As the name indicates Seat belt Pretensioner, it is also known as safety belt pretensioners. It is a device mainly designed to protect the passenger of the vehicle against any harmful movement caused during a collision on a sudden stop and then allow that passenger to come into contact with the inflated airbag in a controlled way. Seatbelts are considered Primary Restraint Systems (PRS), because of their vital role in passenger safety. When the driver and the passenger are traveling with the same speed in motion and suddenly the car stops with a great force then car belt prevents both of them from collision to the interior of the car by applying an opposite force to driver and passenger to the car. By so many crash testing and research, seat belts were improved and pretensioners were added to the basic seat belt for reducing serious injury and save lives of so many people.
Types of Seat Belt Pretensioner
Mechanical Seat Belt pretensioner
It operates mechanically and usually connected to the seat belt buckle assembly with a long powerful sphere that is compressed in a tube and is kept compressed by a latching mechanism. If any collision occurs the latch releases the spring which pulls on the buckle assembly and tightens the seat belt.
Electronic/Pyrotechnic Seat belt pretensioner
It works with an ECU and a gas generator system similar to the airbag. It usually works together and may use the same ECU as the airbag and that increases its effectiveness. For producing a volume of gas, it uses an explosive charge and this volume of gas produces a pressure which acts on a mechanical linkage to pull the seat belt. This pretensioner is usually located in the B-pillar. Both pretensioners have been designed to slacken off slight time lapse which allows the passenger to have a controlled contact with the airbags.
Working of Seat belt pretensioner
When a car hits something, it starts to decelerate (lose speed) very speedily and airbag sensor detects the change of speed. If the deceleration is great enough, the airbag sensor sends a signal to the ACU. The ACU passes an electric current through a heating element and it ignites a chemical explosion. The older airbags used sodium azide as their explosive chemical and newer ones use different chemicals. As the explosive burns, it generates a huge amount of harmless gas that quickly fills up the nylon airbags and as the bag expands, it blows the plastic cover off the steering wheel and inflates in front of the driver. The bag is coated with talcum powder to help it unwrap smoothly. The driver (moving forward because of the impact) pushes against the bag and the beg deflate as the gas it contains escapes through small holes around its edges. At last car stops and airbags deflated.
Like a valiant but tragic hero, the seat belt pretensioner sacrifices its life in an accident in order to save ours. Hence it is necessary to have seat belt pretensioner in the vehicle to avoid hazardous accidents and to protect our loved ones.