What is Seat Belt Pretensioner | Types and Working Principles

What is Seat Belt Pretensioner | Types and Working Principles

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.

Seatbelt pretensioners are a component of the seatbelt system which locks the seatbelt in place during a crash.

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 three types of seat belt pre-tensioners:


The mechanical pretensioner consists of an inertial wheel combined with a pendulum which moves to lock the belt into place during sudden deceleration. Most drivers have experienced the frustration of pulling a seatbelt from its reel only to have it lock unexpectedly or to be driving down the road and experience the seatbelt gradually getting tighter and tighter. This is the action of the mechanical pretensioner at work.

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.


The electric pretensioner replaces the pendulum with an electrical sensor. This prevents the false locking condition that is commonly observed with a mechanical pretensioner. The electrical sensor can also be interconnected with other systems on the vehicle, such as rollover sensors or airbag sensors. This connectivity ensures that the seatbelts will lock even if the sudden deceleration that is needed to lock a mechanical pretensioner does not occur.


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.

The pyrotechnic pretensioner is the most sophisticated type of pretensioning device. The electronically triggered pyrotechnic device not only locks the seatbelt in place but tightens the belt to take up any slack that may be present, minimizing the movement of the occupant within the vehicle during a crash situation. Once a pyrotechnical pretensioner is activated it must be replaced.

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.

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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Sachin Thorat

Sachin is a B-TECH graduate in Mechanical Engineering from a reputed Engineering college. Currently, he is working in the sheet metal industry as a designer. Additionally, he has interested in Product Design, Animation, and Project design. He also likes to write articles related to the mechanical engineering field and tries to motivate other mechanical engineering students by his innovative project ideas, design, models and videos.

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