reference.
*T means the time of collision
- T + 0.003
↓second—Sensors detect the impact
- As the satellite sensors (collision sensors) attached to the vehicle detects the collision, a signal is 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.)
【Key Point!】When you pull the seat belt webbing suddenly from the housing during normal use, it also locks. However, this is due to the operation of a separate webbing sensitive locking feature. Retractors are equipped with both vehicle sensitive and webbing sensitive locking features to further enhance safety.
- T + 0.015
↓second—Evaluation of the impact
- The ECU 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 pretensioners (gas emitting devices) attached to the seat belt.
- T + 0.020
↓second—The Pretensioners go into action.
- Next, the pretensioners activate and retract some of the seat belt webbing. By pulling the seat belt webbing back, the pretensioners remove some of the slack between the passengers and the belts, thereby, restraining the passengers in their seats more effectively. Removing this slack greatly enhances the passenger protection function of the seat belt system. By using the crushing of the car body efficiently, the system helps absorb the energy of motion that comes to bear on the passengers.
- T + 0.040
↓second—The load limiter goes into action as the occupant moves forward.
- The shock resulting from the accident reaches the passengers and the inertial force moves them forward. At this very moment when a specified level of weight is transferred to the webbing, 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.
【Key Point!】Before load limiters were developed, more of the energy from the accident was absorbed by the occupant as the belt stretched. Today, most vehicles are equipped with load limiter functions.
- T + 0.060
↓second—Energy of movement is absorbed.
- At this moment the seat belt has positioned the occupant to take advantage of the supplemental restraint system (airbags). 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
↓second—Absorption of the energy of passengers’ movement is completed.
- 【Key Point!】Seat belts are the primary device for passenger safety. The “airbag” is a “supplemental restraint system” (SRS). 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.