Introduction:
In modern vehicles, all seat belts at the front seats are equipped with a belt tensioner. This belt tensioner tightens the belt in the event of a frontal or side impact (at an angle of up to 30 degrees relative to the vehicle’s centerline).
When the belt is pulled hard, in any design – including systems without a belt tensioner – the retractor will lock. The belt can then no longer be pulled out further. (The retractor should not be confused with the belt tensioner.) As soon as the belt is released slightly, the retractor unlocks and the belt can be pulled out again. This is easy to test with any belt in the vehicle. This is also checked during a major service and the periodic vehicle inspection (APK). If the retractor does not lock, wearing the belt is theoretically of little use.
In a frontal or side impact (at an angle of up to 30 degrees), the vehicle experiences heavy deceleration. As a result, the belt retractor locks, preventing the occupant from being thrown forward and injured. With systems that have a belt tensioner, the belt is pulled even tighter. After the collision, a small explosive charge ensures that the belt buckle is pulled down within 30 milliseconds. This tightens the belt significantly. At that moment, the retractor locks immediately, just as when the belt is jerked sharply. The occupant’s freedom of movement is further restricted, reducing the risk of injury.
The belt tensioner can be located at the buckle section (next to the front seats) or at the retractor in the B‑pillar. Both types are explained below.
Belt tensioner at the buckle section:
In a collision, the airbag control unit applies voltage to the gas generator, igniting the pyrotechnic element. The working piston is propelled to the left by this explosion (see image below) and remains trapped in the left section. Because the cable is connected to both the buckle section and the working piston, the buckle section is pulled downward with it. As a result, the belt is tightened by a distance of approximately 80 mm (8 cm).
Belt tensioner at the retractor:
This belt tensioner is mounted in the B‑pillar (roughly next to the seat, behind the interior trim). Here too, the airbag control unit applies voltage to the gas generator after a collision. The powder (shown in red) is ignited. The blue plunger forces the balls downward with great force. The recesses of the gear wheel are exactly the same size as the balls. Because the balls move downward with great force, the gear wheel starts to turn. The belt is wound onto the extended section of the gear wheel, which can also be seen in the illustration.
By winding the belt, it is shortened. With this tensioner, the belt is wound in by about 80 mm, and this happens within 30 milliseconds after the collision.
After activation:
When a car is involved in a collision and the belt tensioner is activated, it becomes unusable and must be replaced. Belt tensioners are designed so that after activation the connection between two metal contacts is broken. The airbag control unit continuously monitors the resistance of the system and will record a fault in the event of an interruption (infinitely high resistance). In that case, the airbag warning light will come on. The fault code cannot be cleared as long as a defective or missing belt tensioner is fitted.
With some older types of belt tensioners, it is possible to visually check whether the system has been activated. At the end of the metal tube, in the direction in which the piston moves, there is a colored plastic cap that is ejected after activation. This cap serves as an indicator that the tensioner has deployed. In addition, after activation the buckle mechanism sits noticeably lower than normal, making it more difficult to reach.
It is important that after every collision in which a belt tensioner or airbag has deployed, all components of the safety system are inspected and replaced in accordance with the manufacturer’s specifications.