Seat belt#Pretensioners and webclamps

The belt tensioner counts how the airbag also the so-called passive safety system of a vehicle. He has the task of streamlining in an accident the seat belts so that the occupant participates formerly of the total delay of the vehicle. For this, the belt is tightened in about 10-15 milliseconds to up to 15 cm. This is useful, especially if thick clothing prevents the belt from fitting tight to the body.

The belt tensioner in the belt retractor may either be integrated below the seat belt buckle and / or on the anchoring of the strap. As an energy supplier is either a stretched spring or a small explosive charge used.

When used correctly, bring current pretensioners about 2 kN force on the shoulder when the strap already abutted tightly. This results in proper use only in exceptional cases to injuries. In order not to let grow too strong after the tightening of the belt in the Vorverlagerungsphase the occupant forces, the so-called force limiters has been developed.

Process

The airbag control unit will trigger the belt tensioner if it gets on the sensory information about a collision. The triggering threshold may lie below that of the air bag, for example at low speed. Thus, there may be the case that, while triggering the pretensioner, the air bag does not. In modern cars Check sensors in addition, if someone is in the seat and / or the belt is being worn. If this is not the case, remains at the trigger.

There are several modes of operation of belt tensioners:

• A relatively common principle is the so-called Rotary tightening. The control unit triggers the seat belt retractor from a pyrotechnic propellant charge, the gas pressure accelerates steel balls in the retractor. This release their kinetic energy to a gear which is located on the shaft of the retractor. Result: The belt is further rolled up and is thus tightly fitting the passenger. In Gurtschlossausführung the gas pressure of the propellant charge to a flask under the seat is made, which abruptly attracts a steel cable, the buckle rearward. The result is again a tightening of the belt.

• Other functionalities include: rope tensioner, pipe tensioner and tensioner Wankel ( a pump, which is constructed similar to the Wankel engine ). Instead of a pyrotechnic propellant charge may apply the necessary force alternatively a compressed spring.

• A modern, but relatively expensive model is the reversible Gurtstraffer.Dabei an electric drive in addition to the pyrotechnic belt tensioners used, which can tighten the belt with a smaller force ( for example, 100-200 N). The reversible belt tensioners can be used because of its multiple usability even during a Gefahrensitutation without it actually has to come to the accident. Currently ( 2010) are mainly luxury vehicles equipped with reversible belt tensioners, such as the BMW 7 Series, Audi A8 and Mercedes- Benz S-Class.

Disadvantages

A pyrotechnic belt tensioner, as the airbag can be used only once. A triggered belt tensioners is displayed in some car manufacturers by a yellow or red tab on Gurtstraffungsmechanismus.

Special safety rules for disassembly must be observed as with the airbag in the scrapping of a vehicle, so do not accidentally the Gurtstraffungsmechanismus is triggered and this leads to injuries. Also in the patient-friendly rescue, a belt tensioner be problematic because he when cutting through the B-pillar with scissors triggers under certain circumstances and can injure patients or emergency personnel. The B-pillar should be cut as high as possible (above the belt tensioner ).

In principle, the seat belt in the car should be applied as tightly as possible and shall in no twisted. It may cause by the belt in an accident injury.