A seat belt pre tensioner is a device that is used to hold a passenger firmly in their seat just before and at the
start of a collision (especially in a front or head on collision) and then allow that passenger to come into contact with
the inflated airbag in a controlled way.
The pre tensioner is an addition to the basic seat belt. While the basic has been to reduce serious injury and save
lives, research and crash testing has shown that the seat belt in itself is not perfect and
that it can be improved.
With a seat belt there is still a small space between it and the passenger. If a frontal collision occurs, the passenger's
body moves forward quickly but the seat belt stiffens up to restrain the passenger from any further forward movement. The
passenger's body can also compress against the seat belt in the slight forward movement.
This process is very good compared to someone who is not wearing a seat belt but it is not the perfect scenario as the
possibility exist that the passenger can still be severely shaken up and it may not always work perfectly with the airbag.
How the seat belt pre tensioner works is by tightening the seat belt as soon as the collision occurs or even before by
monitoring sudden hard application of the brakes. The belt is pulled tightly against the
passenger and holds them firmly in the soft seats. Some milliseconds later it releases the passenger in a timely manner to hit
the airbag at the correct time.
There are two types of seat belt pre tensioners:
Mechanical Pre Tensioner
This pre tensioner is operated mechanically. It is usually connected to the seat belt buckle assembly with a long
powerful spring that is compressed in a tube and is kept compressed by a latching mechanism. If an accident occurs the impact
causes the latch to release the spring which pulls on the buckle assembly and tightens the seat belt.
Electronic/Pyrotechnic Pre Tensioner
This pre tensioner operates with an ECU and a gas generator system in a similar
way to the airbag. In fact the electronic pre tensioner usually works together with the airbag and may even use the same ECU
as the airbag and that increases its effectiveness.
An explosive charge is used and causes the gas generator to produce a volume of gas. This volume of gas produces a pressure
which acts on a mechanical linkage to pull the seat belt. This pre tensioner is usually located in the B-pillar.
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Both pre tensioners are designed to slacken off slightly around the same time the airbag deploys, this can be around 30
milliseconds after the collision. This slight lapse allows the passenger to have a controlled contact with the airbag.
Both pre tensioners are intended to be used once. Once a pre tensioner has been deployed the entire mechanism including the
seat belts should be replaced. The seat belt pre tensioner can be found on the front and rear seats to provide all round safety and
protection.
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