Only a seat belt can “slow down” a person in the event of a collision and an airbag deployment, provided that it is tightly tensioned - much more than with a quiet ride. Therefore, in the late 80s, the so-called pretensioners appeared.
The essence of the idea is that an asterisk is put on the axis of the spring-loaded coil winding the belt, which rotates with a set of balls. They are pushed through the tube by powder gases from a special squib. The latter works in parallel with the pillow holder, instantly choosing the slack of the belt. And so that the rider does not catch his breath, the axis with a certain effort on the tape (about 400 kgf) is twisted like a torsion bar, limiting the amount of tension.
Such pretensioner schemes have been used for quite some time (and now they are still not a thing of the past), but the designers found flaws in them too. The three-point seat belt, as you know, consists of two branches - waist and diagonal. Moreover, the tape passes twice through the loop (on the stand and in the buckle), bending around them at an angle of about 180 ° and also fits snugly on the clothes. Therefore, a single pretensioner does not provide uniform tension on both branches. So there were schemes (in particular, on Renault cars) with two linear pretensioners.
Here, the cartridges are fired into the pistons with which the belt lock and a special unit with a lower attachment point are connected.
Recently, a number of companies have been introducing hazardous systems (Pre Safe). If the sensors detect a threat of collision, they will pull the riders to the seats in advance. But not with a disposable squib, but with an electric motor in the inertial coil mechanism. Then the belts will relax to their original state, without restricting your movements.
