The legal obligation to wear seat belts - at least in Central Europe - is to a large extent, adhered to on motorways and country roads. There are still some incessant drivers who try to get away with driving unbelted in urban areas. It is recommended that they check out the test where a collision, at 11 kmh, with a solid wall is shown. Indeed, the seat belt is sometimes too loose and not taut across the body. The fault lies in either clothing which is too light, careless behaviour or the retractor is too lax. The smaller airbag, particularly in motor cars up to mid-range models, only offers the necessary passive security together with the three-point-belt. However, even if the belt is taut across the body, when a collision, at normal urban speeds occurs, the head and body of the passengers are not effectively kept away from the steering wheel or dashboard. The belt also stretches when strained and the more belt that is rolled up, the further it must unroll before it locks. This is where the tensioner comes into play.
This is where pyrotechnics come into the picture. A mini-explosive device is installed right next to the belt-retractor. It is ignited by the control device according to the same criteria that trigger the airbag, in addition it checks whether or not the seat belt is actually in use. The effect on the passengers however, occurs earlier. Only one of the various construction types is described here. In the above picture, several balls are shown, their movement causes a windlass to rotate, this tightens the belt, the balls then fall into a container where they cannot do any damage. The windlass retracts a pre-defined amount of seat belt, thus causing it to tighten up. This construction only affects the (upper) shoulder- or sash belt. If, pyrotecnically seen, the complete seat belt roller is shifted to the rear, then the lap belt is also tightened. Newer constructions are now combined with a belt-force limiter. To make access to the rear seats easier in two- or thee door vehicles, the retractor point can be movable. In this case, the pyrotechnics can also be located in the belt buckle (see bottom picture). In the meantime, there are also electrically driven belt tensioners.
For adults and children who have outgrown the child safety seat age, there are usually no rear seat belt tensioners, only Mercedes and Volvo offer them for their vehicles, other manufacturers in certain models only, or an extra charge. Doctors claim that the remarkably high number of abdominal injuries from accident victims on the back seat would decrease, if there would also be mandatory seat belt tensioners, after all, a cost of more than 100 €. in the series. Incidentally, it's not so easy to identify whether your car is equipped so rear, certainly not by asking the car dealer.
function of the belt tensioner has been explained using the ball-tensioner as an example. The same result can be obtained with other contruction types, e.g., the webclamp, the tubular retractor or the one with a small pump, similar to the Wankel (rotary) engine. By the way, did you know that, depending on the vehicle, the purposely annoying sound-signals which are given out when the seat belt is not used, should only be heard when a speed of 25 kmh is exceeded? 03/12