The pretensioner mechanism consists of the retractor shaft, pretensioner shaft, drum, wire, drive plate, piston, cylinder, inflator, etc.
The pretensioner shaft is directly installed on the retractor shaft (for seat belt winding) and goes inside the drum. Since there is clearance between pretensioner shaft and drum in the normal condition, these do not contact each other.
There is a spring portion on the drum. As a result, the drum contracts inward by the force produced when the wire that is wound around the drum is drawn out.
The drive plate is installed so that it turns together with the drum.
The wire is wound around the drum.
An end is fixed on the drive plate and the other is fixed on the cylinder via the piston.
When an impact that exceeds the specified value occurs, the inflator is ignited according to the signal from the center airbag sensor and generates high-pressure gas. This high-pressure gas strongly pushes the piston in the cylinder. As a result, the wire is retracted. Then the drum contracts in the center direction of the clearance and is pressed on the pretensioner shaft to be a unit with it. After that, the share pin that fixes the drive plate is sheared off, causing the drum, drive plate, and pretensioner shaft to rotate in the winding direction as a unit and the seat belt to retract to restrain the passenger.
The stroke about twice the moving distance of the piston is the length that the wire is drawn.
There are various types of pretensioners, such as one that uses a rack instead of a wire and drives by a gear, etc.
Force limiter mechanism
The retractor, force limiter and the spool are combined and generally they rotate as a unit.
If a larger load than the specified value is applied to the seat belt because of the movement of the passenger in a collision, the force limiter plate alters the shape (absorbs the energy) by the rotational force of the spool and winds around the shaft. As a result, the seat belt is released.
The force limiter plate can be deformed until the spool makes approximately 1.3 turns.
There are various types of force limiters, such as one that uses a torsion bar instead of a force limiter plate, etc.