Seatbelt pretensioners are a component of the seatbelt system which locks the seatbelt in place during a crash. There are three types of seatbelt pretensioner: mechanical, electrical, and pyrotechnic.
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.
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.
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.