Purpose and principle of operation of the belt tensioner and limiter

The use of a seat belt is mandatory for every driver and passengers. To make the belt design more efficient and comfortable, the developers have created devices such as a pretensioner and stopper. Each performs its own function, but the purpose of their use is the same - to ensure maximum safety of each person in the passenger compartment of a moving car.

Belt tensioner

The pretensioner (or pretensioner) of the seat belt ensures a secure fixation of the human body on the seat, and in the event of an accident, preventing the driver or passenger from moving forward relative to the movement of the car. This effect is achieved by reeling in and tightening the seat belt.

Many motorists confuse the pretensioner with a conventional retractable coil, which is also part of the seat belt design. However, the tensioner has its own scheme of action.

Due to the actuation of the pretensioner, the maximum movement of the human body upon impact is 1 cm. The response speed of the device is 5 ms (in some devices this indicator can reach 12 ms).

Such a mechanism is installed on both the front and rear seats. Most often, the device is included in the package of more expensive cars. However, sometimes the pretensioner can be seen in the maximum trim levels of economy cars.

Types of devices

Depending on the principle of operation, there are several main types of belt tensioners:

• cable;
• ball;
• rotary;
• rack-and-pinion;
• tape.

Each of them is equipped with a mechanical or automatic drive. The operation of the mechanism, depending on the design, can be carried out autonomously or in the complex of a passive safety system.

Principle of operation

The work of the pretensioner is pretty simple. The principle of operation is based on the following sequence:

• The power wires are attached to the belt, which, in an emergency, activate the igniter.
• If the impact energy is high, the igniter is triggered simultaneously with the airbag.
• After that, the belt is instantly tensioned, providing the most effective fixation of the person.

With this scheme of work, the human chest is subjected to high loads: the body, by inertia, continues to move forward, while the belt is already trying to press it as much as possible against the seat. To reduce the impact of the strong belt strap, designers began to equip cars with seat belt restraints.

Belt stops

During an accident, strong overloads inevitably occur, which affect not only the car, but also the people inside it. In order to reduce the resulting load, belt tension limiters are used.

On impact, the device releases the belt strap, providing the smoothest contact with the deployed airbag. Thus, at first, the tensioners fix the person on the seat as tightly as possible, and then the force limiter slightly weakens the tape to such an extent as to reduce the load on the bones and internal organs of the person.

Types of devices

The most convenient and technically simple way to limit the tension force is a loop-sewn seat belt. Extremely high loads tend to break the seams, which increases the length of the belt. But the reliability of retaining the driver or passengers is preserved.

Also, a torsion limiter can be used in cars. A torsion bar is installed in the seat belt reel. Depending on the applied load, it can twist to a greater or lesser angle, preventing peak impacts.

Even seemingly insignificant devices can increase the safety of people in the car and mitigate injuries sustained in an accident. The simultaneous action of the pretensioner and the restraint in an emergency helps to firmly fix the person on the seat, but not unnecessarily squeeze his chest with a belt.